petrografia

ABSTRACT

The Sacha field is a prolific producer of hydro-carbons from the Cretaceous Hollin and Napo for-mations in the Oriente basin, Ecuador. To under-stand the depositional origin of these reservoirs,we did a detailed sedimentological study using 516 ft(157 m) of conventional core from seven wells. Thisstudy reveals seven lithofacies: (1) cross-beddedsandstone with erosional base (fluvial channels),(2) heterolithic facies with erosive-based, cross-bedded sandstone (tidal channels), (3) heterolithicfacies with cross-bedded sandstone showing full-vortex structures, crinkled laminae, sandy rhyth-mites, and double mud layers (tidal sand bars), (4)heterolithic facies with flaser-bedded sandstone(tidal sand flats), (5) muddy rhythmites with siltylenticular beds and double mud layers (subtidalmud flats), (6) bioturbated glauconitic sandstone(sandy shelves), and (7) bioturbated and laminatedmudstone (muddy shelves).

Based on the presence of mud drapes on bedforms, heterolithic facies, double mud layers, bidi-rectional (i.e., herringbone) cross-bedding, sandyrhythmites, thick-thin alternations of silt and clay lay-ers showing cyclicity (muddy rhythmites), crinkledlaminae, and deepening-upward (i.e., transgressive)

652 AAPG Bulletin, V. 84, No. 5 (May 2000), P. 652–682.

©Copyright 2000. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received September 5, 1997; revised manuscript receivedAugust 31, 1998; final acceptance October 30, 1999.

2Department of Geology, The University of Texas at Arlington, Box19049, Arlington, Texas 76019; e-mail: [email protected]

3Mobil New Exploration and Producing Ventures, P.O. Box 650232,Dallas, Texas 75265-0232.

4Petroproducción, Unit of Research and Laboratories, Quito, Ecuador.We thank Manuel Berumen (Mobil) for assistance during core and

outcrop examination; Joe Hayden (Mobil) for seismic interpretation; JorgeMontenegro and Carlos Huaman (Petroproducción, Quito, Ecuador) fordiscussion; R. J. Moiola, J. B. Wagner, M. Berumen, D. W. Kirkland, and P.L. Kirkland for reviewing an earlier version of the manuscript; J. E. Kruegerfor managerial support; and M. K. Lindsey for drafting. We wish to thankPetroproduccion, Amoco, and Mobil for granting permission to publish thispaper. We thank Bulletin Associate Editor J. A. May for his critical commentsthat considerably improved the manuscript, Bulletin reviewers H. J. White andK. W. Shanley for their helpful reviews, and AAPG Editor N. F. Hurley for hisconstructive comments.

Tide-Dominated Estuarine Facies in the Hollin and Napo (“T” and “U”) Formations (Cretaceous), Sacha Field,Oriente Basin, Ecuador1

G. Shanmugam,2 M. Poffenberger,3 and J. Toro Álava4

successions, we interpret the cored intervals of theHollin and Napo formations to represent tide-dominated estuarine facies. We propose four stagesof deposition for the Hollin Formation (oldest toyoungest) following the regional uplift and erosionof the Misahualli volcanics: (1) the first stage (duringdeposition of the lower Hollin) represents minor flu-vial channels (low-sinuosity streams) and commontide-dominated estuary, (2) the second stage (duringdeposition of the lower and upper Hollin) repre-sents a well-developed tide-dominated estuary, (3)the third stage (during deposition of the upperHollin) represents drowning of a tide-dominatedestuary, and (4) the final stage (during deposition ofthe upper Hollin) represents well-developed shelfenvironments in the Sacha field area. During Napo“T” and “U” deposition, stages two, three, and fourwere repeated.

Previous interpretations that the Hollin andNapo formations represent fluvio-deltaic environmentsare not supported by this study. A tide-dominated estu-arine setting is proposed instead. An importantaspect of our work is that tidal sand bars interpret-ed in the Sacha area are predicted to trend east-west, paralleling the direction of sediment trans-port. In contrast, the conventional fluvio-deltaicmodel would predict north-south–trending dis-tributary mouth bars with an easterly sedimentsource. Outcrop, core, seismic, or well data do notcorroborate an incised valley-fill model that wasapplied to the Hollin and Napo formations by otherworkers. Estuarine facies are quite complex, as thisstudy shows, and may not always fit into a generalincised valley-fill model.

INTRODUCTION

The Sacha oil field of the Oriente basin is locatedabout 180 km east of the capitol city of Quito,Ecuador (Figure 1). Texaco discovered the field inFebruary 1969 and it went on production in July1972 (Canfield et al., 1982). Through 1995 theSacha field had produced over 530 million barrels

of oil. To enhance further oil production, it isimportant to gain a clear understanding of thereservoir in terms of its depositional origin.

The primary purpose of this study was to devel-op a viable sedimentological model to predict thedistribution of the Cretaceous Hollin and Naporeservoirs in the Sacha field. Our objectives wereto (1) describe cores and interpret depositionalprocesses, (2) calibrate depositional facies withwireline logs, (3) establish sand-body geometriesusing stratigraphic correlations of well logs andseismic data, and (4) develop a depositionalmodel by integrating core, outcrop, log, and seis-mic data.

The principal reservoir, the Lower CretaceousHollin Formation (Figure 2), traditionally has beenconsidered as braided fluvial deposits with sheet-like geometries (Canfield et al., 1982; White et al.1995). Macellari (1988) proposed a fluvio-deltaicenvironment for the Hollin Formation. White et al.(1995) interpreted the overlying Upper CretaceousNapo Formation as f luvio-deltaic deposits in anincised valley-fill setting. Our study, based on con-ventional cores from the Sacha field area, showsthat tidal processes were much more importantthan fluvio-deltaic processes in depositing sands ofboth the Hollin and Napo formations. A possiblereason for this difference in interpretation is that

Shanmugam et al. 653

Figure 1—Location maps (two inset maps) showing structural features and distribution of producing fields (blackpatches) in the Oriente basin, Ecuador (compiled from Canfield et al., 1982; Dashwood and Abbotts, 1990; White etal., 1995), and the outline of the Sacha field showing line of a north-south well-log cross section (Figure 26), posi-tion of an east-west seismic profile (Figure 28), and cored wells used in this study.

we were able to document the occurrence of somekey sedimentary features, such as double mud lay-ers (also known as mud couplets), tidal rhythmites,full-vortex structures, and crinkled laminae. Be-cause some of these features are subtle and difficultto observe at core scale, they might be overlookedor even mistaken for something else (e.g., crinkledlaminae might be misidentified as stylolites). Incross-bedded sandstone, failure to recognize thesetidal features in cores can result in misinterpretingthe sandstone as possible braided fluvial deposits.Kuecher et al. (1990) reported cases in which sand-stones of tidal origin have been misinterpreted asdeposits of fluvial origin in the United States. Ourobservations and interpretations have importantimplications for developing alternative sedimento-logic models for the Hollin and Napo formationswith different orientations of sand bodies(Shanmugam et al., 1998).

GEOLOGIC SETTING

The Oriente basin, which covers about 100,000km2, lies between the Andes on the west and theGuyana shield on the east (Figure 1). The basinextends northward into the Putamayo basin inColombia, and southward into the Maranon basin inPeru (Figure 1). These basins are part of the sub-Andean foreland zone that stretches from Venezuelato southern Chile (Gansser, 1973).

The Sacha field is a large, very low relief structurethat lies in the axial region of the Oriente basin. Theproducing structures are north-south–trending anti-clines, usually faulted on one flank (Canfield et al.,1982). Oil accumulations in the Oriente basin arefound in the Cretaceous sandstones of the Hollinand Napo formations. Local stratigraphic subdivi-sions of the Hollin and Napo formations are shownin the type log from the Sacha 130 well (Figure 3).

654 Oriente Basin, Ecuador

Figure 2—Generalizedstratigraphic column, Oriente basin, Ecuador(from Smith, 1989).

Sandstones of the Hollin and Napo formations arebelieved to be derived from the east, perhaps fromtwo intrabasinal highs (Figure 1), the Aguarico archto the north and the Cononaco arch to the south(White et al., 1995).

Oil accumulations in the Hollin Formation arestructurally controlled, whereas oil accumulationsin the Napo Formation are both structurally andstratigraphically controlled (Canfield et al., 1982).The source rocks for these reservoirs are consideredto be organic-rich shales of the Napo Formation(Dashwood and Abbotts, 1990). Biological markerdata of the oils show very good correlation with bio-logical marker data of organic extracts from theNapo Formation (Mello et al., 1995). Geochemicalanalyses indicate that the oil migrated into thesestructures from Cretaceous source rocks in the east-ern Cordillera and southernmost Oriente basin(Dashwood and Abbotts, 1990). The oil is trapped instructures of Cretaceous–Oligocene age (Canfield etal., 1982).

CORE STUDY

We described 516 ft (157m) of conventional corefrom the Sacha field (Figure 1, Table 1). All coredwells are straight holes. The cored intervals are com-posed of consolidated fine-grained sandstone andmudstone. Cores were examined for (Figure 4) (1)bedding contacts, (2) bed-thickness variations, (3)grain-size variations, (4) lithologic variations, (5) pri-mary physical sedimentary structures, (6) biologicalsedimentary structures, (7) syndepositional andpostdepositional sedimentary structures, and (8) oilstaining. Core depths are measured depths in feet.Seven lithofacies are described in the cored inter-vals, and each type is interpreted to represent a spe-cific depositional facies (Tables 2–4).

Cross-Bedded Sandstone with Erosional Bases(Fluvial Channel)

DescriptionCross-bedded sandstone with erosional bases is

present in the lower Hollin (well SA 133) and in theupper Hollin (well SA 130) intervals but is absent inthe Napo “T” and “U” intervals. This facies is com-posed of dark gray (oil-stained), fine- to medium-grained sandstone. Sand grains are moderately wellsorted and subangular to subrounded. Depositionalmatrix is generally low because sand usually com-prises 100% of this facies. The most diagnostic fea-ture is cross-stratification. Planar cross-stratificationis common, with dips of cross-beds ranging from 10to 20°. In the lower Hollin, the basal part of this

Figure 3—Type log and lithostratigraphy from the Sacha130 well showing cored (hachured) intervals.

facies contains quartz granules about 3 mm in size.Rare mudstone clasts are also present. The cross-bedded sandstone is interbedded with very finegrained sandstone with mud layers (i.e., muddrapes) in the lower Hollin (well SA 133). Thisfacies has a thickness of up to 5 m in core andexhibits a “blocky” motif in wireline logs (Figure 3).

InterpretationOn the basis of cross-stratification and basal lags,

this facies is interpreted as high-energy fluvial chan-nels with traction structures. A lack of interbeddedfine-grained (levee) facies suggests low-sinuositystreams. The interbedded sandstone units withabundant mud drapes indicate some tidal inf lu-ence. The vertical gradation of these fluvial chan-nels (e.g., 9930.5 ft, 3028.8 m, well SA 133) intotidal channels likely indicates a transgressivecoastal plain setting.

Heterolithic Facies with Erosive-Based Cross-Bedded Sandstone (Tidal Channel)

DescriptionA heterolithic facies with erosive-based, cross-

bedded sandstone is present in the lower Hollin

(well SA 133), upper Hollin (well SA 122), andNapo “U” (well SA 119) intervals. This facies iscomposed of light brown (oil-stained), mud-draped, cross-bedded, fine-grained sandstone.Cross-beds dip up to 18°. Some intervals showbidirectional cross-beds. Sand grains are poorlysorted and subrounded. Depositional matrix islow to moderate because sand comprises90–100% of this facies. This facies is 1.5 m thick inthe SA 122 well and shows a fining-upward trendwith a basal erosional surface and a basal lag com-posed of carbonaceous and mudstone clasts.Carbonaceous fragments are common throughoutthis facies.

InterpretationCross-beds, erosional bases, basal lags, and fining-

upward trends provide evidence for channel depo-sition. Bidirectional cross-beds and foresets withmud drapes indicate deposition in tidal channels.Elliott (1986) considered cross-bedded sandstonewith an erosional base, basal lags, and fining-upward trends in association with heterolithicfacies and flaser bedding to represent estuarine tidalchannels. Shanley et al. (1992) interpreted cross-beds with mud drapes as tidally influenced fluvialstrata. In this study, the main difference between


Table 1. Cored Wells Used in This Study

Interval Thickness Total ThicknessFormation Well* (ft) (m) (ft) (m) (ft) (m)

Napo “U” SA 119 9450–9510 2882–2900 60 18Napo “U” SA 126 9425–9455 2874–2883 30 9Napo “U” SA 129 9577–9608 2920–2930 31 9.4Napo “U” SA 132 9393–9454 2864–2883 61 18.6Total Napo “U” Thickness 182 (35.3%) 55

Napo “T” SA 126 9652–9682 2943–2953 30 9Napo “T” SA 129 9768–9795 2979–2987 27 8.2Napo “T” SA 130 9650–9675 2943–2950 25 7.6Napo “T” SA 133 9685–9715 2953–2963 30 9Total Napo “T” Thickness 112 (21.7%) 34

Upper Hollin SA 122 9831–9886 2998–3015 55 16Upper Hollin SA 126 9825–9885 2996–3014 60** 18Upper Hollin SA 129 9935–9953 3030–3035 18 5Upper Hollin SA 129 9965–9993 3039–3047 28 8.5Upper Hollin SA 130 9870–9901 3010–3019 31 9.4Total Upper Hollin Thickness 192 (37.2%) 58

Lower Hollin SA 133 9910–9940 3022–3031 30 9Total Lower Hollin Thickness 30 (5.8%) 9

Total Thickness All Formations 516 (100%) 157

*SA = Sacha.**2 ft (0.6 m) of core is missing in the core box.

tidal and f luvial channels is that tidal channelsexhibit cross-beds with mud drapes, whereas cross-beds in fluvial channels do not typically show muddrapes.

Heterolithic Facies with Cross-BeddedSandstone, Full-Vortex Structures,Rhythmites, and Double Mud Layers (Tidal Sand Bar)

DescriptionA heterolithic facies with cross-bedded sand-

stone, full-vortex structures, rhythmites, and dou-ble mud layers is common in the upper Hollin(wells SA 122, SA 126, and SA 130), Napo “T”(wells SA 126 and SA 133), and Napo “U” (wellsSA 126, SA 129, and SA 132) intervals. This faciesis composed of light brown to dark brown (oil-stained), fine- to medium-grained, cross-beddedsandstone with abundant mud drapes (3–4 mm to8 cm in thickness) (Figure 5). Sand layers com-monly vary in thickness from 3 mm to 3 cm. Sandgrains are moderately to poorly sorted and sub-rounded. Depositional matrix is generally lowwith visual estimates of sand near 100%. Mud

drapes are ubiquitous, resulting in a heterolithicfacies (Figures 5, 6). Rhythmic alternation of thesandstone and mudstone layers (i.e., sandy rhyth-mites) is a diagnostic feature. Thick-thin alterna-tions of successive sand layers (i.e., bundles) arewell developed in some intervals. Double mud lay-ers are common (Figures 5, 6). Mud offshoots(i.e., top-truncated drapes) in ripples also arecommon.

Dips of cross-beds range from 15 to 36°. In rarecases, bidirectional (i.e., herringbone) cross-bedding is present. Some cross-bedded units dip22° (well SA 126) and show normal grading alongforesets. Graded beds are 2–3 cm in thickness.Internal truncation surfaces (i.e., reactivation sur-faces) generally dip at lower angles than dips ofassociated cross-beds (Figure 7). Some cross-bedsshow mud-draped tangential toesets and fanning(i.e., thickening) of the foresets (full-vortex struc-tures) (Figure 8A). Crinkled laminae are common,and they are conformable to ripple bed forms(Figure 8B). Small carbonaceous mudstone clastsare present in some intervals (well SA 126, 9859 ft,3006.9 m). Individual sandstone beds range inthickness from 5 cm to 2 m. Amalgamated units are5–10 m thick (e.g., Figure 9).


Figure 4—Symbols used in sedimentological logs (for Figures 9, 15, 21, and 22).

InterpretationThick-thin alternations of successive sand layers

or bundles reflect (semi-) diurnal tidal inequality(de Boer et al., 1989). Mud drapes along pauseplanes, reactivation surfaces, and crinkled laminaelikely indicate slack-water periods (Terwindt, 1981;Banerjee, 1989). During periods of higher energy,current activity maintained ripples and cross-beds(i.e., avalanching phase), whereas during periodsof low energy, mud was deposited from suspen-sion. Mud offshoots in the rippled sands representlow-angle foresets caused by settling of mud from

suspension (i.e., nonavalanching phase) on the leeside of ripples. Double mud layers have beenascribed to alternating ebb and flood tidal currentswith extreme time-velocity asymmetry in subtidalsettings (Visser, 1980). The thick sand units likelyreflect deposition during dominant tides, whereasthe thin sand units are probably products of subor-dinate tides.

The crinkled laminae tend to mimic stylolites; how-ever, they are not stylolites. When a section that istransverse to the megaripple foreset passes through thecrest line of the ripple trains, the mud-draped ripples


Table 2. Depositional Facies in the Hollin Formation as a Percentage of Cored Interval*

Lower Hollin Upper HollinWell SA 133 Well SA 122 Well SA 126 Well SA 129 Well SA 130

Facies (30 ft, 9 m) (55 ft, 16 m) (58 ft, 17 m) (46 ft, 14 m) (31 ft, 9.4 m)

1. Fluvial Channel (%) 67 – – – 392. Tidal Channel (%) 13 9 – – –3. Tidal Sand Bar (%) 13 49 21 2 614. Tidal Sand Flat (%) 7 29 38** – –5. Tidal Mud Flat (%) – 13 17 – –6. Shelf Sand (%) – – 21 87 –7. Shelf Mud (%) – – 3 11 –

* In well names, SA = Sacha.**Contains less than 1% marsh facies.

Table 3. Depositional Facies in the Napo “T” as a Percentage of Cored Interval*

Well SA 126 Well SA 129 Well SA 130 Well SA 133Facies (30 ft, 9 m) (27 ft, 8.2 m) (25 ft, 7.6 m) (30 ft, 9 m)

1. Fluvial Channel (%) – – – –2. Tidal Channel (%) – – – –3. Tidal Sand Bar (%) 23 – 12 174. Tidal Sand Flat (%) 40 – 36 405. Tidal Mud Flat (%) 37 – 52 106. Shelf Sand (%) – 67 – 337. Shelf Mud (%) – 33 – –

*In well names, SA = Sacha.

Table 4. Depositional Facies in the Napo “U” as a Percentage of Cored Interval*

Well SA 119 Well SA 126 Well SA 129 Well SA 132Facies (60 ft, 18 m) (30 ft, 9 m) (31 ft, 9.4 m) (61 ft, 18.6 m)

1. Fluvial Channel (%) – – – –2. Tidal Channel (%) 5 – – –3. Tidal Sand Bar (%) – 33 22 534. Tidal Sand Flat (%) – 67 39 235. Tidal Mud Flat (%) – Trace 10 166. Shelf Sand (%) – – – –7. Shelf Mud (%) 95 – 29 8

*In well names, SA = Sacha.

appear as a series of small-amplitude crinklets(Terwindt, 1981; Banerjee, 1989). Crinkled laminaesuch as these are conformable to ripple bed forms intidal facies, but stylolites are not conformable to bed-form surfaces.

The general absence of burrows in this faciessuggests that the rate of sedimentation was highand therefore hostile to infaunal burrowers.Rhythmic alternation of sand and mud layers pro-vides evidence for tidal deposition (i.e., sandy tidalrhythmites). Other features of the inclined het-erolithic facies are analogous to those described fortidal sand bars (e.g., Dalrymple et al., 1992). Thepresence of tangential basal contacts, steeply dip-ping foresets (up to 36°), and fanning of the fore-sets may be equivalent to the full-vortex part oftidal bundles described by Terwindt (1981) for

mesotidal deposits of the North Sea (Figure 10).Tidal bundles represent a lateral succession ofcross-strata deposited in one event by the dominanttide (Terwindt, 1981). In the upper Hollin, small-scale tidal bundles are recognized, which may becomparable with sigmoidal tidal bundles describedby Mutti et al. (1985). We interpret the full-vortexstructures to be products of migrating megaripples,which are common in tide-dominated estuaries(Nio and Yang, 1991; Harris, 1988). Reactivationsurfaces similar to those in the cross-bedded sand-stones also have been reported in tidal sand bars(Klein, 1970). In the Hollin Formation, the occur-rence of tidal sand bars above fluvial channels sug-gests a transgressive phase of deposition (Figure 9).


Figure 5—Core photograph of heterolithic facies show-ing cross-bedded sandstone with double mud layers(arrow). Note rhythmic alternation of thick and thinsand layers. Each mud layer represents a period ofslack-water deposition. Tidal cyclicity is poorly devel-oped because of merging of mud layers (black). Tidalsand bar facies. Upper Hollin, 9871.5 ft (3010.8 m),Sacha 130 well.

Figure 6—Core photographs of fine-grained sandstoneshowing horizontal stratification with double mud lay-ers (arrow). Sand layers range in thickness from 3 mmto 1 cm. Upper Hollin, 9846 ft (3003.0 m), Sacha 122 well.

Heterolithic Facies with Flaser-BeddedSandstone and Rhythmites (Tidal Sand Flat)

DescriptionHeterolithic facies with flaser-bedded sandstone

and rhythmites is rare in the lower Hollin (well SA133) but common in the upper Hollin (wells SA122 and SA 126), Napo “T” (wells SA 126, SA 130,and SA 133), and Napo “U” (wells SA 126, SA 129,and SA 132) intervals. This facies is composed oflight gray, very fine grained, ripple-bedded sand-stone with abundant mud drapes (2–3 mm to 1 cmthick). Sands are poorly sorted and subrounded.Depositional matrix varies from moderate to highbecause sand comprises 50–100% of this facies.Flaser bedding is diagnostic of this facies (Figure11). Rhythmic bedding (rhythmites) of sand andmud layers is common (Figure 12). Double mud lay-ers (Figure 13), wavy bedding (Figure 13), andlenticular bedding are also common. Reddishbrown elongate mudstone clasts (siderite?) are alsopresent (Figure 14A). Dimensions of clasts are upto 7 cm long and 1.5 cm thick. Carbonaceous, glau-conitic, and micaceous fragments are dispersedthroughout. In some intervals, there is a concentra-tion of carbonaceous fragments and plant resins(Figure 15). These resin (i.e., amber) particles aregolden yellow in color and vary in size from a fewmillimeters to a centimeter.

Crinkled laminae are also associated with thisfacies and are common in some upper Hollin inter-vals (Figure 14B). Merging of crinkled laminae ispresent (Figure 14B). Individual sandstone bedsrange in thickness from 3 to 35 cm (well SA 126,upper Hollin). Amalgamated units show a thicknessof up to 5 m (well SA 126, Napo “U”).

Bioturbation is common, and some intervals inthe Napo “T” contain Rhyzocorallium (well SA 133,9690 ft, 2955.4 m; well SA 126, 9660 ft, 2946.3 m)and Ophiomorpha trace fossils (well SA 133, 9698ft, 2957.8 m). Napo “U” cores exhibit Skolithos (wellSA 132, 9411 ft, 2870.3 m) and Ophiomorpha tracefossils (well SA 132, 9418 ft, 2872.4 m). In somecases, this lithofacies grades vertically into bioturbat-ed glauconitic sandstone (i.e., sandy shelf).

InterpretationThe common occurrence of double mud layers

indicates a subtidal environment (Visser, 1980). Theelongate mudstone clasts in this facies may have orig-inally been emplaced as double mud layers, whichwere later broken up by tidal currents. Elongate mud-stone clasts have been reported from tidal sandsheets (Banerjee, 1989). Flaser bedding, wavy bed-ding, and lenticular bedding are also evidence of atidal-f lat environment (Reineck and Wunderlich,1968). We interpret this facies to be a tidal sand flat.

Associated intervals of concentrated carbonaceousfragments with resin particles may be interpreted asa marsh environment (Figure 15); however, we givelittle importance to marsh facies because it compris-es less than 1% of all cored intervals. Also, evidenceof rooting is lacking. Conceivably, these carbona-ceous fragments were transported onto the tidal flat.

Mudstone with Lenticular Bedding andRhythmites (Subtidal Mud Flat)

DescriptionMudstone with lenticular bedding and rhythmites

is present in the upper Hollin (wells SA 122 and SA126), Napo “T” (wells SA 126, SA 130, and SA 133),and Napo “U” (wells SA 126, SA 129, and SA 132)intervals. This facies is composed of medium to darkgray, silty mudstone. Lenticular bedding caused bystarved ripples of silt are common (Figure 16).


Figure 7—Core photograph showing sandstone withmud-draped reactivation surface (arrow). Note steeplydipping cross-stratification below reactivation surface.Tidal sand bar facies. Upper Hollin, 9887 ft (3015.5 m),Sacha 130 well.

Mud-draped silty ripples and double mud layers,composed of clay laminae 2–3 mm thick, are pres-ent (Figure 16). Thick-thin alternations of silt andclay layers show cyclicity (Figure 17); they arecalled “rhythmites.”

Carbonaceous fragments are common, andsiderite layers and pyrite nodules are rare. Thisfacies ranges in thickness from several cm to 3 mand is commonly associated with tidal channel andtidal sand bar facies.

InterpretationLenticular bedding is common in tidal-flat envi-

ronments (Reineck and Wunderlich, 1968), as well

as other environments. Because of the presence ofdouble mud layers and rhythmites, this facies isinterpreted as a subtidal mud flat (Nio and Yang,1991). Thick-thin alternations of silt and clay layersshowing cyclicity have been interpreted to repre-sent tidal influence on inner estuarine sediments(Kuecher et al., 1990). The silt layers represent trac-tion deposition from ebb and flood tides, whereasthe clay layers represent deposition from suspensionduring slack-water periods. The thin layers are inter-preted to be deposits of neap tides and the thick lay-ers to be deposits of spring tides. The absence ofburrows in this facies suggests that either the rate ofsedimentation was too high or the salinity was toolow to support burrowing organisms. We envision a


Figure 8—(A) Core photograph showing cross-bedded fine-grained sandstone with tangential lower contacts andmud-draped toesets. Note fanning of the foreset or full-vortex structure (i.e., double-headed arrow). Also note thelower bounding surface with wavy mud drapes. Tidal sand bar facies. Upper Hollin, 9880 ft (3013.4 m), Sacha 130well. (B) Core photograph showing fine-grained sandstone with multiple crinkled laminae composed of mud layers(horizontal arrow). Note crinkled laminae are conformable to ripple bed forms (vertical arrow). Tidal sand barfacies. Lower Hollin, 9932 ft (3029.2 m), Sacha 133 well.

relatively level area of mud (silt and clay) accumula-tion along the margins of an estuary. The marginalarea, which we call a subtidal mud flat, was likelycovered by shallow water.

Bioturbated Glauconitic Sandstone (Sandy Shelf)

DescriptionBioturbated glauconitic sandstone is present in the

upper Hollin (wells SA 126 and SA 129) and Napo “T”(wells SA 129 and SA 133) intervals (Figure 18). Thisfacies is composed of greenish gray to light brown(oil-stained), very fine to fine-grained sandstone. Sand

grains are moderately to poorly sorted and subround-ed. Some intervals are argillaceous, with high deposi-tional matrix (>10%). Sand comprises 80–100% ofthis facies. Bioturbation is ubiquitous (Figure 18).Glauconite content is up to 40% in the Napo “T” (wellSA 133, 9688 ft, 2954.8 m). We observed faint planarcross-stratification in the upper Hollin (well SA 129,9951 ft, 3035 m). Mudstone clasts (4 cm) and mudlayers are also present. Calcareous pelecypod frag-ments and Rhyzocorallium and Ophiomorpha tracefossils are evident. Pyrite and siderite nodules are rare.Individual depositional units are difficult to recognizebecause of bed destruction by bioturbation. Thisfacies reaches thicknesses of 6 m or more due toamalgamation.


Figure 9—Sedimentologicallog of core from the Sacha130 well showing tidalsand bar facies overlyingfluvial channel facies,indicative of a transgressivephase. Lower to upperHollin. Note that thesecored facies show blockylog motif (see Figure 3).See Figure 4 for explanation of symbols.

InterpretationThe glauconite in these cores is interpreted to be

mostly in situ in origin. Glauconitic deposits arewidespread on present-day continental shelves andslopes at water depths from 50 to 500 m (Odin,1985). On the basis of the abundance of glauconiteand extensive bioturbation, we interpret this facies tobe a sandy shelf environment. The term “shelf” isdefined here as an open, shallow-marine setting.Many intervals of the Napo Formation contain trans-ported glauconite that has been attributed to accre-tional origin by which inorganic bodies grow largerby the addition of fresh particles to the outside (Lopezand Vera, 1992). There is, however, no evidence ofwave processes in this facies, perhaps because of bio-turbation or deposition below wave base.

Bioturbated and Laminated Mudstone (Muddy Shelf)

DescriptionBioturbated and laminated mudstone is present

in the upper Hollin (wells SA 126 and SA 129),Napo “T” (well SA 129), and Napo “U” (wells SA119, SA 129, and SA 132) intervals. This facies iscomposed of dark gray, silty mudstone. Thin inter-vals of skeletal wackestone, composed of pelecy-pods, are present in the Napo “U” (Figure 19). Fainthorizontal laminae and lenticular silt layers are alsopresent. We observed rare synaeresis cracks in theupper Hollin (well SA 129, 9988 ft, 3046.3 m).

Burrows and Teichichnus are present in this facies.Scattered carbonaceous fragments and pyrite nod-ules are also present. Some intervals show fissility.Approximately 15 m of this facies occurs in the Napo“U” interval.

InterpretationWe interpret this facies to represent a muddy shelf

environment. Pelecypod fragments appear to haveundergone minor transport in the shelf environment.

OUTCROP STUDY

Cross-Bedded Sandstone with RhythmicBedding and Double Mud Layers (Fluvial toTidal Channels)

DescriptionThe basal Hollin Formation is exposed at Hollin

Loreto Coca Road, located nearly 70 km southwestof the Sacha field (Figure 1). At this location, theHollin Formation is separated from the underlyingMisahualli volcanics by a well-developed angularunconformity (Figure 20). Oil seeps are extensive.The basal Hollin is composed of reddish brown, peb-bly sandstone with a matrix ranging in size fromcoarse to fine grained and in sorting from medium topoor. Quartz pebbles are up to 7 mm in size. Planarcross-stratification, trough cross-stratification, andhorizontal stratification are common (Figure 21).


Figure 10—A model fortidal bundles. The term“mud couplet” refers todouble mud layers. Corephotographs (e.g. Figure8A) in this paper may becompared with the probable view in core outlined by the threeboxes. Simplified from Terwindt (1981) and Banerjee (1989).

Tangential lower contacts of cross-beds are evident.Mud drapes, double mud layers, and rhythmic bed-ding are present. Some siltstone intervals containquartz granules and Planolites. Carbonaceous andresin (i.e., amber) fragments are common.Carbonaceous mudstone clasts are up to 80 cmlong and 10 cm thick. There are no basal pebblelags at this locality.

InterpretationThe measured interval of the Hollin Formation is

interpreted to represent f luvial- to tidal-channelfacies. This portion of the outcrop is somewhatanalogous to the cored interval of the lower Hollinin the SA 130 well (Figure 9). De Souza Cruz (1989)also studied this outcrop. There are both similari-ties and differences in interpretations between DeSouza Cruz (1989) and our study (Table 5):

(1) De Souza Cruz (1989) and this study agreewith the upper Hollin being interpreted as a tidalbar facies.


Figure 11—Core photograph of very fine grained sand-stone showing flaser bedding (arrow). Tidal sand flatfacies. Napo “U,” 9408 ft (2869.4 m), Sacha 132 well.

Figure 12—Core photograph showing rhythmic alterna-tion of sandstone and mudstone (arrows) units. Notemud-draped ripples (flaser) in sandstone. Tidal sand flatfacies. Napo “U,” 9416.5 ft (2872.0 m), Sacha 132 well.

(2) There is also agreement that the NapoFormation represents deposition on a transgressiveshelf.

(3) De Souza Cruz (1989) identified eolian faciesin the lower Hollin; however, we did not recognizeeolian facies in the basal part of the outcrop.

(4) Unlike the study by De Souza Cruz (1989),we recognize tide-dominated estuarine faciesthroughout the Hollin and Napo formations.


Figure 13—Core photograph of very fine grained sand-stone showing wavy bedding. Note rhythmic alternationof mudstone (horizontal arrows) and sandstone. Alsonote double mud layers (vertical arrow). Tidal sand flatfacies. Napo “T,” 9663.5 ft (2947.3 m), Sacha 126 well.

Figure 14—(A) Core photograph showing fine-grained sandstone with elongate mudstone (sideritic?) clasts (arrow).Tidal sand-flat facies. Upper Hollin, 9841.5 ft (3001.6 m), Sacha 126 well. (B) Core photograph showing fine-grainedsandstone with crinkled laminae (arrows). Note merging of crinkled laminae in the middle. Tidal sand flat facies,upper Hollin, 9870 ft (3010.3 m), Sacha 126 well.

DEPOSITIONAL ENVIRONMENTS ANDMODELS

Evidence for Tidal Processes

Tidal processes and related depositional featureshave been discussed by many workers (e.g., Klein,1970; Visser, 1980; Terwindt, 1981; Banerjee, 1989;Nio and Yang, 1991). Sedimentary features indica-tive of tidal processes in the Hollin and Napo for-mations include the following (see Table 6): (1)heterolithic facies, (2) rhythmic alternation of sand-stone-shale couplets (sandy tidal rhythmites), (3)thick-thin alternations of silt and clay layers showing

cyclicity (muddy tidal rhythmites), (4) double mudlayers, (5) cross-beds with mud-draped foresets, (6)bidirectional (herringbone) cross-bedding, (7) reacti-vation surfaces, (8) crinkled laminae, (9) elongatemudstone clasts, (10) full-vortex structures, (11)flaser bedding, (12) wavy bedding, and (13) lentic-ular bedding.

Diurnal inequality and tidal cyclicity are consid-ered to be diagnostic properties of clastic tidaldeposits (de Boer et al., 1989; Kuecher et al., 1990;Nio and Young, 1991). Most areas of the Earthexperience semidiurnal (i.e., two tides per day)periodicity (de Boer et al., 1989). One key elementof the tidal system is that alternating high and low


Figure 15—Sedimentologicallog of core from the Sacha 126 well, upperHollin. See Figure 4 forexplanation of symbols.

peak current velocities are represented by alternat-ing thick and thin sand layers or bundles, respec-tively. This alternation of thick and thin sand bun-dles ref lects alternating ebb and f lood episodesknown as diurnal inequality (de Boer et al., 1989).Thick-thin alternations of sand bundles, which areunique to the tidal regime (de Boer et al., 1989),are evident in the upper Hollin (Figure 5).

In addition to diurnal inequality, clastic tidaldeposits also exhibit cyclicity (de Boer et al., 1989).Tidal units tend to thicken progressively to a maxi-mum (spring tide), then thin to a minimum (neaptide), and then thicken to a next maximum (springtide), resulting in a complete cycle every 14 days;therefore, sediments deposited over a period of 14days should ideally be composed of 28 sand bun-dles in a semidiurnal tidal regime (i.e., two tides aday), or 14 sand bundles in a diurnal tidal regime(i.e., one tide a day). In some cases, sand deposi-tion may not occur during neap tides, resulting in aless than ideal number of bundles due to mergingof clay layers.

In the upper Hollin there is evidence for tidalcyclicity in some of the mudstones (Figure 17). Thethick silt-rich and thin clay-rich mud layers areinterpreted to be products of spring and neap

tides, respectively. Although the exact number ofmud layers is difficult to count because of merging,the deposition of mudstone probably took placeunder a semidiurnal regime. This observation isbased on the thick-thin alternations of sand bun-dles, typical of semidiurnal regime, observed in theupper Hollin sandstone (Figure 5).

Fluvial vs. Estuarine vs. Deltaic Environments

Tidal features that we documented in this studycan be interpreted to occur in more than one set-ting (e.g., tide-dominated deltas, bayhead deltas,tide-dominated estuaries). In arriving at a reason-able interpretation of depositional setting for theHollin and Napo formations, we considered the


Figure 16—Core photograph showing mudstone withlenticular bedding (horizontal arrow). Note double mudlayers near the top (vertical arrow). Tidal mud flatfacies. Napo “T,” 9665 ft (2947.8 m), Sacha 126 well. Figure 17—Core photograph of mudstone showing

alternating silt and clay layers exhibiting thick-thincyclicity. This pattern may indicate tidal rhythmites.The silt layers are interpreted to represent tractiondeposition from ebb and flood tides, whereas the claylayers are interpreted to represent slack-water deposi-tion. The thin silt layers are interpreted to be deposits ofneap tides, and the thick silt layers are interpreted to bedeposits of spring tides. Subtidal mud flat facies. UpperHollin, 9882 ft (3014.0 m), Sacha 122 well.

three common environments (i.e., fluvial, estuar-ine, and deltaic).

Earlier workers suggested a fluvial environmentfor the Hollin Formation and coastal plain environ-ments with bayhead deltas for the Napo Formation inthe Oriente basin (e.g., White et al., 1995). Macellari(1988) proposed a river-dominated deltaic environ-ment for the Hollin Formation. Core data from thisstudy, however, provide evidence that both theHollin and Napo formations in the Sacha field areawere deposited in a tide-dominated estuarine envi-ronment (Table 7). A possible reason for this differ-ence in interpretation is that we were able to docu-ment some key sedimentary features, such as doublemud layers (also known as mud couplets), crinkledlaminae, and full-vortex structures in cross-bedded

sandstone units. Even in the lowermost Hollinexposed at the Hollin Loreto Coca Road, there is evi-dence for tidal influence. Thus, tidal environmentspersisted throughout the deposition of the Hollinand Napo formations in the Sacha area.

The distinction between estuarine and fluvio-deltaic environments has important implicationsfor petroleum geology, including the distribution ofsand bodies. Estuarine tidal sand bars are alignedparallel to depositional dip, whereas delta-frontsands are typically aligned along strike. This differen-tial distribution of sand bodies is critical in mappingsubsurface trends. To help distinguish between theestuarine and fluvio-deltaic interpretations, charac-teristics of these two environments are summarizedin the following paragraph.

Characteristics of an estuary typically include thefollowing (modified after Dalrymple et al., 1992):

• Represents the seaward portion of a drownedvalley system

• Receives sediment from both f luvial andmarine sources

• May contain tidal, wave, and fluvial facies• May represent bidirectional sediment trans-

port (i.e., seaward and landward)• Can exist only during rising sea level (i.e.,

transgressive)• Commonly exhibits deepening-upward

successions• Fills during falling or stable sea level• Can become sites of river-dominated deltas

only after the estuaries get filled completely


Figure 19—Core photograph showing wackestone withcalcareous shell (pelecypod) fragments. Muddy shelffacies. Napo “U,” 9459 ft (2884.9 m), Sacha 119 well.

Figure 18—Core photograph showing bioturbated glau-conitic sandstone with an Ophiomorpha trace fossil.Sandy shelf facies. Upper Hollin, 9984 ft (3045.1 m),Sacha 129 well.

Characteristics of a river-dominated delta includethe following (modified after Dalrymple et al.,1992; Boyd et al., 1992):

• Represents seaward protrusion of the coast-line of fluvial origin

• Receives sediment from both f luvial andmarine sources

• Contains fluvial (dominant), wave, and tidalfacies

• Represents unidirectional sediment transport(i.e., seaward)

• Can exist only when sediment supply exceedssea level rise

• Exhibits a progradational trend

To form river-dominated deltas in our study area,the estuary had to have been filled completely withsediment prior to deltaic progradation. Filling of anestuary is normally indicated by shallowing-upwardsuccessions, commonly capped by fluvial or marshfacies; however, there is no evidence for shallowing-upward successions and abandonment in the coredintervals of the Sacha field. In fact, fluvial facies areextremely rare in the cored intervals of the upperHollin interval and absent in the cored intervals ofthe Napo “T” and Napo “U” intervals. Except for thefluvial facies recognized in the lower Hollin, there is

no evidence of progradation. The vertical distribu-tion of facies in the lower Hollin (well SA 133), upperHollin (well SA 130), Napo “T” (well SA 133), andNapo “U” (wells SA 129 and SA 132) intervals showsa deepening-upward trend, suggesting transgressivedeposition (Figures 22, 23); therefore, the deltaicprogradation and fill model is not a viable model.

Another possibility is that the Hollin and Napo for-mations may represent tide-dominated deltas.According to Galloway (1975), who introduced theconcept, tide-dominated deltas represent mainlyestuarine settings. As has been mentioned, estuarineand deltaic environments differ from one another.There is a question as to whether tide-dominateddeltas are true deltas (i.e., progradational systems).Walker (1992) even advocated abandoning the con-cept of tide-dominated deltas. In light of these prob-lems, we do not consider tide-dominated deltas as aviable depositional setting.

Proposed Tide-Dominated Estuary Model

Although the common perception is that all estu-aries are tide dominated, Dalrymple et al. (1992)and Zaitlin et al. (1994a, b), using physical process-es and facies, made a formal distinction of estuaries


Figure 20—Outcrop photograph showing an angular unconformity (arrow and dashed line) between the basal con-tact of the Hollin Formation and the underlying Misahualli volcanics. Hollin Loreto Coca Road (see Figure 1 forlocation). The basal part of the Hollin Formation exhibits features of both fluvial and tidal channel facies (see Fig-ure 21 for a measured section).

into two end members (Figure 24): (1) wave-dominatedestuary and (2) tide-dominated estuary. Bay-headdeltas, central basins, flood-tidal deltas, washovers,and barrier bars, although they may be influenced bytides, characterize wave-dominated estuaries. In theHollin and Napo formations of the Sacha area, we donot recognize any of these wave-dominated deltaicfacies.

We do recognize, however, major facies of a tide-dominated estuary. Evidence for a tide-dominated estu-ary in the Hollin, Napo “T,” and Napo “U” includes(1) an erosional unconformity at the base of theHollin, (2) tidal channels and associated fluvial chan-nels, (3) tidal sand bars, (4) tidal sand flats, (5) subtidalmud flats, (6) sandy and muddy shelves, and (7) deep-ening-upward (i.e., transgressive) successions. In par-ticular, the preservation of delicate mud drapes indi-cates a protected environment, such as an estuary.

The depositional model proposed for the Hollinand Napo formations is a modified version of the

general model proposed for a tide-dominated estuaryby Dalrymple et al. (1992). We propose the followinggeneral stages of deposition for the Hollin Formation(oldest to youngest) following the regional uplift anderosion of the Misahualli volcanics (Figure 25):

Stage 1: Minor f luvial channels (low-sinuositystreams) and common tide-dominated estuary dur-ing lower Hollin deposition.

Stage 2: Well-developed tide-dominated estuaryand shelf environments during lower and upperHollin deposition.

Stage 3: Drowned tide-dominated estuary duringupper Hollin deposition.

Stage 4: Well-developed shelf environments (i.e.,complete drowning) with glauconitic sands and mudsduring the final phase of upper Hollin deposition.

During Napo “T” deposition, stages 2, 3, and 4were repeated. Following deposition of the “B”limestone and overlying shales, stages 2, 3, and 4


Figure 21—Sedimentologicallog of the outcrop at HollinLoreto Coca Road (see Figure 1 for location). Note base Cretaceousunconformity and its angular relationship withunderlying volcanics. The measured interval is interpreted to be composed of mixed fluvial and tidal channelfacies. See Figure 4 forexplanation of symbols.

were repeated again during deposition of the Napo “U”interval. Finally, deposition of the “A” limestone took place(see Figure 3). The carbonate intervals signify regional trans-gressive deposition. Because we interpret the upper Hollin,

Napo “T,” and Napo “U” formations to be tide-dominat-ed estuarine facies, we suggest that the tidal environ-ment persisted throughout the deposition of the Hollinand Napo formations (i.e., time transgressive).


Table 5. Comparison of Interpretations of Depositional Facies for the Hollin Loreto Coca Road Outcrop by DeSouza Cruz (1989) and by This Study

Study De Souza Cruz (1989) This Study*

Data Outcrop and core descriptions Core and limited outcrop descriptionsNapo “U” Transgressive shelf Tide-dominated estuary and shelfNapo “T” Transgressive shelf Tide-dominated estuary and shelfUpper Hollin Estuarine tidal bars Tide-dominated estuary and shelfMiddle Hollin Lacustrine delta front (Gilbert type) –Lower Hollin Braided fluvial and eolian Tide-dominated estuary and distal fluvial**

*Palynological study of limited shale samples from the basal Hollin exposed at the Hollin Loreto Coca outcrop (Petroproduccion internal report) suggestscontinental environments.

**Lower Hollin interpretation is based on outcrop study; the remaining interpretations are based on core.

Table 7. Summary of Features Recognized in the Hollin and Napo Formations*

Feature Figure Number (This Paper) Related Reference

Heterolithic facies 5 Terwindt (1981)de Boer et al. (1989)

Rhythmic alternation of 5 and 6 Nio and Yang (1991)sandstone-shale couplets (sandy rhythmites)

Thick-thin alternations of silt and clay layers 17 Kuecher et al. (1990)showing cyclicity (muddy rhythmites)

Double mud layers 5 and 6 Visser (1980)Cross beds with mud-draped foresets 5 Terwindt (1981)Bidirectional cross-bedding – Terwindt (1981)Reactivation surfaces 7 Klein (1970)Crinkled laminae 8B and 14B Terwindt (1981)Elongate mudstone clasts 14A Banerjee (1989)

Feines and Tastet (1998)Full-vortex structures 8A Terwindt (1981)Flaser bedding 11 Reineck and Wunderlich (1968)Wavy bedding 13 Reineck and Wunderlich (1968)Lenticular bedding 16 Reineck and Wunderlich (1968)

*Many of these features are considered to be indicative of deposition from tidal processes.

Table 6. Comparison of Interpretations of Depositional Facies for the Hollin and Napo Formations by White et al.*and by This Study

White et al. (1995) This Study

Data type Core and outcrop descriptions Core and limited outcrop descriptionsNapo “U” Incised valley-fill fluvial and deltaic Tide-dominated estuary and shelf

estuary parasequences shelfNapo “T” Incised valley-fill fluvial and deltaic Tide-dominated estuary and shelf

estuary parasequences shelfUpper Hollin Coastal plain tidal shoreline Tide-dominated estuary and shelf

parasequencesMain Hollin Alluvial braid plain coastal plain Tide-dominated estuary and fluvial

*White et al. (1995).

Although both the Hollin and Napo formationsexhibit similar depositional facies and episodes ofdrowning, there is an important difference. The basalHollin is marked by a major angular unconformity,indicating erosion prior to deposition. In contrast, theNapo “T” and “U” formations rest on shelf facies with-out any evidence for erosion. Stratigraphic correla-tions show that the shelf facies beneath the tidal faciesin the Napo maintains a uniform thickness regionally,indicating a lack of incision prior to Napo deposition(Figure 26).

Tide-dominated estuaries commonly occur inmesotidal and macrotidal ranges (Harris, 1988). Davies(1964) defined microtidal, mesotidal, and macrotidalranges as 0–2 m, 2–4 m, and >4 m, respectively; there-fore, we infer that the tidal range for the Sacha area waslikely more than 2 m. Double mud layers in cross-bedsof the Hollin and Napo resemble those of the modernOosterschelde estuary in the Netherlands (Visser,1980). The mouth of the Oosterschelde estuary is 7.4km wide, and its tidal range is 3.5 m. In terms of depo-sitional processes, large-scale bed forms observed in


Figure 22—Sedimentologicallog of core from the Sacha 132 well Napo “U” sand showing a transgressive (deepening-upward)phase. Note that thiscored interval, composed of multipledepositional facies, shows a fining-upward log motif (see Figure 23).In the absence of core,this interval could be misinterpreted as a channel-fill facies basedon a fining-upward logmotif. See Figure 4 forexplanation of symbols.

the modern Bristol Channel estuary (UnitedKingdom) may also be considered an analog to thetidal sand bars in the Hollin and Napo formations.The linear bed forms in the Bristol Channel estuaryare 2–10 m in thickness, hundreds of meters inwidth, and several kilometers in length. Their longaxes are aligned parallel to the tidal flow (Harris,1988). The mouth of the Bristol Channel estuary is38.8 km wide and its tidal range is 8 m (Harris,1988). Individual tidal sand bar units in the Hollin

reach up to 2 m in thickness. Conceivably, widthsof paleoestuaries in the study area may have beenin the range of tens of kilometers, exceeding thesize of the Sacha field (see Figure 25).

An important outcome of the proposed tide-dominated estuarine model is that tidal sand bars inthe Sacha area are predicted to align in an east-westdirection, paralleling the direction of sedimenttransport (Figure 25). In contrast to the proposedmodel, the conventional f luvial-deltaic modelwould predict north-south–trending distributarymouth bars with an easterly sediment source.

Difficulties of an Incised Valley-Fill Model

In a sequence stratigraphic framework, the con-cept of incised valley-fill systems is quite popular(Zaitlin et al., 1994b). White et al. (1995) interpretedthe Hollin Formation to represent fluvial paleovalleydeposits associated with coastal-plain deposits. Suchvalley-fill successions may be considered to be coastal-plain incised valley systems (Zaitlin et al., 1994b);however, there are some difficulties in advocating anincised valley-fill model of Zaitlin et al. (1994b) for theHollin and Napo formations in the Sacha area.

An incised valley-fill system is characterized by abasal, regional, erosional surface forming a sequenceboundary (Zaitlin et al., 1994b) (Figure 27). Thepresence of an angular unconformity at the base ofthe Hollin Formation exposed at the Hollin LoretoCoca roadcut indicates a regional surface of erosion;however, an erosional surface does not necessarilymean a deep incision.

Incised valley systems may reach lengths inexcess of hundreds of kilometers, widths of tens ofkilometers, and depths to hundreds of meters(Zaitlin et al., 1994b). If so, evidence for significantincision may be established from seismic data andregional correlations. Seismic data clearly showtruncated reflections, suggesting regional erosionat the base of the Hollin Formation (Figure 28).Seismic data, however, do not show clear evidencefor laterally confined, deep incised valley systemsin the Sacha area. Again, we make a distinctionbetween erosion (shallow) and incision (deep) interms of depth.

The Hollin and Napo formations exhibit overallparallel and continuous reflection patterns (Figure28). This could be interpreted to mean that deposi-tion of the Hollin and Napo took place on a nearlyflat erosional surface without a recognizable incisedvalley morphology in the Sacha area. Another possi-bility is that the incised valley in the Sacha area, ifpresent, is below the seismic resolution.

Detailed stratigraphic correlations are helpful inrecognizing erosional and incisional features; how-ever, a stratigraphic cross section does not show


Figure 23—Fining-upward wireline log motif of coredinterval (hachured) in the Sacha 132 well, Napo “U”sand (see Figure 22).

discernible incision at the base of Napo “T” andNapo “U” reservoirs (Figure 26). In fact, both theNapo “T” shale and Napo “U” shale maintain theirthickness through the entire length of the north-south cross section of the Sacha field (Figure 26).These underlying shale units should exhibit a lateralchange in thickness, if they had indeed experienceda major downcutting or incision. This is perhaps themost convincing evidence for lack of incisionbeneath the Napo “T” and Napo “U” sands; howev-er, if an incised valley is much wider than the lengthof our stratigraphic cross section, then we cannotestablish the existence of an incised valley withoutadditional seismic data and a regional study.

The regional sequence boundary in an incisedvalley-fill system is related to relative sea level fall(Zaitlin et al., 1994b). In the Sacha field area, theangular unconformity at the base of the Hollin is

considered to be the result of uplift and erosionassociated with the early Mesozoic tectonic activityin the Oriente basin (Dashwood and Abbotts, 1990;Balkwill et al., 1995). Although most sequencestratigraphers consider uplift under the all-inclusiveterm “relative sea-level fall,” we wish to emphasizethe tectonic origin of the base Cretaceous uncon-formity. Estimated tectonic subsidence rate duringdeposition of the Hollin and Napo formationsranges from 3.714 to 9.143 m/m.y. for the middlenorth of the Oriente basin (Jaillard, 1995), indicat-ing that the basin was tectonically active during theCretaceous deposition.

The current trend in sequence stratigraphy is toexplain most coastal erosion and deposition relatedto incised valley-fill systems by allocyclic processes,such as sea level changes (Zaitlin et al., 1994b); how-ever, there is no evidence for subaerial exposure in


Figure 24—Morphologicalcomponents in plan viewof two end members ofestuarine models. (A) Spatial distribution offacies in a tide-dominatedestuary. (B) Spatial distribution of facies in awave-dominated estuary.Vertical dashed lines show facies boundaries.Wave-dominated estuarinefacies are considered to be the typical fill of anincised valley (see Figure27). Simplified from Dalrymple et al. (1992).Reprinted with permis-sion from SEPM.


Figure 25—Interpretedpaleogeography of theHollin Formation showingfour stages of evolutionfrom bottom (older) to top(younger). (1) Depositionalmodel for the lower Hollinshowing fluvial and tide-dominated estuarinefacies. During this time,deposition took placeabove the unconformitythat separates the Hollinfrom the underlying Misahualli volcanics. (2) Depositional model forthe Lower to upper Hollinshowing tide-dominatedestuarine and shelf facies.Note well-developed sandbars and sand flats in theestuary. During this time,tidal facies were depositedover fluvial and tidal facies.(3) Depositional model forthe upper Hollin showingdrowned tide-dominatedestuarine facies with well-developed open shelf facies. During thistime, shelf facies weredeposited over tidal facies.(4) Depositional model forthe upper Hollin showingcomplete drowning of thearea by transgression andestablishment ofwidespread shelf facies.Approximate position ofthe Sacha area is shown by a rectangle to illustratedominant facies encountered in the cores.


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the Sacha cores that would indicate a lowering of sealevel. In contrast, there is ample evidence for deepen-ing or drowning in the lower Hollin, upper Hollin,Napo “T” (Sacha 133), and Napo “U” intervals, indi-cating possible rises in sea level. Autocyclic process-es, such as lateral migration of facies, can also explainthe vertical changes in facies.

An incised valley-fill system is characterized by abasinward shift in facies (Zaitlin et al., 1994b). Thisis typically the result of a fluvial valley incising intothe exposed shelf during falling sea level. As a result,

fluvial deposits overlie shallow-marine parasequences(Figure 27); however, such a relationship is absent inthe study area because tidal and fluvial facies of theHollin unconformably overlie the Misahualli vol-canics, not shallow-marine deposits (Figure 21). Moreimportant, there is no evidence in the core for fluvialerosion within the Napo Formation in the Sacha area.In fact, the fluvial facies are completely absent in theNapo Formation in our study area.

Coarsening-upward parasequences are one charac-teristic of incised valley-fill systems, often occurring


Figure 27—Generalized model for incised valley-fill system. Note that the incised valley is filled with wave-dominatedestuarine facies (see Figure 24B). Also note coarsening-upward trends of parasequences. From Zaitlin et al. (1994b).Reprinted with permission from SEPM.


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both beneath the incision and within the valley fill(Figure 27). In describing properties of parase-quences, Kamola and Van Wagoner (1995, p. 29) stat-ed, “In siliciclastic, shallow-marine settings, parase-quences are composed of beds and bedsets thatrecord continuous, gradual upward shallowing. . . .Most parasequences are marked by an upward-coars-ening change in grain size, an upward decrease inmud, an upward increase in thickness of the beds,and an upward change in the beds and the trace fos-sils reflecting an upward decrease in water depth.The grain-size and bed-thickness trends are reversedin parasequences composed of subtidal-tidal f latdeposits, where deposition still records an upwarddecrease in water depth.” As one might expect,parasequences do not occur in the underlying pre-Hollin Misahualli volcanics. More important, coarsen-ing upward and thickening-upward trends are absentin the Hollin and Napo formations. Although parase-quences in subtidal environments may show fining-upward trends, the Hollin and Napo formations donot show shallowing-upward trends that are charac-teristic of some parasequences.

An incised valley-fill system is generally charac-terized by a vertical association of wave-dominatedestuarine facies composed of fluvial, bayhead delta,central basin, and barrier beach in an ascendingorder (Figure 27); however, wave-dominated estu-arine facies are absent in the Sacha core.

Outcrop, core, seismic, or well data do not cor-roborate an incised valley-fill model, applied to theHollin and Napo formations by other workers.Estuarine facies are quite complex, as this studyshows, and may not always fit into a general incisedvalley-fill model.

RESERVOIR FACIES

In the core and outcrop examined, we recognizefour reservoir facies on the basis of lithofacies (i.e.,sand percent), inferred sand-body geometry, bedcontinuity and connectedness, and depositionalpermeability barriers. All four facies types generallyexhibit similar porosity values (15–20%), but theirpermeability values differ.

Reservoir Facies I

Reservoir facies I comprises tidal sand bars thatare dominantly composed of fine- to medium-grained sand. This facies is most common in theouter estuarine setting (Johnson and Levell, 1995),closely associated with tidal sand flats (reservoirfacies III). Sand percentage in this facies is common-ly 100%. The sand is clean and devoid of deposition-al matrix because strong tidal traction processes

winnowed the associated fines. Measured perme-ability (horizontal) ranges from 1 to 9300 md.

Although mud drapes are ubiquitous in thisfacies, they are not considered to be major perme-ability barriers because they tend to be too thin(i.e., 3–4 mm thick) and discontinuous. This faciesis expected to have good vertical and lateral com-munication.

This reservoir facies is characterized by an elon-gate bar geometry, parallel to depositional dip.Vertical amalgamation of sand packages is a diag-nostic feature of this facies (e.g., well SA 130,upper Hollin), where multiple bars are stackedboth laterally and vertically, and the deposit is moresheetlike. Individual bars range in thickness from 5 cmto 2 m. Amalgamated units show a thickness of upto 10 m.

As indicated by the gamma-ray curve, this faciesis characteristically clean and exhibits a blockycharacter; however, the wireline-log motif alone isnot a reliable indicator of depositional facies. Forexample, the blocky log motif in the Sacha 130 well(Figure 3) represents both fluvial channel and tidalsand bar facies (Figure 9).

Reservoir Facies II

Reservoir facies II comprises both tidal and flu-vial channels. This facies commonly occurs in theinner estuarine setting. Although both fluvial andtidal channels are cross-bedded, the tidal channelsare distinguished by their mud drapes. This facies iscomposed dominantly of medium- to fine-grainedsand. Sand percentage is commonly 90–100%.Sands in this facies are not as clean when comparedto tidal sand bars because of moderate matrix con-tent; therefore, reservoir quality is somewhatlower. Measured permeability (horizontal) rangesfrom 30 to 3400 md.

We infer that most of these channel sands arelenticular in geometry. The tidal channel facies is1–2 m thick. The fluvial channel facies is up to 5 min core and up to 10 m in outcrop. Fluvial channelstend to exhibit slightly better reservoir quality thantidal channels.

Comparing the wireline logs to the core, thisfacies exhibits fining-upward and blocky motifs.Again, caution must be exercised in using log motifsto interpret depositional facies because both trans-gressive sequences and fluvial sequences can show afining-upward log motif. For example, the coredinterval of the Napo “U” in the Sacha 132 well showsa transgressive phase of deposition in which tidalsand bar facies grades vertically into shelf facies (seeFigure 22). This transgressive interval is seen as a fin-ing-upward trend in wireline logs (see Figure 23). Inthe absence of core, this fining-upward trend, based


solely on its wireline log motif, could be misinter-preted as a fluvial channel facies.

Reservoir Facies III

Reservoir facies III is composed of tidal sand-flatdeposits and comprises dominantly fine-grainedsand. The most diagnostic feature of this facies is itsflaser, wavy, and lenticular bedding. Sand percent-ages of this facies range from 50 to 100%. Sands arefine grained and have a high matrix content. Thesands thus are not as clean as sands of tidal sand barsand channels; therefore, the reservoir quality of thisfacies is lower than the tidal sand bar and channelfacies. Measured permeability (horizontal) rangesfrom 0.1 to 1000 md.

Reservoir facies III is one of the most commonfacies in the Hollin, Napo “T,” and Napo “U” inter-vals (Tables 2–4). Although this facies can occurthroughout the estuary, it commonly occurs land-ward of the tidal bars. Sand bodies are likely to belaterally extensive (i.e., sheetlike geometry) andare often connected to tidal sand bars. The sandbars tended to migrate over the sand flats duringmegaripple formation. As a result, these connect-ed facies may behave as single reservoir f lowunits. In the tidal-f lat facies, neap bundles withhigher numbers of crinkled laminae (i.e., mud lay-ers) may be of poorer reservoir quality than springbundles with fewer crinkled laminae. Individualsandstone units range in thickness from 3 to 35cm. Vertically amalgamated units show thickness-es of up to 5 m. In wireline logs, this faciesexhibits an irregular character in comparison tothe other two sand-rich facies (i.e., reservoirfacies I and II) because it contains more intercalat-ed finer grained material.

Reservoir Facies IV

Reservoir facies IV is composed of shallow-marine shelf sands. This reservoir facies is consid-ered to be the least important of the four typesbecause of (1) high glauconite content, which canresult in considerable compaction and reduction inprimary porosity, (2) high depositional matrix,which can occlude primary porosity, and (3) highbioturbation, which can mix sand and mud result-ing in a poorly sorted texture. Measured permeabil-ity (horizontal) ranges from 0.06 to 150 md. Thesandstones are very fine to fine grained. The sand ismoderately to poorly sorted and subrounded. Someintervals are argillaceous because of the high depo-sitional matrix. Sand content ranges from 80 to100%. Glauconite content is up to 40%. Calcareousshell fragments and bioturbation are ubiquitous.

This facies varies from bars to sheetlike geometry.Thicknesses of individual units are difficult to deter-mine, but this facies can reach a thickness of 6 m ormore due to vertical amalgamation. Commonly, thereis a gradation between this facies and the three tidalreservoir facies.

CONCLUSIONS

The Cretaceous Hollin and Napo formations inthe Sacha field are prolific producers of hydrocar-bons in the Oriente basin, Ecuador. To enhance fur-ther oil production, it is important to gain a clearunderstanding of the reservoir in terms of its depo-sitional origin. A sedimentological analysis using516 ft (157 m) of conventional core from sevenwells established seven depositional facies, namely(1) f luvial channels, (2) tidal channels, (3) tidalsand bars, (4) tidal sand flats, (5) subtidal mud flats,(6) sandy shelves, and (7) muddy shelves. Theseven depositional facies can be grouped into fourreservoir facies: (1) tidal sand bars with excellentreservoir properties (i.e., 100% sand, low matrix,elongate bar geometry), (2) fluvial and tidal chan-nels with good reservoir properties (i.e., 90–100%sand, moderate matrix, lenticular geometry), (3)tidal sand f lats with moderate properties (i.e.,50–100% sand, high matrix, sheet geometry), and(4) shelf sands with relatively poor properties (i.e.,80–100% sand with high matrix and glauconite, barto sheet geometry).

Based on the presence of mud drapes on bedforms, heterolithic facies, double mud layers, bidi-rectional (i.e., herringbone) cross-bedding, sandytidal rhythmites, muddy tidal rhythmites, crinkledlaminae, flaser bedding, wavy bedding, lenticularbedding, and deepening-upward (i.e., transgres-sive) sequences, we interpret the cored intervals ofthe Hollin and Napo formations to represent tide-dominated estuarine facies.

We propose four stages of deposition for theHollin Formation (oldest to youngest) following theregional uplift and erosion of the Misahualli vol-canics: stage 1 (lower Hollin deposition) representsminor fluvial channels (low-sinuosity streams) andcommon tide-dominated estuary; stage 2 (lower andupper Hollin deposition) represents a well devel-oped tide-dominated estuary; stage 3 (upper Hollindeposition) represents a drowning of tide-dominatedestuary; and stage 4 (upper Hollin deposition) rep-resents well-developed shelf environments in theSacha field area. Stages 2–4 are repeated duringNapo “T” and “U” deposition.

An important aspect of the proposed model isthat tidal sand bars in the Sacha area are predict-ed to align in an east-west direction parallelingthe direction of sediment transport, whereas the


conventional fluvio-deltaic model would predictnorth-south–trending distributary mouth bars withan easterly sediment source.

Outcrop, core, seismic, or well data do not cor-roborate an incised valley-fill model, as applied tothese deposits by other workers. Estuarine faciesare quite complex, as this study shows, and may notalways fit into a general incised valley-fill model.

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G. (Shan) Shanmugam

Shan Shanmugam received his Ph.D. in geology fromthe University of Tennessee in 1978 and joined Mobil thesame year. His publications (1 book, 75 papers, and 65abstracts) include AAPG Bulletin articles on secondaryporosity (1984, 1985), oil generation from coal in Australia(1985), unconformity-related porosity in Alaska (1988),submarine-fan lobes (1991), bottom-current reworkedsands in the Gulf of Mexico (1993), sandy slump anddebris-flow reservoirs in offshore Norway (1994), reinter-pretation of classic turbidites of the Jackfork Group inArkansas (1995), basin-floor fans in the North Sea (1995),replies to six discussions on turbidite controversy (1997),and tide-dominated estuarine facies in Ecuador (thispaper). He has been included in the Millennium Edition(2000–2001) of Marquis Who’s Who in Science andEngineering among 470 geologists chosen from 40 coun-tries. In January 2000, he retired from Mobil and joinedThe University of Texas at Arlington as an adjunct profes-sor of geology.

Mike Poffenberger

Mike Poffenberger is currently employed by Mobil NewExploration and Producing Ventures as a senior staff geolo-gist in Dallas, Texas. He received his B.S. (1983) and M.S.(1986) degrees in geology from Louisiana Tech University.He joined Mobil Oil in 1985 and has worked numerousproducing and exploration projects domestically andinternationally including the U.S. Gulf Coast, Mexico,Ecuador, and circum-Mediterranean. He is currentlyassigned to exploration studies in Tunisia.

Jorge Toro Álava

Jorge Toro Álava received a degree in geotechnicalengineering (1994) from the Escuela Politecnica Nacionalin Quito (Ecuador) and a M.Sc. degree in geology from theUniversite Joseph Fourier in Grenoble (France). Heworked in soil and rock mechanics, natural hazards, andmicroseismic volcanology, but mainly in geodynamicscharacterization and basin analysis of the Cretaceous andTertiary basins located in the back-arc, Andean arc, andforeland of Ecuador. Since joining Petroproducción (filialof Petroecuador) in 1994, he worked in sedimentology,stratigraphy, regional geology, and reservoir characteriza-tion of the Cretaceous sediments of the Oriente basin.

ABOUT THE AUTHORS

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