FAULT-RELATED CALCITE CEMENTATION: IMPLICATIONS FOR TIMING OF HYDROCARBON GENERATION AND MIGRATION AND SECONDARY POROSITY DEVELOPMENT, BARROW SUB BASIN, NORTHWEST SHELF

Abstract

Calcite is a common authigenic mineral in subsurface sandstones of the Barrow Sub-basin, North West Shelf. It is present in several formations from different stratigraphic horizons, ranging from Permian to Cretaceous. It occurs as poikilotopic cement and fracture-fill particularly concentrated along one of the major listric faults in the eastern part of the sub-basin. A detailed petrographical and geochemical study was performed on the Early Cretaceous calcite cements in an attempt to provide information on their origin, distribution and effect on reservoir quality. Calcite cements are Ca-rich, Mg-poor with considerable amounts of Fe and are characterised by bright orange to yellow luminescent colours. The δ13C and δ180 values vary considerably, δ13C ranging from −2.0 to −23.5 %o PDB (average of −10.2 %o, ± 4.8 PDB), whilst δ180 values range from 19.3 to 25.4 SMOW (average of 21.1 %o, ± 1.8 SMOW). Calcite cements are characterised by elevated 87Sr/86Sr ratios with a range of 0.71029 to 0.71058 (average of 0.71043 ± 0.00012). The elemental and stable isotope compositions of the calcite cements indicate cementation from meteoric pore-waters, with the same source and timing of occurrence.

Calcite cements formed in the mid-diagenetic history below 45°C. The carbon isotopic composition of calcite cements is interpreted to be sourced from bicarbonate and carbon dioxide generated by thermal decarboxylation of kerogen and oxidation of the early-generated oil. 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 C02 to form pervasive calcite-cemented zones. These zones may reach up to 8 m thick and occlude the intergranular 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 to locally enhance reservoir quality.