AAPG Bulletin, Vol. 83 (1999), No.12

SCHOLLE, P. A., and D. ULMER-SCHOLLE, Southern Methodist University, Dallas, TX; and M. W. JEPPESEN and L. SIMONSEN, Maersk Oil Qatar AS, Doha, Qatar

Abstract: Timing of Porosity Generation in Lower Cretaceous Reservoirs (Shuaiba and Kharib Formations), Block 5, Offshore Qatar

Fine-grained Shuaiba and Kharaib carbonate reservoirs of Block 5 offshore Qatar are characterized by extensive secondary porosity, including biomoulds, leached intergranular calcite cements (resurrecting primary porosity), solution-enlarged fractures, and centimeter-sized vugs. Extensive secondary porosity, ascribed to early meteoric diagenesis, has been described from the UAE and Oman. Core observations, stable isotope geochemistry, and petrography, however, indicate that the Block 5 reservoirs were deposited in deeper-shelf settings that were not exposed to meteoric diagenesis. Early diagenesis was dominated by marine cementation, burial diagenetic aragonite dissolution or neomorphism, extensive calcite cementation, localized multi-stage dolomitization, and substantial small-scale fracturing. Early calcite cements contain single-phase water-filled inclusions, indicating formation at temperatures of max. 35<deg>C. The main phase of porosity development post-dates early diagenesis and appears to be related to large-scale throughput of highly corrosive pore fluids. Extensive corrosion of earlier cements, development of solution-enlarged fractures and vugs, and the presence of two-phase fluid inclusions in cements in secondary pores support a late origin of this porosity. Primary fluid inclusions were formed at 55-60<deg>C (slightly higher than present-day ambient temperatures); secondary inclusions reveal the short-lived presence of much hotter (120-140<deg>C) waters. Abundant hydrocarbon-filled inclusions in some cements indicate leaching by hot brines that may have just preceded or accompanied hydrocarbon migration. Such fluids may have been episodically expelled from nearby overthrust belts (Zagros or Oman Mtns).

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SCHOLLE, P. A., D. ULMER-SCHOLLE, M. W. JEPPESEN, and L. SIMONSEN