Influence of original fabric on subsequent porosity evolution: an example from the Corallian (Upper Jurassic) reefal limestones, the Weald Basin, southern England

The Corallian reefs of the Weald Basin, southern England, developed over a prograded oolitic sandbody during a marine transgression. The diagenesis and porosity evolution of these reefs is directly related to their original fabric, a reflection of original environment of formation. A variety of facies-related diagenetic processes and products has been recognized including early marine cementation, skeletal diagenesis and burial cementation. The early marine peloidal cements are abundant in the Corallian subsurface reefs and have played a key role in the overall reservoir quality. These cements, by virtue of occurring in stromatolitic crusts and having abnormally light, stable isotopic values, are interpreted as microbial products. The skeletal diagenesis is mainly controlled by skeletal and matrix permeability with dissolution predominantly occurring in the upper, relatively high-energy part of the reefs and neomorphism commonly developing in the lower part of the reefs which grew in a low-energy setting. Burial cementation has involved both calcite and dolomite precipitation over a wide range of burial depths. Various forms of calcite and dolomite cements have been recognized including relatively early ferroan drusy/poikilotopic calcite and zoned dolomite, and relatively late saddle dolomite and non-ferroan coarsely crystalline calcite. These carbonate cements, by virtue of post-compactional distribution and having depleted δ 18O values ( δ 18 O PDB = −6 to −9‰ ) are interpreted as having formed under burial conditions, perhaps in the range of 600 to 2000 m. The porosity evolution is essentially controlled by the combined effects of these diagenetic processes with the best reservoir potential developing in the upper part of the reefs (keep-up highstand systems tract) which underwent extensive skeletal dissolution. Porosity was poorly developed in the middle part of the reefs (catch-up highstand systems tract) due to the extensive early marine cementation. The lower part of the reefs (transgressive systems tract) virtually has no reservoir potential due to the relatively fine-grained sediment texture and intensive compaction.