THE ORGANIC GEOCHEMISTRY OF OIL AND GAS GENERATION AND ITS APPLICATION TO BASS STRAIT AND THE NORTHWEST SHELF

Abstract

A study of the chemical reactions occurring within the geological environment can throw new light on the occurrence of petroleum and natural gas deposits. The principles of chemical kinetics enable extrapolations to be made so that the depths and temperatures of typical hydrocarbon-forming reactions can be deduced when intervals of geological time are available. In general, temperature is the crucial parameter with corrections being possible for shorter or longer heating times.

Living organic matter available for incorporation in sediments is of four basic types: carbohydrates, proteins, lignin and lipids. Only the last two are significant precursors of organic matter (kerogen) in consolidated sediments. Lipid-derived material is the principal source of oil, while lignin-related kerogen gives methane as its main gaseous hydrocarbon product. Generation occurs in zones where the rate of cracking of the source material into volatile hydrocarbons due to increasing temperature becomes significant on the geological time-scale. During and after migration of oil from its source to reservoirs (usually at shallower depths and lower temperatures), other processes can alter the initial light oil. In particular, biodegradation by microorganisms can result in a heavier naphthenic or aromatic crude by the loss of normal alkanes.

Application of organic geochemistry to specific basins involves the measurement on uncontaminated core samples of certain parameters (organic carbon, extractable hydrocarbons, normal alkane distributions, reflectance of vitrinite, H/C ratio of kerogen and spore coloration). These data, together with activation energies for specific chemical reactions, geothermal gradients, rates of subsidence and knowledge of the geological history of the basin enable a general view of generation, migration and preservation of oil and gas to be obtained. Such an overview is given for the Gippsland Basin, where generation of both oil and gas in the lower Latrobe Group is indicated, and for the Rankin Platform of the Northwest Shelf where a deep source at greater than 4000 m is suggested for the gas and condensate.