Diagenetically Controlled Porosity Modification Within a Sour Gas Reservoir, UAE

Abstract

An Upper Jurassic reservoir in Field X, onshore UAE, is currently being developed for gas. The reservoir is a >500ft thick, upward-shallowing succession of lime mudstone, oolitic grainstone and interbedded dolomite and anhydrite that was buried to depths of +2km. In the Late Cretaceous, basin inversion led to structural closure and oil emplacement. During reburial in the Miocene there was a secondary charge of gaseous hydrocarbons, H2S and CO₂ before the field was uplifted and tilted to the north-east. This study focuses on the controls on pore type and distribution in the main reservoir unit, dominated by an association of oolitic pack-grainstones deposited in an active parabolic shoal on the margins of an intrashelf basin. Field-scale variability in porosity has been seismically mapped and although reservoir quality varies between facies, porosity also varies in response to position on structure and proximity to faults. Porosity is entirely diagenetic; interparticle macropores are cemented by non ferroan calcite, and ooids recrystallized by microcrystalline calcite. Intra-ooid micro- and mesoporosity dominates, supplemented by minor bio- and oomouldic macropores. Reservoir connectivity is facilitated by short, sub-vertical fractures. Integrated core, petrographical and geochemical data demonstrates that recrystallization of ooids created microporosity that has been enhanced by subsequent leaching. Despite evidence of platform emergence at the top of the reservoir layer, there is little indication of the involvement of meteoric water in this stabilisation process. More likely it was contemporaneous with stabilisation of aragonitic allochems during burial. There is a strong correspondence between the extent of ooid recrystallization and interparticle calcite cementation; macroporosity is best preserved within sweet-spots mostly at the top of the reservoir where ooids have not been altered. Within the lower 5m of the main reservoir, fractures are partially occluded by calcite-replaced anhydrite, saddle dolomite, bitumen and pyrite, suggesting localised intra-reservoir thermochemical sulphate reduction. Leached halos and vugs are common around these fractures. The results of this study demonstrate the complexity of pore networks, within an apparently sedimentologically simple reservoir and have informed static and dynamic modelling, well planning and completion during the earliest stages of field development.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016