Sedimentology and hydrocarbon potential of the Dinantian ... · Jon Gluyas, University of Liverpool. Present Address; B.P. Petroleum Develop ment Ltd., London. Carboniferous (Dinantian)

Sedimentology and hydrocarbon potential of the Dinantian oil-shales of Northern Britain

Report of a meeting sponsored by the British Sedimentological Research Group

and held at the Grant Institute of Geology, University of Edinburgh, 4 April

1986*.

Introduct ion

John Cater (Convenor), University of Edinburgh

The BSRG workshop was convened as a result of an upsurge of interest in the Dinantian oil-shales of northern Britain expressed at the 1984 A.G.M. of BSRG in St. Andrews. The workshop coincided both with a drastic reduction of oil prices and with the announcement of the Department of Energy's awards of onshore exploration licences in Fife, Lothian and throughout the Northumberland Trough.

Industrial interest in the Scottish onshore oil-shales relates to their very high hydrocarbon source-rock potential, and dates from their first commercial mining in 1885. Their exploitation has provided core material and other subsurface data, much of which is now accessible to academic scientists. Recent advances in such diverse fields as organic geochemistry, structural geology, carbonate diagenesis and micropalaeontology are improving our understanding of this material. It was appropriate that the Edinburgh workshop attracted a very wide audience ranging from oil company Chief Geologists to commercial fossil hunters, with nearly half of those present being oil industry employees.

The discussions, both in the lecture theatre and in the field, reflected this healthy diversity and showed that there is no concensus on many basic aspects of the Scottish oil-shales, from the salinity of their depositional environment to their subsurface structure. They also emphasised the value of a greater exchange of information between industry and academia, and I hope the Edinburgh workshop helped to encourage this.

Oil-shale depos i t ion: an o v e r v i e w

A. J. Fleet, BP Research Centre, Sunbury-on-Thames

Oil-shales can only be sensibly defined in economic terms, as organic-rich mudstones (ORM) which, on pyrolysis, yield liquid petroleum in commercial quantities. Current crude oil prices not withstanding, this means that an ORM must yield a minimum of 20-35 kg of oil per metric tonne of rock (25-42 litres per metric tonne: Tissot & Welte (1984). Oil-shales are therefore the immature equivalents of rich oil-prone petroleum source rocks. The total organic carbon contents of recognised oil shales varies from about 10-75%.

Hydrogen-rich kerogen which yields oil on pyrolysis or on natural maturation in the case of petroleum source rocks, is predominantly derived from aquatic organic matter, that

* These abstracts have been the subject of editorial review aided by the meeting's convenor, but not to external refereeing.

Scott. J. Geol. 22, (3), 417-429, 1986

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418 CONFERENCE REPORT

is from freshwater to saline phytoplankton: certain components of higher plants, e.g. the waxy cuticle of foliage, may contribute. Oil-shales will have accumulated, therefore, in environments where such material is preserved in abundance. Aquatic organic matter is preserved under oxygen-deficient or -depleted (ODD) conditions which inhibit benthic activity and prevent bioturbation. Higher plant detritus seems to make a contribution to the kerogen of recognised oil-shales where it is preserved in aquatic environments.

ODD bottom conditions obviously develop where oxygen demand exceeds or is nearly equivalent to oxygen supply. Organic matter degradation creates the demand. The demand is therefore a function of organic productivity and organic matter supply to the bottom. Oxygen is supplied by sinking surface waters, the oxygen contents of which come from exchange with the atmosphere and photosynthesis. Oxygen supply is therefore, a function of water circulation, being least when permanent stratification is approached. The environments in which ODD conditions may develop in response to restricted water circulation and the demands of organic productivity are: 1. Stratified lakes, 2. Stratified restricted basins (geographically and/or bathymetrically enclosed), 3. Some continental shelves beneath areas of coastal upwelling, 4. Continental shelves impinged on by intense oxygen minima.

Both large and small lakes seem to provide the most favourable depositional environments for oil-shales e.g. Dinantian, Midland Valley; Permian Irati, Brazil; Eocene Green River, U.S.A; Tertiary, Thailand. This may reflect three factors: namely, the relatively pristine state of the organic matter input and its hydrogen-rich nature, the lack of sulphate reduction and the permanency of lake stratification. The prevalence of oil-shales from lacustrine environments in the geologic record may also reflect the preservation of immature lacustrine ORM in epicontinental basins as opposed to the burial of marine ORM within continental margins. Oil-shales are also known from the marine environment a) from restricted basins—e.g. Triassic Serpiano, N. Italy; b) from upwelling regions—e.g. Permian Phosphoria, U.S.A.; c) from shelves beneath oxygen minima—e.g. Toarcian, Western Europe.

Geographically oil-shales are generally restricted to lower latitudes, reflecting the depositional conditions under which they were deposited. There seems to be no stratigraphic control on their distribution apart from lack of erosion and burial favouring their presence in Tertiary sequences.

Organic matter oxidation and carbonate generation during early diagenesis of Dinantian mudrocks (SW Scotland)

Jon Gluyas, University of Liverpool. Present Address; B.P. Petroleum Develop

ment Ltd., London.

Carboniferous (Dinantian) sedimentary rocks crop out along the south and east coasts of Kirkcudbrightshire, which during Carboniferous times formed the northern margin of the Northumberland Trough. Deposition of sandstones, mudstones and limestones took place in a variety of sedimentary environments from fluvial to fully marine.

Nodular limestone beds and individual limestone nodules are abundant throughout the sequence and occur mainly in the mudrocks. In addition to carbonate cements, the nodular limestones commonly contain a few weight percent authigenic pyrite and apatite. The carbonate minerals identified include: calcite, ferroan calcite, dolomite, ferroan dolomite, ankerite and siderite. Most of the nodules investigated contained at least two cementing carbonate minerals.

Each of the nodular limestones suffered a complex diagenetic history only fully revealed by analysis of their authigenic mineral distribution, carbonate chemistry and stable isotope composition (both carbonate and pyrite).



DINANTIAN OIL-SHALES 419

Bedding lamination within the enclosing mudstones compacts around the nodules. This is taken to indicate that the nodules began to grow before appreciable compaction of the host muddy ooze, and hence in the upper few tens of metres of the sediment column. In addition, since the precipitation of carbonate during nodule growth probably fills pores without too much displacement or replacement of detritus (Lippmann 1958; Oertel and Curtis 1972), the carbonate content may be used as an estimate of porosity at cementation. The carbonate content of the nodules ranges between about 80 wt. % in their centres to about 40wt.% at their margins (cf. host mudstones, c. 10wt.% carbonate). This indicates that the nodules grew over a considerable compaction history.

Stable isotope (<513C) data are interpreted as indicating that the nodular limestones formed from the bacterial oxidation of organic matter during the processes of sulphate reduction and methanogenesis (Gluyas 1983). For the three nodular limestones which were isotopically analysed, the total range in <513C (PDB) is from -13-0 %o to +0-9%o; although the range for individual nodules was smaller. This range corresponds to an input of sulphate reduction carbonate of between 35% and 70% and methanogenic carbonate of between 65% and 30%.

The calcium, magnesium, iron and other minor elements included in the carbonate phases were supplied by the host sediment and its attendant pore water. The anion composition of the pore water controlled the relative importance of the sulphate reduction and methanogenesis reactions. Changes in pore water composition, from marine to non-marine, probably resulted from secular variations in the depositional environment.

Nucleation of the nodular limestones occurred on horizons enriched at deposition in organic matter. Such horizons probably accumulated during conditions of temporary anoxia and/or suboxia.

Since most of the carbonate in the nodules was produced from organic matter oxidation, the carbonate content of the nodules, together with their abundance, may be used to estimate the minimum quantity of organic matter oxidised during their growth. On average, a minimum of 0-5wt.% organic carbon must have been removed from the mudstone sequence to sustain nodule growth.

The Burdiehouse Limestone Formation: a guide to oil-shale depositional environments

Guy W. F. Loftus, Shell Internationale Petroleum Maatschappij BV., The Netherlands.

Environmental analysis of oil-shales per se is obscured by the equivocal nature of their primary and diagenetic constituents. Oil-shales commonly occur in association with carbonates in the Oil-Shale Group, the thickest and best known of which is the Burdiehouse Limestone Formation. Limestones lend themselves much more readily to environmental interpretation than do the oil-shales with which they are associated. The Burdiehouse Limestone Formation is used as an allusory tool for oil-shale deposition.

Oil-Shale Group sediments of the eastern Midland Valley of Scotland accumulated in a series of north-south trending, tectonically and volcanically active basins. For the most part, the basins were land-locked to the north, south and southwest and periodically became barred in the east to marine influence such that they became the site of a group of restricted standing bodies of water. The relief, geological make-up, climate and drainage affecting the surrounding land areas determined the nature of the sediments deposited within the bodies of water and resulted in complex interactions between freshwater and marine conditions. A range of lacustrine, transitional lagoonal and fully marine conditions is recorded in the succession. Sedimentation is expressed as a series of cycles reflecting alternating periods of expansion and contraction of a standing body of water.




Each sedimentary cycle is resolved into four phases which are viewed in the context of delta progradation and retreat. These include a Transgressional phase (grey shales, cementstones and rare, thin marine bands), a Lagoonal/Lacustrine phase (oil-shales and carbonates), a Progradational phase (coarsening upward siliciclastic sequences) and an Aggradational phase (coals and seatearths or "fireclays"). The Lagoonal/Lacustrine phase of each cycle is represented by oil-shales that are commonly overlain by laterally impersistent dolomitic algal limestones and are also occasionally accompanied by ostracodal limestones, exemplified by limestone members of the Burdiehouse Limestone Formation. A study of these limestones has yielded the following interpretations concerning their depositional environments: (1) the limestones were deposited in a series of basins separated by volcanically active regions of minimum subsidence; (2) they accumulated in a closed system of freshwater lakes sustained by continuous runoff in a wet tropical climate; (3) the lakes were broad and shallow fringed by prolific vegetation with margins sloping gently towards deeper water (<100m) which was probably meromictic; (4) sedimentation was dominated by the presence of algae which both supplied and modified the calcitic sediments. Six facies are recognised in the limestones called here marginal-paludine, paludine, supralittoral, littoral, sublittoral and profundal. An environmental synthesis of the Burdiehouse limestones is used to demonstrate that both oil-shales and limestones, which occupy the same positions in the depositional cycles, were deposited from a series of freshwater lakes, known collectively as Lake Cadell.

The algal-dominated constitution of the areally extensive Burdiehouse Limestone Formation is fundamental to an appreciation of the palaeolimnology of Lake Cadell in which algal oozes, consisting essentially of phytoplanktonic algae related to the modern form Botryococcus braunii, accumulated.

Geophysical constraints upon Carboniferous sedimentary basins in the Midland Valley of Scotland.

M. C. Dentith,1 A. Conway,2 J. J. Doody1 and J. Hall1, University of Glasgow1

and University of Birmingham.2

Interpretation of two c. 100 km long upper crustal seismic refraction lines (MAVIS) across the Midland Valley is presented. The experiment was funded by the Tricentrol Oil Corporation and designed to (a) act as a "framework" for reflection data, (b) obtain maximum thicknesses for the Carboniferous strata, and (c) provide data on the Bathgate gravity and magnetic "high" and Midland Valley upper crustal structure.

Time-distance graphs for the lines allow the recognition of four branches interpreted as due to four crustal layers: layer 1 - Carboniferous and Upper Old Red Sandstone (ORS), 2 - Lower ORS and Lower Palaeozoic, 3 - top crystalline basement, and 4 - higher velocity crystalline basement.

Direct arrivals give curved time-distance branches indicating a gradual increase in velocity with depth. Wiechert-Herglotz-Bateman (WHB) inversion was used to determine the velocity structure within the upper 1-5 km of the top crustal layer. Depth sections were constructed using the plus-minus method. These profiles were refined by ray-trace modelling, using the SEIS81 program, which also allowed the velocity distributions obtained using the WHB inversion to be refined and extended to greater depth.

Layer 1 varies in thickness from 2 to 4 km and is about 2 km thick in the area of the Lothian oil-shale fields. Layer 2 varies from 0 to 5 km thick and layer 3 from 2 to 4 km thick. There is a seismic velocity "high" interpreted as due to the northward continuation of the igneous rocks exposed in the Burntisland Anticline. There is no obvious correlation of the Bathgate anomaly with the velocity structure, or with any basement feature.

Preliminary 2D gravity modelling along the profiles suggests that at least part of the




source of the Bathgate "high" lies within layer 1. A more detailed 3D interpretation is currently being undertaken.

Modelling of the magnetic anomaly produced two models representing the shallow and deep extremes. The shallow model involves an upright cone of basic rocks extending to a depth of 5 km. The deep model involves an ultra-basic intrusion extending from 10-14 km depth. These models must be considered as end members of a series of possible solutions though the shallower basic source is thought to be more likely.

Layer 1 (Carboniferous and Upper ORS) was investigated in more detail using 20-40 km long refraction lines with quarry-blast sources. The results confirm those from the MAVIS experiment, the layer being about 2 km thick, with no increase in seismic velocity associated with the Bathgate "high".

The geophysical data suggest that the part of the source of the Bathgate anomaly lying above the ORS unconformity is anomalously dense and magnetised but has "normal" seismic velocity. This can be explained in terms of a basic lava pile where boles reduce seismic velocity whilst not significantly reducing density or magnetisation. Such an explanation is supported by the presence of basic lavas at the surface near Bathgate and at depth in the Rashiehill borehole.

The presence of lavas of probable early Carboniferous age at Bathgate and continuing north from Burntisland would probably have controlled the distribution of the oil-shales as is seen elsewhere in the Midland Valley.

Scottish oil-shale basin tectonics and sedimentation in the context of Carboniferous North Atlantic rifting

R. S. Haszeldine, University of Strathclyde.

Late Devonian sediments in the Midland Valley record NE axial drainage of large, low sinuosity rivers, supplied by alluvial fans from the faulted NW basin margin, and which fed into marginal marine environments in Fife. Sinistral transcurrent stress controlled this geography. Carboniferous drainage changed completely, with shallow evaporitic Cement-stone facies merging diachronously up into the Oil-Shale Group, when major rivers flowed south into the Midland Valley from the NE and north. Drainage remained similar to this throughout the Carboniferous.

Loftus (1984) states that Oil-Shale Group basins were north-south oriented; similar orientations can also be seen in the overlying Lower Limestone Group, Limestone Coal Group and Westphalian Basins, which have a 50 km spacing. Within these second-order depocentres, third-order sub-basins adjacent to active east-west normal faults may form sets with north-south alignments. East-west extension must have been persistent, in addition to north-south tension.

Whilst Oil-Shale Group sedimentation was proceeding, alkali basalt rift volcanism in the west of Scotland produced 1 km of Clyde Plateau Lavas within 10 m.y. These formed part of a major alkali basalt province across most of the Midland Valley and throughout the northern British Isles, often marking the start of rapid basin subsidence.

Marine faunas are rare in the Oil-Shale Group, and show a limited marine connection into a rapidly subsiding, starved basin, which accumulated 1-6 km of sediment within 12 m.y. Faunal abundance increases rapidly towards the top of the Group, as part of the northward transgression of the equatorial Rheic Ocean. Extreme salinites in fluid inclusions from the Lower Carboniferous Tyndrum Pb-Zn deposit, and halite pseudomorphs in north Fife were produced by sabkha evaporation at the northern limit of this Rheic transgression.

Mid-Carboniferous clastic rejuvenation was supplied by doming around a fractured rift line to the west and north of Britain (Haszeldine 1984). The resulting coastal plain progradation infilled basins and excluded the equatorial sea by the mid-Westphalian C. During the late Westphalian C, north-south trending basins ceased to subside. British




FIG. 1. The north Atlantic in late Carboniferous times, (by M. J. Russell).

basins with east-west axes continued to subside rapidly until after the Westphalian D, culminating in the 300 Ma quartz dolerite dyke swarm running from Scotland to the North Sea Central Graben. The Oslo Graben experienced similar volcanism at the same time. Rupture of the crust between northern Spain and the northern Rockall Trough (Fig. 1) had resulted in stress reorientation from east-west to north-south. Dyke intrusion marked the final rupture across the Faeroe Basin, which permitted spreading to form the 220 km width of oceanic lithosphere in the Rockall Trough. This newly-formed rift formed the route for southwards transgression of the Arctic Ocean. Distinctive Boreal Faunas occur in the late Carboniferous of the Oslo Graben and in the Permian of the North Sea and NW England. The Viking Graben could, therefore, contain Lower Carboniferous oil-shale facies 1 km thick, which could have generated hydrocarbons early in the North Sea subsidence history, preserving reservoir porosity.

T h e classification o f the L o w e r Carboniferous in Fife and Lothian

M. A. E. Browne, British Geological Survey, Edinburgh.

The new BGS classification of the Lower Carboniferous Calciferous Sandstone Measures (including the stratigraphical position of the Upper and Lower Oil-Shale Groups) is presented (Fig. 2). Isopachyte maps for the Courceyan to early Brigantian (Fig. 3) and



SERIES

VISEAN

TOURNAISIAN

FAMENNIAN

STAGE

Brigantian

Asbian

Holkerian

Arundian

Chadian

Courceyan

GONIATITE

ZONES

P2

PI

B 7

MIOSPORE

ZONES

VF

NM

TC

Pu

CM

OLD CLASSIFICATION

LOWER LIMESTONE GROUP

CALCIFEROUS

SANDSTONE

MEASURES

UPPER OIL-

SHALE GROUP

LOWER OIL-

SHALE GROUP

CEMENTSTONE

GROUP

UPPER OLD RED

SANDSTONE

NEW GROUP

CLASSIFICATION

(Paterson and

Hall 1986)

LOWER LIMESTONE

GROUP

STRATHCLYDE

GROUP

(Lothian Oil-

Shale Group)

INVERCLYDE

GROUP

STRATHEDEN

GROUP

FORMATIONS

Pathhead

Sandy Craig

Pittenweem

Anstruther

Fife Ness

Balcomie

Ballagan

Kinnesswood

Knox Pulpit

Dura Den

Glenvale

Burnside

MAXIUM

THICKNESS

m (FIFE)

240

220

670

220+

810+

230+

50+

240

120

180

40

450

150

MAJOR

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BANDS

8

6 1

4

4

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PHASE

Curvirimula

Naiadites

Paracarbonicola

Modiolus

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FIG. 3. Courceyan to lower Brigantian, conjectural isopachytes for the Midland Valley.

late Brigantian (Lower Limestone Group, Fig. 4) have been prepared as part of the Department of Energy low enthalpy geothermal resources programme (Browne et al. 1985). Palaeogeographical maps have also been constructed (Browne et al., in press). Periods of tectonic activity are recognised based on the occurrence of areas of non-deposition, the presence of pedogenically influenced formations (Kinnesswood, Balcomie, Sandy Craig), and the presence of unconformities (Pathhead and Sandy Craig formations variously resting on the Ballagan and Kinnesswood formations). The Burdiehouse Limestone appears to lie stratigraphically at the changeover in non-marine faunas from the Naiadites to the Curvirimula phase. On these grounds, the occurrence of Naiadites above the limestone on Inchkeith suggests that is is not a correlative of the Burdiehouse Limestone. Corals have been found in the Pittenweem Marine Band, the Pumpherston Shell Bed equivalent, at Elie in east Fife which markedly alters the marine faunal distribution of the Coral-Brachiopod fades presented by Wilson (1974, fig. 3) for the Dinantian of SE Scotland.




FIG. 4. Upper Brigantian (Lower Limestone Group), conjectural isopachytes for the Midland Valley.

Hydrocarbon compositions of bitumens in Lower Devonian volcanics,

Ochil Hills: relationship to Oil-Shale Group bitumens

J. Parnell,1 N . Robinson,2 G. Eglinton,2 S. C. Brassell,2 Queen 's University, Belfast1 and University of Bristol,2

Solid and viscous hydrocarbons occur in fractured lavas of the Lower Devonian Ochil Volcanic Formation at Alva, Ochil Hills. Gas Chromatography and Mass Spectrometry studies have been used to compare bitumen extracts from these occurrences with those from the Oil-Shale Group outcrop of the eastern Midland Valley, and with plausible source rocks from the Oil-Shale Group and from the Lower Old Red Sandstone Jaminites in Angus.




Samples of Pumpherston Shale (Oil-Shale Group) and Lower ORS both yielded abundant n-alkanes but with different distributions: a greater range and abundance of long chain homologues were extracted from the oil-shale. Polycyclic hydrocarbons are much more abundant in the Lower ORS. Both source rocks showed characteristics of mature shales. As expected, hydrocarbon samples from the Oil-Shale Group yield similar gas chromatogram distributions to the Pumpherston Shale sample. Several Oil-Shale Group hydrocarbons show evidence for distillation rather than generation by burial. These analyses match their field occurrence in the vicinity of igneous intrusions. Differences between samples reflect variations in source material or degree of biodegradation, e.g. hydrocarbons in a quartz dolerite sill from a 54 m deep core section at Hilderston are less biodegraded than those from the margin of a quartz dolerite dyke in Linhouse Water. The Alva hydrocarbons yield a very high abundance of long chain n-alkanes, with a sharp increase in abundance at C2o, which suggests a distillation cut produced by thermal 'event'. This is possibly the intrusion of quartz dolerite along the adjacent Ochil Fault: hydrocarbons occur in the Carboniferous sequence on the downthrown side of the fault. Polycyclic hydrocarbons were not detected. This suite of hydrocarbons suggests that a Carboniferous source for the Alva hydrocarbons is more likely than a Lower ORS source. The Alva extract similarly resembles the extract obtained from the Carboniferous torbanite of Lothian.

Organic maturation of Scottish Dinantian oil-shales: a study using biomarkers

J. F. Bateson1 and R. S. Haszeldine,2 Britoil pic,1 and University of Strathclyde2

We report here on the distribution of biological compounds (biomarkers) sensitive to organic source and thermal maturity in several Scottish Dinantian oil-shales. These sterane and triterpane compounds have been extracted from oil-shale samples, and then analysed by gas chromatography-mass spectrometry.

An 80 cm core of Upper Oil Shale Group Mungle shale, obtained from 1 km north of Broxburn shows a similar C27, C28 and C29 sterane distribution in each sub-sample up the core (Fig. 5). This indicates a constant mix of organic debris during deposition with little terrestrial input. Sterane and triterpane biomarker ratios were also measured; such ratios are sensitive to maximum temperature rather than organic debris input. These ratios are identical throughout this core and approximate to a vitrinite reflectance of 0-4-0-5 percent, showing that the shale in this core is immature for oil generation.

At Mid Calder-Linhouse Water [NT 079 671], a 300 m east-west fault throws the sandstones, limestones and mudstones of the Lower Oil-Shale Group against the shales and marls of the Upper Oil-Shale Group. Analysis of C27, C28 and C29 steranes from both Lower and Upper Oil-Shale Group mudstones (Fig. 5) shows a consistent terrestrial input into all sediments, which was slightly higher than into those of the Mungle shale.

A 2 m wide, 300 Ma dyke post-dates the faulting and parallels the fault plane. Samples collected from traverses away from each side of the dyke, and which also crossed the fault plane, were analysed to assess the heating influence of the dyke (Fig. 6). A preliminary interpretation suggests that the Lower and Upper Oil-Shale Group sediments have a similar background maturity away from the heating influence of the dyke. There is no distinction between the maturity of the Lower and Upper Oil-Shale Group sediments. As the biomarker reactions are irreversible, such ratios will reflect the maximum maturity reached. This may be interpreted in two ways: either this technique is too insensitive to "pick out" maturity differences that result from the 300 m fault throw (i.e. 10°C), or, more probably, that the regional maturity post-dates faulting.

Ratios shown on Figure 6 suggest a trend of decreasing maturity maximum from the dyke to a distance of at least 100 m. This indicates that the dyke emplacement (300 Ma)




VARIATION IN THE PROPORTION OF THE C27, C28 AND C29

5a(H), 146(H), 176(H) (S + R) STERANES

C27

100%

MID CALDER LOCALITY / \ / \ • MUNGLE SHALE SAMPLES (LOWER OIL SHALE GROUP) MID CALDER LOCALITY / \ / \ / \ COAL BENEATH MUNGLE (UPPER OIL SHALE G R O U P ) / — X — A — A SHALE

CAMPS SHALE

C28 100%

FIG. 5. Precursors to the steranes, the sterols, show a strong bias towards C29 in extracts of higher terrestrial plants. This fact is routinely applied to interpreting rock extracts. Here, with some overlap, the tendency is for a greater input of higher terrestrial plants into the Mid Calder-Linhouse Water samples and Camps shale sample, than into the Mungle shale samples.

post-dates the time when regional maturity maximum was reached. However, samples within a few centimetres of the dyke show anomalously low maturity. Further work is in progress on additional samples to address this problem. In addition, the abnormally high C32 hopane ratio (Fig. 6) and the effect of variations in organic matter input on the 18or(H) -r- [17ar(H) + 18ar(H)] trisnorhopane ratio are also being investigated.

Drawing these data together, the following maturity history can be proposed for the area around Mid Calder-Linhouse Water. We assume here that the biomarkers were sensitive to 300 m of fault throw. 1. Deposition and burial of Lower and Upper Oil-Shale Group Sediments was followed by

faulting. 2. Further subsidence occurred. A regional maturity maximum was reached, so masking

the differences between the Lower and Upper Oil-Shale Group sediments. This was pre 300 Ma.

3. Emplacement of the dyke has locally raised the maturity levels. The background regional maturity is still comparatively low with sediments having never attained the oil generation threshold.

Acknowledgement: J. F. B. acknowledges Britoil's permission to publish this abstract.




VARIATION OF SELECTED TRITERPANE RATIOS AWAY FROM DYKE, MID CALDER LOCALITY

o C27 triterpane 18a(H)/(17a(H) + 18a(H))

. C31 hopane 17a(H), 210(H) S/(S + R)

. C32 hopane 17a(H), 218(H) S/(S + R)

.2 .4 .6 .8 1.0 RATIO

FIG. 6. All three ratios plotted are of the form B/(A + B) where B is the product and A the reactant. Thus all three ratios show an increase as the dyke is approached except for the anomalous data close to the dyke-sediment contact. Different starting and end values of the ratios relate to the different reaction kinetics involved in each transformation.

Facies analysis of a Scottish Dinantian Lower Oil-Shale Group sedimentary sequence, from Collinswell, Burntisland, Fife

S. J. Maddox, University of Edinburgh.

A 12 m Dinantian (Asbian) Lower Oil-Shale Group sedimentary rock sequence crops out within the Burntisland district of Fife. The lowest exposures are 8 m of tabular cross-bedded sandstones containing fossil plant remains. Evidence of bioturbation is sparse, except within the uppermost sandstone beds, which pass upward into a i m dark shale. The shale contains fossil fish remains, ostracodes, one species of bivalve, plant remains and coprolites. This passes upward into a 4 m, buff-yellow, laminated dolostone, which is dominantly dolomicritic and oolitic. The dolostone also contains numerous intraclasts which occasionally form coarse breccias. Mud cracks and small-scale folds are also present. The topmost dolostone beds are interbedded with thin sandstones, shales and limestones. These beds are fossiliferous and contain fish remains, ostracodes, plant remains and corprolites. The sedimentary sequence is terminated by a picrite sill (Maddox and Andrews, in press).

Petrographic study reveals that the laminated dolostone fabric is composed of alternating dolomicritic and dolosparitic laminae. Oolitically-coated grains are abundant and some display a radial texture. Dolomicritic and laminated intraclasts are present, the

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dolomicritic intraclasts occasionally containing filament moulds of cyanobacteria. Intraclasts are sometimes coated with dolomicritic and dolosparitic laminae.

The lower sandstone beds are interpreted as the deposits of a fluvially-dominated delta. The uppermost, bioturbated sandstone beds indicate a waning of current energy, culminating in the deposition of fine-grained clastic sediment, now represented by the overlying shale. The fossils within the shale are suggestive of a non-marine palaeoenviron-ment, such as a brackish lagoon. The laminated dolostone is interpreted as a cryptalgal laminite, deposited around the peri-littoral zone of the lagoon. Mud cracks and intraclastic breccias resulted from periodic exposure and subsequent desiccation of the partially lithified cryptalgal laminite. The absence of fossils, with the exception of cyanobacterial filament moulds, is indicative of biologically stressful conditions. The small-scale folds are somewhat enigmatic, possibly forming in response to seismic shocks associated with local volcanicity. The fine preservation of radial ooid textures and cyanobacteria] filament moulds is probably indicative of early diagenetic dolomitisation. The topmost dolostone beds, interbedded with thinly-bedded, fossiliferous sandstones, shales and limestones represent a partial return to clastic sedimentation. This part of the sequence may have been deposited within the sub-littoral zone of the lagoon.

Although the exact stratigraphic position of the dolostone is unknown, it may be correlated with a thinner, yet lithologically similar, laminated dolostone directly above the Pumpherston Shales of the Lothians.

REFERENCES

BROWNE, M. A. E., HARGREAVES, R. L. and SMITH, I. F. 1985. The Upper Palaeozoic Basins of the Midland Valley of Scotland. Invest. Geotherm. Potent. UK., Br. Geol. Surv.

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MS. accepted for publication 5th October 1986



Documents

Sedimentology and hydrocarbon potential of the Dinantian ... · Jon Gluyas, University of Liverpool. Present Address; B.P. Petroleum Develop ment Ltd., London. Carboniferous (Dinantian)