Organic petrology and geochemistry of organic-rich rocks of the Late Triassic and Early Jurassic Sandilands and Ghost Creek Formations, Queen Charlotte Islands, British Columbia

Abstract

Latest Triassic and Early Jurassic marine mudrocks on the Queen Charlotte Islands comprise a composite interval up to 600 m thick of organic-rich potential petroleum source rocks. These strata, assigned to the Sandilands and Ghost Creek Formations, are the only known significant potential petroleum source rocks on Canada's western continental margin. Regionally, the strata range from immature (0.33 R o,rand.) on northern Graham Island to overmature (>5 R o,rand.) on Moresby Island. Anomalously high levels of maturity exist in sediments adjacent to intrusive rocks and dyke swarms. The weight per cent total organic carbon (TOC) content of the strata ranges up to 6.1% and the organic matter as defined by Rock-Eval is predominantly types I and II, with hydrogen indexes ranging up to 589 mg HC/g C org. Petrographically, the organic matter is predominantly algal in origin, although minor to major amounts of vitrinite occur in many samples. Lateral variations in TOC closely reflect lateral variations in maturity. Based on the n-alkane distribution of the saturate fraction, the strata of the Sandilands Formation are divisible into two groups: (1) samples characterized by high concentrations of C 11 to C 31 with C 15 to C 17 dominant; and (2) samples characterized by n-alkanes of higher molecular weight with C 20 to C 23 dominant. In most samples, pristane is the dominant isoprenoid, but in all samples the pristane to phytane ratio is less than 2. Gas chromatography-mass spectrometry (GC-MS) of representative samples indicates the presence of both diasteranes and regular steranes with a predominance of C 29 regular homologues and a series of extended tricyclic terpanes from C 19 to at least C 35. There is a predominance of terpanes over steranes, the rearranged to regular sterane ratio is less than 1 and the 18α(H) trisnorhopane/17α(H) trisnorhopane + 18α(H) trisnorhopane ratio varies from 0.49 to 0.62. Samples from the Ghost Creek Formation have a similar n-alkane distribution to group 1 from the Sandilands Formation and GC-MS of two samples suggests less abundant terpanes and steranes than found in the Sandilands Formation. The carbon isotope composition of the organic fraction of all the strata is relatively homogenous, ranging from δ 13C, of −25 to −30% with a mean value of −28.8%; there is no consistent stratigraphic trend in δ 13C, nor is there significant correlation between δ 13C and any of the other geochemical indexes studied. In general, vitrinite-rich, terrestrially-derived organic matter is isotopically heavier than strata dominated by marine organic matter, the reverse from that found today, but similar to that reported during some oceanic anoxic events. Both organic petrology and geochemistry indicate that the organic matter is comprised of a mixture of terrestrial and marine organic matter with marine organic matter of algal origin predominating in most samples. The observed geochemical variation between samples is considered to reflect variations in maturity, relative abundance of terrestrial and marine organic matter and, to a lesser extent, biodegradation.