Kerogen facies and maturity of the Kimmeridge Clay Formation in southern and eastern England

Kerogen facies and maturity of the Kimmeridge Clay Formation in southern and eastern England

Kerogen facies and maturity of the Kimmeridge Clay Formation in southern and eastern England* I. C. S c o t c h m a n t A m o c o (UK) Exploration Co,...
Download PDF
2MB Sizes 0 Downloads 50 Views
Report

Kerogen facies and maturity of the Kimmeridge Clay Formation in southern and eastern England* I. C. S c o t c h m a n t A m o c o (UK) Exploration Co, A m o c o House, West Gate, London W5 1XL, UK

Received 2 June 1989; revised 26 September 1990; accepted 3 October 1990 The Upper Jurassic Kimmeridge Clay Formation of southern and eastern England was deposited under anoxic or dysaerobic conditions in a stratified shelf sea. Anoxic conditions developed in the intra-shelf Weald and Central Channel Basins of southern England and the Cleveland Basin of eastern England, with generally dysaerobic conditions in the flanking shelf areas. Only in the basin margins, such as in northern Scotland, were oxic bottom water conditions prevalent. The maturity and distribution of the kerogen facies, which can be explained in terms of the palaeogeography and burial history of the Upper Jurassic of southern and eastern England, are discussed along with the effects of their variation on commonly used geochemical parameters. Relationships to clay mineralogy are also examined. Kerogens are largely Type II amorphous sapropels of marine origin with variable amounts of terrestrially derived Type III material. Type III kerogens predominate in the basin margin areas whereas Type II kerogens are restricted to the basin centres. Over the intermediate shelf areas mixed Type II-Type III kerogens are prevalent. Both the sedimentation rate and differential organic matter preservation can be demonstrated to be the major controls on the kerogen facies of the Kimmeridge Clay Formation. Variations in the kerogen facies appear to be responsible for the often spurious and scattered maturity measurements from the Kimmeridge Clay Formation using vitrinite reflectance, spore coloration and Rock-Eval pyrolysis parameters, the results often conflicting with geological models for the post-Jurassic burial history. This appears to be due to the highly variable content of terrestrial and recycled Type III kerogens from older sediments in the mudstones, causing the conventional maturity indices to often not reflect the true maturity of the autochthonous marine Type II kerogens.

Keywords: Kimmeridge Clay Formation, UK; kerogen facies; maturity

Introduction The Upper Jurassic Kimmeridge Clay Formation, with its outcrop and extensive subcrop in southern and eastern England (Figure 1), has, in recent years, been the subject of extensive study due to its large resources of oil shale (Macleod-Matthews, 1975; Gallois, 1978; 1979). A detailed stratigraphy has been erected (Gallois, 1979, and references cited therein), providing a framework for regional diagenetic and organic geochemical studies (Scotchman, 1989; 1991; and Ebukanson and Kinghorn, 1985; Williams, 1986; Scotchman, 1987a; 1987b, respectively). These studies show that depositional features, in particular the sedimentation rate, exerted a strong influence on the sedimentology, the lithological character and the kerogen facies of the mudstones, and their subsequent style and extent of diagenesis. Organic geochemical studies have shown the onshore mudstones to be * Presented at the 'Regional geology of the southern United Kingdom: petroleum exploration perspectives' session of the Seventh Meeting of the Geological Societies of the British Isles, held at University College London on 21-23 September 1988 i" Present address: Enterprise Oil plc, 5 Strand, London WC2N 5HU, UK

immature to marginally mature (Gallois 1979; Williams 1986; Scotchman, 1987a; 1987b), but extremely oil prone, the kerogen types being largely independent of the cyclic lithology (Williams and Douglas, 1983). The aim of this paper is to re-examine the organic geochemistry of the Kimmeridge Clay Formation and, in particular, to compare the maturity parameters and study the effect of kerogen facies variations.

Geological background The Kimmeridge Clay Formation is a laterally extensive, organic-rich mudstone deposited during the Late Jurassic transgression over much of north-west Europe (Tyson, 1987). Deposition took place in a relatively shallow epeiric sea (50-100 m deep) (Myers and Wignall, 1987) with a narrow northern seaway connection to the Boreal Ocean and limited connections in the south to the Tethyan Ocean (Gallois, 1978; Oschmann, 1988; Figure 4 of Dor6 et al., 1985). North-south trending rift grabens, positive axes and low lying islands divided the Kimmeridgian sea into several sub-basins. Current depositional models suggest that the waters of this epicontinental sea were stratified with a

0264-8172/91/030278-18 ©1991 Butterworth-Heinemann Ltd 278

Marine and Petroleum Geology, 1991, Vol 8, A u g u s t

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman mudstone units of the basin being present on the swells,

Figure 1 Upper Jurassic palaeogeography of the UK area showing the distribution of the Kimmeridge Clay Formation and its offshore North Sea time equivalents

predominant, sluggish, southerly water flow (Oschmann, 1988). Miller (1988) suggests that stratification was due to salinity variations in the water column with evaporative brine, formed in arid shallow embayments flanking the Tethyan Ocean, sinking and moving northwards to fill the grabens and basins. Oschmann (1988), however, proposes a wind-driven, temperature-stratified model with the seasonal development of water stratification. He suggests that in summer, colder, oxygen-poor, nutrient-rich bottom waters flowed southwards from the Arctic Ocean, promoting the development of anoxia; in winter the flow was reversed with the resultant breakdown of the thermocline, except in the graben structures where cold bottom waters existed all year. This model by itself fails to explain the lateral facies variations within the basinal areas; bituminous mudstones should be laterally continuous except on the basin flanks. Also, it does not account for the thick oil shale units outside the graben areas, where annual cycles of bituminous and calcareous mudstones are predicted. Wignall (1989) argues for a storm-limited, temperature-stratified depositional model where water depth was the principal control on the distribution of bituminous shales and subsidence controlled their thickness and influenced their organic richness. The deposition of bituminous shale requires a stratified water column with anoxic bottom water conditions for the large-scale preservation of organic matter, particularly of Type II kerogens. In the shallower waters of the swells the influence of storms was greater than in the deeper water basinal environments and the development of a stratified water column occurred less frequently. This resulted in only the main organic-rich

where calcareous mudstones with mixed Type II-III kerogens predominate. The mudstones have a generally cyclic character with small-scale rhythms, typically 2 - 3 m in thickness, and a number of idealized cyclothems have been proposed (Cox and Gallois, 1981; Aigner, 1980; Tyson et al., 1979; House, 1985). These are compared by Wignall (1989, his Figure 16) who noted that, whereas any of these cyclic schemes can occur, the cyclicity is mainly an alternation of calcareous and bituminous mudstones. Some rhythms from the upper part of the Lower Kimmeridge Clay and from the Upper Kimmeridge Clay show an upwards increase in carbonate and decrease in kerogen content from bituminous mudstones or oil shales passing up into dark grey mudstones and finally calcareous mudstones, often with diagenetic carbonate bands (Cox and Gallois, 1981). Tyson et al. (1979) envisage the different lithologies as the result of the rise of the anoxic-oxic interface from within the sediment column (deposition of clays), through the sediment-water interface into the bottom waters (bituminous mudstone deposition), reaching its maximum height at the storm wave base (oil shale deposition) where storm activity caused its eventual overturn and the breakdown of the water stratification (deposition of coccolithic sediments). Superimposed on the cyclic sequence are regional facies trends related to the geometry of the depositional basin and to the effects of variable water depth and sedimentation rate. Sediments laterally become calcareous and coarser grained on the shallower shelf areas and swells flanking the basins (Figure 2) and are generally condensed sequences with minor erosion

AREA OF NONDEPOSITION SILTY,ORGANIC RICH MUDSTONE

ORGANIC-RICH"--FMUDSTONE [~

CALCAREOUS MUDSTONE

+

FAULTS

MEAN TOC

~LL.T#

(wt %)

SWELL AXES

Figure 2 Variation of lithology within Bed 32 across southern and eastern England. Mean TOC values are plotted to illustrate variations in organic content

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, Vol 8, A u g u s t

279

Kerogen facies and maturity o f the Kimmeridge Clay: surfaces. The basinal facies organic-rich mudstones laterally become organically leaner and pass into a series of carbonate-rich mudstones with thin organic-rich horizons (Figure 2). Approaching the basin flanks, the carbonate-rich mudstones predominate over the organic-rich layers. Overall this change is due to the greater amounts of detrital clay and carbonate available to dilute the organic content of the mudstones in the proximal regions of the basin, coupled with poorer preservation of organic matter due to the limited and transitory nature of the stratified anoxic bottom conditions in these areas. Anoxic bottom waters only spilled over into the shallows of the basin flanks during periods of major development of water stratification and were soon overturned by storm activity. Thus oil shale horizons were only poorly developed. The post-depositional burial history of the Kimmeridge Clay Formation was variable across the outcrop and subcrop (Scotchman, 1987a; 1987b; 1989). Maximum burial depth was attained in the Weald and Central Channel Basins of southern England and the Cleveland Basin of North Yorkshire, where burial was continuous, until the onset of inversion and uplift in the Late Cretaceous associated with the Alpine Orogeny (Kent, 1980; Hemingway and Riddler, 1982; Chadwick, 1985; Lake and Karner, 1987). Other areas, including the northern flanks of the Weald and Central Channel Basins, the south Midlands and eastern England were subject to uplift and erosion at the end of the Jurassic to the Early Cretaceous which removed much, and in some cases all, of the Kimmeridge Clay, followed by a second phase of burial in the Middle-Late Cretaceous. A final phase of uplift and

DLE PIT ~OUGH

L C. Scotchman Table 1 Analytical procedures

Whole rock analysis: Total carbon and total organic carbon (TOC) (LECO) for all samples Rock-Eval (using LECO THA) for all samples with TOC > 1.0% Bitumen extraction (SOXTEC) on representative samples Kerogen isolates on representative samples

Bitumen extract analysis: Gas chromatography Separation into saturate and aromatic fractions on representative samples (not discussed in this paper)

Isolated kerogen analysis: Elemental analysis (C, H, N, O) on representative samples Visual kerogen analysis (by Karl Schwab of Geostrat Inc., Houston TX, USA) Vitrinite reflectance measurement (by Karl Schwab) Fourier-transform infrared analysis on selected samples

Saturate/aromatic fractions analysis: Gas chromatography-mass spectrometry analysis (not presented in this paper) Carbon isotope determination (not presented in this paper)

erosion began in the Tertiary. The maturity of the Kimmeridge Clay is therefore constrained by the overall configuration of the depositional basin, which, as noted earlier, also had a major influence on the kerogen type and preservation and its subsequent diagenesis.

)

Sampling and analytical techniques

[~

AREA OF NONDEPOSITION

~

KIMMERIDGE CLAY FORMATIONOUTCROP

+

KIMMERIDGE CLAY FORMATION SUBCROP

---i--

FAULTS

SWELLAXES

Figure 3 Kimmeridge Clay Formation outcrop-subcrop map showing the location of boreholes 280

Sampling was from 13 cored boreholes drilled along the onshore Kimmeridge Clay outcrop and subcrop (Figure 3) by the British Geological Survey, mostly for an oil shale study sponsored by the Department of Energy (Gallois, 1978; 1979). Bed 32, consisting of a sequence of interbedded organic-rich and calcareous mudstones situated at the top of the A. eudoxus zone (Scotchman, 1987a, his Figure 2), was sampled from each borehole. Data from the Kimmeridgian Helmsdale Boulder Beds at Lothbeg Point and Portgower, Sutherland and the Staffin Shale, Staffin, Isle of Skye are also presented. To compare and contrast the onshore Kimmeridge Clay section with that occurring offshore (Figure 1), maturity data from the East Shetland Basin are plotted. Geochemical analyses were made on samples from Bed 32 at each location following the analytical programme in Table 1. Estimates of maximum burial depth for each location are given in Table 2 and sedimentation rate data appear in Table 3 of Scotchman (1989).

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, Vol 8, A u g u s t

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman Table 2 Maximum burial depth for Kimmeridge Clay Formation ](modified from Scotchman (1989)] Location

Burial depth (kin)

Evidence

Kimmeridge Bay BH

2.0

Encombe BH

2.0

Based on burial curve of Penn et aL (1987) and reconstruction of Wytch Farm oilfield, south Dorset (Colter and Havard, 1981). The south Dorset burial history curves of Stoneley (1982) and Stoneley and Selley (1986) suggest 1.7 km burial for the base of the formation

Portesham BH

1.2

Reconstruction of Wessex Basin by Gallois (1979) suggests burial depth of 1-1.5 km

Tisbury BH

1.0

1 km burial depth based on burial history reconstruction for Winterbourne Kingston (Ebukanson and Kinghorn, 1986)

West Lavington BH

0.8

Burial curve for Vale of Pewsey Basin (Penn et aL, 1987)

Swindon BH

0.75

Maximum burial depth of 1 km around western flanks of London Platform (Gallois, 1979)

Hartwell BH

0.5

600 m burial depth estimated for Calvert-Bletchley area by Jackson and Fookes (1974) for Oxford Clay

Warlingham BH Denver Sluice BH

1.5 1.0

North Runction BH

1.0

North Wootton BH Donington-on-Bain BH

1.0 1.1

Reighton BH

1.6

Estimate of 0.6 km post-Cretaceous uplift in the adjacent Hunmanby-1 well by AIIsop and Kirby (1985), added to regional uplift of 1 km (Green, 1989a). Williams (1986) estimates 1 km maximum burial by geological reconstruction

Marton BH

2.5

AFTA suggests uplift of 3 km for the centre of the Cleveland Basin (Green, 1989b; P. F. Green, personal communication). Estimate of maximum burial depth of 1.4 km by geological reconstruction (Williams, 1986); AIIsop and Kirby (1985) record 1.29 km post-Cretaceous uplift in the adjacent Malton-1 well

Portgower

1.0

Robertson Research (1985a) estimate 0.6-1.2 km of section eroded from the offshore Beatrice Oilfield

Staff in

1.5

Robertson Research (1985b) estimate maximum burial of 1-2 km prior to uplift in the Tertiary

Jackson and Fookes (1974) estimate 330 m burial for the Oxford Clay to the west at Peterborough. However, apatite fission track analysis (AFTA) shows that the Eastern England Shelf has been subjected to Tertiary regional uplift of 1 km (Green, 1989a) AFTA of the nearby Biscathorpe -1 well indicates uplift of 1.1 km (Green, 1989a). AIIsop and Kirby (1985) estimate 0.6 km uplift from sonic velocity of Bunter Shale-Upper Permian Marl

Results Analytical data are presented in Tables 3-5. The Bed 32 samples are generally organic-rich with a total organic carbon (TOC) content ranging from 1.2 wt% at Hartwell to 32.0 wt% at Kimmeridge Bay (Table 3). The carbonate content ranges up to 70 wt% in the mudstones and attains 83.5 wt% in the septarian nodule C134.05 from Marton. The kerogens are generally amorphous (Table 4), consisting largely of amorphous debris (finely disseminated and massive or fluffy types, _+71%), mixed with moderate amounts of palyniferous material (_+16%) comprising spores, pollen, algal cysts and foraminiferal linings. Vitrinite (+7%) and inertinite (+7%) occur in lesser amounts. Bitumen fibrils occur in sample C132.75 from Marton Borehole. The visual kerogen analyses (Table 4) were used to determine the palynofacies variations along the section. The abundances of land-derived kerogen (phytoclasts) and marine (amorphous) kerogens were calculated by combining the visual analyses with TOC data using the method of Tyson (1989). The amount of phytoclasts (phyTOC) were calculated from the percentages of plant tissue, vitrinite and inertinite multiplied by TOC whereas the amorphous marine component (amexOC) was determined from the mean percentage of fluorescent kerogen multiplied by TOC. The

distribution of phyTOC and amexOC data for each location are plotted in Figure 4 and are used in the determination of palaeoenvironmental settings (Tyson, 1989). High phyTOC values indicate more proximal environments whereas high amexOC values represent good quality, oil-prone source rocks deposited under anaerobic conditions. AmexOC appears to be controlled by the sedimentation rate (Tyson, 1989). The highest phyTOC values occur in the basinal samples from Kimmeridge Bay and, particularly, Marton, suggesting that increased terrigenous kerogen inputs are associated with high sedimentation rates. Unexpectedly, the more proximal shelf and swell samples have the lowest phyTOC values. A sedimentation rate control on kerogen facies is therefore suggested. AmexOC is highest in the basinal Kimmeridge Bay samples but is generally low in the Cleveland Basin samples from Marton where low fluorescent terrestrial kerogens predominate. The cyclical nature of the sequence is highlighted by the wide range of amexOC values in the shelf and swell samples with high values in the organic-rich mudstones and low values in the calcareous intervals. The colour of spores and pollen (Table 4) varies from light yellow to orange-brown, and the thermal alteration index (TAI) (Schwab, 1990) ranges from 2.33 to 3.00, indicating that the samples are immature

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, V o l 8, A u g u s t

281

Kerogen facies and maturity of the Kimmeridge Clay: L C Scotchman Table 3 Lithology and geochemical and clay mineral data Sample No.

Carbonate (wt%)

TOC (wt%)

$1 (ppm)

S1/TOC

Kimmeridge Bay BH: G59.25 BIk. m/st G59.50 BIk. m/st G60.00 BIk. m/st G60.25 Gy. m/st G60.50 Dk.gy, m/st G62.00 Dk.gy. m/st G62.75 BIk. m/st G63.25 BIk. m/st G63.50 Dk.gy.sh m/st G63.64 Dk.gy.sh m/st G63.75 Dk.gy.cocc m/st G64.13 BIk. m/st G64.50 BIk sh m/st G64.75 Dk.gy.m/st G65.00 Dk.gy.sh m/st

20.4 2.5 7.3 7.7 21.5 16.2 4.2 9.7 6.4 26.8 27.8 2.0 14,1 13.1 17.6

6.4 6.0 4.1 4.1 4.2 9.0 5.6 4.6 10.1 32.0 17.9 4.6 6.5 5.1 9.9

560 570 300 330 300 1300 500 390 1990 5270 4550 380 520 460 2080

Kimmeridge Bay Outcrop: DC/89-29 BIk,dol.I/stb DC/89-30 BIk.blky m/st DC/89-31 BIk,lam m/st DC/89-32 BIk.blky. m/st DC/89-33A BIk,blky m/st DC/89-34 BIk,blky m/st DC/89-35 81k,blky m/st DC/89-36A BIk,blky m/st DC/89-36B BIk.blky m/st DC/89-39 BIk.blky m/st DC/89-40 BIk.blky m/st DC/89-41 BIk.blky m/st DC/89-42 BIk.lam. m/st DC/89-43 BIk.lam. mist DC/89-44 BIk.lam. m/st

90.0 30.5 2.8 0.1 0.3 0,8 10.2 11.3 13.3 0.8 5.0 11.5 13.4 13.9 14.1

1.6 10.6 8,1 7.0 7.1 4.3 9.1 9.4 10.3 4.9 10.7 9.2 13,7 13.8 17.9

Portesham BH: K134.49 Dk.gy.sh m/st K139.49 Dk.gy,calc m/st K144.49 Dk.gy.sh. m/st

13.6 5.3 5.0

Tisbury BH." F228.00 F231.25 F231,56 F232.00 F235.75 F236.00 F236.50 F237.50 F238.00

Dk.gy.sh m/st Dk.gy m/st Dk.gy m/st Dk.gy m/st Dk.gy m/st Dk.gy m/st Dk.gy m/st Dk.gy.sh m/st Dk.gy m/st

Vitrinite reflectance (%Ro) Tmax Productivity (°C) index (Sl/SI+S2) Averagea Minimum Maximum Readings

S2 (ppm)

Hydrogen index

0.01 0.01 0.01 0.01 0.01 0,02 0.01 0.01 0.02 0.02 0.03 0.01 0.01 0.01 0.02

45 220 41 460 21 330 22 040 21 440 65 620 36 220 30 160 104800 279 520 213 500 29440 38 670 34 590 109 200

700 690 520 540 510 820 650 660 1040 870 1200 640 700 670 1100

424 424 428 426 431 419 423 427 419 405 415 424 425 424 416

200 1660 1210 700 760 350 1250 1440 1750 440 1720 1260 2200 2100 3310

0.01 0,02 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.02

8200 93 930 69 730 50 540 53 540 25 220 76 560 78 600 92 590 24780 91 390 73 920 130 600 140 630 185 380

500 890 860 730 750 590 840 840 900 510 850 810 980 1020 1040

5.9 2.3 5.4

370 100 310

0.01 <0,01 0.01

39 740 5650 26430

12.0 3.3 1.2 1.7 19.3 14.3 22.8 15.0 3.1

4.5 4.9 4.3 4.4 4.2 4.7 4.5 7,4 3.1

260 270 180 230 200 230 230 390 100

0.01 0.01 <0.01 0.01 <0.01 <0.01 0.01 0.01 <0.01

West Lavington BH: J153.25 Gy.sh. m/st J156,25 Dk.gy.sh m/st J159.50 Gy.ah. m/st

-14.2 5.8

5.9 13.2 3,9

330 1910 180

Swindon BH." L77.25 Dk.gy.sh m/st L79.60 Gy.sh m/st L81.35 Gy, m/st

5.6 15.4 13.8

3,6 8.4 5,9

Hartwell BH: B48.25 B48.75 B49,25 B49.50 B49.75 B50.25 B50.75 850.80

5.3 16.3 18.0 43.8 69.5 15.9 9,3 2.1

Lithology

<0.01 <0,01 <0.01 <0.01 <0.01 0.02 <0.01

0,49 0.42 . . 0.39 . .

.

.

.

.

.

.

.

.

.

<0.01

0.37 0.40 .

.

.

.

414 430 414

<0.01 0.02 <0.01

0.45 . 0.46

23 480 27 060 16 470 22 540 22 630 27 260 27 060 53 740 11 610

520 560 390 510 540 580 610 730 380

425 417 416 418 428 427 430 419 422

<0.01 <0.01 <0.01 <0.01 <0,01 <0,01 <0,01

0.42

<0.01

0.54

0,01 0.01 <0.01

26 630 125 900 15 810

460 950 400

418 393? 423

0.01 <0.01 <0.01

0.47 0.42 0.49

100 860 260

<0.01 0.01 <0.01

7870 62 870 24460

220 750 420

420 411 421

<0.01 <0.01 <0.01

0.46

3.5 2.8 5.7 3.1 1.2 8.9 6.9 3.9

140 110 230 100 70 960 370 140

<0.01 <0,01 <0.01 <0.01 0.01 0.01 0,01 0.01

10 290 7400 25 020 11 570 2920 76 290 41 600 16 150

300 260 440 380 250 860 600 410

423 426 418 424 429 407 414 419

<0.01 <0.01

0.38

<0.01

0.39

5.5 14.8 4.3 15.8

4,2 1,7 5.6 4.3

240 80 370 350

0.01 <0.01 0.01 0.01

18 340 2700 31 180 23 740

430 160 560 550

420 429 414 423

<0.01 0.03 <0.01 <0.01

0.53 0.43 0.48

Denver Sluice BH: 116.70 Gy,calc.sh. m/st 119.30 Gy.calc.sh. m/st

11.8 36.0

6.6 12.7

440 2100

0.01 0.02

32 560 12 300

500 960

415 410

<0.01 0.02

0.40

North Runcton BH: H45.25 Lt.gy.calc. m/st H48.50 Ltgy.calc.sh. m/st H48.75 Dk.gy m/st H51.50 Lt.gy.calc.sh m/st

7.3 12.3 8.6 8.1

2.2 6.4 4.5 6.0

110 440 190 340

<0.01 0.01 <0.01 0.01

6610 42440 24 130 34 750

300 670 540 580

427 422 416 414

0.02 <0.01 <0.01 <0.01

0,52 0,47

20.4 2.5 7.3 7.8

7.5 6.4 5.4 3.1

660 560 310 190

0.01 0.01 0.01 0.01

55 560 39 980 26 290 14090

740 630 490 450

417 419 415 421

<0.01

0.43

<0.01 <0.01 <0.01

21.5 16.2 4.2

12.1 9.6 5.3

1040 1240 280

0.01 0.01 0.01

95 170 85 680 27 690

790 900 520

415 412 419

<0.01 <0.01 <0.01

0.02

0.02 0.02 0,02

<0.01 0,02

<0.01

<0.01 0.02 <0.01 <0.01 <0.01

0.20 0.22

.

.

.

. 0.23

.

.

.

.

.

.

.

0.38

. 75

1.57

1.07

63 --

85 95

1.95 1,75

1,20 0.93

---

1.61

1.06

--

1.55

1.29

--

1.70

0,87

--

1.47

1.22

--

1.69

0.89

--

2.43 1.17

0.72 1.27

.

.

.

.

.

0.23

.

90 .

.

85 .

.

85 .

1,14

.

. 85

.

.

.

.

2.89

. . 85

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0,25 . 0.36

.

0.43

1.09

70

1.41

75

--

--

62 63 65

80

2.37

0,64

64

2.18

0.31

61

2.51

0.53

--

2.73

033

71

75 85 90

2.04 4.01 2.72

1.14 0.74 1.11

46 48 53

75

1.72

1.34

61

.

0.36 .

1.68 .

.

0.36

.

1.43

.

81

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

. .

.

.

. .

.

1,42

.

.

.

.

.

.

.

.

0.35 0.44 .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

135 1.31

.

.

.

.

.

.

.

.

.

.

.

.

0.26

80

2.40

0.23

51 51

80

2.98

0.21

43

2.53 2.51

0.24 1.26

37 54 55

80 80 80

-1.91 7.71

-0.95 0.88

65 67 68

46

4.18

1.41

61 71

68 80

1.88 2.34

1.54 0.89

59 --

.

.

.

1.46

.

60

65

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0.38

0.22

62

.

1.40

0.30 0.26

.

85 71

1.86 1.09 1.15

0.32

.

.

1.69 1.18

.

.

.

1.76

.

0.37 0.23 0.30

.

.

1.44 .

0.32 0.38

.

.

.

0.39 .

.

1.71 .

0.35 .

.

. 65

1,40 1.19 1.34

0.40 .

.

.

. .

0.40 0.21 0.22

.

.

. . 0.40

---

.

1.20 .

1,26 1.10

.

.

0.24

2.52 1.67

.

.

0.30 .

0.40

--

.

1.68 .

0.61

.

1.93 .

61 --

.

.

0.23 .

70 75

1.10 1.84 .

1,21 1.22

.

. 0.96

0.23 .

0.50 .

.

.

.

0.49 .

.

0.21 0.25 .

0.43 .

0.93 1.11

1.48 1.47 . . 2.42 .

.

. .

670 250 490

Marine and Petroleum Geology, 1991, Vol 8, August

.

. .

0.41 0.43

282

.

.

0.02 0.02 0.02 <0.01 <0.01 <0,01 0.02 0.02 0.02

North Wootton BH." A60.30 Lt.gy.calc,sh m/st A60.60 Ltgy,calc.sh m/st A60.90 Lt.gy.calc.sh m/st A61.20 Lt.gy.calc. m/st A61.50 Lt.gy.calc. cocc m/st A61.80 Lt.gy.calc.sh/mst A62.10 Gy.sh. m/st

.

.

.

420 418 420 423 426 427 419 423 429 424 421 427 423 414 418

m/st m/st m/st m/st

.

. .

75 75 . . 80 .

0.02 0,02 0.02 <0.01

. 0.39

Warlingham BH." N829.06 Dk.gy. N830.58 Dk.gy. N832.10 Dk.gy. N834.34 Dk.gy.

.

2.36 0.80 . . 1.08 .

<0.01

<0.01 0.02

Lt.gy.calc.sh m/st Lt.gy.calc.sh m/st Lt.gy.calc.sh m/st Gy.calc.sh m/st Lt.gy.calc.sh m/st Gy.ah. m/st Gy.sh. m/st Gy.sh. m/st

. .

0.41 0.24 . . 0.21 .

CPI

Pristane: phytane % ratio Illite

1.58

1.97

0.88

59

1.66

0.92

--

.

65

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman Table 3 - - Continued Vitrinite reflectance (%R0) Sample

Tma x

index

(°C)

Productivity index ($1/$1+$2)

3810 67 680 68 770 96 940 14 440 78 950 22 550 16730

200 830 760 940 430 860 480 411

424 404 403 409 413 422 421 421

0.02 0.02 <0.01 <0.01 0.02 <0.01 <0.01 <0.01

. . . 0.42 . . . 0.35

0.01 <0.01 0.01

28 370 13 240 4350

500 330 230

427 424 423

<0.01 <0.01 0.03

0,45 0.53 0.45

<0.01 <0.01 0.01 <0.01

31 490 4260 46 280 5740

550 200 590 250

420 427 417 427

<0.01 <0.02 <0.01 <0.01

0.44 . 0.41 0.52

34370 10 400 30 460 8890 81 390 51 920 14 120 37 400 34800 150400 179 000

590 310 550 310 940 550 550 580 630 1090 850

430 438 430 432 421 437 429 426 429 418 421

0.02 0.02 0.02 0.02 0.03 0.08 0.02 0.03 0.02 0.04 0.04

0.41 . . . 0.36 0.46 0.48 . . 0.41 0.45

<0.01

3970

50

420

0.03

<0.01 <0.01

3490 12 210

70 210

418 412

0.02 0.01

30

NR

0.33

.

.

40

432

0.05

.

.

TOC

(wt%)

(wt%)

$1 (ppm)

9.7 6.4 26.8 27.8 2.0 14.1 13.1 17.6

1.9 8.2 9.0 10,4 3.4 9.2 4,7 4.1

70 1350 1000 1340 250 880 250 180

<0.01 0.02 0.01 0.01 0.01 0.01 0.01 <0.01

Dk.gy m/st Dk.gy m/st Dk.gy m/st

13.3 12.5 26.9

5.7 4.0 1.9

290 130 130

Gy.sh. m/st Gy.sh. m/st Gy.sh. m/st Dk.gy.sh. m/st

14,5 21.6 14.1 6.0

5.7 2.1 7.8 2,3

270 90 490 80

Dk,gy.sh m/st BIk sp.sh m/st BIk sp.sh m/st BIk. m/st Dk.gy. m/st Dk.gy. m/st Dk.gy.cem m/st BIk sp.sh m/st BIk.sp.sh m/st BIk.sp.sh m/st BIk.sp.sh. m/st

20.6 2.4 5.9 2.5 27.6 -83.5 15.2 20.1 11,2 16.0

5.8 3.3 5.5 2.9 8.7 9.4 2.6 6.4 5.6 13.8 21,0

610 220 570 190 2410 4800 360 1030 770 5830 6890

2.7

7.3

110

---

4.8 5.7

60 100

70.3

0.4

80

0.01

160

15,3

5.1

120

<0.01

2180

NO.

Lithology

A62.70 A63.00 A63.30 A64.50 A64.80 A65.10 A65.70 A66.30

Hydrogen

Carbonate Lt.gy.calc m/st Lt,gy.sh m/st Lt.gy.calc.sh m/st Gy,sh. m/st Gy m/st Gy m/st Gy.sh. m/st Gy.calc, m/st

S1/TOC

$2 (ppm)

Avera9 ea Minimum . . .

Maximum . . .

0.30

. . . 1.56 . . . 1.49

0.23 0,29 0,38

1.25 1.32 1.58

0.21 . . .

. . .

Readings . . .

CPI . . .

65 . . .

Pristane: phytane ratio

% Illite

. 50 . 1.95

. . .

0.75

--

. . .

67

3.74

1.15

62

90 90 70

2.18 1.97 1.61

0.44 0.36 1.23

47 -61

1.04

45

1.75 1.60

0.86 1.41

-60

1,55

1.65

1.39 1,09 1.31

1.45 1.45 1.39

80 80

1.27 1.27

1.28 1.54

21 35 39 39 --46 46 39 57 --

Donington-on-Bain BH: D86.25 D90.50 D94.75

Reighton BH: El16.43 El17.75 E120.50 E124.75

0.26 .

. 0.22 0.33

0.97 . 1.69 1,21

90 .

1,99 .

80 80

.

Marton BH: C122.30 C126.50 C127.15 C128.63 C132.15 C132.75 C134.05 C135.85 C137.28 C139,29 Ct40.00

0.01 0.01 0.01 0.01 0.03 0.05 0.01 0,02 0.01 0.04 0.03

0.33

0.37 0.38

1.68 . . . 1,88 1.28 1.35 . . 1.76 1.49

0.41

0,22

3,29

85

4.16

1,61

--

0.45 0.49

---

---

21 24

---

---

---

1.30

--

. . .

. . . 0.35 0,26 0.32

. .

. .

75 . . .

. . . 80 60 80

. .

. .

Lothbeg Point Outcrop: LBP-1

Ibd sst/blk.m/st

Portgower Outcrop: KCS-1 KCS-2

Staffin

Ibd sstJblk mist BIk. mist

Outcrop:

CST-01 CST-02

Lt.gy, m/st BIk.ind.sp. sh. m/st

. .

. .

.

.

. 3.32

a Average of in situ population

b Flats Stone Band {NR) Not reliable; (m/st) mudstone; (sst) sandstone; (gy) grey; (dk) dark; (It} light; (cocc) coccolithic; (calc) calcareous; (blk) black; (ibd) interbedded; (sh) shelly; (sp) sparsely; {dol) dolomite; (blky) blocky; (lam) laminated; (ind) indurated; (cem) cemented

to marginally mature. Under ultraviolet and blue light

irradiation, the kerogens, except those at Marton, show good fluorescence, which suggests that the organic material has a moderate to good hydrogen content and is oil-prone. (%) ~-

I

~ F

:ge

I----

Bay

~L

West L o v / n ~ t o n

8

I

I

10

I

.....

-)~ . . . . . . . . .

F. . . . . . . . ~

6

I

? ........

, Tisbury /

I

I_

.........

I

12 I ~

:_%.

4

h-. . . . . . . . . . .

_u~__~ . . . . . . . . . . .

I

I

Sw]ndon

I

Hartwell Warlingham

h......

X....

4

I

Denver Sluice

I

Runcton

~-. . . . .

¢

I North Wootton j Donington-on-Bain

4

~-. . . . . . . . . . . .

F----X-

RM•M

X......

X. . . . . . . . . .

-I

ariCjh t on

I

ton

¢

............... I F

X---I

-X- . . . . . . . . . . . . . .

-4

phyTOC (range and mean) a m e x O C (range and mean)

Figure 4 PhyTOC and amexOC distribution for each location

The vitrinite particles are generally small, not very abundant and occasionally show signs of oxidation. The reflectance values show a wide range and the histograms for the samples are illustrated (Figure 5). This wide range suggests that many of the vitrinite particles have been reworked from older sediments, making the picking of in situ populations very difficult. The reflectance values that most closely agreed with the visual kerogen data were chosen. The problematic nature of vitrinite reflectivity as a maturity indicator are illustrated by the Marton Borehole sample C132.75, which contains bitumen filaments and is the most mature, but has low reflectivities of 0.36 to 0.48% R0. Gas chromatograms of bitumen extracts are presented in Figure 6. The pristane:phytane (pr:ph) ratio and carbon preference index (CPI) data (Table 3) have ranges between 0.21 and 1.65 and 1.09 and 7.71, respectively. The general low level of maturity of the samples is further illustrated by the Rock-Eval Tm~xtemperatures of between 403 and 437°C and the hydrogen:carbon (H:C) and oxygen:carbon (O:C) ratios from elemental analysis which plot in the immature to marginally mature zone on the Van Krevelen diagram (Figure 7). The samples are generally of kerogen Type II or mixed kerogen Type II-III as defined by Tissot et al. (1974) and Thomas et al. (1985) (Figure 7). The mixed Type II-III composition is predominant in the swell areas where the kerogen composition shows a much wider range than the narrower Type II kerogen

Marine and Petroleum Geology, 1991, Vol 8, August

283

Kerogen facies and maturity of the Kimmeridge Clay: I. C. Scotchman Table 4 Visual kerogen data Organic constituents (%)

Spore colour

AmexOC

TAI

Fluorescence (%)

PhyTOC

Pyrite

10 5 6 5 5 6

TR TR TR TR TR TR

PY-YO PY- YO PY-YO PY-YO PY-YO PY-YO

2.80 2.50 2.67 2.67 2.50 2.67

81 - 100 66-80 66-80 81-100 81-100 81-100

1.9 1.3 0.8 5.1 5.4 2.3

5.7 4.4 3.1 9.1 16.1 8.9

5

10

R

PY-YO

2.80

81-100

2.7

8.5

18 18

6 6

6 11

TR R

PY-YO PY-YO

2.50 2.80

66-80 81-100

1.1 0.9

4.3 2.1

65 65 70

20 25 15

5 5 5

10 5 10

TR TR R

PY-YO PY-YO PY-YO

2.50 2.67 2.80

26-40 11-25 81 - 100

1.1 2.2 1.9

1.5 0.8 6.7

0 0 0

70 77 67

15 12 18

5 6 11

10 6 6

R R TR

PY-YO PY-YO PY-YO

2.86 2.50 2.50

81-100 81-100 66-80

1.5 3.2 0.9

5.3 11.9 2.9

0

60

15

5

20

R

PY-YO

2.80

66-80

1.3

2.6

0 0 0 0

55 65 69 70

27 18 20 10

11 12 5 6

6 6 5 12

TR TR TR TR

PY-YO PY-YO YO PY

2.33 2.50 3.00 2.67

11 - 25 26-40 41-65 81 - 100

1.0 0.4 1.9 2.1

0.6 0.4 4.7 6.2

0 0 0

65 71 73

23 18 14

5 6 7

10 6 7

TR TR R

PY-YO YO YO

2.67 2.80 2.80

81-100 81-100 81 - 100

0.6 1.0 0.9

1.5 5.0 3.9

0

70

20

5

5

TR

PY-YO

2.80

81 - 100

2.5

11.4

0 0

71 67

12 18

6 11

12 6

R TR

PY-YO PY-YO

2.80 2.50

81-100 81-100

0.8 1.5

2.0 5.8

0 0 0 0

71 75 71 68

12 12 18 12

12 6 6 13

6 6 6 6

TR TR TR TR

PY-YO PY-YO PY-YO PY-YO

2.50 2.50 2.50 2.50

81-100 81 - 100 81-100 81 - 100

1.8 2.9 2.5 1.0

6.8 10.9 9.4 3.7

0 0 0

71 68 66

18 18 14

6 6 7

6 6 13

TR R R

PY-YO PY- YO PY-YO

2.50 2.67 2.80

66-80 81-100 81-100

1.0 0.7 0.6

4.2 3.6 1.7

0 0 0

73 66 67

14 21 12

7 7 11

7 7 11

R R R

PY-YO PY-YO PY-YO

2.50 2.50 2.80

66-80 66-80 81 - 100

0.8 2.1 0.9

4.2 5.7 2.1

0 0 P 0 0 0

71 78 77 85 70 85

12 11 12 5 20 5

12 6 6 5 5 5

6 6 6 5 5 5

TR TR TR TR R TR

YO-OB YO-OB YO-OB YO-OB YO-OB YO-OB

2.67 2.67 2.80 2.67 2.50 2.67

5-10 5-10 81-100 5-10 11-25 26-40

1.7 2.5 3.4 1.2 4.8 10.5

0.5 0.7 8.5 0.2 2.5 6.9

Sample Bitumen

Amorphous

Palyniferous

Vitrinous

Inertinite

68 68 73 90 80 72

20 15 18 0 10 18

5 11 6 5 5 6

0

66

20

0 0

68 66

0 0 0

(%)

(%)

Kimmeridge Bay BH: G59.25 G59.50 G60.50 G63.50 G63.75 G65.00

0 0 0 0 0 0

Kimmeridge Bay Outcrop DC/89-36A

Portesham BH: K134.49 K139.49

Tisbury BH: F228.00 F231.50 F237.50

West Lavington BH: J153.25 J156.25 J159.50

Swindon BH: L77.25

Hartwell BH: B48.25 B49.25 B50.25 B50.75

Warlingham BH: N830.58 N832.10 N834.24

Denver Sluice BH: 119.30

North Runcton BH: H45.25 H48.50

North Wootton BH: A60.30 A61.50 A64.50 A66.30

Donington-on-Bain BH: D86.25 D90.50 D94.75

Reighton BH: El16.43 E120.50 E124.75

Marton BH: C122.30 C132.15 C132.75 C134.05 C139.29 C140.00

(TR) Trace; (R) rare; (P) present; (YO) y e l l o w - o r a n g e ; (PY) pale yellow; (OB) o r a n g e - b r o w n

composition range of the basinal samples (Figure 7). Type III kerogens occur in the basin margins in the Helmsdale Boulder Beds at Portgower and Lothbeg Point and in the Staffin Shale. The kerogen facies distribution in southern and eastern England, based on the kerogen elemental analysis and phyTOC data, is

284

shown in Figure 8. The maturity distribution based on Rock-Eval pyrolysis and vitrinite reflectance data are shown in Figure 9. The hydrogen indices (HI) show a relatively high remaining hydrocarbon generation capability with variations related to facies rather than to maturation

Marine and Petroleum Geology, 1991, Vol 8, August

Kerogen facies and maturity of the Kimmeridge Clay: I. C. Scotchman analysis further point to the immaturity of these level, as suggested by the positive relationship between oil-prone kerogens. hydrogen index and TOC (Figure 10), the oil-prone samples (HI > 200) having the highest TOC. The pyrolysis S1/TOC, the production index [S1/(SI+S2)] Discussion and the wide range of the H:C ratios from elemental The analytical data show that the kerogens from Bed 32 Table 5 Kerogen elemental analysis data have a wide range in composition and are immature to marginally mature, the relative levels of maturity being Atomic ratio Normalized elemental analysis (wt%) Recovery difficult to determine. Further studies utilizing Sample C H O N (% ) O/C H/C biomarkers are in progress. Kimmeridge Bay BH: G59.50 G60.50 G63.50 G63.75 G65.00

81 80 81 81 81

7.7 7.8 8.8 8.6 8.4

9.2 10.4 8.5 8.4 8.0

2.3 2.3 2.2 2.3 2.2

74 71 84 81 82

0.09 0.10 0.08 0.08 0.07

1.14 1.17 1.30 1.27 1.23

79 80 80 81 80 80 80

8.2 8.4 7.8 7.7 8.3 7.8 8.4

11.3 9.7 9.9 9.4 10.1 10.0 9.7

1.8 2.1 1.9 2.0 2.1 1.9 2.1

83 84 81 79 76 80 81

0.11 0.09 0.09 0.09 0.09 0.09 0.09

1.24 1.25 1.16 1.13 1.25 1.16 1.26

78 78

8.1 6.9

11.9 12.6

2,2 2,1

78 84

0.12 0.12

1.24 1.05

75 75 76 75

7.5 7.1 7.8 7.5

15.0 16.0 14.5 14.9

2.3 2,1 1.6 2,2

71 68 76 83

0.15 0.16 0.14 0.15

1.18 1.14 1.22 1.19

75 75

8.3 7.4

15,1 15,3

1.8 1.9

82 76

0.15 0.15

1.33 1.16

76

7.1

15.3

2.0

77

0.15

1.11

75 74 73 74

6.4 6.7 7.5 7.8

16.8 16.8 17.9 16.9

2.1 2.0 1.7 1.7

72 78 73 84

0.17 0.17 0.18 0.17

1.02 1.08 1.22 1.27

77 77

8.0 7.8

13.3 13.4

1.9 2.0

79 81

0.13 0.13

1.24 1.22

74 73

8.1 7.7

15.9 17.5

1.5 1.6

85 84

0.16 0.17

1.30 1.27

74 75

7.3 8.5

17.1 14.8

1.7 1.8

80 77

0.17 0.15

1.28 1.35

A60.30 A61.50 A64.50 A66.30 A66.30 (Dup.)

75 73 75 75 73

8.4 8.6 8.6 7.6 7,2

14.9 16.7 14.9 15.8 18.0

2.0 1.6 1.7 1.7 1.6

81 82 78 71 82

0.15 0,17 0.15 0.16 0.18

1.34 1.40 1.37 1.21 1.17

Donington-on-Bain

BH:

Kimmeridge Bay Outcrop: DC/89-29 DC/89-30 DC/89-32 DC/89-34 DC/89/36A DC/89-39 DC/89-42

Portesham BH: K134.49 K139.49

Tisbury BH: F228.00 F231.50 F236.00 F237.50

West Lavington BH: J156.25 J159.50

Swindon BH: L77.25

Hartwell BH: B48.25 B48.75 B50.25 B50.75

Kerogen facies The kerogen facies within Bed 32 tend to correspond with the sedimentation rate and lithological facies trends described previously and are predominantly mixed Type II-Type III (Figure 8). Amorphous kerogens approaching the composition of the Type II end-member predominate in the basinal areas whereas Type III end-member kerogens predominate in the basin margins. A wide range of kerogen composition occurs in the swell area samples, from Type II to mixed Type II-Type III kerogen. The facies distribution follows the lithology and organic richness of the mudstones (Figure 2), reflecting variations in sedimentation rate and the topography of the depositional basin (Scotchman, 1989). The mixed Type II-Type III facies indicates a marine, predominantly dysaerobic environment with a terrestrial kerogen input. Fluctuating anoxic-oxic boundary levels in the depositional waters also appear to have been a major control on facies (Demaison et al., 1984; Ebukanson and Kinghorn, 1985), largely by influencing kerogen preservation. The preservation of Type II kerogens is favoured by anoxic conditions (Demaison et al., 1984), thus a relatively constant flux of amorphous marine

Warlingham BH: N832.10 N834.24

Denver Sluice BH: 119.30 119.30 (Dup.)

0

0.5

1.0

1.5

i

2.0 0

0.5

i

10

i

15

21o

North Runction BH: H45.25 H48.50

76 76

0

05

10

15

20 o

13.8 14.4

2.3 2.1

80 76

0.14 0,14

1.20 1.21

,S ° z

~, ~o5

~I

10

15

05

1.0

7

n = BO 7.7 7.7

10

15

20

0.5

10

15

21o

7

North Wootton BH:

D86.25 D90.50

i

05

" 2i0

0

1.5

21.0 0

n = 540

~ '05

r~nh, n ' ' 10 15

n = 68

2'°

N0 5

10

n,15

20

i

L

2to

Reighton BH: El16.43 E120.50 E124.75

78 78 78

7.6 7.9 6.7

12.2 11.9 13,5

2.2 2.1 1.9

72 76 87

0.12 0.11 0.13

1.17 1.21 1.02

81 81 81 81 81

7.2 7,9 8.0 8.5 8.2

9.7 8.9 8.8 8.7 8.4

2,3 2,3 2.2 2.3 2.3

73 73 85 67 78

0.09 0.08 0.08 0.08 0.08

1.06 1.17 1.18 1.26 1.20

80

3.9

15.4

0.5

90

0.14

0.58

74 78

4.1 5.2

20.1 15.5

1.4 1.5

87 75

0.20 0,15

0.66 0.80

83

3.3

12.2

1.2

85

0.11

0.47

LJ

Marton BH: C122.30 C132.15 C134.05 C139.29 C140.00

. . . . . . . . . . .

Lothbeg Point Outcrop: LBP-1

10!05 ~

Portgower Outcrop: KCS-1 KCS-2

l

........

'. . . . . . . . .

nn[l132.75n lC = 60

L

21o

VITRINITE REFLECTANCE (% Ro)

Figure 5 Vitrinite reflectance histograms showing the wide

Staffin Outcrop: CST-02

'. . . . . . . . . . . . .

s p r e a d of data, T h e p o p u l a t i o n is e v i d e n t

difficulty

in i d e n t i f i c a t i o n

of t h e in s/tu

Marine and Petroleum Geology, 1991, Vol 8, August

285

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman

Portesham BH K134.49

Kimmeridge Bay BH G59.50

Tisbury BH F231.50

West Lavlngton BH J156.50

Swindon BH L86.75

Hartwell BH B50.75

Warlingham BH N832.10

Denver S l u i c e 119.30

North Runcton BH H48.50

North Wootton BH A64.50

Figure 6 Representative bitumen gas chromatograms from each location. Figure 6 is continued on the following page

286

Marine and Petroleum Geology, 1991, Vol 8, August

BH

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman Donington-on-Bain BH D90.50

~E

Marton BH C139.49

-

Reighton BH El16.43

Lothbeg Point outcrop T,BP-I

i Staffin outcrop CST-02

~

L

J

I

£

........

Figure 6 cont.

20 IMMATURE MARGINALLYMATURE MATURE POST-MATURE

f

t~Pr i 15

O O2

I

10

(D ©

0,5 enver Sluice BH orth Runcton BH orth Wootton BH onlngton on Boin elghton BH lot ton BH othbeg Point O/C or tgower O/C toffin O/C

0

I

0

005

O.10 ATOMIC

0 15

0.20

0 25

0 / C RATIO

Figure 7 Van Krevelen diagram showing the predominant Type II - Type III composition of the kerogens. Note the anomalous composition of kerogens from the Helmsdale Boulder Beds and Staffin Shale.

KEROOENFAC,ES

Q

TYPE, ~

TYPE,,/~,, ~

[~

~

FAULTS ~

AREASWITHINCREASED TERRESTRIALKEROGENCONT.

AREAOF NON DEPOSITION

Amoxoc~hyTOC

Figure 8 Kerogen facies distribution map. High phyTOC values indicate increased terrestrial input while high amexOC indicates oil-prone kerogen. See Figure 3 for basin topography

Marine and Petroleum Geology, 1991, Vol 8, August

287

Kerogen facies and maturity of the Kimmeridge Clay: I. C. Scotchman

IMMATURE

~

MEANVITRINITEREFLECTANCE(~Ro) MEANTrnex (C)

MARGINALLYMATURE

AREA OF NON DEPOSITION

T

FAULTS

Figure 9 Maturity map for the onshore Kimmeridge Clay Formation. See Figure 3 for basin topography

organic matter into the depositional basin is suggested (Williams and Douglas, 1985), with dilution by variable amounts of structured terrestrially-derived kerogen towards the basin centres where higher sedimentation rates increased the relative proportions of the terrestrial components (Figure 11). The influence of bottom water oxicity on kerogen facies is illustrated by the samples from northern Scotland. The high TOC values indicate a general anoxic depositional setting, favouring kerogen preservation. However, periodic influxes of oxygenated bottom waters associated with deposition of the turbiditic quartz sandstone laminae appear to have caused oxidation of Type II kerogens prior to burial (MacLennan and Trewin, 1989), leaving residual poor quality kerogens of Type III-Type IV composition. The CPI data (Bray and Evans, 1965) are greater than 1.0, indicating an odd carbon preference. CPI values are affected both by organic matter type and by maturity (Tissot and Welte, 1978). Higher values at a constant maturity level are due to terrestrial input whereas low values suggest a marine source. A predominantly marine kerogen input is indicated in the samples, except those from Scotland where an increased terrestrial component is indicated by the higher values, although this may also reflect pre-burial oxidation. Lithology variations may also affect the CPI values; higher values appear to be related to increased carbonate content. The pr:ph ratio has been shown to be a kerogen source indicator (Lijmbach, 1975; Hunt, 1979) and can also reflect the depositional environment (Powell and

SEA LEVEL

OXIC

ox

................................................................

::::::::

1400 ~

20--

~_

g ~Z

e~ z

O"

1200

N

/

/

tJ

// /

0 0 1000

~

/

/'.f"

A

800

0

[]

~

~

9i

',

" :

i

:

t ...................

6O

8

(D c~ ~'~ 6 0 0 p~ IZ

N

S 8o

5 o 0

200

••

[]

Kimmeridge Bay BH

X

Kimmeridge Bay O/C Porteshom BH Tisbury BH

<~

West Lavington BH

V []

i/

/,

II

II --

lit

L

hy

400 --

q:

@* Denver Sluice BH # ~ • ~) []

~

STRONGLY

~

OIL -

[] •

Lothbeg Point O/C Portgower O/C

Warlinghdm BH

•

Stoffin O/C

I

I

I

10

20

50

GAS

o

North Runcton BH North Wootton BH Donington-on Bain Relghton BH Morton BH

Swindon BH Hartwerl BH

MODERATELY OIL PRONE TO GAS PRONE

PRONE

}SUB_BAS I I SHELF I TECTONIC SETTING

III --

IV

GAS PRONE TO NONSOURCE I SWELL

i

PREDOMrHANT DIAGENETICREACTIONS (Scotchman, 1989) i~i

High terrestrial kerogen input associated with increased sedlmentotion rates

!~::.~'. O~ze~~erogen~ 40

TOO (wt %) Figure 10 Hydrogen index and TOC plot, which indicates the richest samples to be the most oil-prone

288

j,.,"i

I

Z

0

40-

Z&

/

X

Z

[]// /~/

Figure 11 Depositional model for the onshore Kimmeridge Clay Formation (modified from Scotchman, 1989), showing the levels of diagenesis attained and kerogen type and quality in relation to basin topography. A high terrestrial kerogen component is often associated with high sedimentation rates while oxidized kerogens predominate in basin margin settings

Marine and Petroleum Geology, 1991, Vol 8, August

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman 1400

McKirdy, 1973). Low values (<1) indicate a marine origin and a reducing environment whereas high values (>3) indicate an oxidizing environment with kerogen of terrestrial origin. The samples have low pr:ph ratios, indicating a general anoxic depositional environment, many showing a predominant marine kerogen input with values <1. Increased terrestrial kerogen components are shown by the higher ratios in the Marton samples and, particularly, in the basin margin samples from the Helmsdale Boulder Bed sequence.

The organic maturity indicators (visual kerogen analysis, vitrinite reflectance, Rock-Eval pyrolysis Tma×, gas chromatography and elemental analysis) analysed from Bed 32, show that the kerogens are immature to marginally mature, but often show no consistent trends either with estimated burial depth or amongst themselves.

KimmerldgeBey BH

A

Kimmeridge Boy O/C

+ 1200

PorteshGmBH

o

--

\\

O

\

[]

\ 1000

\N

[] []

',* k \

O

k

[]

A

\\

E3 Z 800

Maturity indicators

0

Z LJ (.D 0

O

N

\

[]

Z

7,®

×

0

$

N \ •+\

[] A

400

Tisbury BH West Lovington BH Swindon BH [] HertweH BH -~ Warlingham BH ~* Denver Sluice BH x North Runcton BH ~" North Wootton BH [ ] Donington on-Bain ( ~ Reighton BH [] Morton 8H [~ Lothbeg Point O/C • Portgower O/C • Staffin O/C

O N• \

~D~ 600

x

[]

&

×

[] $

Visual kerogen analysis. The spore coloration data show little variation along the outcrop, ranging from light yellow at Warlingham and Hartwell through yellow-orange at most locations to yellow orange-orange brown at Marton, indicating all of the samples to be immature or marginally mature. In most of the samples a second population is apparent with organic material yellow-orange to orange-brown in colour. This higher maturity population is interpreted as being recycled material. The TAI data (Schwab, 1990), based on the in situ population, also show little range in maturity. Vitrinite reflectance. The results of vitrinite reflectance analysis are difficult to interpret due to the wide scatter of the data and the numerous populations GO

[]

~l~

[]

V©

©

t.O

©

[]

[] ++ @

2.0

O

OO

X@

X

O

X

@,4

OA

z

E LJ

[]

c]

rr

[]

[]E~

[]

30

40

0

Kimrnerldge Bay BH

~

Denver Sluice BH

S + X

Kirnmeridge Bay O/C Portesham BH fisbury BH West Lavington BH

x ~ [] ~) []

North Runcton BH North Wootton BH Donington-on-Baln Reighton BH Morton BH

Swindon BH Hartwell BH Warlingham BH

[] • •

Lothbeg Point O/C Portgower O/C Staffin O/C

[] X 5.0

f

03

0.4

1

05

J 06

VITRINITE REFLECTANCE (% Ro) Figure 12 Vitrinite reflectance shows no relationship with maximum burial depth

® 200

[]

I 0 5

I

I

O4 VITRINITE

I O5

REFLECTANCE

(%

O6 Ro)

Figure 13 Vitrinite reflectance s h o w s an inverse relationship with h y d r o g e n index, suggesting retardation in the m o r e oil-prone s a m p l e s

present in each sample (Figure 5). Interpretation of the vitrinite data was made using the spore coloration and TAI data as a constraint and the lowest population was considered to represent the true level of maturity. The reflectance populations show no relationship with the estimated maximum burial depth (Figure 12). This suggests that much of the vitrinite, particularly in the more proximal areas of the basin, has been reworked from older sediments although, as southern England and the Cleveland Basin have suffered strong Alpine-age inversion, reconstruction of accurate burial history is problematic (Cornford et al., 1987). Thus the relative levels of maturity of these samples cannot be determined from vitrinite reflectance data alone. The effect of kerogen facies on vitrinite reflectance can be seen in Figure 13 where a poor negative relationship is apparent, the reflectance values being lowest in the most oil-prone samples, suggesting that the kerogen facies has a considerable influence on the measured vitrinite reflectance values. The most mature samples from the basins, e.g. Marton and Kimmeridge Bay, tend also to be the most oil-prone and retardation of vitrinite reflectance, as suggested by Price and Barker (1985), appears likely.

Ymax values. The Rock-Eval pyrolysis Tmax, the temperature at which the peak of the $2 curve occurs, can be used as a maturity indicator (Tissot and Welte, 1978, Espitalie et al., 1985). Tma× shows poor relationships with Ro (Figure 14) and burial depth (Figure 15), with a wide variation, particularly at shallow depths. This is due to the influence of kerogen type and rock matrix (Tissot and Welte, 1978; Espitalie et al., 1985; Espitalie, 1986), in particular the proportion of terrigenous structured kerogens to

Marine and Petroleum Geology, 1991, Vol 8, August

289

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman 0.60

0 A

Kimmeridge Boy BH

+

Portesham BH

X ©

Tisbury BH

1400

Kimmeridge Bay O/C \ 1200

West Lovington BH

055

Swindon BH

X

Hat twell BH

¢ LD Z

<:

[]

Don)ngton- on-Boin

\


X Ld d3 Z

Reighton BH ©

~
+

Lothbeg Point O/C

~

Portgower O/C Staffin O/C

[]

I

!/

El"

\\

¢

0 A ÷ X ©

600

°x

/

®/

[] 400 --

I[

0

¢

[]

/ /

/

[]

/ 0.35

200

/

@

[]

/

[]

/ / /

390

T 410

r 420

F

F

430

440

\\

West Lovington BH Swindon BH Hortwell BH Warlinghom BH Denver Slu;ce BH

+X

marine amorphous kerogens and the clay content of the rock matrix. The richness of the sample as well as the amount of oil-prone kerogen appears to affect the measured Tmax value (Figure 16), Tm~xshowing an inverse relationship with both TOC and hydrogen index. The basinal

=

,, "\

\

North Runcton BH North Wootton BH Donington-on-Btain Relghton BH Morton BH Lothbeg Point O/C Portgower O/C Staffin O/C

\

[]\ •

~

~

>~ ~

®\

\ \..

~

\

\\

r 400

i 420

410

i 430

440

Tmax ( o C) Figure 16 Relationship between Tma x and hydrogen index, showing the reduction of Tma× in the more oil-prone (high HI) samples. The basinal samples appear to follow a different trend from the shelf samples.

samples from Kimmeridge Bay and Marton show a slightly different trend between Tma x and hydrogen index from the shelf samples (Figure 16), probably due to their increased content of amorphous kerogens. The clay content of the matrix appears to cause a minor

0.0 --

12 0

[]

~',

~

0

~

[] []

+

®~m 8 --

(3".,

2,0--

ND/'f~

I

x

£1_ DJ Eb

N

(D 0

\ \

N

"

;"

[] ! .....

,

6 --

J= O_

~

" 4 --

x %

O

Kimmeridge Bay BH

~

Denver Sluice BH

/~

Kimmeridge Bay O/C

>k

North Runcton BH

-F

Port°sham BH

~

North Wootton ~H

Tisbury BH West Lavington BH

@

Donington on Bcfin Reighton BH

[]

Marton BH

Swindon BH Hortwell BH

[] •

Lothbeg Point O/C Portgower O/C

X ~' []

Worlinghom BH

5.0 -- - - T 390

400

\

\

T

410

420

~t;::~ ...... ~"":'~ ~ : : : ~ ' i ' i

', ,// \\

[] \

Ae

\ \

\

D0

\

<>

\\

[]

\

[]%

\

\

k\

I--430

v \

[] Staffln O/C

r

0 \

,.

4.0 --

Reighton BH Morton BH Lothbeg Point O/C Portgower O/C Stoffin O/C

~7 [] X-

+

~ ® ~,,~

£I3

[] [] • •

', O W O 10

3.0--

>k ]~' Eli

Kimmeridge Boy BH Kimmeridge Bay O/C Porteshom BH Tisbury BH West Lavington BH Swindon 8H Hortwell BH Worlingham BH Denver ShJice BH North Runcton BH North Wootton BH Donington-on-Bain

+ X

1.0 --

440

OxL []

[]

l 450

390

Tmax ( ° C) Figure 15 Tmax versus maximum burial depth. The East Shetland Basin maturity trend is shown for comparison

290

,,'-"~

~'~

X [] ~XP¢~

Tmox ( o C) Figure 14 Tma. and vitrinite reflectance show a poor correlation in these samples

X A~

Kimmeridge Bay O/C Portesham BH Tisbury BH

390

450

A \

Kimmeridge Boy BH

0

T 400

A

~ \

Z

rY C:l

A ~

800

[]

0,30

\[] A

,

Uo O

~,,

0,40

]G÷

\ \

Marton BH •

1000 -

/

North Wootton BH

Ld 0.45 o~

\\\\1

/

North Runcton BH

\o/

/

®//

'~

®

I

~

Denver Sluice BH

>~

o.5o

m

Warlingham BH

l

r

]

T

r

400

4 10

420

430

440

45C

Tmax ( o C) Figure 17 Relationship of Tma. to kerogen type (phyTOC) showing a negative relationship

Marine and Petroleum Geology, 1991, Vol 8, August

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman increase in Tmax whereas increased carbonate content o.o causes a slight decrease (Espitalie, 1986). The amount of terrigenous structured kerogen, as defined by phyTOC, also has an effect on the Tm~ values, showing an inverse relationship (Figure 17) which can be best seen in the Marton Borehole 10 samples. Thus Tm~x appears to be dependent largely on kerogen facies (Huc et al., 1985), particularly the proportions of amorphous oil-prone and terrestrial kerogen and, at the low levels of maturity experienced by the onshore samples, appears to largely reflect ~" variations in facies. ~ 2.0 Bitumen-extract gas chromatograms for each location (Figure 6) have an immature character, the Marton sample showing the highest maturity. The CPIs (Bray and Evans, 1965) are all greater than 1.0, suggesting immaturity of the samples (Hunt, 1979), with the Marton Borehole samples showing the highest ~ s.0 maturity with values of 1.5-1.1. The pr:ph ratio appears to be unaffected by maturity.

::i::!:~::";~.~;i~"~::'~!}}:~:~:~*i~i',:~*::~:~&li

I

~

1200

Kimmeridge Bay BH Kimmeridge Bey O/C + Portesham BH X Tisbury BN

0

West LavingtonBH

~ ~0 ® 1000

dS~ Z 800-

g

I

Z :~

0

+~

O

--

~' [] ?X 4~

Sw~ndonBH HortwellBH W(~rlingham BH DenverSluice BH

>~ ]~ [] (~ []

North Ru~cton BH North Wootton BH Donington-on-B~in Reighton BH MartonBH

[] •

Lothbeg Point O/C Portgower O/C

•

Staffin O/C

+

400 I []

200

0

Kimmeridge Bay BH

q-

Portesham BH

Kimmeridge Bay O/C

(2t2~i X

Tisbury BH West Lavington BH

V

Swindon BH

[]

Hartwell BH

X-

Warlingharn BH Denver Sluice BH North Runcton BH

\

North Wootton BH \\\\\\

[]

4.0

\ [ii! r::W"~ *':':." "

\

~,~

\

"4\ 50

\

X k

I 1.5

@

Reighton BH

[]

Matron BH

[]

Lothbeg Point O/C

•

Portgower O/C

•

Staffin O/C

I

1.0

ATOMIC

Donington on Bain

0.5

H/C

RATIO

Figure 19 Atomic hydrogen:carbon ratio versus depth with the East Shetland Basin kerogen maturitytrends for comparison

comparison purposes, the H:C data from the Upper Jurassic of the East Shetland Basin are plotted to illustrate typical maturation trends for Type II and Type III kerogens. Type III kerogens are prevalent in the basin margin samples from the Helmsdale Boulder Beds and the Staffin Shale. The unusually low H:C and high O:C ratios of the Helmsdale Boulder Bed samples, which comprise thin (millimetre to centimetre scale) interbeds of black mudstone and clean quartz sandstones, suggest the kerogens have suffered post-depositional oxidation (MacLennan and Trewin, 1989). The sandstone interbeds appear to be of turbiditic origin and oxidation of the kerogens, deposited under anoxic conditions, is postulated to have occurred during short-lived oxygenation events. The unusual kerogen elemental composition of Staffin Shale sample CST-02 may be due to the effects of intense localized heating as it is from a location 0.5 m below an igneous sill. The H:C ratio is a facies dependent maturity indicator (Tissot and Welte, 1978; Hunt, 1979) and the data suggest that the H:C ratio in the onshore samples largely reflects kerogen type variations (Figure 18) rather than the effects of maturity.

0

600

I I

ratio shows only a poor correlation with depth (Figure 19) due to the presence of different kerogen types. For

1400

E] q_

I

I-I

Elemental analysis. Kerogen elemental analysis data can be used to determine maturity either by the Van Krevelen diagram (Durand and Monin, 1980) or by the H:C ratio (Laplante, 1974; Harwood, 1977). The Van Krevelen diagram (Figure 7) illustrates the variation in kerogen facies and maturity within the samples. The H:C ratio shows a positive relationship with the hydrogen index for the bulk of the samples of Type II-Type III composition (Figure 18) but clearly shows the different composition of the basin margin kerogens from northern Scotland. The H:C ratio therefore strongly reflects the kerogen facies (Tissot and Welte, 1978). Like the other kerogen maturity indices, the H:C

U.

I m~

-

,;a

-

[] 0

16

I

I

I

I

I

14

4.2

4.0

0.8

0.6

ATOMIC H / C

•

0.4

RATIO

Figure 18 Atomic hydrogen:carbon ratio hydrogen index showing the very different composition of the basin margin kerogens from northern Scotland

Inorganic indicators. The clay mineral transformation of smectite through random ordered illite-smectite to ordered illite-smectite is a temperature dependent reaction (Hower et al., 1976) and has been used as a maturity indicator (Monnier,

M a r i n e and Petroleum Geology, 1991, Vol 8, August

291

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman O0

~o O

W

O

I

P

1.0

~+

I

©

® 800 -

E

,~

AX O z~

(2)0

2.0

a~

I

E~ 600

[]

~:~ D C[]

[]

a-.~

o,o

'E? /;°

3.0

I ~e,#~SN~I~N~ ~ # / 0 +

40

X

Kimmeridge Bey BH

~

Denver Sluice BH

Kimmerldge Boy O/C Portesham BH /isbury BH

:~ ]~ []

West Lovington BH

[]

North Runcton BH North Wootton BH Donington on Bain Reighton BH Morton BH

Swindon BH HQrtwell BH

[] •

Lothbeg Point O/C Portgower O/C

•

St~ffin O/C

@

[]

~X- Worlinghom BH 5,0

q 0

20

F

L

80

60

100

1982; Pearson et al., 1982; Burtner and Warner, 1986). Illite-smectite was studied in the samples (Scotchman, 1984; 1987a) and the results confirm this reaction as a relatively insensitive measure of palaeotemperature, particularly as it appears to be affected by the source composition and other parameters during diagenesis. The depth relationship of the illite content of illite-smectites (Figure 20) is poor, the onshore samples differing from the relationship determined from the Upper Jurassic of the northern North Sea by Pearson et al. (1982). Two explanations can be offered:

i E~lm

COv

o% q

@@

i

,

®

®~

q i

"E

2

i i i i

i

ck

I

I

15 1.0 0.5 0 0 ATOMIC H/C RATIO

I

50 100 ELITE (%)

r

40

Figure 20 Variation of the illite content of illite-smectite with maximum burial depth. The Viking Graben trend of Pearson et al. (1982) is shown for comparison

N

TmQx ( ° C )

.0

Figure 22 Comparison of maturity indices with kerogen facies (hydrogen index)

ILLITE (%)

NNN

001,00 ,;0

O2 VITRIN REFLECT.(% Re)

I

G

either the burial depth of southern and eastern England was 1.5-2 km greater than estimated or, more likely, the data are affected by the presence of detrital illite. The ordered illite-smectites appear to be of a largely detrital origin and therefore are not indicative of high burial temperatures.

Comparison of maturity indicators. T h e

various

maturity indicators can be compared on the basis of burial depth (Figure 21) and kerogen facies (hydrogen index, Figure 22). Data from the East Shetland Basin have been included for comparison purposes; trend lines have been added to these figures to attempt an interpretation of the onshore data and are not statistically meaningful. With burial depth, vitrinite reflectance and Tmaxshow a poor, positive relationship whereas the H:C ratio decreases. The illite content of the associated illite-smectites shows no trend. Figure 22 shows that vitrinite reflectance and Tmax are both inversely related to hydrogen index. The H:C ratio data are shown to be facies dependent and the clay mineral data suggest that much of the illite may be of detrital origin. The 'traditional' organic maturity indicators therefore appear to be strongly affected by kerogen facies, making the determination of relative maturity levels difficult.

[] NN[][]

Synthesis: variation of source rock parameters with burial depth and facies

i

o3

I 11 0.2 04 06 350 400 450 2 0 0 VITRIN REFLECT.(% Ro) Tmox ( ° C ) ATOMIC H/C RATIO

I 50 ILLITE (%)

Figure 21 Comparison of maturity indices with maximum burial depth. The East Shetland Basin data are shown for comparison

292

The data from Bed 32 show that the organic facies exerts a strong control on the geochemical analyses. This has been demonstrated in other source rocks such as the Carboniferous of the Midcontinent, USA (Wenger and Baker, 1986; 1987). The significant reworked terrestrial kerogen component in these samples appears to have affected the maturity determination using vitrinite reflectance, resulting in the poor, often inconsistent, estimation of maturity. Tma× appears to be a better maturity indicator and comparison with biomarker data from autochthonous

Marine and Petroleum Geology, 1991, Vol 8, August

K e r o g e n facies a n d kerogens (Williams, 1986) confirms the general immaturity of the onshore section (Farrimond et al., 1984). Reliance on allochthonous kerogens, particularly their vitrinite reflectance values, to determine the maturity has led other workers to conclude that the Kimmeridge Clay shows a general trend of increasing maturity northwards (Gallois, 1979; Dypvik et al., 1979; Williams and Douglas, 1979; 1981), which is not seen in this study. Interestingly, the Cleveland Basin section at Marton appears to be the most mature with a mean Tmaxof 428°C, and the in situ formation of bitumen fibrils in one sample. Its vitrinite reflectance, however, suggests a much lower maturity. Thus geochemical parameters based on bulk kerogens are representative only when a small proportion of structured, allochthonous kerogen is present. This in itself causes problems as the kerogen maceral most commonly used for maturity measurement, vitrinite, is a structured kerogen. For predominantly Type II kerogen-rich rocks such as the Kimmeridge Clay Formation, molecular parameters (e.g. Mackenzie, 1984; Mackenzie et al., 1988; Radke, 1988) appear to be necessary to accurately determine the levels of maturity.

Conclusions 1. Geochemical parameters measured on bulk kerogens are strongly dependent on kerogen facies and, by inference, the sedimentation rate and depositionai setting (Scotchman, 1989). Variations in these parameters can reflect facies changes rather than trends in maturity. 2. Vitrinite reflectance shows a wide scatter with up to five populations in each sample. It is a poor measure of maturity in these rocks and shows no relationship with burial depth. Vitrinite reflectance appears to be retarded in the more oil-prone samples. Tmax from Rock-Eval pyrolysis is also a poor measure of maturity in these samples and is affected by kerogen facies and to a lesser extent by the rock matrix. Kerogen facies thus exerts a strong control on maturity measurements such that variations in these parameters tend to represent facies changes rather than differences in maturity. 3. Kerogen facies appear to be strongly controlled by the sedimentation rate and by the palaeogeography of the depositional basin. Terrestrial kerogen inputs are highest in the basins, particularly the Cleveland Basin, where they are associated with high sedimentation rates, and in the basin margins. Mixed Type II-Type III kerogens of marine origin predominate in the basin flank and shelf areas, with variable terrestrial input whereas oxidized kerogens occur in the basin margins.

Acknowledgements This work forms part of an ongoing research project, begun at the University of Sheffield/Institute of Geological Sciences (now British Geological Survey) under the auspices of a NERC studentship, which is gratefully acknowledged. Geochemical analysis were made at the Amoco Production Company's Tulsa Research Centre under the supervision of R. L. Ames and L. M. Ross. Amoco are thanked for permission to publish this paper and for assistance in its preparation.

m a t u r i t y o f the K i m m e r i d g e Clay: L C, S c o t c h m a n Drs M . D . L e w a n , R. L . A m e s and R. J. H a r w o o d are

thanked for discussions on the interpretation of the data.

References Aigner, T. (1980) Biofabrics and stratinomy of the Lower Kimmeridge Clay (U. Jurassic, Dorset, England) IV, Jb, GeoL Palaont. Abh. 159, 324-338 AIIsop, J. M. and Kirby, G. A. (1985) Estimates of Post-Cretaceous uplift in eastern England derived from study of sonic logs. In: Geophysical Logs in British Stratigraphy, Special Report No. 18 (Eds A. Whittaker, D. W. Holliday, and I. E. Penn) Geological Society, London, p, 18 Bray, E. E. and Evans, E. D. (1965) Hydrocarbons in nonreservoir-rock source beds: Part 1 Am, Assoc. Petrol GeoL Bull. 49, 248-257 Burtner, R. L. and Warner, M. A. (1986) Relationship between illite/smectite diagenesis and hydrocarbon generation in Lower Cretaceous Mowry and Skull Creek Shales of the Northern Rocky Mountain area Clays Clay Miner. 34, 390-402 Chadwick, R. A. (1985) Permian, Mesozoic and Cenozoic structural evolution of England and Wales in relation to the principles of extension and inversion tectonics. In: Atlas of Onshore Sedimentary Basins in England and Wales: Post-Carboniferous Tectonics and Stratigraphy (Ed. A. Whittaker) Blackie, Glasgow, pp. 9-25 Colter, V. S. and Havard, D. J. (1981) The Wytch Farm Oilfield, Dorset. In: Petroleum Geology of the Continental Shelf of NW Europe (Eds L. V. Illing and D. G. Hobson) Heyden, London, pp. 594-503 Cornford, C., Christie, O., Endresen, U., Jensen, P. and Myhr, M.-B. (1987) Source rock and seep oil maturity in Dorset, Southern England. In: Advances in Organic Geochemistry, 1987. Org. Geochem. 13, 399-409 DeMaison, G., Holck, A. J. J., Jones, R. W. and Moore, G. T. (1984) Predictive source bed stratigraphy: a guide to regional petroleum occurrence. North Sea basin and eastern North American continental margin. Proceedings of the 1lth World Petroleum Congress, Wiley, Chichester, Vol. 2, pp. 17-29 Dore, A. G., Vollset, J. and Hamar, G. P. (1985) Correlation of the offshore sequences referred to the Kimmeridge Clay Formation - - relevance to the Norwegian Sector. In: Petroleum Geochemistry in Exploration of the Norwegian Shelf (Eds B. M. Thomas et al,) Graham and Trotman, London, pp. 27-38 Durand, B. and Monin, J. C. (1980) Elemental analysis of kerogens In: Kerogen (Ed. B. Durand) Editions Technip, Paris, pp. 113-142 Dypvik, H., Rueslatten, H. G. and Throndsen, T. (1979) Composition of organic matter from North Atlantic Kimmeridgian Shales Am. Assoc. Petrol GeoL Bull.. 63, 2222-2226 Ebukanson, E. J. and Kinghorn, R. R. F. (1985) Kerogen facies in the major Jurassic mudrock formations of Southern England and the implication on the depositional environments of their precursors J. Petrol. Geol. 8, 435-462 Ebukanson, E. J. and Kinghorn, R. R. F. (1986) Maturity of organic matter in the Jurassic of southern England and its relation to the burial history of the sediments J. Petrol. Geol. 9, 259-280 Espitalie, J. (1986) Use of Tma× as a maturation index for different types of organic matter: comparison with vitrinite reflectance. In: Thermal Modelling in Sedimentary Basins (Ed. J. Burrus) Editions Technip, Paris, pp. 475-496 Espitalie, J., Deroo, G. and Marquis, F. (1985) La pyrolyse Rock-Eval et ses applications Rev. Inst. Fran. Petrol 40, 563-579 and 755-784 Farrimond, P., Comet, P., Eglington, G., Evershed, R. P., Hall, M. A., Park, D. W. and Wardroper, A. M. K. (1984) Organic geochemical study of the Upper Kimmeridge Clay of the Dorset type area Mar, Petrol. Geol, 1,340-354 Gallois, R. W. (1978) A pilot study of oil shale occurrences in the Kimmeridge Clay Report of the Institute of Geological Sciences No. 78/13 Gallois, R. W. (1979) Oil shale resources in Great Britain Institute of Geological Sciences Open File Report

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, V o l 8, A u g u s t

293

Kerogen facies and maturity of the Kimmeridge Clay: I. C. Scotchman Green, P. F. (1989a) Thermal and tectonic history of the East Midlands shelf (onshore UK) and surrounding regions assessed by apatite fission track analysis J. GeoL Soc. London 148, 755-773 Green, P. F. (1989b) Uplift and erosional history of Cleveland Basin and surrounding areas revealed by apatite fission track analysis. Abstract 28th Int. GeoL Congr,, Washington D.C., USA, July 9-19, 1989, pp. 585-586 ~larwo0d, R. J. (1977) Oil and gas generation by laboratory pyrolysis of kerogens Am. Assoc. Petrol GeoL Bull. 81, 2082-2102 Hemingway, J. E. and Riddler, G. P. (1982) Basin inversion in North Yorkshire Trans. Inst. Min. Metal/. B91, 175-186 House, M. R. (1985) A new approach to an absolute time scale from measurements of orbital cycles and sedimentary microrhythms Nature (London) 316, 721-725 Hower, J., Eslinger, E., Hower, M. E. and Perry, E. A. (1976) Mechanism of burial metamorphism of argillaceous sediments: 1. mineralogical and chemical evidence Bull. GeoL Soc. Am. 87, 725-737 Huc, A. Y., Irwin, H. and Schoell, M. (1985) Organic matter quality changes in a Upper Jurassic shale sequence from the Viking Graben. In: Petroleum Geochemistry in Exploration of the Norwegian Shelf (Eds B. M. Thomas et aL) Graham and Trotman, London, pp. 179-183 Hunt, J. M. (1979) Petroleum Geochemistry and Geology, Freeman, San Francisco Jackson, J. O. and Fookes, P. G. (1974) The relationship of the estimated former burial depth of the Lower Oxford Clay to some soil properties Q. J. Eng. GeoL 7, 137-179 Kent, P. E. (1980) British Regional Geology: Eastern England from the Tees to the Wash, Second Edition, HMSO, London Lake, S. D. and Karner, G. D. (1987) The structure and evolution of the Wessex Basin, southern England: an example of inversion tectonics Tectonophysics 137, 347-378 Laplante, R. E. (1974) Hydrocarbon generation in Gulf Coast Tertiary sediments Am. Assoc. Petrol GeoL Bull. 58, 1281-1289 Lijmbach, G. M. G. (1975) On the origin of petroleum. In: 9th World Petroleum Congress, Proceedings, Applied Science Publishers, London, Vol. 2, pp.357-369 MacKenzie, A. S. (1984) Organic reactions as indicators of burial and temperature histories of sedimentary sequences Clay Miner. 19, 271-286 MacKenzie, A. S., Leythaeuser, D., Altebaumer, F. J., Disko, U. and Rullkotter, J. (1988) Molecular measurements of maturity for Lias & shales in N.W. Germany Geochim. Cosmochim. Acta 52, 1145-1154 MacLennan, A. M. and Trewin, N. H. (1989) Palaeoenvironments of the Late Bathonian-mid-Callovian in the Inner Moray Firth. In: Northwest European Micropalaeontology and Palynology (Eds D. J. Batten and M. C. Keen) Ellis Horwood, Chichester, pp. 92-117 MacLeod-Matthew, A. F. (1975) UK oil shales - - post and possible future exploitation Department of Energy, Energy Paper, No. 1, HMSO, London Miller, R. G. (1988) Palaeoceanographic approach to the Kimmeridge Clay Formation Abstr. Am. Assoc. Petrol GeoL Bull. 72, 1013-1014 Monnier, F. (1982) Thermal diagenesis in the Swiss Molasse Basin: implications for oil generation Can. J. Earth Sci, 19, 328-342 Myers, K. and Wignall, P. (1987) Understanding Jurassic organic-rich mudrocks: new concepts using gamma-ray spectrometry and palaeoecology: examples from the Kimmeridge Clay of Dorset and the Jet Rock of Yorkshire. In: Marine Clastic Sedimentology: Concepts and Case Studies (Eds J. K. Leggett and G. G. Zuffa) Graham and Trotman, London, pp. 172-189 Oschmann, W. (1988) Kimmeridge Clay sedimentation - - a new cyclic model Palaeogeogr. PalaeoclimatoL PalaeoecoL 65, 217-251 Pearson, M. J., Watkins, D. and Small, J. S. (1982) Clay diagenesis and organic maturation in northern North Sea sediments. In: International C l a y Conference 1981. Developments in Sedimentology Series No.35 (Eds H. van Olphen and F. Veniale) Elsevier, Amsterdam, pp. 665-675 Penn, I. E., Chadwick, R. A., Holloway, S., Roberts, G., Pharoah, T. C., AIIsop, J. M., Hulbert, A. G. and Burns, I. M. (1987) Principal features of the hydrocarbon prospectivity of the Wessex-Channel Basin, UK. In: Petroleum Geology of North

294

West Europe (Eds J. Brooks and K. Glennie) Graham and Trotman, London, pp. 109-118 Powell, T. G. and McKirdy, D. M. (1973) The effect of source material, rock type and diagenesis on the n-alkane content of sediments Geochim. Cosmochim. Acta 37, 623-633 Price, L. C. and Barker, C. E. (1985) Suppression of vitrinite reflectance in amorphous rich kerogen - - a major unrecognised problem J. Petrol GeoL 8, 59-84 Radke, M. (1988) Application of aromatic compounds as maturity indicators in source rocks and oils Mar. Petrol GeoL 5, 225-236 Robertson Research (1985a) The Moray Firth area, North Sea: stratigraphy, petroleum geochemistry and petroleum geology Non-exclusive report, Robertson Research, Llandudno Robertson Research (1985b) A petroleum geochemical evaluation of the Jurassic sediments outcropping on the Island of Skye and from boreholes around the Hebridean Basin, Scotland Report No. 5622 P/D, Robertson Research, Llandudno Schwab, K. W. (1990) Organic petrology seminar (visual kerogen assessment). Presented at Zeist, The Netherlands, 10-13 January, 1990 Scotchman, I. C. (1984) Diagenesis of the Kimmeridge Clay Formation PhD Thesis, University of Sheffield, UK Scotchman, I. C. (1987a) Clay diagenesis in the Kimmeridge Clay Formation, onshore UK, and its relation to organic maturation Miner. Mag. 51,535-551 Scotchman, I. C. (1987b) Relationship between clay diagenesis and organic maturation in the Kimmeridge Clay Formation, onshore UK. In: Petroleum Geology of North West Europe, (Eds J. Brooks and K. Glennie) Graham and Trotman, London, pp. 251-262 Scotchman, I. C. (1989) Diagenesis of the Kimmeridge Clay Formation, onshore U.K.J. GeoL Soc. London, 146, 283-304 Scotchman, I. C. (1991) The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England Sedimentology 38, 79-106 Stoneley, R. (1982) The structural development of the Wessex Basin J. GeoL Soc, London 139, 543-554 Stoneley, R. and Selley, R. C. (1986)A fieldguide to petroleum geology of the Wessex Basin, Imperial College, London Thomas, B. M., Moller-Pedersen, P., Whitaker, M. F. and Shaw, N. D. (1985) Organic facies and hydrocarbon distributions in the Norwegian North Sea. In: Petroleum Geochemistry in Exploration of the Norwegian Shelf (Eds B. M. Thomas et aL) Graham and Trotman, London, pp. 3-26 Tissot, B., Durand, B., Espitalie, J. and Combaz, A. (1974) Influence of the nature and diagenesis of organic matter in formation of petroleum Am. Assoc. Petrol GeoL Bull. 58, 499- 506 Tissot, B. P. and Welte, D. H. (1978) Petroleum Formation and Occurrence, Springer-Verlag, Berlin Tyson, R. V. (1987) The genesis and palynofacies characteristics of marine petroleum source rocks. In: Marine Petroleum Source Rocks (Eds J. Brooks and A. J. Fleet) Spec. PubL GeoL Soc. London No. 26 pp. 47-67 Tyson, R. V. (1989) Late Jurassic palynofacies trends, Piper and Kimmeridge Clay Formations, UK onshore and northern North Sea. In: Northwest European Micropalaeontology and Palynology (Eds D. J. Batten and M. C. Keen) Ellis Horwood, Chichester, pp. 135-172 Tyson, R. V., Wilson, R. C. L. and Downie, C. (1979) A stratified water column environmental model for the type Kimmeridge Clay Nature (London) 277, 377-380 Wenger, L. M. and Baker, D. R. (1986) Variations in organic geochemistry of anoxic-oxic black shale - - carbonate sequences in the Pennsylvanian of the Midcontinent, U.S.A. In: Advances in Organic Geochemistry, 1985. (Eds D. Leythauser and J. Rullkotter) Org. Geochem. 10, 85-92 Wenger, L. M. and Baker, D. R. (1987) Variations in vitrinite reflectance with organic facies - - examples from Pennsylvanian sand cyclothems of the Midcontinent, U.S.A. Org. Geochem. 11,411-416 Wignall, P. B. (1989) Sedimentary dynamics of the Kimmeridge Clay: tempests and earthquakes J, GeoL Soc, London 146, 273- 284 Williams, P. F. V. (1986) Petroleum geochemistry of the Kimmeridge Clay of onshore southern and eastern England Mar. Petrol GeoL 3, 258-281

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, V o l 8, A u g u s t

Kerogen facies and maturity of the Kimmeridge Clay: L C. Scotchman Williams, P. F. V. and Douglas, A. G. (1979) A preliminary organic geochemical investigation of the Kimmeridgian oil shales. In: Advances in Organic Geochemistry 1979 (Eds A. G. Murchison and J, R. Maxwell) Pergamon Press, Oxford, pp. 531-545 Williams, P. F. V. and Douglas, A. G. (1981) Kimmeridge oil shale: a study of organic maturation. In: Organic Maturation Studies and Fossil Fuel Exploration (Ed. J. Brooks) Academic Press, London, pp.255-269

Williams, P. F. V. and Douglas, A. G. (1983) The effects of lithologic variation on organic geochemistry in the Kimmeridge Clay of Britain. In: Advances in Organic Geochemistry 1981 (Eds M. Bjor0y et aL) Wiley, Chichester, pp. 568-575 Williams, P. F. V. and Douglas, A. G. (1985) Organic geochemistry of the British Kimmeridge Clay. I. Composition of shale oils produced from Kimmeridge sediments Fuel 64, 1062-1069

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, Vol 8, A u g u s t

295