ABSTRACT: Calcite cementation and timing of hydrocarbon generation and migration, Barrow Sub-Basin, Australia

Kraishan, Ghazi and Nicholas Lemon , NCPGG, The University of Adelaide, Adelaide, Australia

A detailed geochemical study on calcite cement from the Barrow Sub-basin provided information of its origin, distribution and its effect on reservoir quality. The low Mg calcite with trace amounts of Fe is characterised by bright orange to yellow luminescent colours. The d¹³C values vary considerably, from -2.0 to -23.5 o/oo PDB (average of -11.0 o/oo, PDB), whilst d¹⁸O values range from 18.9 to 23.6 o/oo SMOW (average of 20.8 o/oo SMOW). The elemental and stable isotope compositions of the calcite indicate the involvement of meteoric pore-waters, with the same source and timing of occurrence. Calcite formed in the mid-diagenetic history (~45°C) from a mixture of marine and meteoric waters. The carbon isotopic composition of calcite is interpreted to be a mixture of bicarbonate/carbon dioxide generated by oxidation of an early-generated oil and thermal decarboxylation of kerogen. The model for calcite formation involves fluids rich in organic carbon having migrated up dip along faults to be trapped and mixed with meteoric-derived CO₂ to form pervasive calcite-cemented zones. These zones may reach up to 8 m thick and occlude the primary porosity. Subsequent tectonic reactivation and maturation of organic matter has resulted in late acidic water invasion to partially or completely dissolve the calcite cement and locally enhance the reservoir quality.

This study reveals the likely controls of faults on the distribution of calcite and biodegraded oil, and shows the use of calcite in determining the timing of hydrocarbon generation and migration pathways.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia