PATTERNS OF DIAGENESIS IN SOME AUSTRALIAN SEDIMENTARY BASINS

Abstract

Three characteristic types of Australian sedimentary basins can be recognized on the basis of depth-reflectance curves. These may be designated as the Cooper, Sydney, and Gippsland Basin types. Characteristic depth-reflectance curves allow an assessment of the depositional and tectonic histories of sedimentary basins to be made. If the geological history and especially the stratigraphy of a basin is well known, it is possible to estimate the maturity which coal or kerogen would have attained at any past or present time. This maturity would be expressed by particular reflectance values of vitrinite. By making actual measurements of vitrinite and kerogen reflectance, the model of maturation can be tested. In this way it has been possible, for a number of basins, to estimate the geological period in which liquid hydrocarbons were generated and the rate at which the maturation process proceeded. The formation of an accumulation of oil is a matter of balance between the supply and loss of oil to and from traps, as well as persistence of traps and migration paths through geological time. The lapse of time after oil generation and the rate of generation are thus most important. The above-mentioned three types of sedimentary basins differ in these respects. Reflectance data can therefore be used not only to estimate the present state of organic maturity, but also in conjunction with the history of sedimentation in the basin, to interpret hydrocarbon generation activity in geological time.