CALCITE-CEMENTED ZONES IN THE EROMANGA BASIN: CLUES TO PETROLEUM MIGRATION AND ENTRAPMENT?

Abstract

The occurrence of calcite cementation zones in oil- bearing sequences of the Jurassic-Cretaceous Eromanga Basin is of importance to petroleum exploration. The erratic distribution and thickness of these calcite-cemented intervals is problematic for both prediction of subsurface reservoir quality and structural interpretation of seismic data due to velocity anomalies.

Carbon isotope signatures suggest the carbonate cements may form by dissipation of carbon dioxide upward from the Cooper Basin into the calcium-bearing J-aquifers of the Great Artesian Basin of which the Eromanga Basin forms a part. The model is feasible if the pH of the Eromanga Basin aquifer waters is buffered externally, by generation of organic acid anions during kerogen maturation or aluminosilicate reactions.

Hydrocarbons are likely to have migrated up-dip along the same conduits as the carbon dioxide. Consequently, delineation of massive calcite-cemented zones in the Eromanga Basin reservoirs using well log and seismic data may aid in the identification of petroleum migration pathways, and sites of hydrocarbon entrapment.