Michael J. Pearson

Source rock palaeoenvironments and controls on the distribution of dibenzothiophenes in lacustrine crude oils, Bohai Bay Basin, eastern China

Abstract The applicability of sulphur aromatic parameters, based on ratios of dibenzothiophene (DBT) and its alkylated homologues, as indicators of oil maturity has been investigated in 27 oils from the Damintun and Zhuanhua Depressions, Bohai Bay Basin. The oils were characterised by bulk composition and environmentally-sensitive aliphatic biomarkers and were derived from a wide range of lacustrine source depositional environments ranging from highly reducing hypersaline (sulphur-rich oils) to terrestrial input-dominated freshwater (very high wax oils) with maturities ranging from immature to probable mid-mature. Dibenzothiophene-based maturity parameters are not well-correlated with sterane isomerization ratios, and immature oils, in particular, could not be discriminated by these aromatic maturity indicators. In contrast, DBT-based parameters are strongly-correlated with pristane to phytane and Ts/Tm ratios. High methyldibenzothiophene ratios (MDR) are associated with fresh water source deposition whilst low MDRs are found in oils of hypersaline source affinity. Unusually high relative concentrations of 2+3-MDBT were observed in some of the immature oils inferred to be sourced by lacustrine organic laminites. The data provide further evidence that thermal stress is not the sole factor controlling the distribution patterns of dibenzothiophenes in oils and that source depositional environments can exert a dominant influence when their range of variation is large. Absence of DBT in some biodegraded oils is tentatively attributed to its aqueous solubility during associated water washing.

A Devonian auriferous hot spring system, Rhynie, Scotland

The Early Devonian Rhynie hot spring system is the oldest known and is of the low sulphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radio-metric (396 ± 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin. The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks. The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300–440°C) magmatic fluids occurred with δD–65‰ and δ18O around +8.5‰. δ34S (pyrite) and initial 87Sr/86Sr ratios (vein calcite) lie mainly within the ranges +3.4‰ to +8.5‰ and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. δ13C values for caliche and vein calcite imply derivation of carbon from non-organic sources. The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (δ18Ochert+ 13.1‰ to +16.5‰) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1± 0.6) significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere.

Modeling petroleum generation in the Southern Muglad rift basin, Sudan

Maturity and petroleum generation have been modeled in a 120-km-long northeast-southwest cross section of the Southern Muglad basin, Sudan. The section passes close to the Unity and Kaikang areas, which provide well control on the structure interpreted from gravity and seismic reflection profiles. Modeling of geotherms from corrected bottom-hole temperatures indicates present heat flows around 60 mW/m2 in the well control areas. Using measured vitrinite data as calibrant, maturity can be modeled equally well with a constant paleoheat flow similar to the present day or with a complex heat flow pattern related to rifting. A geologically realistic complex heat flow model was employed to calculate the timing of generation and expulsion from the Lower Cretaceous lacustrine source rocks of the Abu Gabra and Sharaf formations. Kinetic (Easy% Ro) calculation of vitrinite maturity indicates a projected oil preservation depth limit around 4000 m shallowing to around 3500 m northeast of Unity near the basin margin.

Bicadinanes and other terrestrial terpenoids in immature Oligocene sedimentary rocks and a related oil from the Surma Basin, N.E. Bangladesh

Abstract The aliphatic fractions and one aromatic fraction of outcropping Oligocene rocks from the NE Surma Basin of Bangladesh were studied by GC-MS and compared with their subsurface equivalents and one oil (Sylhet) from boreholes further south in the basin. Terrestrial biomarkers frequently encountered in these rocks whose biomass is dominantly woody include the triterpanes oleanane and bicadinane, several tetracyclic terpanes and the sesquiterpenoids cadinane and cadalene. The bicadinanes and their aromatized counterparts are found as major triterpenoid components only in the Upper Jenam Formation where the richest horizons are interpreted to contain locally-derived concentrations of dammar-type resin excluded from Dipterocarpaceae or similar tropical angiosperm trees. Vitrinite reflectance and geochemical maturity parameters indicate pre-oil window maturity for the Upper Jenam outcrop, allowing a temperature of less than about 90°C to be estimated for the formation of bicadinanes from resinous polymer. Cis-cis-trans -bicadinane, which is abundant in Sylhet oil, was not positively identified in any of the rock extracts, although the distribution of bicadinanes is otherwise very similar.

Onocerane and other triterpenoids in Late Cretaceous sediments from the Upper Benue Trough, Nigeria: tectonic and palaeoenvironmental implications

Abstract The aliphatic hydrocarbon bitumen fraction has been investigated in Late Cretaceous outcrop samples from the Upper Benue Trough, inundated by the sea only during the maximum mid-Cretaceous transgression. Thermal maturity in nearly all formations is pre-oil window, indicating only limited, mainly Tertiary burial. The Santonian deformation, a significant thermal influence to the south-west, appears to have had little effect here. Onocerane I is the dominant triterpenoid in the Pindiga Formation but is absent elsewhere. Its origin is uncertain but it appears to be associated with deposition in a restricted basin, possibly a brackish lagoon, receiving dominantly terrestrial organic input. A review of possible natural product precursor compounds suggests α-onoceradiene from a fern-like plant was a likely source. Even carbon number predominance of n -alkanes in the range C 16 to C 22 in some formations is tentatively interpreted to indicate hypersalinity.

Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills

Abstract Carbonate concretion bodies, representing a number of morphological types, and associated calcite fracture fills, mainly from New Zealand, have been studied both organically and inorganically. Extracted organic material is dominated by a complex polymeric dark brown highly polar fraction with a subordinate less polar and lighter coloured lipid fraction. The relative proportion of the two fractions is the principal control on the colour of fracture fill calcites. Concretions are classified mainly by reference to their carbonate stable carbon and oxygen isotope and cation composition. Typical subspherical calcitic septarian concretions, such as those in the Paleocene Moeraki and the Eocene Rotowaro Siltstones, contain carbon derived mainly by bacterial sulfate reduction in marine strata during early diagenesis. Other concretions, including a septarian calcitic type from the Northland Allochthon, have a later diagenetic origin. Siderite concretions, abundant in the non‐marine Waikato Coal Measures, are typically dominated by methanogenic carbon, whereas paramoudra‐like structures from the Taranaki Miocene have the most extreme carbon isotope compositions, probably resulting from methane formation or oxidation in fluid escape conduits. Lipids from concretion bodies and most fracture fill calcites contain significant concentrations of fatty acids. Concretion bodies dominated by bimodally distributed n‐fatty acids with strong even‐over‐odd preference, in which long chain n‐acids are of terrestrial origin, have very low hydrocarbon biomarker maturities. Concretion bodies that lack long chain n‐acids often have higher apparent biomarker maturity and prominent α‐ω diacids. Such diacids are abundant in fracture fill calcites at Rotowaro, especially where calcite infills the septaria of a siderite concretion in the non‐marine Waikato Coal Measures, and support the view that fluid transport resulted in carbonate entrapment of the fracture‐hosted acids. Diacids also occur in Northland calcite concretion bodies, but not in their septarian fracture fill. Release from kerogen into migrating pore fluid during an early organic maturation stage is suggested as a plausible origin of the diacids. Their site of entrapment may have been serendipitous, depending on the timing of concretion body and fracture fill carbonate precipitation.

Petroleum maturation modelling, Abu Gabra–Sharaf area, Muglad Basin, Sudan

Abstract Structural and maturation studies in the northwest of the Muglad Basin, Sudan showed a thick sedimentary section up to 9000 m in the deep sub-basins and an average of about 6000 m in the Sharaf–Abu Gabra Ridge. The isopach map of the Lower Cretaceous Abu Gabra–Sharaf formations, which contain source rocks, showed a thickness of sedimentary section between 2000 and 5000 m. The burial history showed high subsidence rates during the first rifting phase with relatively low subsidence rates during the following two phases in this part of the basin. Significant erosion of the sedimentary section occurred in the ridge area during Miocene and Cenomanian–Albian times. The geothermal gradients in the modelled wells range between 18 and 27.5 °C/km corresponding to the relatively wide range of present day heat flow of between 37 and 63 mW/m 2 , averaging 50 mW/m 2 . Heat flow history curves consistent with vitrinite reflectance data were adopted with elevated high heat flow (75 mW/m 2 ) during the first rifting phase followed by minor peaks (average 60 mW/m 2 ) during the second and the third rifting phases. Application of a kinetic vitrinite maturation model resulted in a present day oil window between 2000 and 4500 m. The source rock of the Abu Gabra (AG) and Sharaf (SH) formations, which has an average total organic carbon of 1.0% and hydrogen index (HI) of 280, was modelled for hydrocarbon generation. The section was divided into four layers; AG-1 (Albian), AG-2 (Aptian), SH-1 (Barremian) and SH-2 (Neocomian). Conditions for hydrocarbon generation are present throughout the Sharaf formation and in AG-2. The generation model suggests an average generation amount of 4 mgHC/g rock in the lower three modelled layers with a timing range between 120 Ma and the present. Total calculated oil volume is about 370 US billion bbls and the expected volume expelled and migrated (40%) is 150 US billion bbls. The total gas expelled and migrated (90%) is 1600 US trillion cu ft. Retention of hydrocarbons may be adversely affected by long-continued faulting and lack of good seals.

An integrated thermal and isotopic study of the diagenesis of the Brent group, Alwyn South, U.K. North Sea

Abstract The petrography, fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore-fluid history of the Middle Jurassic Brent Group reservoir sandstones in the Alwyn South area of the U.K. North Sea. The study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels. The cement assemblage of kaolinite, quartz and illite has resulted in severe deterioration of otherwise good reservoir quality. Early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation. Temperature estimates for kaolinite precipitation of 80°C andδ18O of ≈ + 15‰ (±3‰) suggest co-existing fluids ofδ18O ≈ −3‰. Quartz cementation overlapped both kaolinite and illite formation. Fluid inclusion data indicate that quartz cementation took place at temperatures of 109±7°C. Pore fluid salinities were ≈4 wt% NaCl with a H2O O isotopic composition of ≈ -1 %o ± 0.5‰ SMOW. The fluids which precipitated coexisting illite were compositionally homogeneous with equilibriumδ18O water compositions of +0.5‰ SMOW. Illite SD values range from −33 to −50‰ SMOW. These fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform, marine signature. This is consistent with the predominantly marine to paralic depositional context of the Brent Group in Alywn South. Illite precipitation was followed by hydrocarbon emplacement between the Middle Eocene and Lower Oligocene.

Subsidence and erosion in the Pennine Carboniferous Basin, England: lithological and thermal constraints on maturity modelling

Forward modelling of maturity against vitrinite reflectance and chemical data for a number of onshore boreholes was used to test a range of subsidence and thermal histories for the Pennine Carboniferous Basin. The preferred models suggest maximum palaeotemperatures were reached during late Cretaceous or possibly early Tertiary burial, consistent with the outcome of previous apatite fission‐track studies. Models in which maximum palaeotemperature was attained in the late Carboniferous are considered less likely: rapid burial under a normal heat flow requires an unrealistic thickness of missing late Westphalian strata, whilst high regional heat flow is incompatible with the observed vitrinite reflectance gradients. The lithologies assumed for missing strata have a significant influence on the range of estimates of eroded thicknesses. Preferred estimates of Cenozoic erosion range from around 1.6 km in Mesozoic basinal areas to around 2.8 km within areas of present Carboniferous outcrop.

Sulphur incorporation in Jurassic marine mudrocks and their bitumens at low thermal maturity, Cleveland Basin, England

Abstract The distribution of sulphur between pyrite, kerogen, elemental sulphur, and the aromatic fraction of mudrock bitumens has been quantitated in two cored sections of immature marine Kimmeridgian strata which have experienced different levels of thermal stress. Strata in the two boreholes (Ebberston and Reighton) were deposited in different basinal settings (basinal marine and platform, respectively) and occasional euxinic (laminated) horizons are present at Ebberston, otherwise dysoxic deposition was the norm at both localities. Pyrite, the dominant sulphur carrier in all the mudrocks, has a sulphur isotopic composition consistent with mineralisation in a system mainly open to sulphate diffusion. Kerogens are mostly low sulphur (atomic S C ) and account for between 5 and 10% of rock sulphur although occasionally up to 30%. Kerogen sulphur is closely correlated with TOC and highest in laminated mudrocks consistent with most effective sulphurization of kerogen under anoxic conditions. Although kerogen compositions in both cores vary with organic richness, no systematic differences in organic input between the borehole localities are recognised geochemically. Some thermal effects on sulphur geochemistry are, therefore, interpreted from a comparison of stratigraphically equivalent sections from the two cores. Bitumen yields and abundances of aromatic sulphur in the more thermally-stressed Ebberston section, although variable, are generally much higher than in the Reighton section and are partly related to kerogen sulphur content. This is tentatively attributed to thermal release of additional extractable bitumen from macromolecules. Elemental sulphur (S0) in bitumen is unrelated in abundance to other sulphur species and isotopically heavier than coexisting pyrite. Overprinting of minor preserved diagenetic S0 by secondary oxidation of pyrite appears likely. The distributions of individual sulphur-aromatic compound classes have features related both to maturity and stratigraphy.