AAPG Middle East Region Geoscience Technology Workshop

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Distribution Patterns and Origin of Calcite Cement in Sandstone Successions: Case Studies

Abstract

Predicting and understanding the distribution patterns of diagenetic calcite cemented zones and their impacts on fluid flow within a reservoir unit is a key goal of reservoir characterization. Two case studies of different ages and depositional environments will be presented and discussed. In the first case study, a summary model is proposed that integrates the spatial and temporal distribution of various textural habits of calcite in Upper Jurassic siliciclastic shoreface sediments outcropping in NW France into the sequence stratigraphic framework. The model emphasizes the distribution of cemented vs. non-cemented sandstones and the cementation pattern and texture of calcite cements. In the transgressive (TST) and highstand (HST) systems tracts, which are dominated by mudrocks and very fine-grained sandstones, calcite cement has a microcrystalline texture and occurs mainly as continuously cemented layers (15-20 cm thick). The stable isotopic values of microcrystalline texture in TST and HST indicate calcite precipitation from largely marine pore waters. In the sandstones of the forced regressive wedge (FRWST) and lowstand (LST) systems tracts, calcite cement has a poikilotopic texture and occurs as stratabound concretions (15 cm to 2 m in diameter). Precipitation of poikilotopic calcite cement with relatively low δ18O values suggest incursion of meteoric waters into the shallow-marine sandstones, which occurred during the relative sea level lowstand. The second case study of currently ongoing research project investigates the possible origin of calcite cemented concretions in fluvial sandstones of the upper Cretaceous Wasia Formation in central Saudi Arabia. Detailed geochemical analysis undertaken on slabs and thin sections of many concretion samples representing different stratigraphic intervals indicate an eogenetic origin of the calcite cement. This interpretation is supported by the precompactional nature of the poikilotopic texture and the isotopic compositions of calcite cement. The outcomes of these two outcrop studies are useful for understanding the origin and predicting the distribution of irregularly to continuously cemented zones, hence may provide better upscaled reservoir quality models in similar subsurface reservoirs.