The North Rankin Gas Field, Carnarvon Basin, Australia—Late Authigenic Pyrite Evidence of Early Oil Entrapment and Oil-Charged Fluid Flow

Abstract

The multi-TCF North Rankin gas and condensate field in Early Jurassic to Late Triassic sandstone reservoirs is one of numerous gas-dominated hydrocarbon accumulations on the Rankin Trend of the Dampier Sub-basin in the Carnarvon Basin. However oil shows in Late Cretaceous carbonate and early Tertiary sandstone above these gas-bearing reservoirs and oil shows below the gas-water contact indicate a more oil-dominated past. The presence of extensive late authigenic pyrite cementation in the reservoirs of the North Rankin Field provides further compelling evidence of a large palaeo-oil accumulation that has leaked and/or has been displaced by subsequent gas charge. The pyrite cementation developed in these reservoirs by reduction of formation water sulphate in the presence of migrating and/or entrapped oil followed by the reaction of iron with the resultant hydrogen sulphide. Sulphate reduction is considered to be most likely by sulphate-reducing microorganisms, which utilised hydrocarbons for their metabolism. Development of pyrite cementation in the sandstone was therefore dependent on the presence of sulphate- and iron-rich formation water. In the North Rankin A-18 well thick zones of abundant pyrite cement within the present gas column are interpreted to indicate significant sulphate reduction at both stable oil-water contacts and through the reservoir during oil charge. This late authigenic pyrite is disseminated, in fractures and in solution fronts and provides a fingerprint of the mechanisms involved in oil-charged and sulphate- and iron-rich fluid migration into and through the reservoirs. The shear abundance of pyrite points to an external source of iron with fluid migration inferred to have been at least in part vertically via dilated fractures into the reservoir. Late authigenic pyrite in conjunction with oil shows is therefore considered to provide evidence of the presence of a large North Rankin palaeo-oil field and a more oil-dominated hydrocarbon charge of the Rankin Trend structures in the past. Furthermore the pyrite provides a new methodology to understand and identify one mechanism of oil charged fluid flow into a structure.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015