Thursday, February 22, 1-00pm

Dr. Chris Cornford

IGI Ltd., Hallsannery, Bideford, Devon EX39 5HE, UK

 
The oldest Petroleum System in the World?
 

Abstract

The 'carbon' in the gold-bearing conglomerate reefs of the Archaean of the Witwatersrand basin of South Africa is recognised as a rather unusual petroleum accumulation. Though the fracture-hosted bitumen seams at the base of 'reefs' have been erroneously described as 'kerogen' derived from algal mats, Gray et al. (1998 JGS Vol. 155, pp.39-59) pointed out an abundance of evidence for a petroliferous origin. A Petroleum System (PS) comprises all generated, migrated and entrapped hydrocarbon from a single volume of mature source rock. A somewhat abbreviated definition of the petroleum system approach (Magoon and Dow, 1994, AAPG Memoir #60) is " to account in terms of volumes, composition and process efficiencies for all petroleums expelled from a single pod of mature source rock". This paper considers the application of the petroleum system concept to the Archean bitumens of the Witwatersrand basin. The now greenschist facies rocks involved range from 3,200 Ma to 2,600 Ma, and as such may constitute the oldest described petroleum system to date. The petroleum system feeding the 'fractured reservoirs' of the Witwatersrand Basin can be described in terms of source rocks, maturation, expulsion, migration entrapment and survival to the present day. Though uniformitanarian principles are applied, the age of the rocks introduces some unusual features to the system.

The Witwatersrand Supergroup comprises coarse clastics interbedded with a number of conventional and magnetite-rich shales. The most significant sources within the basin are the thicker shale sequences, such as the Booysens (Central Rand Group) and Jeppestown Shales (West Rand Group). These are found in close stratigraphic proximity to carbon-rich reefs, such as the Basal Reef, Vaal Reef, and Carbon Leader. An oil-prone kerogen type is not in dispute since organic evolution restricts the contributing groups to simple algae and bacteria. The remnant organic matter is very fine grained (<15?m), amorphous in texture, and characteristic of bacterial-algal kerogen in an advanced state of thermal maturity. Maturation is confirmed with 1-D basin modelling. High bitumen reflectances (2.13 to 2.69%Ro) explain surviving TOC values below 1% (mean TOCs <0.3%), as being more indicative of efficient expulsion than of poor initial source rock properties. For example, the low residual TOC levels in the Booysens Shale can be extrapolated to pre-maturation levels of 2% or more. With the interbedding of source rock shales and coarse clastic reservoirs, migration would have been efficient. The initial reservoirs may well have been the sands and conglomerate of the alluvial fans of the Rand Group foredeep basin. From a petroleum system standpoint, lack of seal would have been the highest exploration risk.

However, with compression opening bed-parallel fractures at the base of the reefs, the oil remigrated (inefficiently - see flyspeck) to fill this newly formed low-pressure fracture porosity. Flux of a progressively warming hydrothermal fluid through this same braided fracture network appears to have cracked the oil to pyrobitumen and precipitated uraninite. The radiation from uraninite must have cross-linked and stabilised the pyrobitumen so that with rising temperatures columnar mesophase was formed. With precipitated particulate gold, this is as seen some 2.6Ga later.

Biography of the Speaker

"Chris Cornford has a Joint B.A. (Hons) in Geology & Chemistry (1971) from the University of Keele, UK, and an M.Sc. (1972) and Ph.D. (1977) in organic geochemistry from the University of Newcastle-upon-Tyne, UK. Following 3 years of mainly DSDP-related research at the KFA, Germany (1975-78), he worked for BNOC/Britoil as a petroleum geochemist and latterly acting Head of the Stratigraphic Laboratory (1978-83). In 1983 he resigned from Britoil to set up Integrated Geochemical Interpretation Ltd. (IGI), a consultancy covering petroleum geochemistry, basin modelling and software development. Since 1983, the company has grown to 6 employees and has gained an international reputation in using bulk, molecular and isotopic data to produce innovative interpretations based on geological models. In addition to NW Europe, Chris Cornford has undertaken projects worldwide, with special expertise in the South Atlantic (Africa & S. America), the Arctic, India, China, Asia and Australia.

In addition to having marketed and provided technical support for the BasinMod family of software in Europe Africa and Middle East, Chris has initiated the development and marketing of a unique geochemical interpretation package (p:IGI), together with a company intra-net resource covering petroleum geochemistry (igNET).

As a backup to IGI's other activities, Chris provides industry training in geochemical interpretation and basin modelling. These well-received courses are increasingly taking the form of workshops, emphasising the use of the attendees' own data in the context of both theory and sound industry practice. Following his early academic work, Chris Cornford has continued publishing. He has recently authored the definitive review of Source Rocks and Oils in the North Sea, contributed a key chapter to the AAPG Petroleum Systems Memoir, presented invited keynotes on Risking Hydrocarbon Charge and Prospect Evaluation, and co-authored a seminal paper investigating the links between oil and gold-uranium mineralisation."