Dynamic interplay of Sedimentation and Diagenesis on Miocene Clastic Reservoir Properties, Niger Delta Basin, Nigeria

Abstract

The D7 sand is one of the hydrocarbon bearing reservoirs horizons penetrated by wells in Gbaran field, Central Swamp Depobelt onshore Niger delta Nigeria. Detailed examination of cores taken from the reservoir section in the Gbaran-6 and -7 wells, led to the recognition of thirteen lithofacies units which were then grouped into distinctive genetically related facies associations. The facies associations were grouped into upper and lower depositional sequences separated by a marine mudstone layer. The lower depositional sequence of the sands was interpreted as deposits of a prograding wave dominated shallow marine shoreface while the sediments of the upper depositional sequence were interpreted as transgressive estuarine deposits. Petrographic analysis of thin-section samples from the reservoir show that the sands are mainly quartz arenites and the main diagenetic effect on the reservoir is cementation due to quartz overgrowths. Calcite cements also occur locally in some sections, especially among the fine to silty grained transgressive sands of the upper reservoir sequence. Calcite cementation within the reservoir sequence was interpreted to be biogenic in nature and facies constrained. The calcitic cement was observed to have been dissolved and re-precipitated from carbonaceous shell fragments and peloids originally deposited within the sediment. Analysis of the petrophysical data from core plugs and observations from petrographic study showed that carbonate cementation had a more detrimental effect on reservoir quality within the reservoir than the silica cemented interval.