Diagenetic Control of Reservoir Quality of Araba and Naqua Diagenetic Quartzarenites (Cambrian), Gebel Araba-Qabeliat, Southwest Sinai, Egypt

ABDEL-WAHAB, ANTAR, Tanta University, Kafir El-Sheikh, Egypt, and EARLE F. McBRIDE, University of Texas at Austin, Austin, TX

Sandstones of the marine Lower Cambrian Araba Formation and the overlying fluvial Upper Cambrian(?) Naqus Formation in Gebel Araba-Qabeliat, southwest Sinai (the eastern side of the Gulf of Suez) were studied to evaluate the major factors controlling potential hydrocarbon reservoir quality. The formations have a composite thickness of 873 m and overlie Precambrian granite and metamorphic rocks and underlie Cretaceous marine strata. The framework composition of both sandstones is almost entirely quartz with trace amounts of muscovite, K-feldspar, and heavy minerals. Up to 21.5% oversize pores, some filled with younger cements, attest to extensive dissolution loss of detrital grains, chiefly feldspar. Because the final mineralogical maturation of these quartzarenites was through diagene is, they are "diagenetic quartzarenites."

Following deposition, the introduction of thin coatings of infiltered clay was followed by the precipitation of 6.3% unhomogeneously distributed quartz cement. Some sandstones then were cemented completely by calcite cement, and compaction in them ceased. The amount of quartz cement in calcite-free sandstones was insufficient to prevent compaction, and porosity was reduced in them during burial to approximately 29.9%. These sandstones have tight packing, as measured by several different parameters, and nearly half their grain contacts are either long, concavo-convex, or sutured. These events were followed by extensive dissolution loss of carbonate cement, detrital feldspars, micas, and heavy minerals; by the formation of local patches of kaolinite and dickite (average = 8.4%); and by he formation of extensive iron-oxide cement, including specular hematite. These suggest extensive invasion of meteoric water and exposure to oxidizing conditions. The time of this event can only be dated as post-Cambrian(?) and pre-Cenomanian. These sandstones were buried between 3200 and 3600 m based on values of CI and PCP.

Some outcrop samples contain pore-occluding gypsum cement or mixtures of gypsum and halite cement. Sr/Sr ratios of seven samples of gypsum cement have values (0.7077 to 0.7083) that are equal to Miocene and slightly younger seawater. Evaporites apparently were leached by modern meteoric water from nearby exposed Miocene and younger marine evaporite-bearing strata, transported in surface and groundwater to the topographically low terrain where the Cambrian sandstones crop out, and were precipitated by evaporation of surface water.

These Cambrian sandstones have excellent reservoir potential (mean thin section porosity = 25.7%) because they contained few ductile grains to enhance compaction, and they developed significant amounts of secondary pores by both dissolution of calcite cement and unstable detrital grains. Kaolinite and dickite are potential problems for hydrocarbon production in some beds.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)