Abstract

A compilation of average porosity and permeability data for petroleum reservoirs in the Permian–Triassic Khuff Formation and the Jurassic Arab Formation shows that most Khuff reservoirs have an average porosity of less than 12%, whereas most Arab reservoirs have an average porosity of 12–26%. Higher porosity correlates with shallower depth, suggesting that burial diagenesis is the main cause of the overall porosity difference between these units. Deeper burial of Khuff reservoirs is inferred to have resulted in greater porosity loss by chemical compaction and associated cementation. A broad correlation also exists between average porosity and average permeability, suggesting that deeper burial and the resulting porosity decrease are also a primary cause of the lower permeabilities of the Khuff reservoirs. In addition to greater burial depth, however, a combination of depositional and early diagenetic factors is also reflected in the lower average porosity and permeability values of the Khuff reservoirs. Khuff strata were deposited on an extensive, poorly circulated, very low-relief shelf and consist in large part of interbedded mudstones and grainstones having relatively fine grain size, with major amounts of depositional calcium sulfate present. Arab reservoirs were deposited under better circulated conditions near platform margins facing deep, intracratonic basins and, thus, have coarser, more grain-dominated fabrics and lesser overall content of chemically precipitated grains, calcium sulfate, and dolomite. Khuff deposits were likely composed of less stable mineralogy than Arab sediments because the Late Permian was a time of aragonite seas, whereas the Late Jurassic was a time of calcite seas. The combined result of these factors is that Arab reservoirs are characterized by greater preservation of primary depositional pore types, more coarsely crystalline dolomite fabrics, and lesser plugging by anhydrite. Finally, a possible factor affecting the average porosity and permeability values is petroleum composition, which is gas in most Khuff reservoirs and oil in Arab reservoirs. Lower economic cutoff values for gas production would favor inclusion of low-permeability zones in Khuff reservoirs, thus reducing average reservoir values.

Two main aspects of these results are innovative. This is the first time that porosity and permeability values for either Khuff or Arab reservoirs have been examined regionally. Second, the conclusion that thermal exposure is the primary control on average porosity and permeability in these units is consistent with previous work from other carbonates, but is new for the Middle East.

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