AUTHORS

Mohamed O. Abouelresh � Institute of Res-ervoir Characterization and Conoco-PhillipsSchool of Geology and Geophysics, University ofOklahoma, Norman, Oklahoma; present address:Faculty of Petroleum and Mining Engineering,Suez Canal University, Suez, Egypt;omaradh@yahoo.com

Mohamed Omar Abouelresh works permanentlyas an assistant professor in the Faculty of Petro-leum and Mining Engineering, Suez Canal Uni-versity, Egypt. He received his M.S. degree andhis Ph.D. in geology from Suez Canal University,

GEOHORIZON

Lithofacies and sequencestratigraphy of the BarnettShale in east-central FortWorth Basin, TexasMohamed O. Abouelresh and Roger M. Slatt

Egypt (1999 and 2005, respectively). His researchinterests have centered on lithofacies, sequencestratigraphy, depositional environments, and petro-physics of different unconventional gas shales(Barnett and Woodford) in the United States. Heis also involved in basin analysis work on Jurassic–Cretaceous subsurface sequence from the north-western desert, Egypt.

Roger M. Slatt � Institute of Reservoir Char-acterization and School of Geology and Geo-physics, University of Oklahoma, Norman,Oklahoma 73072; rslatt@ou.edu

Roger M. Slatt is the Gungoll Family chair professorin petroleum geology and geophysics and directorof the Institute of Reservoir Characterization at theUniversity of Oklahoma (OU). Previous positionswere in both academia and the petroleum industry.He has published more than 100 articles and ab-stracts on petroleum geology, reservoir geology,sequence stratigraphy, clastic depositional systems,and geology of shale. He is author/coauthor/editorof six books, teaches an online course on reser-voir characterization for AAPG, has been an AAPGand Society of Petroleum Engineers DistinguishedLecturer, and presents courses internationally forindustry organizations, in addition to OU.

ACKNOWLEDGEMENTS

We thank Devon Energy Co. for providing the fundingand core for this study and for permission to pub-lish this article. The senior author is on sabbaticalleave from the Faculty of Petroleum and MiningEngineering, Suez Canal University, Suez, Egypt.The AAPG Editor thanks the following reviewers for

ABSTRACT

Ten Barnett Shale lithofacies have been recognized in a 223-ft(68-m)-long core from Johnson County, Texas. Eight of theselithofacies match those previously identified in the main pro-ducing area of the Newark East (Barnett Shale) field in thenorthern part of the FortWorth Basin, but two new lithofacieshave been identified in this core, resedimented spiculitic mud-stone lithofacies and lag deposits, both of which are indicative ofa relatively higher energy environment and downslope resedi-mentation of shallower water deposits.

The recognition of cyclical stacking patterns of the litho-facies, condensed sections (CSs), and transgressive surfaces oferosionwere the keys to establishing the sequence-stratigraphicframework in these fine-grained rocks, which consists of sevenstratigraphic intervals in the lower Barnett Shale and nine strati-graphic intervals in the upper Barnett Shale. Spectral gamma-ray uranium and thorium logs aided in this objective and arerecommended for future sequence-stratigraphic studies of theseand other shales.

The sequence-stratigraphic framework reveals that thelower Barnett Shale in this area was deposited mainly in a low-energy, relatively deep-water environment, somewhat far froma terrigenous source area,which probably lies to the northwest.By contrast, the upper Barnett Shale was deposited in an oxy-genated shallower water environment, which had a sourcearea from thewest and southwest sides of the basin. The higher

their work on this paper: Rick Abegg, Kent Bowker,Carl Steffensen, and an anonymous reviewer.

Copyright ©2012. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received July 9, 2010; provisional acceptance October 14, 2010; revised manuscript receivedDecember 14, 2010; revised manuscript provisional acceptance February 8, 2011; 2nd revised manuscriptreceived March 26, 2011; final acceptance April 26, 2011.DOI:10.1306/04261110116

EDITOR ’S NOTE

A color version of Figures 1–10 may be seen in theonline version of this article.

AAPG Bulletin, v. 96, no. 1 (January 2012), pp. 1–22 1

frequency of sea level fluctuation during develop-ment of the upper Barnett Shale most probablyindicates periodic tectonic activity, perhaps asso-ciatedwith a structural high thatwas susceptible tosea level fluctuations. Alternatively, it could have re-sulted from the onset of glaciations inGondwanalandduring this time. This higher frequencymay indicatethat the upper Barnett is Chesterian in age, becausecyclicity was higher than during the Osagean andMeramecian stages. If so, there may be more high-frequency cycles than recognized in this core.

Siliceous sponge spicules are more common inthis core than in more northerly cores, so morebrittle facies might prevail in the southern part ofthe Fort Worth Basin.

High gamma-ray log responses, which arecaused by a high total organic carbon, and/or in-situphosphate minerals are commonly found in CSsand can be used for regional correlations.However,high gamma-ray phosphatic deposits that havebeen resedimented to downslope positions by sedi-ment gravity flows are an exception to the previousstatement. Correlation of the Barnett stratigraphicintervals now provides a north-to-southeast strati-graphic framework along the Fort Worth Basin.

Relative hydrocarbon potential (RHP) is anorganic geochemical parameter applied to thiscore and found to provide an indicator of marinetransgressions and regressions. We recommendcontinued testing and use of the RHP parameterfor high-frequency sequence-stratigraphic analysisof unconventional gas shales.

INTRODUCTION

The unconventional shale gas play, initiated in ear-nest only a few short years ago, continues to ex-pand with new potential and real discoveries beingannounced frequently. The Barnett Shale is one ofthe fewmature gas shales in theUnited States. TheBarnett Shale was initially tested in the early 1980swith vertical wells. Horizontal drilling commencedin 2003, and today, about 95% of the wells arehorizontal.

Because of its relative maturity, the BarnettShale has become the standard from which other

2 Geohorizon

shale reservoir exploration and production activ-ities and technologies are measured. Although sev-eral articles have been published recently on theBarnett Shale, most have centered on the NewarkEast (Barnett Shale) field of north Texas (Schieber,1998a;Montgomery et al., 2005;Hickey andHenk,2007; Loucks and Ruppel, 2007; Singh et al., 2008,2009; Slatt et al., in press), termed the “dry gas zone”by Montgomery et al. (2005).

This article presents the results of lithofaciesand petrographic and high-frequency sequence-stratigraphic analysis of a continuous 223-ft (68-m)vertical core (Spencer Trussell 1-H well, 7766.8–7989.7 ft [2367.4–2435.3m]) in JohnsonCounty,Texas, to the southeast of the main producing area(Figure 1A). The stratigraphy of this area is not aswell known, nor has the southern area been cor-related with the northerly core area. Thus, theobjectives of this article were to (1) describe thestratigraphy in the southern part of the FortWorthBasin; (2) compare and contrast the lithofaciesdistribution between the north and the south of thebasin; and (3) correlate the stratigraphy across thelength of the basin using a sequence-stratigraphicapproach.

GEOLOGIC SETTING

The geologic setting of the Barnett Shale in the FortWorth Basin has been summarized byMontgomeryet al. (2005), Bowker (2007), and others, so onlyfeatures relevant to this article arementioned here.Paleozoic strata comprise almost the entire fill ofthe basin. The Barnett Shale was deposited duringtheMississippianOsagean toChesterian ages (345–320 Ma) within a foreland basin that had gener-ally poor circulation with the open ocean (LoucksandRuppel, 2007). From long cores in theNewarkEast (Barnett Shale) field, Singh et al. (2008, 2009)documented variations in environments andprocessesfrom quiet-water deposition of muddy, organic-rich,and clay-rich lithofacies to high-energy shallowerwater andmore carbonate-rich lithofacies (Table 1).Loucks and Ruppel (2007) estimated water depthsfrom 400 to 700 ft (122–213 m) during depositionof the Barnett Shale.

Figure 1. (A) The location map for the studied core in the Fort Worth Basin showing the distribution of the Barnett Shale (shaded area)and structural and tectonic features; dot refers to the cored well (modified from Montgomery et al., 2005). (B) The schematic sectionshows an interpretation of the Mississippian stratigraphy. The vertical line refers approximately to the studied well where no Forestburglimestone is present (modified from Montgomery et al., 2005).

Table 1. The Identified Lithofacies with Their Total Thickness and Total Organic Content Percentages Compared with Those Identified in

the Newark East Field, North Fort Worth Basin by Singh et al. (2008)

Percent of Total Thickness of Lithofacies and Organic Richness in Core(Current Work)

Lithofacies and Organic Richness in Devon 7 Sol.Carpenter (Singh, 2008)

No.

Lithofacies Total

Thickness (%)

Average

TOC (wt. %)

Lithofacies

Abouelresh and Sla

AverageTOC (wt. %)

1

Siliceous noncalcareous mudstone 48.6 4.2 Siliceous noncalcareous mudstone 5.6 2 Siliceous calcareous mudstone 24.5 3.22 Siliceous calcareous mudstone 4.2 3 Dolomitic mudstone 7.1 1.79 Dolomitic mudstone 2.3 4 Silty-shaly interlaminated mudstone 3.7 2.2 Silty-shaly (wavy) interlaminated 1.8 5 Concretion horizons 2.9 1.35 Concretion 3.9 6 Calcareous laminae 2.5 0.56 Calcareous laminae 4.2 7 Reworked shelly deposits 0.8 4.39 Reworked shelly deposit 2.9

Micritic/limy mudstone

1.5 8 Phosphatic lithofacies 0.7 3.6 Phosphatic deposit 6.8 9 Resedimented spiculitic mudstone 8.3 2.01 10 Lag deposits 0.9 3.57

tt 3

The Barnett Shale is thickest (700–1000 ft[213–305 m]) to the northeast against the Muen-ster arch and thins to less than 30 ft (<9.1 m) overthe Llano uplift to the southwest (Pollastro et al.,2007) and over the Bend arch to the west (Mapelet al., 1979; Kier et al., 1980; Flippin, 1982;Henry,1982). The Barnett Shale sits on a major unconfor-mity atopEllenburger, Chappel, andViola-Simpsonstrata in various locations (Figure 1B).

LITHOFACIES

Detailed description of slabbed core, comparisonwith spectral core gamma logs, and analysis of 34thin sections identified the following 10 lithofacies:(1) siliceous, noncalcareousmudstone; (2) siliceouscalcareous mudstone; (3) dolomitic mudstone;(4) silty-shaly interlaminated mudstone; (5) concre-tion horizons; (6) calcareous laminae; (7) reworkedshelly deposits; (8) phosphatic deposits; (9) resedi-mented spiculitic mudstone; and (10) lag deposits.The first eight of these lithofacies are the same asthose that occur in themainproducing area (Table 1);

4 Geohorizon

they have been described and interpreted by Singhet al. (2008), so only brief descriptions are presentedbelow.

Significant upper and lower surfaces of bedsand lamination include sharp (Figure 2A), grada-tional (Figure 2B), irregular (Figure 2C), scoured(Figure 2D), and “poker chip” (Figure 2E) surfaces.

A spectral gamma-ray log of the core was ac-quired. This logmeasures the relative contributionof potassium (K), thorium (Th), and uranium (U) tothe natural radioactivity in a sedimentary succession(Hampson et al., 2005). The proportions of K, U,and Th provide insight into the provenance, de-position, and diagenesis of gas shale. The K- andTh-rich muddy deposits are suggestive of morerapid sedimentation and inheritance of mineralswith terrestrial affinity (Paxton et al., 2007). Ura-nium accumulates in phosphates, organic matter,and U-bearing minerals that deposit in muddy sub-strate under conditions of anoxia and slow sedi-mentation. Uranium, however, has a high mobil-ity and may be easily leached and redeposited inthe subsurface, which can make its distributionhighly irregular (Rider 1996). These additional

Figure 2. Core photographs showing different types of lamination surfaces and fractures in the core; (A) sharp (7812.3 ft [2381.2 m]),(B) gradational (7817 ft [2383 m]), (C) irregular (7766.1 ft [2367.1 m]), (D) scoured base (7818.6 ft [2383.1 m]), and (E) poker chip(7874.7 ft [2400.2 m]).

occurences of U should be considered to avoidmisinterpretation.

The Forestburg limestone separates the lowerfrom the upper Barnett Shale in themain producingarea, but pinches out to the south, and is absent atthis core location (Figure 1). In this core, the bound-ary between the upper and lower Barnett Shale ismarked by more carbonate lithofacies above andmore siliceous calcareousmudstone below (Figure 3).

Siliceous Noncalcareous Mudstone

This is the most common lithofacies in the core,particularly in the lower Barnett interval. It is black,massive, organic rich (Table 1), poorly to well lam-inated, and pyritic (Figure 4A, B), and has a sharp togradational boundary. Both detrital and biogenic(sponge spicules) quartz are present (Figure 4C).Other skeletal fragments and organic remains includemollusks, cephalopods, echinoderms (Figure 4D),leaf fragments, and compressed wood chips. Authi-genic minerals include calcite, dolomite, pyrite, andrare phosphatic particles. These characteristics plusrelatively high total organic carbon (TOC) indicatea low-energy, anaerobic depositional environment.

Siliceous Calcareous Mudstone

This lithofacies is found throughout the upperBarnett interval and to a lesser extent in the lowerBarnett. It consists mainly of dark- to light-grayinterlaminationswith sharp to gradational contacts.The light-gray laminae are calcite rich (averages∼20%). Calcite occurs mostly as sparry filling ofskeletal components (Figure 4E), partially replacedby dolomite crystals (Figure 4F). The calcareouscontent and lower TOC in this facies are attributedto relatively shallower water, oxic conditions.

Resedimented Spiculitic Mudstone

This lithofacies occurs only in the upper Barnettinterval (Figure 3). It is massive, instead of lami-nated, (Figure 4G); about 50% of the grains are acombination of silt-size quartz, phosphatic pellets,and glauconite, and the other 50% is composed ofsponge spicules (Figure 4H), some of which have

been replaced by dolomite (Figure 4I). Other skel-etal particles include radiolarians, bryozoans, andechinoderms. The diverse assemblage of grains andbroken skeletal fragments indicates reworking,probably by sediment-gravity flows. In most cases,these deposits are capped with very thin, horizon-tal, black shale laminae (Figure 4G).

Silty-Shaly Interlaminated Mudstone

This lithofacies is well laminated (Figure 4J), in-cluding some cross-laminations. The silt-size grainsare mainly sponge spicules (∼25%), with fewerdetrital quartz grains (3–5%). In transverse section,spicules are oriented and commonly occur in dis-crete laminae (Figure 4K). The repetitive alter-nations of gray and black laminae and occasionalcross-laminations indicate alternating energy levelsof depositional events (Singh et al., 2008).

Dolomitic Mudstone

This lithofacies is restricted to the upper Barnettinterval. It is light gray and consists of abundantfossil fragments recrystallized to ferroan-dolomite,resulting in a marl-like texture (Figure 4L). Theeuhedral shape of the dolomite rhombs implies apostcompactional diagenetic origin (because of thetraces of skeletal material).

Concretion Horizons

Calcareous concretions are common in the BarnettShale core area (Hickey and Henk, 2007; Loucksand Ruppel, 2007; Singh et al., 2008) (Figure 5A).They are less common in this core (Table 1). In-dividual concretions range in thickness between1.5 in. (3.8 cm) and 6 in. (15.2 cm) and exhibitextensive calcite-filled fractures (Figure 5B). In-ternally, the concretion horizons display uncom-pacted bedding and delicate skeletons of cepha-lopods and mollusks (Figure 5C) cemented bymicrosparite, indicating that they formed duringvery early burial, before compaction. Commonframboidal pyrite throughout the spar matrix indi-cates bacterial activity near an oxic-anoxic interface(Wilkin and Barnes 1997).

Abouelresh and Slatt 5

Figure 3. The lithologic and petrographiclog with the corresponding total organiccarbon (TOC) wt. % for the studied core.

6 Geohorizon

Figure 4. (A–D) Siliceous noncalcareous mudstone. (A) Core photograph (7982.3 ft [2433 m]); arrows point to thin pyritic streaks.(B) Low-resolution thin-section photograph showing faint lamination. (C) Thin-section photograph showing very fine quartz andskeletal debris; arrow points to well-preserved Bryozoan skeletal fragments. (D) Thin-section photograph showing echinoderm tests.(E–F) Siliceous calcareous mudstone. (E) Thin-section photograph showing very fine, subrounded quartz grain imbedded in calcite (pink-stained areas; see online version). (F) Thin-section photograph showing that rhombohedral dolomitic crystals prevail within the siliceouscalcareous mudstone. (G–I) Resedimented spiculitic mudstone. (G) Core photo (7841.8 ft [2390.2 m]) showing cross-lamination. (H) Thin-section photograph showing the abundance of glauconitic grains. (I) Thin-section photograph showing preferable dolomitization of a largesponge spicule. (J, K) Silty-shaly interlaminated mudstone. (J) Core photo (7941.4 ft [2420.5 m]) showing the interlaminated relationbetween silt (gray) and shale (black). (K) Thin-section photograph showing the silty-size components (quartz grains and sponge spicules).(L) Dolomitic mudstone. Thin-section photograph showing the abundance of skeletal particles in dolomitic groundmass.

Abouelresh and Slatt 7

8 Geohorizon

Calcareous Laminae

Laminae comprising this lithofacies are less than0.1 ft (<3 cm) thick (Figure 5D) and are interca-lated with other lithofacies. The laminae aremainlycomposed of autochthonous calcite mud and com-pletely micritized microfossils (Figure 5E). Theabundance of micrite indicates warm-water con-ditions conducive to precipitation of carbonates.

Reworked Shelly Deposits

This lithofacies is rich in compacted to flattenedfragments of bivalves, bryozoans, brachiopods, fili-branch mollusks, cephalopods, and echinodermsthat have been transported from their original de-positional setting. This lithofacies is more frequentin the lower Barnett interval. Shells may be im-bricated, coalesced fragmentswithin beds less than0.2 ft (<6 cm) thick (Figure 5F) or aligned becauseof early compaction (Figure 5G). The shelly de-posits commonly interfinger with the siliceous non-calcareous mudstone lithofacies and exhibit sharpupper and lower boundaries, indicating fluctuat-ing energy levels (Loucks and Ruppel, 2007). Re-worked shelly deposits have also been identified inthe Barnett Shale core area (Hickey and Henk,2007; Loucks and Ruppel, 2007; Singh et al.,2008).

Lag Deposits

This facies is thin (0.05–0.3 ft [1.5–9.1 cm]) andinfrequent, with sharp, gradational, and/or irreg-ularly scoured lower and upper contacts. It con-

sists of sand- to silt-size quartz grains, compactedagglutinated forams (Figure 5H), pyritized intra-clasts, phosphatic peloids, and some glauconiticgrains (Figure 5I). They are important indicatorsof the residual accumulation of coarse particles pro-duced by winnowing or erosion (Schieber, 1998b).Hickey and Henk (2007) recognized lag depositswithin Barnett phosphatic lithofacies in Wise Coun-ty, Texas. The presence of framboidal pyrite in-dicates sulfate-reducing bacterial activity resultingfrom the abundance of organic matter during sedi-mentation (Herbin et al., 1993). Proposed originsfor lag deposits such as found in this core include(1) subtle expressions of erosion surfaces (Schieber,1998b), (2) subaqueous erosion (Catuneanu, 2006),and/or (3) transgressive lags (Posamentier andAllen,1999).

Phosphatic Lithofacies

This lithofacies consists of two types: beddedphosphorite and nodular phosphates. The phos-phorite beds are of variable thickness (from 0.1 to0.25 ft [3–7 cm]) and are commonly interbeddedwith carbonatemud. Beds are composed of peloids,ooids, phosphatized fossils, and skeletal fragments(Figure 5J). Detrital quartz grains and microfossilsare also present (Figure 5K). Nodular phosphatesare contained within other facies such as siliceouscalcareous mudstone, resedimented spiculitic mud-stone, and lag deposits. They are brownish to black,spherical to irregularly shaped, mainly phospha-tized pellets and fossil fragments, and phosphate-coated grains of bioclasts, micrite, quartz, and glau-conite (Figure 5L). The reworking processes are

Figure 5. (A–C) Concretion horizons. (A) Core photograph (7899.9 ft [2407.9 m]) showing the sharp contact between concretion andshale. (B) Core photograph (7768.8 ft [2367.9 m]) showing many calcite-filled fractures. (C) Thin-section photograph showing some well-preserved gastropods (lower arrow) with micritic rims; note the framboidal pyrite (upper arrow). (D, E) Calcareous laminae. (D) Corephotograph (7953.8 ft [2424.3 m]); arrows point to calcareous laminae. (E) Thin-section photograph showing the widespread micritic tosparite matrix, leaving ghosts of the preexisting skeletal fragments. (F, G) Reworked shelly deposits. (F) Core photograph (7980.3 ft[2432.4 m]) showing different laminae of reworked shell fragments. (G) Thin-section photograph showing mixtures of strongly com-pacted shelly fragments filled with sparite. (H, I) Lag deposits. (H) Thin-section photograph showing coarse assemblages of quartz grains(upper right arrow), phosphatic pellets, and sponge spicules (middle arrows), compacted agglutinated forams (lowermost arrow). (I) Thin-section photograph showing intraclast of quartz affected with pyritization; lower arrow points to glauconite and upper arrow points tophosphatic grain. (J–L) Phosphatic lithofacies. (J) Close-up of phosphate-coated pellets with calcite cement. (K) Thin-section photographshowing the phosphatic grains with quartz grains and skeletal debris imbedded in calcite cement (stained blue; see online color version).(L) Thin-section photograph showing sponge, quartz, and glauconite grains.

Abouelresh and Slatt 9

indicated by the more circular shape of mostphosphatic grains. However, the phosphatic de-posits could also be flooding surfaces characterizedby low sediment accumulation rates (Bohacs andSchwalbach, 1992).

SEQUENCE-STRATIGRAPHICINTERPRETATION OF THE CORED WELL

Sequence-stratigraphic frameworks of shale are yetpoorly understood, in comparison with those incoarse clastic and carbonate successions (e.g., Bohacs,1998). The criteria for establishing a sequence-stratigraphic framework are limited in Paleozoicrocks because of a lack of high-frequency biostrati-graphic data (Slatt and Rodriguez, 2010) and mustbe developed more on interpretations of environ-ments of deposition of lithofacies and of temporalchanges in environments by lithofacies stacking pat-terns (Bohacs and Schwalbach, 1992). In this arti-cle, our goal is to extend the sequence-stratigraphicframework established in theNewark East (BarnettShale) field (Singh et al., 2009) to the southeast,thus providing a sequence-stratigraphic correlation,and differentiation of the lower and upper BarnettShale where the Forestburg limestone is absent.

The Barnett Shale was deposited during anapproximately 25-m.y. or second-order time spanduring the Mississippian (Osagean to Chesterianages) (Ross and Ross, 1988; Loucks and Ruppel,2007). At least 15 global sea level cycles of approx-imately 0.5- to3-m.y. (third-order cyclicity) durationoccurred during this time interval (Van Wagoneret al., 1990;Haq and Schutter, 2008).Osagean andMeramecian strata show less regional cyclicity thanthe Chesterian.

Within the Barnett Shale in the main produc-ing area, Singh et al. (2009) identified three dif-ferent types of lithofacies stacking patterns basedon a combination of core examination and coregamma-ray log character. These patterns, termed“gamma-ray patterns” (GRPs) for their gamma-raylog response, are (1) “upward-decreasing GRP,”(2) “upward-increasing GRP,” and (3) “constantGRP” (Singh et al., 2009; Slatt et al., in press). Thesethree patterns are interpreted here as representing

10 Geohorizon

two consecutive relative sea level change events:(1) transgression event (TR), whereby shelly car-bonate facies are replaced upward by lower energy,clay organic-rich lithofacies; (2) regression event(RE) of relative sea level, whereby deeper water,organic-rich clays are replaced upward by higherenergy shelly, carbonate lithofacies (note that sucha relative fall could be a result of an absolute dropin sea level or a pushing of the shoreline seawardduring a progradational regressive phase of de-position). Two major surfaces were identified todelineate the stratigraphic intervals and their as-sociated relative sea level events that are trans-gressive surface of erosion (TSE) and flooding sur-face (FS), all as recently discussed by Catuneanuet al. (2008).

Based on these criteria, seven stratigraphic in-tervals, consisting of partial or complete cycles ofGRP-TR andGRP-RE, were identified in the lowerBarnett core and nine were identified in the upperBarnett core.

The Lower Barnett Stratigraphic Intervals 1 to 7

Stratigraphic Interval 1 (7989.7–7974.1 ft [2435.3–2430.5 m])This interval comprises the sediments depositedduring the earliest transgression that generated aTSE on top of the Ellenburger Group unconformity(SB). It is composed of two GRPs: a basal trans-gression stage (GRP-TR.1), which exhibits a typ-ical upward increase in gamma-ray log response,followed upward by a GRP-RE.1, which exhibits atypical upward-decreasing gamma-ray log response(Figure 6). The boundary between these twoGRPsis interpreted as a FS on top of a condensed section(CS) representing deposition during the deepestwater part of this cycle. The CS is a siliceous non-calcareous lithofacies,markedon the spectral gamma-ray log by high U and low Th and K contents andmoderate TOC values.

Stratigraphic Interval 2 (7974.1–7960 ft [2430.5–2426.2 m])The base of this interval is a TSE2 associated witha calcitic hard ground (low spectral gamma ray)(Figure 7A). This interval consists of two GRPs:GRP-TR.2, which is composed mainly of siliceousnoncalcareous mudstone with several calcareous

Figure 6. Sequence-stratigraphic interpretation of the lower Barnett Shale, stratigraphic intervals 1 to 7 (Strat. Int. 1–7) with the as-sociated spectral gamma ray, lithology, and total organic content (TOC). S. L. Inter. = sea level interval; TR = transgression event ofrelative sea level; RE = regression event of relative sea level; CS = condensed section; TSE = transgressive surface of erosion; FS = floodingsurface; F = fracture; C = concretions.

Abouelresh and Slatt 11

Figure 7. (A) Core photograph for submarine surface of erosion, TSE.2. (7974.2 ft [2430.5 m]). (B) Core photograph showing calcareous laminae (7968.6 ft [2428.9 m]), TR.2. (C)Low-resolution thin-section photograph showing silty-shaly mudstone, TR.4. (D) Low-resolution thin-section photograph showing condensed section with an abundance of phosphaticgrains, TR.4. (E) Core photograph showing several lag deposits within RE.4; arrows point to lag deposits (7933 ft [2418 m]). (F) Core photograph showing transgressive surface of erosionTSE.7 (7885 ft [2403.4 m]); note the shale clasts beneath the sharp-based shale laminae (white arrows). (G) Core photograph showing phosphatic laminae (lower arrows) followed bycalcareous laminae (upper arrows) and shale clasts (7863.2 ft [2396.7 m]) represents condensed section at the top of TR.8. (H) Core photograph showing TSE.9 between siliceousnoncalcareous mudstone and siliceous calcareous mudstone facies (7859.5 ft [2395.6 m]). (I) Core photograph showing TSE.10 between stratigraphic intervals 9 and 10 (7846.8 ft[2391.7 m]). (J) Low-resolution thin-section photograph for the TSE.11. (K) Core photograph for scoured surface (white arrows) at the base of TR.15 (7780 ft [2371 m]).

12Geohorizon

laminae (Figure 7B), a concretion interval, and tworeworked broken shelly laminae with an upwardconstant gamma-ray pattern, indicating a relativelyshallow-water environment of deposition associ-ated with the underlying TSE2 and then an upward-increasing gamma-ray trend that is composed ofphosphate-rich siliceous mudstone forming a high-frequency CS with FS-2 at the top (Figure 6). TheGRP-RE.2 is characterized by siliceous noncalcare-ous mudstone at the base, grading upward to sili-ceous calcareousmudstone, representing an upwardshoaling of the depositional environment becauseof either a drop in absolute sea level or migrationof the shoreline seaward during a regressive epi-sode of progradation.

Stratigraphic Interval 3 (7960–7942 ft [2426.2–2420.7 m])The base of this interval is interpreted as a TSE(TSE.3) based on a sharp decrease in a spectralgamma-ray log across the boundary (no core of thisinterval exists to corroborate). The TR.3 is rep-resented by siliceous noncalcareousmudstone withsome intercalated calcareous laminae and exhibitsa typical upward-increasing gamma-ray log pattern,which is in part caused by the presence of an in-terval of transported phosphatic material, giving afalse CS pattern, followed upward by a true CS;then GRP-RE.3, which exhibits a slightly upwarddecrease in gamma-ray log response caused by highamounts of K and Th.

Stratigraphic Interval 4 (7942–7931.5 ft [2420.7–2417.5 m])The contact between siliceous noncalcareous mud-stone (GRP-RE.3) and overlying silty-shaly mud-stone marks the lower boundary (TSE.4) of thisinterval (Figure 6). Pronounced variations in gamma-ray log patterns are present within this interval.The basal part of GRP-TR.4 is highly laminatedsilty-shalymudstone (Figure 7C). It is overlain by aninterval with a very high gamma-ray log response;the lower part of which has no core, but the upperpart of which contains abundant TOC, phosphaticfacies (Figure 7D), and two well-preserved organic-rich intervals separated by an abrupt decreasinggamma-ray surface on which sit reworked shellylag deposits (Figure 6). The GRP-RE.4 consists ofsilty-shaly interlaminatedmudstone capped by one

lamina of siliceous noncalcareous mudstone. Thisinterval contains several lag deposits and trans-ported phosphatic nodules, indicating a relativelyshallow-water energetic environment (Figure 7E),with fluctuations such as might occur near an ac-tive structural high.

Stratigraphic Interval 5 (7931.5–7918.3 ft [2417.5–2413.5m])A sharp decrease in gamma-ray log character(Figure 6), which coincides with a very faint ir-regular surface of erosion, marks the TSE.5. Thebasal unit, GRP-TR.5, consists of siliceous calcar-eous mudstone followed upward by phosphatic-rich, siliceous, noncalcareous mudstone facies, in-dicating a rising sea level. The uppermost part ofGRP-TR.5 is a CS marked by a high U content.The overlying thick GRP-RE.5 is composed of sili-ceous noncalcareous mudstone facies with severalcalcareous laminae, indicating an upward-shoalingof the environment.

Stratigraphic Interval 6 (7918.3–7888 ft [2413.5–2404.3 m])The base of this interval is expressed by a changefrom an upward-decreasingGRP to a constantGRP,which we interpret as TSE.6. The basal GRP-TR.6is characterized by a slightly upward-increasing pat-tern associated with local maximum Th (Figure 6)and consists of siliceous calcareous mudstone faciesgrading upward to siliceous noncalcareous mud-stone alternating with phosphatic laminae, thincalcareous laminae, and concretions. The greaterthickness of this interval relative to older intervalsindicates that the basin received greater amountsof terrigenous calcareous detritus at this time. Thisinterval contains two thin phosphatic laminae re-sulting in a sharp increase in gamma-ray log re-sponse, a local maximumTOC, and high U and Thcontents, all indicating that it is a CS. The over-lying GRP-RE.6 consists of siliceous noncalcareousmudstone overlain by siliceous calcareous mud-stone with low U and high Th.

Stratigraphic Interval 7 (7888–7867.5 ft [2404.3–2398 m])The transgressive surface of erosion TSE.7 lies be-neath strata containing large shale clasts (Figure 7F),indicating high-energy transport and incorpora-tion of rip-up clasts during downslope flow. The

Abouelresh and Slatt 13

overlying TSE.7 is GRP- TR.7, which consists of alower interval mainly composed of siliceous non-calcareous mudstone with slightly upward con-stant GR. Two cycles of high Th content separatedby one interval of low Th occur within this lowerinterval, indicating internal variability not detectedby the core characterization or total spectral gamma-ray profile. An overlying interval is composed ofsiliceous noncalcareous mudstone with two thinphosphatic laminae at the top, corresponding to arelatively high gamma-ray log response. Thin cal-careous laminae associated with shale clasts directlyaboveGRP-RE.7marks another TSE (TSE.8). Thisboundary separates the upper and lower BarnettShale (Figure 7G).

TheUpperBarnett Stratigraphic Intervals 8 to16

The upper Barnett Shale (Figure 8) in this corespans the interval between 7867.5 and 7766.7 ft(2398 and 2367.3 m). It is differentiated from thelower Barnett Shale by a generally higher calcare-ous content and more common vertical lithofacieschanges. Common transgressive surfaces of erosionand flooding surfaces indicate more common sealevel fluctuations that resulted in the developmentof nine stratigraphic intervals.

Stratigraphic Interval 8 (7867.5–7860 ft [2398–2395.7 m])The basal GRP-TR.8 consists of siliceous noncal-careous mudstone overlain by thin phosphatic lam-inae thatmarks the flooding surface at the top of thisinterval. GRP-RS.8 is composed of siliceous calcar-eous mudstone with typically upward-decreasingGRP. The facies change between this facies and itsoverlying dolomitic mudstone facies marks theboundary between intervals 8 and 9 (TSE.9).

Stratigraphic Interval 9 (7860–7846.6 ft [2395.7–2391.6 m])The basal GRP-TR.9 consists of thick dolomiticmudstone capped by thin laminae of silty-shalymudstone and siliceous noncalcareous mudstonewith phosphatic laminae, giving rise to one of thehigh gamma-ray log readings in the core. This in-terval is overlain byGRP-RE.9, which is composedof siliceous noncalcareousmudstone characterizedby high Th and low U (Figure 7H).

14 Geohorizon

Stratigraphic Interval 10 (7846.6–7831.6 ft[2391.6–2387.1 m])This interval overlies TSE.9 and is composed oflower and upperGRPs (Figure 8). The lower GRP-TR.10 exhibits a high Th content and consists ofsiliceous calcareous mudstone and resedimentedspiculitic mudstone facies overlain by siliceous non-calcareousmudstone and exhibits generally upward-increasing GR. An abundance of calcareous spic-ules, glauconite, silt-size quartz grains, phosphaticgrains, and other skeletal fragments characterizethis interval. GRP-RE.10 is more calcareous; a highTh content is suggestive of terrestrial influence.

Stratigraphic Interval 11 (7831.6–7826 ft [2387.1–2385.4 m])A sharp boundary and decrease in gamma-ray logresponse marks TSE.11 (Figure 7I). The GRP.TR.11, the basal part of this sequence, consistsmainly of transported phosphatic materials withinresedimented spiculitic mudstone, giving rise to ahigh gamma-ray log response, low U and high Th,indicating terrigenous input. The GRP.RE.11 ex-hibits upward-decreasing GR because of the sili-ceous calcareous mudstone content that, cappedwith lag deposits, marks TSE.12 (Figure 8).

Stratigraphic Interval 12 (7826–7818.3 ft [2385.4–2383 m])This interval developed on TSE.12 from two rela-tive sea level change events; GRP.TR.12 is com-posed of intercalations of siliceous noncalcareousmudstone, silty-shaly mudstone, and concretionhorizons. Thin phosphatic laminae are present at thetop of this interval, giving rise to the high gamma-ray log response, highU and lowTh contents, and alocalmaximumofTOC.The overlyingGRP-RE.12is mainly siliceous, calcareous, and resedimentedspiculitic mudstone with upward-increasing Th con-tent, suggesting proximity to a source area (Figure 8).

Stratigraphic Interval 13 (7818.3–7804 ft [2383–2378.7 m])The base of this interval is marked by TSE.13(Figure 7J) on which sits GRP-TR.13, which con-sists of siliceous calcareous mudstone intercalatedwith phosphatic-rich resedimented spiculitic mud-stone (Figure 8). Transported phosphatic intervalsgive rise to a high gamma-ray log signature withinGRP-TR.13, which is otherwise mainly siliceous

Figure 8. Sequence stratigraphic interpretation of the upper Barnett Shale, stratigraphic intervals 8 to 16 (Strat. Int. 8–16) with theassociated spectral gamma ray, lithology, and total organic content (TOC). S. L. Inter. = sea level interval; TR = transgression event ofrelative sea level; RE = regression event of relative sea level; CS = condensed section; TSE = transgressive surface of erosion; FS = floodingsurface; F = fracture.

Abouelresh and Slatt 15

noncalcareous mudstone. The GRP-RE.13 containsresedimented spiculitic mudstone. The interval be-tween 7804 and 7791.6 ft (2378.7 and 2374.9 m)was not cored, but TSE.14 is placed somewherewithin this interval.

Stratigraphic Interval 14 (7804–7780 ft [2379–2371.3 m])The basal GRP-TR.14, composed of phosphate-richresedimented spiculites intercalated with silty-shalymudstones, exhibits upward-increasing GR drivenby U content. The phosphatic pellets, which werepresumably transported from upslope areas, also areenriched inTOC(Figure 8). The upper GRP-RE.14consists of alternating siliceous noncalcareous mud-stone and silty-shaly and resedimented spiculites.This relative sea level change event is indicative ofhigher energy levels, at least episodically, when theshelf sediments were transported downslope intothe basin by sediment gravity flows, suggesting arelative sea level fall.

Stratigraphic Interval 15 (7780–7772 ft [2371.3–2368.9 m])The TSE.15 is a scoured, sharp-based surface(Figure 7K) that represents the base of GRP-TR.15,which is composed mainly of phosphate-rich re-sedimented spiculites and dolomitic mudstone,containing some calcite-filled fractures. The highergamma-ray log intervals in GRP-TR.15 are mostlycaused by U associated with two local maximumTOC zones to form a CS capped with a floodingsurface FS.15. The GRP-RE.15 is characterized bya thin interval of dolomitic mudstone with upward-decreasing GR and high Th content. The boundarybetween dolomitic mudstone and siliceous mud-stone facies is associated with a sharp decrease inGR and marks the TSE.16.

Stratigraphic Interval 16 (7772–7766.7 ft [2368.9–2367.3 m])This is the uppermost interval of the Barnett. Itdeveloped on TSE.16. The GRP-TR.16 consists ofsiliceous noncalcareousmudstone. Theupper sharpincrease in GR marks the flooding surface FS.16.TheGRP-FS.12 consists of siliceous noncalcareousmudstone and silty-shaly interlaminated facies. It

16 Geohorizon

has a local maximum TOC zone (∼7%). The up-ward decrease in U and increase in Th indicateshoaling conditions during deposition; however, asharp decrease in the gamma-ray log response as-sociated with an irregular surface between concre-tions and shale laminae marks the upper boundaryof interval 16.

SUMMARY OF SEQUENCE STRATIGRAPHYFROM THE CORED WELL

The siliceous and calcareous contents differentiatethe Barnett Shale into lower (7989.7 up to 7867.5 ft[2435.3–2398m]) andupper (7867.5up to7766.7 ft[2398–2367.3 m]) intervals in this core, separatedby TSE.8. The lower Barnett contains more sili-ceous, noncalcareous mudstone lithofacies imply-ing an overall period of relatively slow, quiet-watersedimentation (Loucks and Ruppel, 2007; Singhet al., 2008). The general lack of silt-size quartz grainsand large fossils indicates that shallower water sedi-ment source areas were some distance from thisdepositional site and most likely to the west andnorthwest (Singh et al., 2009). The high total or-ganic content (average, 4.2%) (Table 1) of this in-terval implies a generally anoxic depositional set-ting during this period.

The upper Barnett Shale contains more cal-careous mudstone, silty-shaly interlaminated mud-stone, calcareous laminae, dolomitic mudstone,and resedimented spiculitic mudstone facies, all ofwhich imply an oxygenated, shallower water en-vironment of deposition with a source area fromthe west and southwest sides of the basin (Singhet al., 2008). Also, resedimented spiculitic mud-stone with abundant silt-size sponge spicules, glau-conite, and phosphatic intraclasts, indicate thatsediment gravity flows were more common duringthis time interval. The closer spacing of gamma-ray-log highs indicates repeated fluctuations inrelative sea level. The higher frequency of sea levelfluctuation during development of the upperBarnett Shale than the lower Barnett Shale corre-sponds with the high rate of fluctuation proposedby Haq and Schutter (2008) for the Late Missis-sippian (Figure 9). This high rate probably indicates

Figure 9. Correlation charts for the identified lithofacies, stratigraphic intervals (Strat. Int.), and relative sea level curve with totalspectral gamma ray (SGR), sea level changes curve (Haq and Schutter, 2008), and relative hydrocarbon potential (RHP) curve. TOC =total organic carbon; S1 = free hydrocarbon; S2 = hydrocarbon potential; HC = hydrocarbon; PD = present day; TR = transgression eventof relative sea level; RE = regression event of relative sea level; TSE = transgressive surface of erosion; FS = flooding surface; F = fracture.

Abouelresh and Slatt 17

periodic tectonic activity, perhaps associated witha structural high that was susceptible to sea levelfluctuations. Alternately, it could have resulted fromthe onset of glaciations in Gondwanaland duringthis time. This higher frequency may indicate theupper Barnett Shale is Chesterian in age, as cyclicitywas higher than during Osagean and Meramecianstages. If so, there may be more high-frequency cy-cles than recognized in this core.

Variations in thickness of the stratigraphic in-tervals probably result from variations in sedimen-tation rates and shifting of dominant source areaswith time as a result of the relative sea level changes.Condensed sections, which are identified by theirhigh gamma-ray log values, provide the best in-dicator of slow sedimentation (Loutit et al., 1988)and likely anoxic bottom waters that allowed or-ganicmatter to accumulate and be preserved (Slattet al., in press).

However, note that not all high gamma-ray-logresponses are indicative of CSs. Transported phos-phatic deposits (e.g., GRP-TR.3, lower BarnettShale; and GRP-TR.13, upper Barnett Shale) pro-vided high gamma-ray log signatures (Figures 5, 7).The abundance of phosphatic deposits in TRs isindicative of shallower upslope source areas andtransport by sediment gravity flows during periodsof high rates of deposition.

TOTAL ORGANIC CONTENT AND RELATIVEHYDROCARBON POTENTIAL

Because of the caveats previously mentioned con-cerning some high gamma-ray-log responses beingcaused by transported phosphatic deposits insteadof enrichedTOC, caution should be exercisedwhendeveloping a sequence-stratigraphic framework fromonly conventional well logs. A useful parameter toaid in the development of a framework is the rela-tive hydrocarbon potential (RHP), which is a pa-rameter obtained from Rock-Eval pyrolysis datathrough the formula (S1 + S2)/TOC; S1 is the freehydrocarbon, S2 is the hydrocarbon potential, andTOC is the total organic carbon. As suggested byFang et al. (1993), RHP data can be used to deter-

18 Geohorizon

mine the organic facies sequence based on the con-cept that an upward-increasing RHP pattern willresult from vertical change of organic facies fromthose characteristic of oxic to those characteristic ofanoxic conditions. In contrast, a decreasing-upwardpattern of RHP will suggest vertical change of or-ganic facies from those characteristic of anoxic tothose of oxic conditions. Knies (2005) has usedsimilar data to interpret climate-induced fluctua-tions in the type of organic matter in interglacialand glacial sediments.

The RHP was applied to this core from 38samples. Although the RHP trend is incompletebecause of uneven sample spacing, the resultingRHP curve shows intervals of anoxic conditionsmatching those of TR samples and intervals of oxicconditionsmatchingREs (Figure 9). At some points,the RHP curve displays a sharp change from oxicto anoxic conditions; these points coincide withsubmarine erosional surfaces in the interpretedsequence-stratigraphic framework (Figure 9). Thiscorrelation provides further support for the useof RHP analyses of unconventional gas shales, assuggested by Slatt et al. (in press).

REGIONAL CORRELATION OF THESTRATIGRAPHIC INTERVALS

It has been possible to correlate the GRPs identi-fied in themain producing area (Singh et al., 2008,2009) to the cored well described in this article(Figure 10). The lower Barnett Shale thins towardthe south. In the southerly direction, south of thetermination of the Forestburg limestone, the up-per Barnett Shale tends to thicken. These variationssupport the interpretation of different source areasbetween the lower and upper Barnett Shale.

The cross section shows that the lower andupper Barnett Shale in the studied well are similarin thickness (126.5 and 96.5 ft [38.6 and 29.4 m],respectively) in contrast with the rest of the sec-tion, where the lower Barnett Shale is thicker thanthe upper Barnett Shale. The lower Barnett Shaleis somewhat thick toward the northern and north-eastern parts of the Fort Worth Basin.

Figure 10. Cross section through the Fort Worth Basin to correlate the identified stratigraphic intervals with gamma-ray parasequences (GRPs) identified in the main producing area bySingh et al. (2008, 2009) (not to scale). Strat. Inter. = stratigraphic interval; GR = gamma ray.

Abouelreshand

Slatt19

The Forestburg limestone is restricted only tothe main producing area of the Fort Worth Basin;it thins and pinches out toward the west and thesouth (Singh et al., 2008) (Figure 10). The upperBarnett Shale shows a striking decrease in thick-ness from the main producing area toward thewestern and the southern parts of the Fort WorthBasin.

CONCLUSIONS

Ten Barnett Shale lithofacies have been recog-nized in a long core (223 ft [68 m]) from JohnsonCounty, Texas. Although the Forestburg limestoneis absent in this core, the upper 100.8 ft (30.7m) ofthe core is much richer in carbonate than the lower122.2 ft (37.2 m), so the stratigraphically lowestsiliceous calcareous mudstone is designated as theregional boundary between the upper and lowerBarnett Shale.

The recognition of cyclical stacking patterns ofthe lithofacies, flooding surfaces, and transgressivesurfaces of erosion were the keys to establishingthe sequence-stratigraphic framework in these fine-grained rocks that consists of seven stratigraphicintervals in the lower Barnett Shale and nine strati-graphic intervals in the upper Barnett Shale.Withinthis context, the transgressive surface of erosionTSE.8 is the contact between the upper and lowerBarnett Shale. Spectral gamma-ray U and Th logsaided in this objective and are recommended forfuture sequence-stratigraphic studies of these andother shales.

The sequence-stratigraphic framework revealsthat the lower Barnett Shale in this area was de-posited mainly in a relatively low-energy deep-waterenvironment, relatively far from a terrigenous sourcearea that probably lay to the northwest. By contrast,the more interbedded character and abundance ofresedimented spiculitic mudstone lithofacies witha high content of silt-size particles of sponge spic-ules, glauconite, and phosphatic intraclasts in theupper Barnett Shale, provide evidence for more sed-iment gravity flows, suggesting repeated fluctua-tions inwater depth in a shallower,well-oxygenated

20 Geohorizon

marine environment. The source area may havebeen more proximal, to the west and/or south-west, and suppliedmore siliceous sponge spiculesthan were supplied to the northern part of thebasin.

The upper part of the upper Barnett Shale ischaracterized by a series of very high gamma-raypeaks, indicating a higher frequency of sea levelfluctuations. This high frequency of fluctuationsmatches the high rate of fluctuation proposed byHaq and Schutter (2008) for the LateMississippian.

High gamma-ray-log responses are commonlyassociatedwithCSs in the Barnett Shale (and othershales) because of their high TOC content, thusmaking them likely candidates for regional corre-lation. However, downslope transport of reworkedphosphatic deposits can also provide high gamma-ray-log responses, so caution should be notedwhenattempting regional correlations with conventionalgamma-ray logs.

The RHP applied to this core shows a generalcorrespondence between interpreted stratigraphicintervals and oxic-anoxic events.

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