Mapping Hydrocarbon Migration Pathways via the Integration of Geophysical and Geochemical Data: Vulcan Sub-Basin, Timor Sea, North-Western Australia

Geoffrey W. O'Brien, E. Phillip Woods

Within localized areas of the Jurassic Vulcan Sub-Basin, Timor Sea, Australia, the Eocene sandstones are very strongly cemented with calcite. These carbonate-cemented sands are recognizable on seismic data as zones of anomalously high velocity, and result in both time 'pull-up' and deterioration of the stack response in the underlying section. We have mapped the distribution of these cemented zones using seismic data, conducted extensive carbon and oxygen isotope, mineralogical and petrologic analysis of the carbonate cements (and gases and oils) within selected wells and integrated the results with ~4,000 km of water column geochemical data.

A causal relationship has been established between the carbonate cementation in the Eocene sandstones and multi-stage hydrocarbon seepage from the underlying section. As such, the areal distribution of the cemented zones, as defined by seismic mapping, is a useful exploration aid in delineating hydrocarbon migration pathways, with their presence or absence over a prospect also providing an indication of seal integrity. Significantly, a correlation can be established between the degree of hydrocarbon leakage from a fault-bound closure (as indicated by the cemented zones and/or water bottom geochemical anomalies), and the degree of the strike-slip component of the fault reactivation. Traps which reactivate in almost pure dip-slip are less likely to leak than those with a significant str ke-slip component in the reactivation.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995