ABSTRACT: Reconstructing Heat-Flow Histories of Sediments Using Organic Reactions

ALEXANDER, ROBERT, Centre for Petroleum and Environmental Organic Geochemistry, Curtin University, Perth, Western Australia, Australia

The past decade has been a period of intense scrutiny of the organic constituents of sediments to identify reactions that can be used to provide information about the heat-flow histories of sediments. Suitable reactions, which can be studied under laboratory conditions to obtain kinetic parameters, are potentially useful in association with computer basin-modeling packages for reconstructing the effective heat flow histories of sediments.

Reaction systems useful for this quantitative approach to sediment maturity need to be dependent only upon the heating time and sediment temperature and should not be affected by changes in the maceral or mineral composition of the samples or by the nature of the depositional environment of the sediments. Preferably, reactions should also be amenable to simple kinetic treatments.

Many of the biomarker ratios that have been used traditionally to infer sediment maturity have failed to meet these criteria and are inappropriate for such quantitative approaches to thermal history. For example, changes in the relative proportions of sterane diastereomers (20S/20R) in sediments of increasing maturity are not the result of simple interconversion, but rather the net effect of a set of complex processes that probably are unsuitable for quantitative applications. Laboratory heating experiments also suggest that some of the changes in the distribution of hopanoid diastreomers that occur with increase in sediment maturity may also be due to selective depletion of one component rather than interconversion. However, reactions such as the cyclization of ortho alkylbiphenyls a d the elimination of alkenes from alkylesters meet most of the requirements for processes suited to quantitative thermal heating approaches and offer new avenues for assessing the thermal history of sediments. These procedures are illustrated using examples from some Australian sedimentary basins.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)