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. 335 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/2/335/4886495/gsjgs.155.2.0335.pdf by guest on 28 September 2021 336 J. E. A. MARSHALL 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/2/335/4886495/gsjgs.155.2.0335.pdf by guest on 28 September 2021 DEVONIAN SOURCE ROCKS, INNER MORAY FIRTH 337 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.

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