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Evaluation of the role of anoxia in the origin of petroleum source rocks: a critique and attempt to reconcile modern and ancient evidence.

Lookup NU author(s): Dr Richard Tyson


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Geologists have traditionally associated the deposition of petroleum source rocks with dysoxic to anoxic environments. This association has been based upon the strong correlation between high TOC contents and petroleum potentials, and fine-grained sediment facies which are finely laminated and/or have benthic macrofaunas, microfaunas, and ichnofaunas and geochemical characteristics indicative of dysoxic-suboxic palaeoxygenation or true anoxia at the sediment-water interface. The higher content and quality of the organic matter (OM) deposited in these sediments has been widely attributed to enhanced preservation of the organic matter due to reduced exposure to oxygen and thus lower utilisation by benthos and less efficient microbial degradation (e.g. Demaison & Moore 1980). Empirical geological data shows these facies are undoubtedly the best petroleum source rocks, regardless of how exactly they were formed. Calvert (1987), Pedersen & Calvert (1990), and Calvert & Pedersen (1992) have recently used modern sediment TOC data to suggest that TOC is mainly a function of carbon flux, the sediment accumulation rate (SAR), and the sediment grain size, and is not enhanced under dysoxic-anoxic conditions. Calvert et al. (1992) and Pedersen et al. (1992) have also used Hydrogen Index (HI) data from modern dysoxic-suboxic OMZ sediments to challenge the prevalent geological view that dysoxia-anoxia results in the preservation of OM of higher (oil-prone) quality. While Pedersen and Calvert (1991) stated that their 1990 AAPG paper was not concerned with source rock deposition per se, their publications clearly have major potential implications for source rock depositional models and therefore require close scrutiny. Upon detailed examination, I believe the arguments advanced by Calvert et al. have a number of important weaknesses that are likely to diminish the differences between "oxic" and "anoxic" facies and thus result in misleading conclusions: (a) Most of the modern data discussed are from dysoxic to suboxic (2.0-0.2 ml/l oxygen), not "anoxic", OMZ environments; (b) Near-bottom dissolved oxygen measurements often fail to determine the conditions at the actual sediment-water interface; (c) Oxic vs. anoxic differences in relative degradation rates do not need to be large to be significant, and are inevitably much lower than differences in relative preservation rates; (d) short-term (less than 2 year) laboratory carbon losses during microbial diagenesis are dominated by the less redox-sensitive fate of the metabolisable OM fraction, and the oxygen conditions are often poorly controlled during such experiments; (e) Oxic degradation (associated with greater decrease in HI) is not limited by oxygen until at least suboxic conditions are attained, and thus OMZs should not necessarily be expected to have especially high HI values; (f) Sediment fabrics can be very sensitive to short-term variations in bottom water oxygenation, and bioturbation does not necessarily coincide with a major increase in oxic degradation of the OM, especially at high SAR's; (g) The available modern carbon burial efficiency data suggest higher preservation under dysoxic-anoxic conditions, especially at low SAR's; (h) Comparisons based on modern sediments with high SAR's cannot properly distinguish between the effects of pore-water anoxia and dysoxic-anoxic conditions in the lower water column: when carbon burial is already high for "other" reasons, any additional effect of anoxia is inevitably small; maximum oxic vs. anoxic differences occur at low SAR values; (i) Comparisons have not always been made on the basis of post-diagenetic TOC values, or adjusted for variations in OM source; (j) Source rock deposition is mostly associated with (relatively shallow) shelf settings and dysoxic-anoxic conditions; at these depths carbon flux is unlikely to be a major limiting factor on TOC or the development of dysoxia-anoxia; (k) calculation of the maximum potential TOC of deep-sea sediments (based on mass fluxes and 100% preservation) indicates that the TOC of even deep-sea sediments is not limited by the carbon flux, but by carbon preservation; (l) A variety of observations suggest that TOC values may increase by up to 2-5 times under dysoxic-anoxic conditions, but that any greater increase is likely to be due to very low dilution of organic matter at low siliciclastic SAR's.

Publication metadata

Author(s): Tyson RV

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: Exploration Update ’94 - Western Canadian and International Expertise (CSEG & CSPG joint national convention)

Year of Conference: 1994

Pages: 90-91

Publisher: Canadian Society of Exploration Geophysicists & Canadian Society of Petroleum Geologists

Notes: Although rather old now, this abstract is provided here because it will not be easily available elsewhere, and because it contains some important conclusions not previously published elsewhere. The paper won the "Best Geological Paper" award for the meeting.