Post-compactional porosity in a hydrocarbon-bearing carbonate succession: Potential timing and processes controlling porosity creation in the Aptian Shuaiba Formation, Oman

Abstract

Porosity formed after compaction has been recognised for many years in carbonate rocks, but there is little consensus as to the origin of the fluids responsible for dissolution, and the processes by which pores are generated. In particular, it is often hard to identify the source of fluids that are below calcite saturation, and in sufficient volumes, to explain the apparent volume of porosity. This study directly addresses this challenge through analysis of core data from a number of oil fields across north Oman, which produce from the lower Cretaceous (Aptian) Shuaiba Formation. The Shuaiba Formation in all these fields exhibits high total porosity, comprising pores that range in size from less than a micron to centimetres in diameter. There is an increase in the effective porosity upwards within the succession; permeability is highest in the upper 15 m of the formation, beneath the top-Shuaiba unconformity. Since this unconformity represents a hiatus of over 5 million years (My), caused by platform emergence, previous studies often interpreted porosity modification to be due to dissolution by groundwater during penecontemporaneous exposure of the platform. Nevertheless, paragenetic relationships indicate that most of the secondary porosity that was formed early in the burial history was either occluded by drusy calcite cements, or destroyed by compaction. Furthermore, many of the drusy calcite cements are etched, indicating that dissolution took place after cementation. Consistent with this, many of the pores formed on the margins of tectonic fractures and stylolites, indicating that some pore-formation post-dates compaction.The top of the Shuaiba Formation in all fields is between 600 and 1600 m, following uplift in the late Cretaceous and early Oligocene by inversion along deep-seated, strike-slip faults. Possible causes of dissolution are cooling of upward-rising brines fluxed along faults and corrosion by organic acids prior to hydrocarbon emplacement. It is unclear, however, as to the extent to which these fluids could dissolve carbonate before reaching equilibrium. A third possible fluid is groundwater. Sulphur isotope data from paragenetically late pyrite, is consistent with bacterial sulphate reduction, indicative of relatively cool temperatures (<60-80 °C). Porosity enhancement might, therefore, have been facilitated by mixing of groundwater with highly saline reservoir fluids, resulting in undersaturation with respect to calcium carbonate. Overall, the data reveal a complex history of porosity modification, which relates to rock fabric, stratal architecture and tectonic history and suggests multiple controls on post-compactional porosity modification. Most significantly, the data strongly suggest that dissolution occurred during orogenic uplift and telogenesis, providing a new explanation for how porosity formation can occur after lithification, along stylolites and fractures, from a volumetrically significant source of carbonate-undersaturated fluid.