Organofacies and Early Maturation Effects in Upper Jurassic Sediments From the Inner Moray Firth Basin, North Sea

Abstract

Summary A study of oil source rock characteristics and the degree of organic maturity has been carried out on a thick succession of Upper Jurassic sediments from the inner part of the Moray Firth Basin. The sequence, which attains a thickness of up to 1000 m, consists of organic-rich mudstones and silty shales with periodic intercalations of turbidite sands. Samples representative of finer grained lithologies covering a range of present burial depths from 800 to 2400 m were selected from four exploration wells, one each in Blocks 12/21 to 12/24, immediately northeast of the Beatrice Oil Field. Kerogen preparations from all samples are dominated by amorphous algal sapropel, but contain appreciable coarsely particulate terrigenous material originating from vascular plants and probably introduced into the restricted marine basin of deposition by distal turbidity currents. This organofacies is highly rated as a potential oil source, but maturation levels are consistently low within the drilled wells. Independent maturation-specific parameters, including vitrinite reflectance, sporomorph colour, solvent extract and extractable hydrocarbon:organic carbon ratios and the carbon preference index of nC 21 -nC 33 alkanes indicate immature organic matter at depths above 2000 m. A pronounced increase in extract ratios below 2000 m indicates approach to the oil generation threshold at the base of the deepest section. Initially high CPI values in the uppermost samples (except where migrated oil is present in Block 12/23-1) decrease steadily with burial. Significant amounts of steranes and triterpanes are present in the sediment extracts and have been identified by combined gas chromatography-mass spectrometry (GC/MS). A C 28 pentacyclic triterpane (17αH,18αH,21βH-28,30-bisnorhopane) was identified in unusual abundance. Isomerization occurs at one or more chiral centres in the steranes, rearranged steranes and hopane type pentacyclic triterpanes in response to increased burial and temperature (Mackenzie et al. 1980). These changes have been assessed in terms of relative abundances of certain specific isomers and compared with abundances in other basinal sequences with known maturational trends. A consideration of all parameters places the onset of intense oil generation somewhat below the base of the deepest section (Block 12/24-1) and sets limits on the maximum relative uplift experienced by the other sections.