Quantification of Cretaceous-Cenozoic exhumation in the Otway Basin using sonic velocities and implications for hydrocarbon exploration

Abstract

Increasing demand for hydrocarbons is prompting increasing exploration in poorly understood and geologically complex basins. These include basins affected by exhumation, where there is often uncertainty regarding the maximum burial depths of source, reservoir and seal horizons. This uncertainty can be reduced by quantifying exhumation magnitudes using techniques such as sonic velocity analysis. The Cretaceous- Cenozoic Otway Basin of the southern Australian margin is an important oil and gas province whose subsidence history was interrupted by exhumation events during the mid-Cretaceous, mid-Eocene and late Miocene. We have used sonic velocity data from 135 wells to quantify the magnitude and extent of exhumation during these events. Here we present estimates of net exhumation based on analyses of Lower Cretaceous Eumeralla Formation shales, which are important source rocks within the basin. Our approach involves measuring the velocities of overcompacted intra-formational shale units identified from gamma ray logs, and determining their displacement on the depth axis from an empirically derived normal compaction trend based on data from wells where the Eumeralla Formation is at maximum burial at presentday. Our results indicate significant values of mid- Cretaceous net exhumation in the Otway Ranges and Colac Trough, with estimates for Olangolah-1 (~2700 m), Stonyford-1 (~1100 m), Tirrengowa-1 (~860 m) and Ingleby-1 (~1000 m) in good agreement with independent estimates of mid-Cretaceous exhumation for these wells based on palaeothermal data. There is also moderate net exhumation around the Merino High (~700 m) but minor (0-200m) net exhumation around the basin?s northern margins and Penola Trough area and moderate (200-600 m) around the Port Campbell Embayment gas fields. Mid-Cretaceous exhumation is interpreted to have been caused by NW-SE directed compression and inversion of the basin. Our results indicate that potential Eumeralla Formation source rocks in wells where large values of mid-Cretaceous exhumation are recorded are most likely over-mature for hydrocarbon generation, in agreement with previous studies of the thermal history of the basin.