The palaeobiology and geochemistry of Precambrian hydrocarbon source rocks

Abstract

Organic carbon productivity and formation of hydrocarbon source rocks during the Early Precambrian was almost exclusively the product of the growth of microbial mats. Indirect evidence of microbial mats can be traced back to at least 2.6–2.7 Ga (Neoarchaean), with the earliest evidence of mat development in siliciclastic sediments coming from the 2.9 Ga (Mesoarchaean), predominantly marine sedimentary rocks of the Mozaan Group in South Africa. The earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2.75 Ga (Palaeoproterozoic) fluviolacustrine sediments of the Hardey Formation of the Pilbara craton in Western Australia. Well-preserved Proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere. Eukaryotic steranes (biomarker for eukaryotic cells and, therefore, evolved forms of life) are present in the geological record from about 2.7 Ga, but they are not abundant or diverse within Archaean communities, which tend to be dominated by Archaea isoprenoids. Some hydrocarbons have been generated and migrated from Archaean organic-rich shales, but they were probably not of sufficient volume to be of commercial interest. The world's oldest significant hydrocarbon source rocks are Palaeoproterozoic in age and include the shungite deposits (2.0 Ga) in the Lake Onega region of Arctic Russia.There is strong evidence of a global biospheric oxygenation event at c. 1300–1250 Ma (Mesoproterozoic) in conjunction with a first-order positive shift in the marine carbon isotope record. This is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest, unambiguously multicellular eukaryotes at this time. This oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the Neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons. Hydrocarbon source rocks of late Mesoproterozoic and Early Neoproterozoic age are widespread and include highly organic-rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks. By c. 850 Ma, the Neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes, including those of multicellular eukaryotes (red and green algae), as well as molecular evidence for heterotrophic protists. The most widespread hydrocarbon source rocks of mid to late Neoproterozoic age are commonly transgressive organic-rich black shales associated with inter-glacial and post-glacial phases of the Neoproterozoic global scale glaciations. The relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late Neoproterozoic and earliest Cambrian, perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and/or nutrient supply.Precambrian and ‘Infracambrian’ petroleum systems are relatively abundant and widespread. The oldest live oil recovered to date is sourced from Mesoproterozoic rocks within the Velkerri Formation (Roper Group) of the McArthur Basin of northern Australia, dated at 1361 ± 21 Ma and 1417 ± 29 Ma (Re–Os dates) with at least the initial phase of oil generation and migration having taken place before 1280 Ma. However, the geologically oldest commercial production is currently from the somewhat younger mid to Late Neoproterozoic (Cryogenian–Ediacaran) petroleum systems of the Lena-Tunguska province in East Siberia and in southern China, from the latest Neoproterozoic to Early Cambrian Huqf Supergroup in Oman and, potentially in the near future, from the age-equivalent Mawar Supergroup in western India.