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Reservoir Heterogeneity and Quality of Khuff Carbonates (Central Saudi Arabia)


The Khuff carbonates host several main gas reservoirs in the Middle East. These reservoirs are known for their complex heterogeneity, which is both of a sub-seismic scale and beyond the interwell spacing. To get a better insight in these reservoirs, the heterogeneity of the Upper Khartam Member within a high-frequency sequence stratigraphic framework was studied at a lateral resolution of 5 m. The original (unconsolidated) depositional texture of the Upper Khartam Member sediments was dominated by high intergranular porosity and permeability (in the order of up to 42% and 12 D, respectively), which were subsequently obliterated by diagenetic overprinting. Six major diagenetic processes were differentiated in the studied outcrops, namely: dissolution, pore lining and pore blocking calcite cementation, dolomitization, fracturing, and stylolitization. Significantly, cementation has blocked most of the original intergranular porosity. Ten porosity types were differentiated in the studied outcrops, namely: intergranular, intragranular, shelter, dissolution, moldic, vuggy, micro-porosity, and porosity in relation to dolomite–dedolomite, dolomite-leaching, and fracture development. The oolitic grainstones are dominated by dissolution, moldic, and vuggy porosity, whereas intercrystalline and fracture porosity prevails in the recrystallized limestone. Most critically, the reservoir quality of the studied reservoir units along their lateral extension, as defined in their sequence stratigraphic framework, was influenced differentially by diagenesis, which resulted in a lateral segmentation of single bodies into different, hydraulic units. Both XRF and XRD data show a control of certain geochemical associations on the lateral intermittence of the intra-reservoir body. Generally, mineral content has little impact on the reservoir quality, although clay minerals have played a central role in lining the pore space and affecting permeability. SEM observations reveal a textural control on porosity and permeability. The differential mold-filling calcite cements have significantly reduced the porosity and permeability. Overall, the lateral segmentation most likely was caused by a differential diagenetic evolution, and most critically, this evolution seems to have been controlled by stylolites and fractures which respectively acted as barriers to and as conduits for vertical fluid flow and, hence, controlled the differential cementation and dissolution processes.