An Example from Devonian Lacustrine Sedimentary Rocks in the Inner Moray Firth, Scotland

Journal of the Geological Society, London, Vol. 155, 1998, pp. 335–352. Printed in Great Britain.

The recognition of multiple hydrocarbon generation episodes: an example from Devonian lacustrine sedimentary rocks in the Inner Moray Firth, Scotland

J. E. A. MARSHALL Department of Geology, University of Southampton, Southampton Oceanography Centre, European Way, Southampton S014 3ZH, UK (e-mail: [email protected])

Abstract: Burmah well 12/27-1, drilled on a structural high in the Inner Moray Firth, penetrated over 3000 ft (914 m) of Lower Devonian, organic-rich sedimentary rocks. These have been analysed for source-rock potential, and contain a substantial proportion of oil-prone, thermally mature kerogen. A decompacted vitrinite reflectivity profile shows that the lower part of the Devonian section has a steeper thermal maturity gradient, which is interpreted as indicating a high Devonian geothermal gradient associated with basin extension. Solid bitumen reflectivities and organic petrography show that the most recent hydrocarbon generation occurred in the upper part of the Devonian interval during maximum Tertiary burial. Vitrinite reflectivity data from the Jurassic rocks in the Beatrice Field and onshore East Sutherland enable the position of the oil generation window to be predicted in relation to the depth of the top of the Devonian sequence. The main controls on Devonian source rock potential in the Inner Moray Firth are not the depth of Mesozoic and Tertiary burial but the distribution of source rocks, their degree of Permian truncation, and the pre-Permian thermal maturity level.

Keywords: Moray Firth, Devonian, source rocks, vitrinite, bitumen, reﬂectivity.

Although contiguous with the North Sea extensional rift strate an entirely Middle Devonian source, specifically from system, the Inner Moray Firth Basin has so far yielded only the upper part of the Caithness lacustrine sequence. one significant hydrocarbon discovery, namely the Beatrice Underhill (1991) incorporated the dual Devonian–Jurassic Field. This field is located (Fig. 1) close inshore in the Inner origin for the Beatrice oil in a model for basin development of Moray Firth and some distance from the main groups of the Inner Moray Firth. He suggested that the most likely North Sea oil fields which are clustered on the flanks of the centre of oil generation was beneath the Sutherland Terrace Viking and Central graben. The primary oil source rock for during maximum Mesozoic burial, accompanied by long these latter fields is the Kimmeridge Clay Formation (e.g. distance (20 km) migration up dip to charge the Beatrice Cornford 1990; papers in Abbotts 1991), which is buried to structure. The Sutherland Terrace was subsequently uplifted greater depths in the Outer Moray Firth (2.5 km) and the and disconnected from the rest of the Inner Moray Firth by Viking and Central graben (4.5 km) than in the Inner Moray Tertiary movements along the Great Glen Fault system. It was Firth (1.5 km). implied (Underhill 1991) that the necessary conditions, i.e. The Inner Moray Firth basin differs in two important maximum Mesozoic burial of Devonian and Jurassic source respects from the Outer Moray Firth and the Viking and rocks, only occurred in the northern part of the Sutherland Central graben. The degree of Jurassic crustal extension in Terrace. the Inner Moray Firth was much less than in the North Sea, A similar, but more generalized, model was proposed earlier such that no deep grabens developed with accompanying by Trewin (1989), in which hydrocarbon generation in locally high heat flow and fill of rich source rocks. Further- Mesozoic times from Devonian source rocks was envisaged as more, the Inner Moray Firth underwent significant uplift being restricted to the areas of maximum Mesozoic burial in the early Tertiary (Hillis et al. 1994), and so was (Wick and Great Glen sub-basins) that lie immediately south never subjected to the deep Tertiary burial which else- and east of the Wick and Great Glen faults (Fig. 1). where brought the Kimmeridge Clay Formation to oil Invoking such an episode of deep Mesozoic burial circum- generation. Indeed, the Kimmeridge Clay Formation in the vents the major difficulty with plays based on a Devonian Inner Moray Firth is still thermally immature (Pearson & source rock, i.e. the timing of hydrocarbon generation. The Watkins 1983). difficulty is that there is good evidence (Astin 1991) for These contrasts between the Inner Moray Firth and the pre-Permian generation of hydrocarbons from Devonian North Sea graben are of such magnitude that, based on current source rocks, and thus the inevitable loss and/or degradation understanding of North Sea hydrocarbon generation pro- of any generated hydrocarbons during the significant Permian cesses, the Beatrice Field would not have been predicted. After inversion episode. a prolonged debate, however, it is now generally accepted that Thermal-maturity results reported here, from Burmah Oil the distinctive high wax oil of the Beatrice Field was not Exploration Ltd well 12/27-1 (Fig. 1), provide the first direct generated from Upper Jurassic shales. Duncan & Hamilton evidence for an episode of Mesozoic hydrocarbon generation (1988) and Peters et al. (1989) concluded that the Beatrice oil from a Devonian source rock. Significantly, it is demonstrated was sourced by a combination of Devonian and Lower to that Mesozoic generation was likely to have occurred across Middle Jurassic organic-rich shales. However, Bailey et al. the Inner Moray Firth, and not to be restricted solely to (1990), using pyrolysate carbon isotopes, were able to demon- limited areas of deepest burial such as the Sutherland Terrace.

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Fig. 1. Map of the Inner Moray Firth with simplified geology of the adjacent land area. The location of well 12/27-1 and the Beatrice oilfield are shown, together with selected structural elements from the Inner Moray Firth. Note the location of the anomaly (A) beneath the Central Ridge identified as a ‘granite’. BI, Black Isle; BRG, Ben Rinnes Granite; CR, Central Ridge; ER, Easter Ross; GGF, Great Glen Fault; HF, Helmsdale Fault; ST, Sutherland Terrace; WF, Wick Fault. Compiled from Underhill (1991) and Thomson & Underhill (1993) with additional information from Andrews et al. (1990) and other sources.

This in turn demonstrates that the main controls on the Devonian lacustrine strata as a viable source rock are not the extent of Mesozoic burial, but rather the local distribution of organic-rich shales and their pre-Permian level of thermal maturity.

Burmah well 12/27-1 Burmah well 12/27-1 was drilled in 1982–1983 on an exten- sionally rotated fault block at the western margin of the Smith Bank Graben, along structure from the Central Ridge. Comparison of well stratigraphy (Fig. 2) with other Moray Firth sections, together with an immediately adjacent seismic section (Underhill 1991), shows that the Jurassic and Lower Cretaceous sequences are attenuated across this structure. Compared to the rest of the Inner Moray Firth, this represents Fig. 2. Simplified stratigraphical log for Burmah well 12/27-1. All significantly shallower burial. Following penetration of a depths are below KB (kelly bushing). U, significant unconformities. largely clastic Triassic and Permian sequence, Devonian rocks Simplified from the composite log with updated lithostratigraphical were identified at 7693 ft (c. 2340 m) and drilled for some nomenclature from Cameron (1993) and Richards et al. (1993). 3200 ft (975 m) before the well terminated at 10 904 ft (c. 3325 m) without reaching basement (Fig. 2). The cuttings descriptions, mud log and wireline logs (Fig. 3) show that the above this sandy interval. At 10 550 ft (c. 3205 m), the succes- Devonian interval comprises an upper series of alternating sion reverts to alternating siltstones and organic-rich shales for shales and siltstones, dark in colour, with the peaks in the the remainder of the drilled section. gamma log representing organic-rich shales. Below 9900 ft Examination of spores during this study has confirmed the (3018 m), the lithologies become coarser, changing to siltstones Early Devonian age suggested by Richards (1985b) for the interbedded with sandstones, sometimes red in colour. Signifi- Devonian section. The spores are neither well preserved nor cant gas shows and traces of oil were recorded within and abundant, but enough characteristic forms were identified to

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indicate that most of the sequence is of early Emsian age, thus Struie Formation has a low TOC content which can be confirming correlation of the Devonian section in well 12/27-1 attributed to dilution by organically barren Rotliegend Group with the Lower Devonian, organic-rich sedimentary rocks cavings (casing shoe at 7963 ft), and to the in situ oxidation of found onshore in the Strathpeffer area (Fig. 1). organic matter beneath the sub-Permian unconformity. The In the Inner Moray Firth, Lower Cretaceous rocks are TOC rapidly increases below 7900 ft generally to between 1 exposed at the sea bed as the result of early Tertiary basin and 2%, until 9400 ft, where it decreases to <0.5% and then to inversion. The amount of inversion in this area has been <0.2% below 10 500 ft. Between 7900 ft and 9500 ft there are quantified using sonic velocity data (Hillis et al. 1994), which two cycles of increased TOC, with peaks at approximately show that well 12/27-1 has undergone a total erosion of some 8000 ft and 9000 ft. 600 m. A different method using biomarkers (Pearson & On the gamma log, the upper part of the succession contains Duncan 1996) suggests a lower figure of 350 m but with a number of distinct and cyclic peaks which, by analogy with significant uncertainties in its estimation. sections in the onshore Orcadian Basin, are interpreted as representing peaks in TOC. However, the drilling of the Devonian section in well 12/27-1 was unusual as 15 changes of Material and methods drill bit were made in addition to the cutting of three cores. Burmah Oil supplied unwashed 10 ft composite drill cuttings samples This has had the inevitable effect of producing mixed cuttings at 100 ft intervals through the complete Devonian succession of well which have proved impossible to correlate systematically with 12/27-1. As all the cutting, core and wireline log depths are in feet, wireline log based lithologies. An attempt was made to match these are used here with approximate metric equivalents given in the gamma ray peaks (generally >80 to 100 API) to TOC Table 1. In order to extend the vitrinite reflectivity measurements content, but the uncertainty over the relationship between uphole from the Devonian interval, a further set of cuttings samples cuttings and drilled depths, exacerbated by the wide sample were acquired from the DTI core store. However, the small amount of spacing, prevented any consistent or continuous correlation. available material meant that these samples were of necessity compos- Occasional correlations can be made, such as that at 9800 ft ited over 100–200 ft intervals. In addition, available core from 12/27-1 was studied, together with Jurassic core material (Table 2) from the where a single sample shows a marked TOC maximum of 1.4% Beatrice Field wells 11/30a-8 and 11/30a-A29(02). A representative in an otherwise low TOC interval. This TOC peak comprises suite of field samples (Table 3) was collected from the Jurassic AOM, and is coincident with a gamma ray peak. Thus the sediments exposed in the hanging wall of the Helmsdale Fault between cuttings samples should generally only be regarded as pro- Golspie and Helmsdale. The Brora Coal was provided by J. M. Jones viding representative average TOC contents based on 10 ft (University of Newcastle upon Tyne) from a seam pillar removed composited samples. from the Highland Colliery whilst the latter was still operational. The In an attempt to determine the maximum TOC content samples from Beatrice and the onshore section were acquired represented by the gamma ray peaks, the >2 mm cavings specifically to compare burial depths at an equivalent stratigraphical fractions from the cuttings samples were hand picked for rock level, as a test of existing burial models for the Inner Moray Firth. The cuttings samples were initially prepared by sieving at 2 mm and fragments with the darkest lithologies. These selected cavings 64 ìm to remove mud contaminants, the top sieve retaining the larger were then analysed for TOC as single fragments. The resulting caved fragments. The TOC (total organic carbon) determinations were TOC values are higher than those of the equivalent composite made on washed and dried cuttings samples using a Carlo-Erba EA samples, generally double with the highest value being 3.2%. 1108 elemental analyser, and are fully corrected for mineral carbonate In addition to considering the present-day range of TOC, it content. The kerogen isolates were produced using standard HCl/HF is relevant to correct these values for organic carbon loss demineralization procedures, followed by sieving at 20 ìmanda during burial. Using the ratio method of Raiswell & Berner single short treatment in hot concentrated HCl to remove fluoride (1987), the cuttings TOC average of 1.3% for the AOM-rich precipitates. Most of the kerogen isolates were dominated by AOM interval (7900–9300 ft) converts to an original TOC of 2.2%. (amorphous organic matter), to such an extent that structured kerogen Similarly, conversion of the maximum TOC (3.2%) indicates components were relatively rare. In order to increase the phytoclast (terrestrially derived fragmentary plant particles; Tyson 1995) and that some of the richest shale units may have had initial TOC spore abundance to the level needed for effective petrography and values in excess of 5%. photometry, the AOM in these samples was first fragmented using a Kerogen samples, viewed using transmitted white and tunable ultrasonic probe (Sonics & Materials 300W Vibracell) and reflected UV light, show that the organic-rich part of the then removed by sieving. The resulting kerogen isolates, largely devoid Devonian sequence above 9800 ft is dominated by fluorescent of AOM, were mounted as polished thin sections using the method of AOM, with a subordinate component of spores and phyto- Hillier & Marshall (1988). In addition, whole-rock polished blocks and clasts. Below about 10 000 ft, the kerogen content becomes polished thin-sections were prepared from selected samples to sparse and the assemblage composition erratic, with most ff investigate the relationships between the di erent bitumen phases. kerogen assemblages comprising spores and phytoclasts but Vitrinite and solid bitumen reflectivity and spore colour determinations were made as described by Hillier & Marshall (1992), with the with occasional high percentages of AOM. As all these additional use of 0.412% spinel and 0.919% YAG (yttrium aluminium kerogen assemblages originate from cuttings samples with very garnet) synthetic standards. Vitrinite and solid bitumen measurements low TOC, the AOM influxes are interpreted as indicating the were random and spore colours were estimated using the scheme of presence of organic-rich cavings. The presence of significant Pearson (1984). Visual kerogen identifications were made in combined cavings contamination can also be seen clearly in the sudden transmitted white light and reflected white and UV light. Percentages reduction in vitrinite reflectivity and spread in solid bitumen of different components were determined by point counting in reflectivity values at 10 100 ft (Fig. 3). transmitted white light.

Kerogen richness and type: Burmah well 12/27-1 Solid bitumens: Burmah well 12/27-1 Figure 3 and Table 1 show the distribution of TOC in the Bitumen and pyrobitumen are persistent components of most Devonian interval of well 12/27-1. The uppermost part of the samples (Fig. 3), averaging some 8% of the acid insoluble

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organic matter in the AOM-rich samples. Since these two accumulated around the margins of an isolated clastic grain. components are defined chemically, with pyrobitumens being Figure 4, nos 12 and 13 show a rare instance of both relatively insoluble in carbon disulphide, they cannot always hydrocarbons occurring together. Here, at 9900 ft the pyro- be distinguished microscopically. Therefore they are generally bitumen is present within porosity whereas brightly fluorescent referred to here as solid bitumen, but where relevant and ‘liquid’ bitumen occurs within a cross-cutting fracture system. separable are identified as bitumen and pyrobitumen. Such These petrographic relationships show clear evidence for solid bitumen is easily recognized (e.g. Thompson-Rizer 1987) two groups of solid bitumens. A less mature and strongly by its morphology, which frequently shows negative pseudo- fluorescent group is present in the upper part of the Struie morphs after carbonate (Fig. 4, nos 5–7). In transmitted light it Formation, and coincides with the positively skewed low has a characteristic yellow-orange colour, is translucent, and reflectivity solid bitumen populations. The lower group occurs lacks inclusions (Fig. 4, no. 7). In reflected light, it is present in the lower part of the Struie Formation, is non-fluorescent, is (Fig. 4, no. 5) as a very homogenous, low reflectivity material associated with the normally distributed solid bitumen reflec- (in comparison to vitrinite). At very low reflectivity, it has a tivities (specifically pyrobitumen), and is present in porosity bright yellow to orange-brown fluorescence (Fig. 4, no. 6 and as clear inclusions within mineral grains. where it is barely discernible, also more conspicuously in no. 8) and is more properly referred to as bitumen. Solid bitumen is particularly prevalent in the upper part of the AOM-rich interval, where it occurs with distinctive reflec- Thermal maturity: Burmah well 12/27-1 tivity distributions, characterized by a mode of true bitumen at Vitrinite reflectivity values derived from core samples (Table the minimum reflectivity values (0.01–0.02%), with a scatter of 1, Figs 3, 5) are statistically indistinguishable from those higher reflectivities. The range of scatter of these reflectivity measured from adjacent cuttings samples. The vitrinite reflec- values generally declines down-hole. tivity results are parallelled for the Devonian interval by spore At and below 9400 ft, solid bitumen fluorescence is lost, and colour determinations (Fig. 3). reflectivities increase rapidly in value and become normally Within the Mesozoic interval (Fig. 5), the vitrinite reflec- distributed. A marked peak in solid bitumen content occurs at tivity data points, although scattered, define a gradient. The 9900 ft (Fig. 3), coinciding with shows of gas and oil. Exami- scatter is believed to result from the reliance on cuttings nation of polished blocks in reflected light shows the solid samples composited over wide depth intervals. Additional bitumen at 9900 ft to be a migrated hydrocarbon product, factors limiting vitrinite data quality are a preponderance of occurring in siltstone rather than source rock. sedimentary facies representing open marine shelf environ- Examination of polished blocks and polished rock thin ments, which generally have a low phytoclast abundance and a sections in incident UV light, incident white light and trans- low proportion of vitrinite 1 (vitrinite A) in the vitrinite mitted white light (PPL and XPL) shows a number of distinc- populations. In addition, vitrinite reflectivity values from the tive petrographic relationships (Fig. 4) between pyrobitumen, samples beneath the top of the Kimmeridge Clay Formation bitumen and liquid hydrocarbons. Figure 4, nos 1 and 3 are suppressed by the presence of AOM (e.g. Hutton & Cook shows paired views of non-fluorescent pyrobitumen (p) within 1980; Senftle et al. 1993). This effect continues below the porosity at 9900 ft. The pyrobitumen is present as inclusions Kimmeridge Clay through the influence of cavings from such within a feldspar grain. The grain shows authigenic over- an organic-rich unit. Vitrinite reflectivity data quality sub- growth and thus the pyrobitumen represents an original stantially improves towards the base of the Jurassic section, migrated hydrocarbon product trapped by later diagenesis. with phytoclast-rich kerogen assemblages becoming the norm. Carbonate grains are also present within pore spaces (Fig. 4, Particularly reliable are the core samples, including the coal nos 1, 3 and 10), their relationships to the original detrital scare from within the Dunrobin Bay Group at 3762 ft. Only mineral grains and their diagenetic overgrowths showing them one data point, from the Zechstein Group, is available for the to have formed after hydrocarbon generation and feldspar 3500 ft, clastic, red-bed dominated Permo-Triassic sequence. overgrowth. Such carbonate is a common diagenetic product Although originating from cuttings, its position beneath within Orcadian Basin lacustrine sediments, where it forms the casing point at 3500 ft and following some 1500 ft of (Hillier 1993) as a byproduct of the diagenetic evolution of organically barren Heron Group strata makes this sample a chlorite minerals at temperatures within the range 120–260)C. highly reliable data point. Samples from the upper part of the well show bitumens with The vitrinite reflectivity data (Figs 3, 5 and Table 1) for the a strong fluorescence which, where present as solid material, Struie Formation indicate a shallow reflectivity gradient from are contained within porosity, and which also show diagenetic the top of the Struie Formation to 9100 ft over the reflectivity overgrowths by carbonate. More soluble forms of bitumen range 0.5–0.6%. Beneath this, a much steeper reflectivity are present within fracture systems. Their bright yellow gradient occurs down to 1.26% at 10 000 ft, with a group of fluorescence colours (contrast Fig. 4, no. 2 in incident UV light reliable data points (1.5%) in cuttings and core samples with no. 4 in transmitted PPL) and presence within fractures between 10 800 ft and 10 900 ft. indicates a migrated ‘liquid’ form of hydrocarbon. Figure 4, However, in common with the Kimmeridge Clay results, all no. 11 shows this material within the source rock where it has the reflectivity values from the Devonian interval down to

Fig. 3. Source rock data from the Devonian interval of well 12/27-1. The gamma log has peaks which represent discrete intervals rich in TOC and which coincide with an abundance of AOM in the kerogen. The vitrinite reflectivities show a dogleg in the thermal maturity gradient just below 9000 ft. The solid bitumen reflectivity distribution shows a distinctive envelope in the upper part of the well demonstrating a recent episode of oil generation, the magnitude of which diminishes down section to the cusp in the vitrinite reflectivity dogleg. Below this point the normally distributed solid bitumen (specifically pyrobitumen) reflectivities (e.g. 9900 ft) represent dead oil in porosity and were generated during Palaeozoic burial. The core locations are shown. Core sample vitrinite reflectivity data are given in Table 1. Gamma log from the composite log.

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Table 1. Vitrinite reﬂectivity data for well 12/27-1

Depth Depth Sample Stratigraphic

(ft) (m) type unit TOC Rv n sd Comments

1560–1650 475–503 Cuttings Cromer Knoll Gp 0.26 19 0.01 Composited cuttings 1680–1860 512–567 Cuttings Cromer Knoll Gp 0.28 23 0.03 Composited cuttings 1890–2010 576–613 Cuttings Cromer Knoll Gp 0.25 65 0.03 Composited cuttings 2040–2160 622–658 Cuttings Cromer Knoll Gp 0.28 12 0.02 Composited cuttings 2220–2370 677–722 Cuttings Cromer Knoll Gp 0.29 77 0.04 Composited cuttings 2400–2600 732–796 Cuttings Kim. Clay Fm 0.32 36 0.03 Composited cuttings 2640–2850 805–869 Cuttings Kim. Clay Fm 0.28 34 0.03 Composited cuttings 2880–3000 878–914 Cuttings Kim. Clay Fm 0.27 31 0.03 Composited cuttings 3010–3100 917–945 Cuttings Heather Fm 0.24 9 0.03 Composited cuttings 3110–3200 948–975 Cuttings Heather Fm 0.25 86 0.03 Composited cuttings 3210–3300 978–1006 Cuttings Heather Fm 0.32 70 0.05 Composited cuttings 3310–3400 1009–1036 Cuttings Heather Fm 0.33 75 0.06 Composited cuttings 3510–3600 1070–1097 Cuttings Heather Fm 0.36 60 0.05 Composited cuttings 3610–3640 1100–1109 Cuttings Heather Fm 0.38 19 0.03 Composited cuttings 3710 1131 Core#1 Dunrobin Bay Gp 0.38 51 0.03 3762 1147 Core#2 Dunrobin Bay Gp 0.36 89 0.02 Coal scare 5460–5600 1664–1700 Cuttings Zechstein Gp 0.44 80 0.04 Composited cuttings 7600 2316 Cuttings Struie Fm 0.1 7700 2347 Cuttings Struie Fm 0.2 7800 2377 Cuttings Struie Fm 0.1 7900 2408 Cuttings Struie Fm 1.3 8000 2438 Cuttings Struie Fm 1.8 8100 2469 Cuttings Struie Fm 1.4 0.50 6 0.03 8200 2499 Cuttings Struie Fm 1.7 0.55 27 0.07 8300 2530 Cuttings Struie Fm 1.2 0.57 42 0.06 8400 2560 Cuttings Struie Fm 0.6 0.57 40 0.06 8500 2591 Cuttings Struie Fm 0.5 8600 2621 Cuttings Struie Fm 1.2 0.58 14 0.04 8679 2645 Core#3 Struie Fm 0.3 0.59 12 0.03 8685 2647 Core#3 Struie Fm 0.4 0.58 67 0.07 8695 2650 Core#3 Struie Fm 0.6 8700 2652 Cuttings Struie Fm 0.9 0.57 19 0.04 8800 2682 Cuttings Struie Fm 1.5 0.58 38 0.05 8900 2713 Cuttings Struie Fm 0.8 9000 2743 Cuttings Struie Fm 2.2 0.65 9 0.05 9100 2774 Cuttings Struie Fm 1.7 0.60 21 0.04 9200 2804 Cuttings Struie Fm 0.5 0.78 22 0.06 9300 2835 Cuttings Struie Fm 1.0 0.77 10 0.07 9400 2865 Cuttings Struie Fm 0.3 9500 2896 Cuttings Struie Fm 0.5 0.97 19 0.07 AOM/phytoclasts 9600 2926 Cuttings Struie Fm 0.5 9700 2957 Cuttings Struie Fm 0.3 1.02 16 0.07 9800 2987 Cuttings Struie Fm 1.4 0.96 13 0.07 AOM rich 9900 3018 Cuttings Struie Fm 0.4 Solid bitumens in sltst 9963 3037 Core#4 Struie Fm 0.5 1.30 67 0.13 Phytoclast rich 9966 3038 Core#4 Struie Fm 0.2 1.31 86 0.19 Phytoclast rich 9988 3044 Core#4 Struie Fm 1.1 0.82 40 0.10 AOM rich 10 000 3048 Cuttings Struie Fm 0.1 1.26 56 0.16 AOM/phytoclasts 10 100 3078 Cuttings Struie Fm 0.4 1.08 21 0.14 AOM rich 10 200 3109 Cuttings Struie Fm 0.3 10 300 3139 Cuttings Struie Fm 0.2 10 400 3170 Cuttings Struie Fm 0.4 10 500 3200 Cuttings Struie Fm 0.2 10 600 3231 Cuttings Struie Fm 0.2 10 700 3261 Cuttings Struie Fm 0.1 10 800 3292 Cuttings Struie Fm 0.1 1.65 27 0.18 Phytoclast rich 10 880 3316 Cuttings Struie Fm 0.4 10 879 3316 Core#5 Struie Fm 0.1 1.57 40 0.11 Phytoclast rich 10 889 3319 Core#5 Struie Fm 0.6 1.52 71 0.09 Phytoclast rich 10 893 3320 Core#5 Struie Fm 0.1 1.52 84 0.13 Phytoclast rich 10 901 3323 Core#5 Struie Fm 0.2 1.54 64 0.14 Phytoclast rich

Depths are given in both feet and metric conversions. Sample type and outline stratigraphy are also shown. TOC is % Total Organic Carbon Rv is vitrinite reﬂectivity, n is number of measurements and sd standard deviation. A coal scare is a signiﬁcant coal clast not parallel to bedding.

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Table 2. Reﬂectivity data for the two studied Beatrice wells

Vertical Vertical Drilled depth depth depth Sample Stratigraphic

(ft) (m) (ft) type unit Rv n sd Comment

11/30a-A29(02) 7009 2136 10 766 Core#1 Heather Fm 0.65 61 0.06 7044 2147 10 820 Core#1 Heather Fm 0.68 71 0.07 7174 2186 11 019 Core#5 Beatrice Fm 0.35 48 0.02 AOM rich 7174 2186 11 020 Core#5 Beatrice Fm 0.35 85 0.06 7212 2198 11 078 Core#5 Brora Coal Fm 0.53 80 0.04 7226 2202 11 100 Core#5 Brora Coal Fm 0.61 77 0.04 Coal

11/30a-8 6793 2071 Core#1 Heather Fm 0.71 81 0.07 6810 2076 Core#2 Heather Fm 0.59 27 0.03 6969 2124 Core#5 Beatrice Fm 0.40 79 0.04 6972 2125 Core#5 Beatrice Fm 0.30 80 0.04 7074 2156 Core#6 Brora Coal Fm 0.51 87 0.03 Coal 7074 2156 Core#6 Brora Coal Fm 0.13 53 0.02 Exinite 7076 2157 Core#6 Brora Coal Fm 0.53 57 0.05 Coal

All samples are from core. Rv is vitrinite reﬂectivity, n is number of measurements and sd standard deviation. Well 11/30a-A29(02) was a deviated hole, actual depths shown in feet. Vertical depth from composite log.

Table 3. Vitrinite reﬂectivity data from the onshore Beatrice to Helmsdale area

No. Locality Grid ref Formation Member Rv n sd Comments

15 Portgower ND008135 Helmsdale Boulder Beds 0.29 66 0.04 AOM 14 Culgower NC992118 Helmsdale Boulder Beds 0.27 64 0.03 AOM 13 Kintradwell NC925074 Kintradwell Boulder Beds 0.26 85 0.02 AOM 12 South Foreshore NC911035 Brora Argillaceous Fascally Sandstone 0.36 29 0.04 11 South Foreshore NC907032 Brora Argillaceous Fascally Siltstone 0.39 46 0.02 10 Brora Brick Pit NC898041 Brora Argillaceous Brora Brick Clay 0.25 78 0.04 AOM 9 South Foreshore NC905033 Brora Argillaceous Brora Shale 0.25 52 0.03 AOM 8 South Foreshore NC904032 Brora Argillaceous Brora Roof Bed 0.26 95 0.03 Exinite 7 Highland Colliery NC899040 Brora Coal Inverbrora 0.34 79 0.02 Coal 6 Highland Colliery NC899040 Brora Coal Inverbrora 0.39 76 0.03 Coal 5 South Foreshore NC902030 Brora Coal Inverbrora 0.42 56 0.03 Cannel 4 South Foreshore NC902030 Brora Coal Inverbrora 0.37 76 0.04 Cannel 3 Dunrobin NC856009 Dunrobin Bay Lady’s Walk Shale 0.32 67 0.02 2 Dunrobin NC856009 Dunrobin Bay Lady’s Walk Shale 0.30 42 0.03

Rex 1 Helmsdale ND034152 Devonian clast 0.11 121 0.02 Rv=0.5 Rbit 1 Helmsdale ND034152 Devonian clast 0.16 135 0.03

Sample locality numbers are also shown on Figs 8, 9. Rv is vitrinite reﬂectivity, n is number of measurements and sd standard deviation. The data for the Devonian clast from Helmsdale refers to both exinite (Rex) and bitumen reﬂectivity (Rbit).

about 9300 ft have been suppressed because they occur within two sets of values. Vitrinite reflectivity from AOM-suppressed AOM-rich facies. This phenomenon is especially prevalent kerogen is on average reduced by 0.69 times the unsuppressed in Orcadian Basin lacustrine rocks (Hillier 1989; Hillier & equivalent. The correlation line (reduced major axis) has a Marshall 1992), where vitrinite reflectivity values in AOM-rich correlation coefficient of 0.95. This relationship is used (Fig. 5) facies are reduced in comparison with values from interbedded to correct the reflectivity values for the AOM-rich samples sediments (i.e. separated by only metres and usually much less from the Struie Formation, which are moved to higher data of vertical section) which have phytoclast-dominated kerogens. values and thus become coincident with a reflectivity gradient This phenomenon can been seen in core samples from extrapolated from the uphole section. well 12/27-1, where a reflectivity value of 0.82% for AOM- In the samples from the lower part of the steep reflectivity dominated kerogen at 9988 ft contrasts with reflectivity values gradient (9500–10 800 ft), the proportion of AOM in the of 1.30% and 1.31%, respectively, for phytoclast dominated kerogens rapidly diminishes. As these samples are also charac- kerogens from the same core at 9963 ft and 9966 ft. A com- terized by low TOC values (generally <0.5%), most of the pilation (Fig. 6) of suppressed and normal vitrinite reflectivities AOM was probably sourced from AOM-rich and hence from similar interbedded sequences, in the Orcadian Basin and phytoclast-poor cavings. Therefore, the reflectivity values from elsewhere, demonstrates a predictable relationship between the these mixed kerogens are more typical of in situ phytoclast-rich

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kerogen. Thus a relatively confident gradient for this lower reflectivity of 0.61% (Table 2). This well is strongly deviated interval can be drawn through selected core and cuttings and was drilled to test the Beatrice reservoir to the east of the sample data points as shown in Fig. 5. This line intercepts Beatrice Fault from where these samples originate. Figure 7 the corrected gradient from the AOM-rich upper part of the shows both its spud and TD locations. Struie Formation at the original cusp between the uncorrected Figures 8 and 9 and Table 3 show the vitrinite reflectivity gradients. Figure 5 additionally demonstrates how AOM- sample points and data from the onshore Jurassic sequence. suppressed vitrinite reflectivity values in the uphole section of The most reliable data points (Table 3) are those from the coal the Struie Formation, when combined with vitrinite reflectivity seam within the Brora Coal Formation. Two samples gave values from the lowermost samples which are phytoclast-rich, vitrinite reflectivities of 0.34% and 0.39% (average 0.37%). have had the effect of steepening the reflectivity gradient. Somewhat older samples from the Lower Jurassic Dunrobin However, it should be emphasized that conversion (Fig. 5) of Bay Formation have slightly lower reflectivities (0.30–0.32%) the vitrinite gradient for the complete Devonian interval still and are less mature. Vitrinite reflectivities from the Brora retains the pattern of an upper shallow gradient succeeded by Argillaceous Formation (Middle Jurassic, Callovian) are simi- a lower steep gradient. lar to those of the coals, except for values (about 0.25%) from Extrapolation (Fig. 5) of the upper part of the vitrinite AOM- or exinite-rich samples which are suppressed (Table 3). reflectivity gradient to give an intercept with a typical surface The Boulder Beds are similar in being AOM-rich with reflec- minimum reflectivity of 0.2% indicates that some 1000 ft tivities of 0.26–0.29% (Table 3), equivalent to unsuppressed (c. 300 m) of overlying strata have been removed. Although values of about 0.4% (Fig. 6) in phytoclast-rich kerogen facies. not a well-defined gradient, and with uncertainty over the These maturity values are consistent with biomarker data validity of the surface minimum value used (0.2% selected from (Pearson & Duncan 1996) which show no evidence for signifi- compilation in Stout & Spackman 1989), this figure is in broad cant burial of the onshore Jurassic sequence. The apparent agreement with the value of 600 m (1960 ft) determined from anomaly of greatest maturity in the highest part of the shale velocities (Hillis et al. 1994). succession is compatible with geological reconstructions (e.g. Wignall & Pickering 1993) which envisage increased dip-slip displacement northwards along the Helmsdale Fault, with the Vitrinite reflectivity results: Beatrice and pre-Boulder Bed sediments being reworked into the Boulder Brora/Helmsdale Bed facies. Jurassic core samples were studied from two wells (Fig. 7) Bitumens have been reported from large clasts of Devonian within the Beatrice Field. These samples are from bedded coals sandstone which occur in the Boulder Bed facies adjacent to within the Middle Jurassic (Bathonian) Brora Coal Formation the Helmsdale Fault Zone (e.g. Parnell 1983). They are also and overlying units, and were sampled to provide direct known from veins within the Helmsdale Granite (Tweedie correlatives of both the onshore Jurassic succession and well 1979). Such occurrences have been regarded as Mesozoic in 12/27-1. Core samples from Beatrice well 11/30a-8 gave vit- age by Underhill (1991). A significant discovery during this rinite reflectivities of 0.51% and 0.53% for the coal samples study was a Devonian clast rich in solid bitumens within the (Table 2). Lower values for the Beatrice Formation are AOM- Helmsdale Boulder Beds at Helmsdale. Analysis of these suppressed, whilst those from the Heather Formation contain bitumens (Table 3, Fig. 10) permits interpretation of the timing significant amounts of semi-fusinite. The single available coal and extent of hydrocarbon generation from the now removed sample analysed from 11/30a-A29(02) has a higher vitrinite upper part of the Devonian lacustrine succession of Sutherland

Fig. 4. All photographs are #600. The UV incident light images were produced by excitation at 450–490 nm using a 100 W Hg pressure lamp with a 510 nm chromatic beam splitter (ploem dichroic) incorporating a 520 nm long pass barrier filter. PPL plane polarized light, XPL crossed polars, UV ultra-violet light. (1), (3). Pyrobitumen (p) infilling pore spaces from a cuttings sample of siltstone lithology at 9900 ft. (1) is in PPL, (3) in XPL. This pyrobitumen has a relatively high reflectivity and represents the first episode of hydrocarbon generation. The feldspar grain (f) contains inclusions of pyrobitumen which have been overgrown by later authigenic growth. Note how the pyrobitumen inclusions lie along boundaries parallel to the present crystal faces. The original crystal faces can be picked out from the right-angled terminations of the inclusions and the zones of inclusion trails. The carbonate grain (c) is interpreted as secondarily growing into a pyrobitumen filled pore space. (2), (4). Fluorescent bitumen (b) within fractures from an organic-rich mudstone lithology in cuttings at 9400 ft. (2) is in incident UV light and clearly shows the bitumen to have migrated within fractures. (4) shows the corresponding PPL image in which the bitumen veins (arrows as in 2) are barely discernible. (5), (6), (7). A typical bitumen morphology isolated in a demineralized polished thin section from 9100 ft. (5) is in incident light and shows its characteristic very low reflectance. (6) is the corresponding view in incident UV light where it displays a barely discernible dull brown fluorescence. (7) which is in transmitted light (no polars) shows the characteristic homogenous nature of the bitumen which is free of inclusions. Note in all three figures the 60/120) angles (arrowed) to the bitumen particle as a negative pseudomorph of diagenetic carbonate. (8), (9). Bitumen accumulating within porosity in a single cuttings fragment from 8400 ft. (8) is in incident UV light and clearly shows the fluorescent bitumen within the pore space and between the grain boundaries. (9) is the corresponding image in transmitted light and shows how the bitumen has, in this instance, accumulated within a pore space occupied by a diagenetic carbonate (c). (10). Secondary diagenetic carbonate (c) within a (pyro)bitumen (p) filled pore in a single cuttings fragment from 9900 ft. (11). Cuttings sample from 8400 ft showing the accumulation of a highly fluorescent ‘liquid’ bitumen, derived from the organic-rich groundmass, along the margins of a clastic grain. Imaged in incident UV light. (12), (13). Part of a single cuttings fragment from 9900 ft. In the transmitted light view (12) a relatively highly reflective dark pyrobitumen (p) infills the porosity. The corresponding image in incident UV light (13) reveals the presence of highly fluorescent bitumen (b), as both globules and infilling fractures.

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Fig. 5. The vitrinite reflectivity data plotted as means for the complete well section. The data from the uppermost composited cuttings are scattered. Beneath these, values within the Kimmeridge Clay Formation are reduced by AOM suppression. Better constrained data were acquired from Middle Jurassic phytoclast-rich cuttings and core samples, including a coal scare from 3762 ft. The isolated Zechstein Group cuttings sample provides an accurate data point. The vitrinite values from the Devonian Struie Formation show the presence of two reflectivity gradients with a clear cusp. The core and cuttings reflectivity values from the upper shallower gradient have been displaced to lower values through AOM suppression. The indicated line shows the corrected gradient for these upper values. The deeper and steeper gradient has been modified by drawing it through selected data points which are largely phytoclast-rich or have low AOM contents, and so are not greatly reduced through suppression of vitrinite reflectivity. Following these corrections, the Devonian section still shows two distinct reflectivity gradients.

(see discussion, below, of the controls on Devonian source (Fig. 10), with a mean random reflectivity of 0.16%. The rock viability). The solid bitumens occur within porosity minimum and mean bitumen reflectivity values, and their within a siltstone. Bitumen reflectivity is normally distributed normal distribution, contrast with those of the solid bitumens in the upper part of the Struie Formation in well 12/27-1. Spores of Givetian (late Mid-Devonian) age are also present within the clast. The spores are orange in colour (spore colour 6/5) and have an exinite reflectivity of 0.11%, equivalent (Smith & Cook 1980) to a vitrinite reflectivity of 0.5%

Discussion The range of actual and initial TOC values, the dominance of AOM in the kerogen, and the range of vitrinite reflectivities, substantiate assertions (e.g. Richards 1985b) that oil-prone and thermally mature source rocks comprise a significant proportion of the Lower Devonian interval in well 12/27-1. Much more significantly, however, the vitrinite and solid bitumen reflectivity data provide the first direct evidence for both Palaeozoic and Mesozoic/earliest Tertiary generation of Fig. 6. Cross-plot of vitrinite reflected data pairs measured from interbedded AOM-rich and phytoclast-rich samples. Reduced major hydrocarbons from a Devonian source rock. axis correlation gives an intercept of essentially zero and a The vitrinite reflectivity results (Figs 3, 5) show a clear dog-leg with two reflectivity gradients (and consequently two relationship where RvAOM=0.69Rvphytoclasts. The correlation coefficient is 0.95 from 34 data points. The Orcadian Basin data is geothermal gradients), and demonstrate that, for a limited from Hillier (1989) together with published data sources listed period of time, there was a relatively high heat flow into the therein (Buiskool-Toxopeus 1983; Hutton & Cook 1980; Kalkreuth lower part of the section. Since this episode of relatively high 1982; Nuccio & Johnson 1984; Ingram et al. 1983; Newman & heat flow has a significant bearing on the timing of hydro- Newman 1982) with additions (Wenger & Baker 1987; Wilkins et al. carbon generation it is most important to determine its origin. 1992). Jurassic Moray Firth data from Tables 2 and 3. The two most likely causes are as follows.

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Fig. 7. Map of the northern part of the Inner Moray Firth, East Sutherland and Caithness, showing the locations of well 12/27-1, Beatrice and the Brora/Helmsdale area, together with the section lines from Fig. 14. Both the spud ( ) and T.D. (C) locations are shown for well 11/30a-A29(02). Note that the northeast dip on the Sutherland Terrace implies that the Jurassic samples from Brora were buried at a shallow depth. The location of the top of the oil window at maximum burial is indicated in the northeast part of the Sutherland Terrace. The onshore Caithness sections are organic rich but overmature east of the Camster Fault. Organic-rich, mature, lacustrine sediments occur between the Camster and Loch Calder Fig. 9. Simplified Brora succession showing stratigraphical location faults. The succession thins west of the Loch Calder Fault, with and reflectivity values of the vitrinite data points. Note that the basement highs exposed at Dirlot. Compiled, with simplification lithostratigraphical nomenclature of the onshore and offshore from Institute of Geological Sciences (1982) and British Geological Jurassic differs (cf. Fig. 2); the onshore scheme used by Trewin Survey (1995). (1993) and Hurst (1993) is followed.

Fig. 10. Reflectivity histogram for bitumens within porosity from a Fig. 8. Outline map of the Brora and Helmsdale area showing clast of Devonian siltstone within the Helmsdale Boulder Beds at location of studied samples. Location numbers as Fig. 9 and Helmsdale. Sample No. 1 on Fig. 8 and Table 3. Note that it is Table 3. Location of separate maps shown on Fig. 7. Simplified normally distributed and has no values less than 0.1%. It formed from Johnstone & Mykura (1989). from a migrated oil in Palaeozoic times rather than from in situ generation. It demonstrates the presence of pre-Mesozoic oil (a) High heat flow associated with Devonian basin exten- generation from the Devonian lacustrine source rocks. sion, essentially contemporaneous with deposition, to produce a steep thermal maturity gradient which was then partially overprinted during reburial under conditions with a lower turity gradient became overprinted by a steeper reflectivity geothermal gradient. gradient. A candidate for such a heat-pulse could be increased (b) A short-lived heat-pulse into the base of the section heat flow associated with crustal extension during Jurassic sometime after deposition, such that a shallow thermal ma- times in the Inner Moray Firth.

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burial continues, previous maximum burial temperatures are exceeded, and the organic matter in the rocks starts to alter irreversibly, the AOM generating bitumen and the vitrinite increasing in reflectivity. Figure 11 shows that the organic matter which first increases its level of thermal maturation above that achieved during Palaeozoic times will be at the top of the Struie Formation section. As burial increases, lower parts of the Struie Formation will increase in thermal maturity to form a distinctive, apex-down, triangular zone of bitumen formation. (3) Figure 3 shows the reflectivity histograms for the solid bitumens. In the upper part of the Devonian interval the modes occur at very low reflectivities (0.1–0.2%), characteristic of active bitumen generation from the AOM. Once formed, these solid bitumens accumulate, increase in maturity (and Fig. 11. Figure showing how overprinting a steep initial gradient by hence reflectivity) and ultimately decompose to generate liquid a lesser gradient but to a greater burial depth generates hydrocarbons. Thus the strongly positively skewed histograms hydrocarbons in the top of a source rock succession. At maximum in the uppermost part of the interval define a zone of active Palaeozoic burial, the Struie Formation was buried beneath younger bitumen generation within which hydrocarbon generation has Devonian and Carboniferous sediment. Bitumens formed and been occurring for some time. At the base of the upper hydrocarbons were generated. By the beginning of late Permian interval, the histograms show only very low and normally time, the succession had been uplifted and significantly eroded, and distributed reflectivities, indicating that bitumen generation hydrocarbons had been degraded, prior to reburial. During Jurassic has only just commenced at this point. This distribution of times continued reburial raised the temperature within the Struie solid bitumen reflectivities is a consequence of the distinctive, Formation but did not achieve levels attained during the Devonian. Hence the vitrinite did not increase in reflectivity and the previous apex-down, triangular zone of hydrocarbon generation pre- gradient was preserved. During the early Cretaceous, burial was dicted by the simple, overprinted shallow-on-steep thermal sufficient to increase burial temperatures within the top of the Struie gradient model (Fig. 11). Formation above those previously attained. Bitumens were formed The lower steep part of the vitrinite reflectivity gradient and vitrinite reflectivity increased. The interval of active bitumen shows solid bitumen reflectivity distributions which are nor- generation takes the form of a downward pointing triangle. During mally distributed, with the population distribution increasing early Tertiary times, burial was at a maximum and the zone of progressively in value in an approximately predictable re- active bitumen generation had increased in extent. Note that the lationship with the reflectance of vitrinite. These distributions upper part of the zone has had a longer history of bitumen are characteristic of thermal maturation levels at which active generation. bitumen generation from AOM has ceased, so that very low reflectivity material is no longer being added to the population Although examples of both scenario are well established which becomes normally distributed on increasing maturation. (e.g. Robert 1988), convincing arguments for overprinting Such solid bitumens from the lower part of the Devonian during reburial can be made in this instance. interval are presumed to have formed contemporaneously (1) The pattern of hydrocarbon generation in well 12/27-1, with the steep thermal maturity gradient. A significant as revealed by the solid bitumens, shows that the lowest accumulation of solid hydrocarbons is present within porosity reflectivities and significant occurrences of live oil occur in the at 9900 ft, and reveals the presence of dead oil formed during upper part of the Devonian interval, and are coincident with an earlier phase of hydrocarbon generation. It now marks the the shallow vitrinite reflectivity gradient. presence of a zone of gas shows within the well, and exami- (2) The cusps of the solid bitumen and vitrinite reflectivity nation of polished surfaces of cuttings in UV incident light gradients are coincident. This demonstrates that hydrocarbon (Fig. 4, nos 12, 13) also shows the presence of minor liquid generation is directly related to the superimposition of the two hydrocarbons. vitrinite reflectivity gradients. If the other scenario of a short (4) Shallow vitrinite reflectivity gradients, such as that lived heat-pulse into the base of the section had occurred, then determined in well 12/27-1, are typical for the Mesozoic rocks hydrocarbon generation would have been initiated in the lower in the Inner Moray Firth (Pearson & Watkins 1983). Steeper part of the Devonian interval. reflectivity gradients have not been reported from the lower The effect of overprinting is illustrated in Fig. 11. Reburial parts of these Mesozoic sections, providing further evidence superimposes a low geothermal gradient onto a sequence that such thermal maturity episodes are unlikely to be of which had previously been subjected to a steep gradient. As Mesozoic age. vitrinite reflectivity is irreversible and records the maximum Accepting these arguments, it is now possible to deduce a thermal maturity, the resulting composite reflectance gradients more sophisticated burial history for the sequence encountered only record the maximum thermal effects. The model initially in well 12/27-1. shows a steep reflectivity gradient in a Palaeozoic succession. This Palaeozoic sequence is then inverted and eroded during ?latest Carboniferous and early Permian times before under- going late Permian and Mesozoic reburial under conditions of The thermal and burial history of well 12/27-1 a lower geothermal gradient. Initially this has no measurable The Emsian section in well 12/27-1 is characterized by a thermal effect on the vitrinite within the Struie Formation, as significant thickness of sediment of limited lateral extent, it does not raise the temperature in the rocks to levels to deposited over a short time interval. This is indicative of active which they had not already been subjected. However, as contemporaneous extension during deposition which would, in

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addition, be characterized by a high geothermal gradient and probable high heat flow. The occurrence of a high geothermal gradient and high heat flow is supported by conversion of the suppression corrected vitrinite reflectivities to a maximum palaeotemperature gradient using the empirical relationship of Barker & Goldstein (1990):

lnRv=0.00811Th)C"1.26 (1)

where Rv is the mean random vitrinite reflectivity and Th is the homogenization temperature of fluid inclusions in calcite and a good approximation of maximum burial temperature. Extrapolating the steep gradient to the top of the Struie Formation at about 7700 ft gives a vitrinite reflectivity of 0.6%. Combined with the vitrinite reflectivity at 10 900 ft (1.6%), this produces an estimated palaeogeothermal gradient of 166)C km"1. However, the steep gradient was acquired during maximum Palaeozoic burial but the Struie Formation must have undergone further significant compaction during Mesozoic reburial. The effect of this would have been to further steepen the reflectivity gradient whilst preserving the vitrinite reflectivity values attained during maximum Palaeozoic burial. The amount of this compaction needs to be estimated to determine the magnitude of its effect on the Fig. 12. Method used to determine thickness of Struie Formation at vitrinite reflectivity gradient. maximum Palaeozoic burial. As a first step, the extrapolated vitrinite Determination of the vitrinite reflectivity gradient at reflectivity data for the top and base (0.6%, 1.6%) of the present day Palaeozoic burial maximum is not simple. The approach used thickness of the Struie Formation are plotted at maximum burial here was to model decompaction of the Struie Formation depths. This gradient is then separately decompacted by 0.5, 1, 1.5 incrementally, until the extrapolated vitrinite reflectivity and 2 km using a shale compaction curve. Each time a new gradient fulfilled the condition of intercepting the minimum thickness is calculated for the Struie Formation. Extrapolation shows that when uplift is 1.6 km the curve fulfils the condition of reflectivity value of 0.2% at the surface, using the empirically intercepting the surface at a vitrinite reflectivity of 0.2%. This gives a established normal shale compaction curve. Since the Struie decompacted maximum Palaeozoic thickness for the Struie Formation contains less than 10% sandstone and siltstone, it Formation of 1.1 km (3500 ft) and indicates that some 1.2 km was regarded as comprising entirely shale to simplify the (4000 ft) of Palaeozoic strata are missing through erosion. decompaction. The normal shale compaction curve used was that of Baldwin & Butler (1985): Such a gradient is compatible with an early, extension- burial depth in km=6.02S6.35 (2) related episode of high heat flow (Robert 1988). Elsewhere within the Orcadian Basin, there is evidence for such active extension, especially during the Early Devonian where a clear where S is the solidity of the shale, the complement of porosity. association of sedimentation with fault activity has been The equation was used to determine the expansion of the recognized (Rogers 1987). Also important in this respect is the Struie Formation during uplift (Fig. 12). The drilled thickness compilation by Norton et al. (1987) of the likely active (3200 ft, 0.98 km) of the Struie Formation was plotted at its Devonian fault systems in the offshore and onshore Moray estimated maximum early Tertiary burial depths (i.e. current Firth, which shows well 12/27-1 to be located (Fig. 13) within depths plus c. 2000 ft) against the extrapolated vitrinite such a fault system. reflectivity values for its top (0.6% at 9700 ft, 2.96 km) and base (1.6% at 12 900 ft, 3.93 km). This thickness of Struie Formation was then successively modelled for uplift by 0.5, 1.0, 1.5 and 2 km. At each uplift change, the solidity (S)was The role of intrusions calculated for the mid-point of the Struie Formation from the change in burial depth. The solidity value was then used to The recognition of a high palaeogeothermal gradient with recalculate the thickness of the Struie Formation. By replotting active extension raises the possibility that associated intrusion the vitrinite reflectivity gradients for each of these uplift emplacement has locally steepened the vitrinite reflectivity changes, the amount of decompaction required to produce a gradient. Within the Moray Firth there is direct evidence for gradient which intercepts the surface at a vitrinite reflectivity of intrusions from well penetrations which have revealed the 0.2% was determined by interpolation. This value (i.e. uplift of laterally extensive South Halibut Granite (Ross Granite of 1.6 km relative to maximum burial) then allows us to estimate Davies et al. 1996; Fig. 13). This granite has been dated at (Fig. 12) the amount of missing Palaeozoic section (3950 ft, 416&52 (Rb/Sr, in Andrews et al. 1990), a result so poorly 1.2 km) and the decompacted thickness of Struie Formation constrained as to be meaningless. However, the granite must (3500 ft, 1.1 km) at maximum Palaeozoic burial. The corrected pre-date at least part of the Devonian as clasts of it occur vitrinite reflectivity gradient can be used to calculate a palaeo- within an ?Eifelian (early Mid-Devonian) section in 13/24-1 geothermal gradient of 150)Ckm"1, using the method of and, more tentatively (Richards 1985a) in sedimentary rocks of Barker & Goldstein (1990; Equation 1). latest Devonian age in the Buchan Field. There is also a single

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Fig. 13. Map of the Inner Moray Firth showing the location of major faults believed to be controlling Devonian sedimentation. The only half-grabens known to be filled with organic-rich lacustrine sediments are those in the Strathpeffer area, and the one in which well 12/27-1 is located. Some of the onshore faults (e.g. the Brough Fault, BF) are downthrown in the opposite sense to their present day condition. An E–W line marks the approximate boundary between sand dominated Middle Devonian sediments and those which contain a significant proportion of lacustrine sediment. Note that the Great Glen Fault has been palinspastically restored to a Mid Devonian position (Rogers et al. 1989). This locates the Beatrice area opposite Wick and west of the organic aureole of the Caithness Granite. Note the ‘granite’ within the footwall of the half-graben bounding fault south of well 12/27-1, situated beneath part of the Central Ridge. Granite has also been drilled in well 12/21-3 beneath the Smith Bank High. Distribution of faults from Norton et al. (1987). SHG, South Halibut Granite.

separate occurrence of granite in well 12/21-3 (Figs 1, 12) (Bartenstein et al. 1971; Teichmüller et al. 1984; Smart & which shows (Andrews et al. 1990) that part of the Smith Bank Clayton 1985) indicate that an organic aureole with a width of High is cored by granite. 5–10 km around the periphery of the intrusion is probable for There is geophysical evidence for a granite beneath the an intrusion with a diameter of about 10–20 km. Placing a Central Ridge (Dimitripoulos & Donato 1981), indicated by 10 km aureole (Fig. 13) around the postulated intrusion of the gravity and aeromagnetic anomalies (Fig. 1). This ‘granite’ is Central Ridge shows that although well 12/27-1 lies beyond the some 20 km along structure from the location of well 12/27-1. edge of the hypothetical aureole, a sizeable proportion of Geophysical modelling (Thomas 1988) places the depth of its the half-graben will have lost its source potential. Even if the top at 6 km, and gives it a top width of 10 km, increasing with intrusion emplacement was pre-Devonian it would still have depth to 19 km. The composition is modelled as granodioritic. had a significant thermal effect, but only on the immediately Although this putative intrusion has never been drilled, a overlying sediments which make up a much smaller area. This comparable pattern of geophysical anomalies (Flinn 1969) phenomenon is well documented from studies of buried occurs over the exposed Ben Rinnes Granite (Fig. 1) on the Devonian granites in northern England (Creaney 1982; southern side of the Inner Moray Firth. This is dated (Zaleski Smart & Clayton 1985). Although emplaced and exposed in Brown 1991) at 411&3 Ma (Rb/Sr) and thus pre-dates prior to Carboniferous times, these granites still significantly deposition of the Devonian sediments in the Orcadian Basin. increased the thermal maturity of the overlying Carboniferous A similar geophysical anomaly occurs (Flinn 1969; IGS sediments. 1977, 1978) in the Wick area of Caithness and has an associ- The early burial history of well 12/27-1 was therefore one ated thermal maturity high (Hillier & Marshall 1992). This of extension, accompanied by rapid deposition of a thick has been interpreted as a Devonian intrusion (Fig. 13, the sequence of largely fine-grained sediments. Deposition may Caithness Granite). Similar thermal anomaly patterns also have been slowed or terminated by an intrusion rising beneath occur in West Shetland around the Mid–Late Devonian the Central Ridge during Mid or Late Devonian times. The Sandsting Granite (360–370 Ma, Mykura & Phemister 1976), creation of a positive structure in what was previously a very which intrudes directly into sedimentary rocks of late Mid- significant depocentre, and continued inversion in the area of Devonian age (Rogers et al. 1989) and is therefore clearly well 12/27-1 during later Palaeozoic times, may explain the contemporaneous with Devonian deposition in the Orcadian missing Mid-Palaeozoic section (Fig. 12) in well 12/27-1. It is Basin. significant that no Middle Devonian cover is preserved above The postulated intrusion beneath the Central Ridge is of the thick Lower Devonian section, unlike other drilled sections unknown age. Comparative intrusions appear to fall into two in the Inner Moray Firth. age groups. Those which predate deposition of the Devonian Significant sedimentation recommenced in the area of well sediments in the Orcadian Basin and are Late Silurian to 12/27-1 in late Permian times and continued with only minor earliest Devonian in age (430–408 Ma, Thirlwall 1988), and hiatuses, uplift and erosion. However, throughout this time, those of Mid/Late Devonian age. the area of well 12/27-1 persisted as a positive structural If the Central Ridge ‘granite’ was intruded in Devonian feature, against which Permian to Cretaceous sediments were times, it would have thermally altered a significant volume of attenuated. Maximum burial was achieved immediately prior country rock. Comparative data from similar intrusions to the early Tertiary uplift in the Inner Moray Firth. As noted,

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Fig. 14. Cartoon cross-sections from well 12/27-1 through Beatrice to Helmsdale and Lybster. Section ACD (top) between well 12/27-1 and Helmsdale, is the present day section, and shows the inversion within the Beatrice structure. At Helmsdale, the top Devonian is well above the top of the oil window. A projected position of the top Devonian at Lybster is at about the top of the oil window. Section BCD (bottom) shows the cross-section restored to maximum burial, but with the Beatrice structure removed for simplicity. Note that the vitrinite reﬂectivity data for the Beatrice wells now fall in the correct depth reﬂectivity sequence with those of well 12/27-1. The top of the oil window extends across the entire structure. Note the opposing senses of thickening within the Jurassic-Cretaceous and Permo-Triassic units. Compiled from Institute of Geological Sciences (1982); British Geological Survey (1995); Underhill (1991); Hillis et al. (1994).

the magnitude of this uplift for well 12/27-1, constrained using able seismic mapping and well control. Comparison of vitrinite sonic velocities (Hillis et al. 1994) is 1960 ft (c. 600 m), a value reflectivity values from equivalents of the Brora Coal and broadly agreeing with that derived from the truncation of the Dunrobin Bay formations shows significant inversion of vitrinite reflectivity gradient (1000 ft, c. 300 m, Fig. 5). Beatrice compared with well 12/27-1 and the onshore section near Brora. When the top of the Devonian sequence is projected onto the section (offshore of Lybster), this is seen to Hydrocarbon generation in the Inner Moray Firth lie above the position of onset of oil generation as defined by A significant conclusion from this study is that the maximum vitrinite reflectivity in well 12/27-1. depth of Mesozoic/early Tertiary burial at a location on a The top of the Devonian has not been penetrated in the structural high in the Inner Moray Firth was sufficient to have Beatrice area, but has been recorded along structure to the generated hydrocarbons from a Devonian source rock. This south in well 11/30-6. This well gives an indication of the total was despite the location having amongst the most attenuated thickness of Heron, Zechstein and Rotliegend groups present Jurassic and Cretaceous sections in the Inner Moray Firth. beneath the Beatrice area (Fig. 14, ACD), and shows that the Hence there is no necessity to restrict the Mesozoic generation Mesozoic overburden combined with the Heron to Rotliegend of hydrocarbons from Devonian source rocks to limited areas groups places the top of the Struie Formation beneath the zone of deeper burial such as the Sutherland Terrace. The Beatrice of oil generation (as determined by vitrinite reflectivity) for oil could therefore have a much more local origin within the present day depths of burial. However, it should be noted that Inner Moray Firth, thus reducing the necessity to invoke the early Tertiary inversion will have probably terminated the long distance migration of a high pour point (Stevens active oil generation. 1991) crude. It also shows that significant hydrocarbon poten- Restoring the Lybster to well 12/27-1 cross-section (Fig. 14, tial remained until Mesozoic times, despite the effects of BCD) to show maximum early Tertiary burial, establishes that Palaeozoic burial under a high geothermal gradient, followed all the Devonian section lay beneath the top of the oil window. by an episode of early Permian basin inversion and erosion. Note that the vitrinite reflectivity values for the Beatrice area In order to test the conclusion of a more local origin for the are now in the correct vertical position when compared to well Beatrice oil, a cross-section has been drawn (Fig. 14, line 12/27-1, lying between the 0.44% Zechstein Group and 0.8% location on Fig. 7) from well 12/27-1 to Beatrice and then to (suppression corrected) Struie Formation values. Clearly, as the Helmsdale Fault at both Helmsdale and Lybster. Figure 14 Underhill (1991) indicated, the maximum point of burial lies ACD (top) shows an unrestored section compiled from avail- closest to the Helmsdale Fault in the Lybster area. However,

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the NE dip on the Sutherland Terrace, although accentuated Palynomorphs in the clast, which include the spore by differential movement between the Helmsdale and Great Geminospora lemurata, demonstrate a clear Givetian age, i.e. Glen Faults, means that the area of Devonian rocks beneath above the main lacustrine succession. G. lemurata is abundant the oil generation zone is quite restricted. The boundary of this in the John O’Groats Sandstone of northern Caithness and line, which approximates to the 3 km isobath, is indicated on the Eday Group of Orkney (Marshall 1996) and shows that Fig. 7. rocks correlative with these units must have been present in Limited Mesozoic burial is thus not the major constraint on Sutherland until at least Jurassic times. Both exinite reflectivity hydrocarbon generation from Devonian source rocks. More measurements and the spore colours show the part of the significant is the balance between the relative thicknesses of Devonian succession represented by this clast to be immature. Permo-Triassic and Jurassic–Cretaceous rocks. The Permo- The distribution of bitumen reflectivities is also informative Triassic succession thickens westwards across the Inner Moray (Fig. 10) as they are normally distributed with all values Firth (Andrews et al. 1990), whilst the Jurassic and Cretaceous >0.1%. Comparison with bitumen reflectivity histograms from successions thicken eastwards to a maximum against the well 12/27-1 shows that this is a mature, migrated bitumen, and Helmsdale Fault. It is this combination of opposed depocentre is not typical of active accumulation within a source rock. directions that ensures that the Devonian sediments are Thus the clast is significant in that it demonstrates the presence beneath the top of the oil window across the Inner Moray of lacustrine rocks which generated oil in late Palaeozoic times Firth. in the area to the west of the Helmsdale Fault. In the Berriedale outlier (Fig. 7), northeast of Helmsdale, the succession comprises a marginal facies (Donovan 1993), with sediments rich in organic matter absent south of the major The controls on Devonian source rock viability development of lacustrine facies in Caithness. Within the main Accepting that Mesozoic reburial cannot be the limitation on lacustrine succession of Caithness, there are two important sets hydrocarbon generation from Devonian source rocks, it is of N–S-trending faults. The Loch Calder Fault delimits the relevant to readdress the question as to why Beatrice is as yet main western basin margin; east of this line the succession is unique. The distribution of potential Devonian source rocks in thin, as evidenced by both the penetration of basement highs both the Inner Moray Firth and onshore in Sutherland and through cover as at Dirlot (Donovan 1973), and the aero- Caithness is significant in this context. magnetic and gravity data (Institute Geological Sciences 1977, As regards the offshore distribution, there is enough com- 1978; Thomas 1988) which show basement close to surface bined seismic and well data to suggest that Lower Devonian across the area. East of the Brough and Camster Fault system source rocks are limited in their distribution. The work of there is a thick succession of lacustrine sediments rich in Norton et al. (1987) has identified structures most likely to organic matter. However, these were all thermally over- have controlled Devonian sedimentation in the Moray Firth. matured by a heating event attributed to the postulated These are approximately NE–SW-oriented extensional rift Palaeozoic Caithness Granite, and clearly lost all generative systems (Fig. 13) which frequently have onshore terminations capacity prior to Mesozoic burial. A palinspastic reconstruc- as small half-graben structures containing infills of Devonian tion (Fig. 13) of the Moray Firth prior to latest Carboniferous/ sedimentary strata. Well 12/27-1 occurs within such a system, Early Permian movements along the Great Glen Fault places as does the onshore development of Lower Devonian rocks at the Beatrice Field area immediately opposite the Caithness Strathpeffer. Such half-graben structures have restricted distri- Granite and adjacent to its projected organic aureole. This is butions within the Moray Firth and are not necessarily filled significant in that, although there are no available data on the with organic-rich sedimentary rocks. Devonian succession immediately east of the Great Glen Fault In contrast, Middle Devonian lacustrine strata appear to be at Wick, a zone of elevated thermal maturity must be antici- much more extensive; the Achanarras level, for example, pated. The level of thermal maturation seen at the surface in represents the spread of deep lake conditions to virtually all the Wick area is extreme, and if a similar level is reached east areas of the Orcadian Basin, including both the southern and of the Great Glen Fault, there would not only be the loss of eastern shores of the Moray Firth. However, there is clearly a any source rock potential but mineral regrowth would seal any reduction in the thickness and extent of Devonian lacustrine porosity. Hence there is a real possibility that migration units in the southern Inner Moray Firth (Fig. 13). On the barriers occur within the Devonian succession west of the Sutherland coast, the southern extent of lacustrine flagstones is Beatrice Field. This is in addition to the presence of the Great marked by clasts in the Boulder Bed (Johnstone & Mykura Glen Fault itself, which at depth is also likely to be a barrier to 1989) just south of Portgower. Further south, the onshore fluid migration. As shown here, there is no objection to local sections, e.g. in Easter Ross and on the Black Isle (Fig. 1), generation of the Beatrice oil, which circumvents the problem together with offshore data from the limited well penetrations, of long distance migration of a heavy oil across such potential show that fluvial sandstones are the dominant facies. migration barriers. Lacustrine sedimentary rocks are also attenuated to the east by Between the Loch Calder Fault and the Brough-Camster the South Halibut Granite (Andrews et al. 1990). faults, there is an area rich in oil-prone lacustrine source rocks This now eroded Sutherland Devonian lacustrine sequence which includes the major development of the Achanarras has certainly generated petroleum, as evidenced by the silt- Horizon. Onshore, it is at a high but not excessive maturity stone clast from Helmsdale which contains bitumen within level and would be anticipated to have source rock potential porosity. This clast occurs within a thermally immature beyond its southern seaward extent which lies within the Jurassic sequence, so these bitumens were present prior to its Sutherland Terrace. These lacustrine rocks include part of the transportation into the Helmsdale Boulder Bed facies, and are Lower Caithness Flagstone Group which Bailey et al. (1990) not the result of Mesozoic hydrocarbon generation (contrast excluded from contention as a source for the Beatrice oil, Underhill 1991). Furthermore, these bitumens were not describing its source potential as lean. Although locality details generated in situ as the kerogen in the clast lacks AOM. are not given in their paper, it is known that much of the

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Lower Caithness Flagstone Group, when sampled in the type A, T.R. 1991. The Devonian lacustrine sediments of Orkney, Scotland; area (Hillier & Marshall 1992), appears lean in contrast to the implications for climatic cyclicity, basin structure and maturation history. Journal of the Geological Society, London, 147, 141–151. Upper Caithness Flagstone Group through being overmature. B, N.J.L., B,R.&H, G.E. 1990. Application of pyro- In these more southern areas the Lower Caithness Flagstone lysate carbon isotope and biomarker technology to organofacies definition Group is at a lower thermal maturity and will thus still have and oil correlation problems in North Sea basins. Organic Geochemistry, source potential. 16, 1157–1172. A further control on the distribution of potential Devonian B,B.&B, C.O. 1985. Compaction curves. American Association of source rocks is the regional distribution of pre-Permian Petroleum Geologists Bulletin, 69, 622–626. B,C.E.&G, R.H. 1990. Fluid-inclusion technique for determin- maturity. This is particularly relevant where steep reflectivity ing maximum temperature in calcite and its comparison to the vitrinite gradients occur, as the thickness of immature source rocks geothermometer. Geology, 18, 1003–1006. available for hydrocarbon generation during Mesozoic B, H., T¨ ,M.&T¨ , R. 1971. Die Umwandlung reburial is likely to be limited. Furthermore, such thicknesses der organischen Substanz im Dach des Bramscher Massivs. Fortschritte in are likely to be reduced by any early Permian truncation of the der Geologie von Rheinland und Westfalen, 18, 501–538. B G S 1995. Moray Firth (Special Sheet), 1:250,000. Devonian sections. However, such factors are more difficult to British Geological Survey, Edinburgh, Scotland. predict in the absence of a substantial data base of Devonian B, P.E. 1991. Caledonian and earlier magmatism. In:C,G.Y.(ed.) well penetrations for the Inner Moray Firth. Geology of Scotland, 3rd edition. The Geological Society, London, 229–295. Conclusions B-T, J.M.A. 1983. Selection criteria for the use of vitrinite reflectance as a maturity tool. In:B,J.(ed.)Petroleum Geo- The Devonian Struie Formation section in well 12/27-1 is a chemistry and Exploration of Europe. Geological Society, London, Special source rock and has generated oils in both Palaeozoic times Publications 12, 295–307. and subsequently during Mesozoic reburial. C, T.D.J. 1993. 4. Triassic, Permian and pre-Permian of the Central During Mesozoic burial, the top of the Devonian was within and Northern North Sea. In:K, R.W.O’B. & C, W.G. (eds) Lithostratigraphic nomenclature of the U.K. North Sea. British Geological the oil window across the Inner Moray Firth. This is because Survey, Nottingham, 163. Jurassic–Cretaceous and Permo-Triassic sediments thicken in C, C. 1990. Source rocks and hydrocarbons of the North Sea. In: opposite directions. Mesozoic burial is therefore not a factor G,K.E.(ed.)Introduction to the Petroleum Geology of the North Sea. limiting hydrocarbon generation. 3rd Edition. Blackwell Scientific Publications, London, 294–361. Lower Devonian source rocks are limited to discrete half- C, S. 1982. Vitrinite reflectance determinations from the Beckermonds Scar and Raydale Boreholes, Yorkshire. Proceedings of the Yorkshire graben structures. Middle Devonian source rocks are more Geological Society, 44, 99–102. extensive but occur only in the northern part of the Moray D, R.J., S,K.J.&U, J.R. 1996. A re-evaluation of Middle Firth. and Upper Jurassic stratigraphy and the flooding history of the Moray The amount of Devonian source rock available during Firth Rift System, North Sea. 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Received 28 November 1995; revised typescript accepted 6 October 1997. Scientiﬁc editing by Stewart Molyneux.

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