SOURCE, MATURITY AND GAS COMPOSITION RELATIONSHIPS IN THE SOUTHERN COOPER BASIN

Abstract

In the Permian sedimentary sequence of the Cooper Basin, land plants contributed the bulk of the organic matter to the sediments. Inertinite, vitrinite and exinite are common kerogen types present in the organic-rich shales. Coal thickness varies areally.

The geothermal gradient, though varying (from area to area), is everywhere higher than normal for sedimentary basins. The whole of the Permian sequence is mature for hydrocarbon generation. The highest temperature gradients of up to 3.19°F/100’ are measured in the Nappamerrie Trough and are associated with areas of granitic basement. Vitrinite reflectance profiles confirm that the sediments are thermally mature.

Trends of gas composition indicate three distinct regions with gases trapped in:

the Patchawarra Trough tend to be high in CO2 and wet gas;the Nappamerri Trough tend to be high in CO2 and low in wet gas; andthe Tennapera Trough tend to be low in CO2 and moderately high in wet gas.

These differences in gas composition are accounted for by differences in thermal history within structural zones.