DIAGENESIS OF ORGANIC MATTER AND OCCURRENCE OF HYDROCARBONS IN SOME AUSTRALIAN SEDIMENTARY BASINS

Abstract

It is important that petroleum exploration geologists know the critical depth limits where oil is generated from original organic matter in sediments and where the oil changes to natural gas. Organic matter is very sensitive to temperature. The maximum temperature experienced is related to its depth of burial.

CSIRO has used the composition and physical properties of various types of organic matter in shaly rocks as indicators for the degree of diagenesis caused by this heat alteration. The reflectance of vitrinite in associated coals is used as the primary standard, and carbon content of such coals as the secondary parameter to distinguish various stages of oil and gas generation. Depth-reflectance curves are useful 1., for estimating palaeogeothermal gradients, 2., for determining the degree of diagenesis at a particular depth and also 3., for estimating the approximate thickness of sediments subsequently lost after deposition.

The petroleum potential of some Australian sedimentary basins is reviewed in the light of this knowledge. In the Northwest Shelf area and in the Capricorn and Otway Basins, the oil generation zone is deeper than in the Cooper, Galilee and Surat Basins. In the Bowen and Sydney Basins and several other small basins along the eastern coast of Australia, this zone is very shallow, and in some areas the oil generation zone has been completely lost by erosion. The areas most promising for oil fields are those where little erosion of sediments has taken place subsequent to deposition and diagenesis, provided that all other geologic factors for hydrocarbon accumulation are present.