Abstract: Diagenesis and Porosity Evolution in the Shuaiba Formation (Lower Cretaceous) at Ghaba North Field, Central Oman

John D. Humphrey, Abdulrahman Al-Bastaki

The Aptian Shuaiba Formation is a prolific hydrocarbon reservoir in the Persian Gulf region. We have studied Shuaiba reservoir rocks from four cores drilled at Ghaba North Field in central Oman. The Shuaiba produces from matrix microporosity, fracture porosity, and secondary macroporosity, with total measured porosities reaching as high as 43% (average 25-30%). Reservoir holding capacity is controlled primarily by matrix microporosity which occurs as intercrystalline porosity within a diagenetic microrhombic calcite matrix. Development of this matrix microporosity is the key for understanding reservoir development in Ghaba North field.

We have subdivided the Shuaiba into two reservoir zones based on proximity to subaerial exposure surfaces recognized in core. Porosity and permeability decreases with increasing distance downward from these exposure surfaces. Intercrystalline matrix microporosity developed due to meteoric recrystallization of originally aragonitic lime mud to microrhombic calcite. Macroporosity, developed through meteoric leaching of rudist fragments, is enhanced close to the exposure surfaces. Matrix ^dgr¹⁸0 compositions are depleted by 1.5 to 2.O per mil relative to inferred marine precursor compositions. The upper reservoir zone shows a 3 per mil depletion in ^dgr¹³C upward toward the subaerial exposure surface. Although relatively invariant, the lower zone is depleted in ^dgr SUP>13C by at least 2 per mil relative to a marine precursor. Coarse, late-stage calcite cements that partially occlude macroporosity have depleted oxygen compositions, indicative of precipitation from higher temperature burial fluids. Reservoir quality of the Shuaiba Formation at Ghaba North Field thus developed relatively early during contact with meteoric pore fluids; however, later stage cementation acted to reduce total porosity prior to, and perhaps coeval with, hydrocarbon migration.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994