ABSTRACT: An Integrated Analytical Approach for Determining the Origin of Solid Bitumens
GEORGE, SIMON C., SYLVIE M. LLORCA, and P. JOE HAMILTON, CSIRO Division of Exploration Geoscience, North Ryde, Sydney, New South Wales, Australia
Solid bitumens commonly occur in reservoir rocks and can be related to the alteration of precursor liquid hydrocarbons through various processes such as thermal alteration, deasphalting and biodegradation. These processes can be distinguished by an integrated geochemical, isotopic and petrographic approach which also provides information on precursors of the solid bitumens together with constraints on the fluid flow and thermal history of a basin.
Petrography relates the relative timing of authigenic cements with that of the solid bitumens and so may provide information on, for example, the timing of meteoric water influx to a reservoir that results in biodegradation of hydrocarbons. Organic geochemical analysis provides information on the degree of biodegradation of solid bitumens and can distinguish those formed by thermal alteration, which are likely to be predominantly insoluble, from those formed by deasphalting, which are more soluble and contain a higher proportion of asphaltenes and other polar compounds. The carbon isotope ratios of solid bitumens formed by deasphalting are expected to be similar to the precursor oil, whereas those formed by thermal alteration have considerably heavier ratios due to kinetic effects ass ciated with the preferential cracking off of low molecular weight and isotopically light components. Isotope type-curves for the different fractions of a bitumen are also of use. For example, during biodegradation, aliphatic hydrocarbons become more depleted in 12C than the aromatic hydrocarbons, whereas asphaltenes become enriched in 12C.
Case examples from Australia and Scotland will be discussed.
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)