Precambrian Petroleum: A Decade of Changing Perceptions

Abstract

Multidisciplinary studies during the last decade have identified hitherto unrecognized controls on the distribution and composition of organic matter preserved in the Earth’s Precambrian sedimentary rock record (> 0.54 Ga). Carbonaceous shales and cherts containing isotopically inhomogeneous kerogen of prokaryotic origin occur sporadically in Archean metasedimentary sequences. The oldest recognizable petroleum source rocks were deposited in anoxic microbasins where methylotrophic bacteria played a key role in the production of oil-prone organic matter highly enriched in 12C. These late Archean sulfide-rich, black shales (TOC up to 15%) are now thermally overmature. During Early Proterozoic time (2.5–1.6 Ga) epeiric seas developed, allowing widespread epicratonic sedimentation and burial of organic matter from an expanding planktonic biomass. The molecular fossil (or biomarker) record indicates that eukaryotic algae became significant kerogen precursors at least 1.7 Ga ago. Along the southeastern margin of the Canadian Shield Aphebian black argillites (or, at Elliot Lake and Blind River, stratiform uraniferous kerogen layers in the Matinenda Formation) were probable sources for widespread vein pyrobitumens found in ~ 2 Ga-old rocks of the Great Lakes region, USA and Canada. Shungite, another epigenetic pyrobitumen in the Jatulian Series of Karelia, USSR, originated from interbedded carbonaceous, pyritic shales. Indigenous Early Proterozoic dry gas was encountered by mineral drillholes in the Pechenga Series, Kola Peninsula, USSR, and McArthur Group, Northern Territory, Australia. Optimum conditions for the formation of thick, extensive petroleum source beds were first attained in Early Proterozoic intracratonic or rift-related tectonic settings, as exemplified by the organic-rich mudstones of the Franceville Basin, Gabon (2 Ga), and the Pine Creek Geosyncline (1.8–2.2 Ga) and McArthur Basin (1.6–1.8 Ga), northern Australia. These ancient source rocks (or carbonized residues of their expelled crude oil) are variously associated with U, Au, Pb, Zn, Ag and Cu mineralization. Among the oldest sediments currently being explored for hydrocarbon resources are those of the 1.4-Ga-old Roper Group, McArthur Basin, and the 1.1-Ga-old Oronto Group, Mid-Continent Rift, USA. Here, extensive organicrich shales and siltstones were deposited in marine and lacustrine environments, respectively; aliphatic Type I–II kerogen is of appropriate thermal maturity for oil generation; and oil shows are common. The Late Proterozoic (1.0–0.54Ga) was a time of prolonged oceanic anoxia and high rates of accumulation of organic matter in marine sediments (TOC contents locally as high as 20–30 %). Rich source rocks and the world’s oldest commercial oil and gas reserves, much of them in giant fields, occur on the Siberian Platform (Lena-Tunguska region) and Arabian Shield (Oman) where late Riphean and Vendian siliciclastics and carbonates pass upwards into thick evaporite deposits which acted as regional seals. Peritidal dolomitic carbonates of Sinian age in the Sichuan Basin, southeastern China, include basal argillaceous source beds and the main reservoir of the giant Weiyuan gas field. Proterozoic oils and pyrobitumens have light carbon isotopic signatures (δ13C = — 30 to — 45%o). The unusual biomarker distributions of the oils and associated organic-rich sediments (where they contain well-preserved kerogen) cast new light on the membrane and cellular lipids of Proterozoic microbial ecosystems.