The Carboniferous of the Outer Moray Firth Basin, quadrants 14 and 15, Central North Sea

M. R. Leeder Department of Earth Sciences, Universi ty of Leeds, Leeds LS2 9JT, UK

and S. R. Boldy Amerada Hess Ltd, 2 Stephen Street, London Wl P 1PL, UK

Append ix by

R. Raiswell and R. Cameron Department of Earth Sciences, Universi ty of Leeds, Leeds LS2 9JT, UK

Received 9 June 1989; revised 31 August 1989; accepted 14 September 1989

Analysis of 11 wells in the Outer Moray Firth Basin reveals a thick (up to 600 m) sequence of Forth Formation (Dinantian) clastic lithofacies deposited in a variety of fluvial and lake/bay environ- ments with minor marine influences. A tentative lithostratigraphy for the Dinantian sequence of the Outer Moray Firth is based partly on the correlation of a western volcanic horizon. Palynological results indicate the presence of the Asbian and Brigantian stages, although older Dinantian stages may also be present. A western, relatively sand-poor sequence is dominated by fine-grained lake or bay facies and may possibly contain oil shales. A central and eastern sandstone and coal-rich sequence records the interaction between a fluvial system, possibly a fan delta, and a lake or bay environment. Sediment derivation was probably from the nearby uplands of the Halibut Horst granitoid to the WSW, which may have existed in the hanging wall of a basin-bounding normal fault. The Dinantian of the Outer Moray Firth Basin occurs in the core of a large pre-Permian syncline and resembles contemporaneous sequences seen at outcrop in the eastern Midland Valley of Scotland.

Keywords: Outer Moray Firth Basin; lithostratigraphy; Carboniferous

Introduction The Moray Firth area (Figure 1) has long been known to contain a large onshore outcrop and pre-Mesozoic offshore subcrop of Middle Devonian strata, defining the important Orcadian Basin (see Anderton et al.. 1979; Zeigler, 1982 and Mykura, 1983 for general reviews). Palaeocurrent data from outcrops indicate sediment derivation from surrounding uplands sourced in the metamorphic Caledonides with the rivers draining into the vast Lake Orcadia (see Allen and Marshall, 1981 ; Mykura, 1983 and Astin, 1985). Recent analysis of a thick Lower Devonian succession in well 12/27-1 (Richards, 1985) suggests that a lake was also present in early Devonian times in the offshore area, perhaps fed by streams draining out of the onshore Turiff Basin (Figure 1; see also Mykura, 1983).

There is no known outcrop or presently known subcrop of Carboniferous in the Inner Moray Firth. The first indication of an active Carboniferous depocentre in the Outer Moray Firth area came with the release of Occidental's Claymore discovery well 14/19-1 which penetrated 486 m (1594 ft) of Lower Carboniferous clastic strata, including thick mudrocks, thin sandstones, numerous coals and a volcanic horizon towards the top (Deegan and Scull, 1977; Maher, 1980; Maher and Harker, 1987). Deegan and Scull (1977) used the sequence in this well to illustrate the highly

generalized nature of the succession. They commented that successions of similar age occurred elsewhere in the Outer Moray Firth and northern Forth Approaches basins. They also noted that the lithologies present were similar to those of the Limestone Coal Group (although it should be noted that this group is of Namurian age) of the onshore Midland Valley of Scotland. Ziegler (1982) subsequently showed an estimate of the areal extent of the active Outer Moray Firth Basin in Dinantian times, the basin being shown as inverted and uplifted in the Upper Carboniferous. More recently Richards (1985) has given an account of the Upper Devonian succession of the Buchan Field in the NW of Quadrant 21 and quotes the significant observation of Hill and Smith (1979) that the upper part of this succession extends into the Lower Carboniferous.

For some years exploration companies have named the Lower Carboniferous section above Late Devonian red beds as the Firth of Forth or Forth Formation on their completion logs (for example Shell/Esso wells 15/18-1 and 15/18-2 and Monsanto well 15/21A-7). Although none of the lithostratigraphical units in the Upper Palaeozoic of the Outer Moray Firth have been formally defined, this nomenclature (Table 1) is widespread, has been extended in recent publications (O'Driscoll et al., in press) and consequently is

0264-8172/90/010029-09 $03.00 ©1990 Butterworth & Co. (Publishers) Ltd

Marine and Petroleum Geology, 1990, Vol 7, February 29

Carboniferous of the Outer Moray Firth." M. R. Leeder et al.

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o Onshore Devonian outcrop ~ ~ Forth Formation/

u~ Volcanics of Forth age f'L¢:e__ \

Figure 1 Location map of the Moray Firth area to show the onshore Devonian outcrop, the approximate position of the Halibut granitoid intrusion of presumed Caledonian age, the offshore subcrop of the Devonian and Carboniferous Buchan. Tay and Forth Formations below the Mesozoic and the location of the wells studied. 1, 14/19-6A; 2, 14/19-1 ; 3, 15/11-1; 4, 15/16-3; 5, 15/21A-7; 6, 15/17- 1A; 7. 15/18-1; 8, 15/18-2; 9, 15/19-3; 1Q, 15/19-2; 11, 1519-1. Also shown is a tentative boundary to the Forth Approaches Basin which contains both Dinantian and Namurian successions (unpublished data)

followed in tills paper.

W e l l d a t a b a s e

Eleven suites of well data were studied (Table 2). Ten of these are released and onc. 15/21A-7. is unreleased. All of these wells penetrated the Lower Carboniferous Forth Formation in the Outcr Moray Firth. The wells lie akmg a 6(I km E W belt (they include deep penetrat ions below the main reservoir levels of the Claymore. Piper and Tartan fields) that straddles the suspected axis of the presently defined synclinal axis to the depocenlrc (l=ig~re 1) and sample a total of some 2909 m f954(I f t ) of Carboniferous section. Oil has been found in ( 'a rboniferous sandstones in a number of these wells, although usually in non-commercial quantities. Thrcc of the wells have core material, including wells 15/21A-7 (13 m, 42 ft), 15/18-1 ( 7 m , 22 ft) and 14,,'19-6A (23 m, 76 ft) and most wells have both gamma and sonic wireline log traces, some also having resistivit~ logs.

B i o s t r a t i g r a p h y

Biostratigraphic data is sparse and based entirely on palynology. Released and proprietary studies (Table 2

and FTgure 2) imply tha! a fairlx complete middle to upper Dinantian succession ma~ bc present but tile deepest well penetrat ions arc not dated. The majority of released wells arc dated onl\ as Dinantian on the completion logs, with fonr wells dated as undifferentiated Carboniferous ( 7abh, 2~. Well 15/19-3 is "middle" Visean. thus probabl} ot Arundian or Holkerian age. Well 15'lS-I has more precise

Table 1 Lithostratigraphic scheme for the Upper Palaeozoic of the Outer Moray Firth (partly after Deegan and Scull, 1977; O'DiscoN et al., in press}. It seems likely that the fluviatile red bed successions of the Tay Formation are, at least in part, the diachronous equivalents of the lower part of the Forth Forma- tion (Richards, 1985; Unpublished data of the authors)

Name Thickness and Age Environment

Forth Formation Up to 1963 ft Fluvio-deltaic/lake/ Dinantian bay, (some marine?)

with coal

Tay Formation Up to 2856 ft Fluv~atile Fammenian to red beds

?Dinantian

Buchan Formation Up to 1948 ft Fluviatile Fammenian red beds

30 Marine and Petroleum Geology, 1990, Vol 7, February

Carboniferous of the Outer Moray Firth: M. R. Leeder et al. Table 2 Well database, extent and age of Carboniferous penetration and the sandstone/coal percentages for the Outer Moray Firth

Carboni- Well Operator ferous Sand % Coal % Age

penetration feet metres

14/19-1 Occidental 1594 486 18 1.8 Dinantian 14/19-6A Occidental 1090 332 36 1 Asbian/

Brigantian

15/11-1 Occidental 816 249 8.2 3.6 Carboniferous 15/6-3 Texaco 212 65 28 2.4 Carboniferous 15/21A-7 Monsanto 315 96 44 2.9 Asbian/

Brigantian

15/17-1A Occidental 922 281 39 11 Carboniferous 15/18-1 Shetl/Esso 235 72 36 1.7 Asbian/

Brigantian 15/18-2 Shell/Esso 416 127 28 6.3 Dinantian 15/19-3 Conoco 427 130 37 4.2 Middle

Dinantian 15/19-2 Conoco 1961 598 32 13 Dinantian 15/19-1 Conoco 863 263 37 8.5 Carboniferous

assignations of Dl (late Asbian) between 9765 ft and 9746 ft and P2 (Brigantian) between 9684 ft and 9618 ft. Detailed analysis of well 15/21A-7 reveals spore zones N M - V F (top Asbian- low Brigantian) between 12 25(1 ft and 12 300 ft and spore zone TC (lower Asbian) between 12 350 ft and 12 475 ft. The top part

of well 14/19-6A is newly dated as Asbian to early Brigantian.

Forth Formation lithostratigraphy and subcrop

Our interpretation of the regional distribution of the Forth Formation and underlying units is shown as a pre-Permian subcrop map in Figure 1. The concentric nature of Devonian and Carboniferous units in subcrop, based on the biostratigraphic dating of over 40 released wells and tested bv recent proprietary wells, suggests a synclinal basin structure. The distribution and nature of the sections interpreted by us from wireline logs and core is shown m fqgure 3. Internal correlation within the study area is tentatively established as follows. Groups of closely spaced wells in the western (14/19-6A, 14/19-1) and eastern (15/19-1, 15/19-2, 15/19-3) extremities of the basin were compared. A first order correlation is proposed between prominent volcanic tuffs and lavas in the west. It is possible that these might also correlate with volcanic tufts in well 15/19-2. A characteristic sandstone and coal-rich division may be recognized in the lower part of well 15/19-2 and may approximately correlate with similar facies in wells 15/19-3 and 15/19-1. In the central part of the study area further well correlations may be hung from the volcanic unit previously defined, which also occurs in wells 15/11-1 and 15/17-1A. The remaining central wells have a more uncertain stratigraphic position. The biostratigraphic

LITHOSTRATIGRAPHY

MIDLAND V A L L E Y

Passage Group

Upper Limestone Group

Limestone Coal Group

Lower Limestone Group

1 2

WELLS

OUTER MORAY FIRTH

3 4 5 6 7 8 9 1 0 1 1

Z O N E S A N D S T A G E S

FR

KV

G1 Yeadonian

R2 Marsdenian

R 1 Kinderscoutian

H2 Alportian SO

H 1 Chokierian

TK E2 Arnsbergian

Z >

C -11 m

z

F m GO > Z

c~ (1)

u)

t - O

~9 "10 c

co u~

O

O

415

o

Upper

Oil Shale Group

Lower

Oil Shale Group

I

1 I I I

t I I I I I

1 , I

I I

I I I

I i I I

i I I

I I

I I I I

Figure 2 Stratigraphic subdivision of the onshore Scottish Midland approximate extent of equivalent units from the offshore described in

NC

Z NM O h-

t r

o Tc

l- r r

,9 Pu

VF

CM

E1

P2

P1

B2

B1

Pendleian

Brigantian

Asbian

Holkerian Arundian Chadian

Ivorian Hastar ian

O < Z

m Z

z > Z

Valley Dinantian and Namurian (after Francis, 1983), with the this paper. Abbreviations FR, KV, etc. are standard spore zones

Marine and Petroleum Geology, 1990, Vol 7, February 31

Carboniferous of the Outer Moray Firth: M. R. Leeder data (see above) are helpful in placing the top portions of both wells 15/18-1 and 15/21A-7.

Little is known about the volcanics other than the brief descriptions from cuttings on completion logs. Thus well 15/11-1 has a 'few basalt or andesite pebbles'; well 15/17-1A has 'volcanic fragments'; well 14/19-6A has 'volcanics, varicoloured, agglomerate, trace obsidian', well 14/19-1 has 'volcanics'; well 15/19-2 has "agglomerate, fragments of greenish-grey to speckled volcanic material, grey-brown argillaceous matrix, trace chlorite and calcite'.

Sedimentary facies

All available wireline logs, cuttings and core have been examined and subdivided into a simple lithofacies scheme. In uncored wells, by far the majority, wireline log traces and data from cuttings and sidewall cores were used to assess lithology and the nature of lower and upper sand body contacts (Figure 3). When possible these inferences were independently checked from core data where additional information on sedimentary structures and grain size variations were incorporated into the facies scheme. Graphical summaries are given for the three cored wells in Figure 4. Five broad lithofacies have been defined (NB: the thin dolomitic limestone beds interpreted in the completion log of well 14/19-6A are seen in cores as horizons of carbonate veining). Environmental analysis of the lithofacies was aided by a pilot study of carbon/sulphur ratios (Berner and Raiswell, 1983; 1984; Raiswell and Berner, 1987) of four mudrock samples from wells 14/19-6A and 15/21A-7 (see Appendix).

Lithofacies 1 comprises sharp-based, micaceous, medium to very coarse sandstones, pebbly sandstones and fine conglomerates. Units are in the range 1 - > 6 m thick in cored intervals and up to 30 m thick in uncored sections. In the latter cases the facies may be recognized by its characteristic low gamma ray (3(I-60 API units)/low interval transit time log signature. Basal contacts with underlying lithofacies are sharp and sometimes erosive, with mudrock intraclasts concentrated locally and serving to increase the gamma ray log trace above its normal sandstone value. Units may fine upwards, seen on gamma log traces as a positive slope, or be truncated by internal erosion surfaces, thus defining multistorey units. Internal sedimentary structures include rare cross stratified sets up to 30 cm thick and rare horizontal laminations. These latter are well developed in well 14/19-6A (9320 f t -9326 ft 6 in; Figure 4A). Apparently structureless units form the majority of cored sections and are particularly characteristic of well 15/21A-7 (Figure 4B).

The characters of Lithofacies 1 are consistent with subaqueous deposition by probably channelized flows of moderate to high power. It is probable that deposition rates were high and internal structures comparatively rare due to rapid deceleration of flows. The close association of this coarse lithofacies with fine-grained sequences thought to be of lake or bay margin origins (Lithofacies 2 and 3) might further suggest that clastic dispersal occurred in distributive channels on fan deltas, although no positive deductions can be made from the small amount of core material available at the present time.

Lithofacies 2 comprises alternating thinly bedded

et al. micaceous sandstones, siltstones and mudstones with a characteristic 'spikey' log signature. Units are in the range of 0 .5 -4 m thick and frequently succeed, or are succeeded by, units of Lithofacies 1. The thin (0.05-0.5 m) very fine to medium grained sandstones are sharp based and usually fine upwards into micaceous siltstones and bioturbated carbonaceous mudstones, the latter sometimes with abundant well-preserved plant remains (e.g. at 9336 ft in well 14/19-6A; Figure 4A) and rarer rootlets. Internally the sandstones contain small scale trough cross laminations, rare mudstone interlaminae and thin (up to 0.25 m) cross sets.

The lithofacies is interpreted as the product of repeated decelerating flows from a system of distributary channels into a marginal lake or bay wetland environment. The flows may have occurred in crevasse channels or as sheet floods. It is impossible to separate the two alternatives from core data alone. It is possible that the example of this lithofacies in well 15/18-1 (Figure 4C) is a channel abandonment facies related to channel plugging because the sequence overlies an example of Lithofacies 1. One sample of the finer grained part of this facies in well 15/18-1 (Figure 4C) yielded a carbon/sulphur ratio indicative of early diagenesis by non-marine pore waters (see Appendix).

Lithofacies 3 comprises gradational, upward- coarsening sequences ranging in thickness between 1.5-3 m. These may overlie coals (Lithofacies 5 units) and dark grey mudrocks with fine organic-rich laminations (tops of Lithofacies 4 units) and pass upwards into silty mudrocks with fine siltstone interlaminations, micro-current ripple and wave modified current ripple form sets and frequent signs of soft sediment deformation and bioturbation. Units may terminate at this sediment grade (to be overlain by examples of Lithofacies 1 or 2) or continue upwards into micaceous fine to medium grained sandstones with abundant soft sediment deformation. A particularly good example occurs between 9331 ft 6 in and 9328 ft 6 in in well 14/19-6A (Figure 4A). On log traces the lithofacies may be recognized by a steady or irregular negative slope in the gamma profile.

The lithofacies is interpreted as the product of gradual progradation of crevasse minor mouth bars into low energy but wave-influenced lacustrine or brackish bay environments. Samples taken in the muddier portions of this facies have yielded C/S ratios indicative of non-marine early diagenetic pore waters (Appendix).

Lithofacies 4 comprises homogenous dark grey to black mudrocks up to 2 m thick with minor fine siltstone laminations occurring where the units pass up gradationally into Lithofacies 2 or 3. The mudrocks contain plant remains and, in one case, at 9227 ft 4 in in well 14/19-6A Figure 4C), possible fish scales. This latter horizon exhibits a low C/S ratio indicative of early marine diagenesis (Appendix), although the accompanying gamma log trace shows no signs of the uranium enrichment that is so characteristic of anoxic marine mudrocks in the Silesian of the Pennine and Southern North Sea basins (Leeder et al., 1989). Several horizons of this lithofacies may be identified in the uncored wells from their characteristically high and uniform gamma readings (90-14(1 API units) and it is possible that some of these horizons may be marine in origin.

32 Marine and Petroleum Geology, 1990, Vol 7, February

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Carboniferous of the Outer Moray Firth." M. R. Leeder et al.

The lithofacies is interpreted as the product of relatively slow deposition of mud from suspension in marine-influenced bay centre or brackish bay environments.

Lithofacies 5 comprises coal horizons which vary in thickness from a few decimetres to 3 m. No core is available but the lithology is readily identified from sonic log data by its characteristically very high interval transit time. The coals may be simply interpreted as the products of wetland environments, but the absence of core and the lack of evidence concerning the detailed sedimentary context of the coals precludes further analysis at the present time. There is evidence from the log traces of well 15/21A-7 that a thick coal underlies the coarsening-upwards sequence of Lithofacies 3 aspects, interpreted as the product of lacustrine transgression and subsequent progradation of a minor crevasse mouth bar.

Thus, m summary, a variety of fluvio-deltaic, lacustrine and marine-influenced bay environments are interpreted from the Forth Formation. The vertical

KEY

pebbly sandstone ~ current ripples

sandstone [ ~ wave ripples

siltst one ~ plant remains

~ : ~ : 1 mudrock ~ ' - - - - ] roots

horizontal to ~ bioturbation low angle laminations

[ - ~ j carbonate veins/ cross stratification concretions

contorted laminations ~ interlaminations

grain s ize s c a l e

m ; v f ; m ; v c ; p

1 4 / 1 9 - 6 A I

GAMMA RAY API ! SONIC microsecs/ft _ 1_00100 50

' < - - -

(

J

9320 '~ ~ " - ~ i . ~ - ~ J

9330 ~ - ' ~ ~

r I °~'t ----q, Litholog[ Grain size ~'~ Deposltlonal Environments land st ructures]~ ~ i

~ - ~ ,2,3j Mi . . . . . . . asse splay lobes

~ : ~ and lake/bay fines ' ~ 2 i

C / S s a m p l e n o n - m a r i n e I

i gluvio-deltaic distributary I 1 i sandbody

3 Crevasse mouthbar

2 Wetland/crevasse margin

Fluvio-deltaic distributary I !

Vegetated crevasse margin !

1 5 / 2 1 A - 7

GAMMA RAY API 50 100

SONIC microsecs/ft 100 50

tholog r Grain size I ~ p o s i t i o n a l envi . . . . ents land structures L~ ~!

"° ~ - ~ 7 ~ Major channel s ~ n ~ - o d y ~;: ~ - - ~ "2,3] abandoned channel fill

. ~i~ ~ ~ ' ~ 1 t4 I?fandelta channel

. - ~ - ~ ~ minor flood channel

~-~ i L I lake or bay fines ~ , _ ~ , , 3 with distal crevasse lobes

underlain by coal seam from sonic log response

i !50 GAMMA R A Y ~ I 0 0 100

1 5 / 1 8 - 1 ~o~T t ~ o ~ SONIC mic . . . . . . /ft ILithologl G . . . . size i ~ l s01 I and structures I-3~i

' ' i _ _ ' '= =- . - ° j . -

I Depoaitional environments 1

Vegetated wetlands with , c r e v a s s e splays

Fluvio-distributary i channel I

Figure 4 Graphic logs, wireline logs and the interpreted lithofacies of cored sections. (A) Well 14/19-6A; (B) Well 15/21A-7; (C) Well 15/18-1

Mar ine and Petro leum Geology , 1990, Vol 7, February 33

Carboniferous of the Outer Moray Firth: M. R. Leeder succession of facies in the cored interval of well 15/21A-7 Figure 4B) and data from the wireline log suite (Figure 3) establishes the repeated development of medium scale coarsening-upwards 'packages' of progressively coarser and thicker sandstones and fine conglomerates. The thinness of mouth bar facies and the poorly stratified nature of the units suggests rapid emplacement by "event' mechanisms, possibly in rapidly depositing portions of channels on prograding lacustrine fan deltas. By ~ay of contrast the finer grained sequence of '?channel sandstones recorded in the cores from well 14/19-6A (Figure 4A) overlie a coarsening-upwards sequence of mouth bar aspects and arc themselves overlain by fine grained, marine-influenced abandonment facies.

Concerning the lateral and vertical facies trends in the area there is clear evidence for a western sand-poor domain in Blocks 14/19, 15'11 and 15/16, where the successions are dominated by fine-grained facies interpreted as lacustrine or bay mudrocks, coarsening-upwards lake margin sequences and rare sharp-based ?channelized sand bodies (sandstones 8 -36% of total thickness). The latter are most common in well 14/19-6A. Coals are also present, but are not as abundant as in sections to the east. Common 'hot' shales have gamma values in excess of 10fl API units and may be organic-rich lacustrine (or possibly marine) shales. These various features imply the location of a persistent lacustrine system in the western area which repeatedly underwent fine-grained infill from suspension currents and lake margin wetland advance before renewed lake expansion. The lakes may have been productive and stratified, with an oxygen minimum encouraging the development of organic-rich facies. The area seems to have 'avoided' the incursion of fan delta and other fluviatile systems for long periods, analogous to the geographical separation of the contemporaneous Dinantian Oil Shale and Calciferous Sandstone sequences in the Midland Valley of Scotland (see Greensmith, 1965).

In the eastern part of the basin, in Block 15/19, coals occur throughout the succession, but are particularly abundant in the lower and middle parts (4 -13% of thickness). In the lower part of the succession, common channel sand bodies also occur (32-37% of thickness), but coarsening-upwards lake infill sequences are very rare. These various features indicate that the area was dominated by the active and abandoned subaerial lobes of deltas and ?fan deltas during deposition of the lowest sequences and by lake margin wetlands during upper sequence deposition. The presence of abundant channel sandstone units in central areas, higher in the inferred lithostratigraphic sequence, implies that the eastern sand dispersal system was abandoned and relocated westwards during basin evolution.

Petrography, provenance and palaeogeography

Analysis of a suite of fine to very coarse grained sandstone samples from wells 15/21A-7 and 14/19-6A shows them to be moderately to poorly sorted, angular to subrounded quartz arenites or subfeldspathic arenites. Quartz is predominantly monocrystalline and strained whilst feldspar is predominantly alkaline, ranging in abundance from 5 -20%. The abundance of kaolinite (up to 15%) in large angular secondary pores suggests that significant amounts of feldspar have been

et al. removed by dissolution during burial diagenesis. Lithic fragments include rare granitoids and polycrystalline quartz. Heavy minerals include tourmaline and zircon.

Regarding provenance, the above features, together with the proximal fan delta lithofacies present in well 15/21A-7, suggest that an upland area lay close-by, possibly with a substantial granitoid presence. The most likely area for sediment derivation is the Halibut Horst area, a persistent 'high' also throughout Mesozoic and Tertiary times. A granitoid body (probably of Caledonian age) has recently been reported under the horst (Owen, quoted in Richards, 1985; Figure 1), a postulate confirmed by the recovery of granitoid clasts recorded in wells in the area. Another possibility is that the Forth sandstones are second-cycle deposits derived from successions of Buchan Formation type which subcrop the Mesozoic around the postulated Halibut granitoid.

A palaeogeographic sketch (Figure 5) shows sediment transferred from the Halibut Uplands by channelized flows on fan- or braid-deltas into a permanent lake or bay, with some periodic connection to a marine area. The fluvial systems may have originated from the hanging wall or footwall of a normal-faulted extensional basin, similar to many other Dinantian extensional basins which are bounded by Caledonian granitoids (Leeder, 1982; Gawthorpe el al., 1989). The rapid interbedding of ?channelized sandstones and conglomerates with coals and mudrocks suggests that depositional slopes were very low and that the hanging wall model (compare Leeder and Gawthorpe, 1987 with Leeder et al. 1988, see Figure 6) may therefore be more appropriate in the present case.

Dinantian Del tas

i l y Firth

.Y/ , '11 ' ,4

. , U .,/(I

,Y \ )' a

Figure 5 Palaeogeographic sketch and regional palaeo- geographic context of the Outer Moray Firth Basin in the Dinantian. Numbered fiuvio-deltaic systems: 1, Outer Moray Firth; 2, Fife Calciferous Sandstone (see Greensmith, 1965); 3, Scottish Borders/Northumberland Border Groups (see keeder el ol., 1989)

34 Marine and Petroleum Geology, 1990, Vol 7, February

i i I I

i . : . L i i : ?i:.

,4~iiii~isiii ::~i'~i~:. I

- - ¢~Ji i;i iii . : ;ii :!i i i i ~ *~i i i i i i iii" :~?i i!i !~i:: iJ"

¢.ta /iL ii2i i ! i : JfJ i!JJ i!Si : ) " ¢ 1 - /iJ::: ~2~i : :: ili: sffsJ~-

" >' -~?#iiii?~s /~'ess z:is iiis i i ~ ~ ii 'iii;i: f":si?)?~i/i:'iJ~"

° I I L I l i b I I I [

0 1 - 2 - 3 ~, 5 6 7 8

Organ i c ca rbon %

Figure 6 Graph to show the weight percentages of organic carbon and pyrite sulphur for the four mudrock samples from wells 14/19-6A and 15/21A-7. The marine field has been adjusted to a mean value of C/S - 1.7 to allow for carbon losses during burial diagenesis (see Raiswell and Berner, 1987)

Western areas were usually lacustrine or wetland environments with only infrequent incursions by channelizcd sediment feeder systems. During early Forth Formation times eastern areas were periodically bay or lake margin wetlands with prograding fan delta lobes. This clastic input switched to central areas during the early part of the Upper Forth Formation. A basin wide w)lcanic episode (Figure 3) occurred in late Forth times (?Asbian) with the thickest development in western areas. Little is known about the nature of these volcanics other than their brief descriptions as basic volcanics and '?agglomerates in comments on ditch samples noted on completion logs. Few substantial sand bodies occur above the volcanics.

D i a g e n e s i s

Examination of a small number (13) of sandstone samples from wells 15/21A-7 and 14/19-6A shows abundant authigenic vermicular kaolinite (5 -14% of whole rock); replacive illitic clay ( 1 - 4 % ) after kaolinite, mica and feldspar; dolomite and Fe-dolomite (3 -27%) and quartz ( 2 - 6 % ) cements; and pervasive haematite staining on the crystal faces of K-feldspar, siderite and Fe-dolomite.

A paragenetic scheme may be established as follows:

(1) Early pyrite precipitation in pore spaces (2) Feldspar hydrolysis and dissolution (3) Syntaxial quartz overgrowth precipitation (4) Dolomite and ferroan dolomite precipitation (5) Illitization of detrital clays (6) Widespread coarse pore-filling kaolinite and a later

quartz overgrowth cement (7) Haematite precipitation

By comparison with haematite cements from the onshore Old Red Sandstone around the Moray Firth analysed by palaeomagnetic techniques (Robinson, 1986) phase 7 may be of Permo-Triass ic age.

Fifteen determinations of porosity in channel sandstone facies from well 15/21A-7 yield a mean of 12% with a standard deviation of 3.4%. A similar number of permeability determinations yielded a mean of 44.5 md with a standard deviation of 12 md.

Carboniferous of the Outer Moray Firth: M. R. Leeder et al. . . . . B a s i n a n a l y s i s a n d r e g i o n a l s e t t i n g

:iiiiiiii!iiii Marine field It is difficult to determine the pre-Carboniferous evolution of the Outer Moray Firth Basin because no released well in Quadrants 14 or 15 penetrates the entire Dinantian sequence. However, the concentric pattern of subcrop of the Upper Old Red Sandstone and Carboniferous (Figure 1) suggests a synclinal basin structure with a continuous record of sedimentation at least from the Fammenian (Upper Devonian) and probably from the Middle Devonian. It is a matter of conjecture as to whether the Lower Devonian lacustrine facies recorded in block 12/27 extend eastwards into the study area.

Following recent work in the Orkneys (Astin, 1985, pp. 372-374) it seems likely that the Middle Devonian Orcadian Basin was of extensional type, perhaps developing during a period of post-orogenic 'collapse', analogous to the present day Basin and Range Province (McClay elal., 1986) or, more likely in our opinion, the Pannonian Basin of the post-orogenic Carpathians (see Leeder, 1988, p. 551; cf. Royden and Sclater, 1981). Our discovery of geochemical indications of marine conditions in the late Dinantian of well 14/19-6A, although necessarily tentative at the present time, assumes great significance in the light of such geotectonic models because the "collapse" phase must have proceeded to sea level between the Middle Devonian and the late Lower Carboniferous, a period of some 50 Ma.

During late Devonian and early Carboniferous times we can be more certain about the existence of a regional extensional stress regime because there is widespread evidence for such a subsidence mechanism over the whole of the British Isles (Dewey, 1982; Leeder, 1982; Gawthorpe el al., 1989) at this time. In other northern British Carboniferous extensional basins (e.g. Northumberland/Solway) the existence of rapidly unroofing granitoid-rich hinterlands was often linked to particularly high rates of extensional normal faulting close to the granitoid margins. Such a situation seems likely for the Halibut Horst margin to the Moray Firth basin. Also, the occurrence of Asbian volcanics (although of unknown petrogenesis) finds close parallels in the widespread syn-extensional volcanics of both the onshore Midland Valley and Northumberland basins.

The position of the Outer Moray Firth depocentre is highly relevant to reconstructions of the palaeogeography of the northern British Isles and offshore areas during the Lower Carboniferous, because the area is usually shown as being within the erosional hinterlands for the Late Dinantian and Namurian Pennine River and its tributaries (see the classical palaeogeographic sketch by Gilligan (1920), Figure 3 and more recent maps by Leeder, 1987).

Since post-Brigantian strata appear to be absent in the Outer Moray Firth basin we cannot be sure whether basin uplift and erosion was initiated in the Namurian (as postulated by Ziegler, 1982, Enclosure No. 10) or, in common with much of northern Britain, in the late Westphalian or Stephanian. Consideration of regional data favours the latter alternative, although the evidence is not conclusive.

The nearest onshore Carboniferous rocks to those in our study area occur in the eastern Midland Valley, in Fife and the Lothians (Figure 2). Several aspects of DJnantian stratigraphy and sedimentation in these

Mar ine and Pet ro leum Geo logy , 1990, Vol 7, February 35

Carboniferous of the Outer Moray Firth: M. R. Leeder et al. areas may be recognized in the Outer Moray Firth, namely the occurrence of:

(1) Fluvio-lacustrine cycles with coals in the Calciferous Sandstone in Fife (Belt 1975; 1984), although the coals in this area are thinner and less common than in the Outer Moray Firth.

(2) Lacustrine or restricted bay cycles with thin coals and minor sandstones in the Oil Shale portion of the Lothians succession.

(3) Interbedded volcanics. (4) The rare occurrence of marine indicators.

The generalized and highly tentative palaeo- geographic sketch for mid-Dinantian times (Figure 5) shows the regional context of the Outer Moray Firth fluvio-deltaic system in relation to the contemporaneous f luvio-deltaic systems of onshore Fife (Calciferous Sandstone) and the Scottish Borders (Border Groups).

Conclusions Thick sequences of upper Dinantian (Asbian to Brigantian) elastic sedimentary rocks in released and proprietary wells from the Outer Moray Firth have been analysed. The few cores available show evidence for rapid vertical alternations of lithofacies of suspected fan delta, coastal plain, lake and ?marine-influenced bay origins. Poorly known volcanic horizons in several wells may provide a means ol establishing correlations. Sediment derivation was probably from the nearby Halibut granitoid, which may have existed in the hanging wall of a basin-bounding normal fault. The Dinantian of the Outer Moray Firth resembles contemporaneous sequences seen at outcrop in the eastern Midland Valley of Scotland.

Acknowledgements The authors thank the management of Amerada Hess Limited and their co-venturers in Block 15/21, Deminex UK Oil and Gas IJmited, Kerr-McGee Oil (UK) pie and Pitt Petroleum plc for permission to publish this paper. We also thank R. F. P. Hardman of Amerada Hess Ltd for his enthusiatic support of our studies. Drs R. L. Gawthorpe and D. C. Mudgc provided critical and helpful referee's comments, for which we thank them, Rob Gawthorpe suggested to us thai the hanging wall model may be most appropriate in the present case.

Appendix C/S study of palaeosalinitv

Experimental analysis. Four samples of black, finely laminated mudrocks were crushed to pass 120#, treated with 10% HC1 for 24 h to remove carbonate minerals and the residue collected by filtration and dried. Two gently crushed samples of about 5 - 7 mg were combusted in a Carlo Erba elemental analyser to determine duplicate analyses of organic CO>

To determine pyrite sulphur about 0.5 g of sample was decomposed with chromous chloride, and the ew)lved H,S was swept into a reaction vessel containing

Table 3 Analyt ical results for organic carbon and pyr i te sulphur for the Outer Moray Firth mudrock samples

Sample %CO3 % organicC % pyrite S Salinity

Well 14/19-6 9227ft 6in 12.9 2.07 065 Marine 9333 ft 8 in 6.9 1.42 0.02 Freshwater 9336 ft 4 in 8.6 2.20 0.20 Freshwater

Well 15/21-7 12 274 ft 2.2 2.66 0.01 Freshwater

.CuSO4 and the unused Cu is then determined by titration with ED TA using glycine cresol red. Blank determinations are carried out to calibrate the CuSO4 against EDTA. Because batches of four samples are run at one time, we chose to run duplicates of 14/19-6 9333 ft 8 in, 9336 ft 4 in and 9227 ft 2 in, in addition to one blank and a single determination of the sample from 15/21A-7. Results are given in Table 3,

Interpretation The organic carbon/pyrite sulphur (C/S) method

described by Berner and Raiswell (1984) can be used to distinguish marine and freshwater mud or mudrock sediments, subject to the following conditions:

(1) Brackish water sediments deposited under salinities greater than half those of present seawater cannot be distinguished from true marine sediments.

(2) The samples should contain between 1-15"/o organic C and <65% CaCO~.

Under these circumstances a plot of organic carbon against pyrite sulphur shows thai marine sediments fall within an envelope which has a mean C/S ratio of 2.8. Because sulphate is less abundant in freshwater the formation of pyrite is sulphate-limited and much higher C/S ratios are observed. It should be noted, however, that the organic carbon content of mudrocks is progressively lessened during deep burial by thermal decarboxylation reactions (Raiswell and Berner, 1987), In the case of the present samples, whose maturity measured by vitrinite reflectance is around 0.6, the marine C/S ratio is reduced to about 1.7 (f=i~ure 6).

The four unknown samples give clear ('/S signals (Figure 5), one (well 14/19-6. 9227 ft 6 in) appears to have suffered early marine diagenesis whils! the other three are of freshwater origin.

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