SUPPRESSED VITRINITE REFLECTANCE AND ITS EFFECT ON THERMAL HISTORY MODELLING IN THE BARROW AND DAMPIER SUB-BASINS.

Abstract

The Barrow and Dampier Sub-basins form an important offshore petroleum province containing major oil discoveries associated with Triassic to Cretaceous reservoirs and source reeks. Constraining the timing cf oil generation and migration within the sub-basins has been hampered by difficulties in assessing thermal maturity using vitrinite reflectance data. Jurassic-Cretaceous formations of predominantly marine origin yield vitrinite reflectance values that are often lower than expected compared to the present day thermal conditions. The two main explanations put forward by workers for this situation are that:

a recent increase in thermal gradients has occurred; orthe vitrinite reflectance is suppressed, and this suppression is related to the marine environment of deposition.

Thermal history analysis of 65 wells throughout the Barrow and Dampier Sub-basins has confirmed, using multiple maturity parameters, that vitrinite reflectance data are suppressed over large parts of the study area.

Thermal history modelling and the confirmation of vitrinite reflectance suppression has enabled revised estimates of pr esent maturity to be made. Maturity levels based on measured reflectance data and values calculated from thermal information exhibit large differences, related to suppression; up to 1,500 m in the depth to the 0.7 per cent R(1 iso-reflectance surface. Vitrinite reflectance suppression, if not taken into account, also strongly affects modelling of uplift and erosion, and the timing of onset of maturity for petroleum generation. Variations of up to 100 Ma have been noted between histories based on measured vitrinite reflectance and those taking vitrinite reflectance suppression into account.