Two case studies of thermal maturity and thermal modelling within the overpressured Jurassic rocks of the Barrow Sub-basin, North West Shelf of Australia

Abstract

Thermal maturity within the Jurassic thick, overpressured source rocks of the Bambra-2 and Barrow Deep-1 wells, Barrow Sub-basin, Western Australia, was investigated and modelled using a temperature–time-related kinetic model. The aim of this study is to investigate whether overpressure has a determinable influence on thermal maturation. Repeat formation tests (RFTs), drill stem tests (DSTs) and mud weights indicate that the Jurassic section in the Barrow Sub-basin is highly overpressured. It appears that the overpressured zone is associated with high sonic transit times and low resistivity values in the Jurassic fine-grained rocks. Anomalously low measured vitrinite reflectance (Ro) values in the overpressured zone appear to be due to either Ro suppression or inaccurate identification of vitrinite. Anomalously low Tmax values are found to be caused by contamination from drilling-mud additives. A good correlation is found between the reliable maturity data including biomarker maturity and predicted maturity values (Ro between 0.8 and 2.2%) and suggests that the anomalously low Ro and Tmax values are probably not related to the overpressuring (fluid pressure about 40–80MPa at 3000–4600m). These results demonstrate that the true maturity level in the Jurassic overpressured section probably depends on temperature and time alone. We, therefore, conclude that overpressure is not an important factor in thermal maturity and hydrocarbon generation in this region.