GROVER, GEORGE, JR., Saudi Aramco, Dhahran, Saudi Arabia

ABSTRACT: Abqaiq Hanifa Reservoir: Geologic Attributes Controlling Hydrocarbon Production And Water Injection

The Hanifa reservoir at Abqaiq field consists entirely of mud-supported (>90% matrix) limestones that were deposited in the deeper water interior of the Arabian Intrashelf basin. The Hanifa mudstones lack megascopic pore spaces; porosity in the reservoir is in the form of micropore spaces. Microporosity is evidenced by high porosities (5-32%) within these fine-grained rocks, based on porosity logs and core-plug analyses, absence of visible pore

spaces (even in thin sections) to account for these high pore volumes, and 2000x SEM images that show a crystal framework texture composed of micro-rhombic (clay-size) calcite crystals with 2-5 mm-size pore spaces between these calcite crystals. The microporosity is considered to reflect retention of primary intercrystalline spaces within the precursor lime mud sediment, and with little introduction of allochthonous calcite to occlude pore spaces.

Flow meters indicate that the reservoir is capable of producing/injecting large volumes of oil/water. But there is little stratigraphic predictability to the flow, and thin (2-10 ft) low porosity (<15%) intervals can contribute over 60% of the entire flow. These reservoir attributes, coupled with the low "matrix" permeabilities (0.1-10 md) of the reservoir indicate the presence of an apparent permeability that is controlling fluid flow. Core studies have revealed that this apparent permeability is in the form of high-angle fractures. These fractures are <1 mm wide, contain hydrocarbon residue and calcite cement, and many are in close association with high-amplitude stylolites, suggesting a genetic link between stylolitization and fracturing. Borehole imaging logs are critical fo fracture location, abundance, orientation, and size.

The Hanifa is separated from the giant Arab-D reservoir by over 450 ft of fine-grained carbonates of the Jubaila Formation. These two reservoirs, however, are in pressure-fluid communication via a network of fractures through the Jubaila carbonates. This reservoir communication, together with reservoir heterogeneity in the form of micropores and associated fractures, provides a challenge for reservoir geology and reservoir engineering to formulate a development plan, involving horizontal producer and injector wells, to efficiently and effectively extract the reserves within the Abqaiq Hanifa reservoir.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.