Biogenic carbonates; are they a criterion for underlying hydrocarbon accumulations? An example from the Gulf of Suez region, Egypt

Cairo University, Geology Department, Giza, Egypt

The Gulf of Suez petroliferous basin is characterized by the occurrence of a middle-upper Miocene evaporite cap. Field and subsurface investigations in the southwest Gulf of Suez region indicate that the evaporites usually contain two types of biogenic carbonates in addition to sulfur. The first type of carbonate is biogenic-syngenetic in origin and was formed in low-temperature euxinic aquatic environments by the reduction of dissolved sulfates and the oxidation of organic matter by sulfate-reducing bacteria. The resulting carbonate and sulfur are stratigraphically restricted within the evaporite sequence and occur as interlaminated gypsum-calcite-sulfur, or as sulfur ooids, sulfur globules, and stromatolitic sulfur that are enclosed in gypsum or calcite. The biogenic-syngenetic carbonate and sulfur did not originate from petroleum and have a significance of euxinic aquatic settings. The second type of carbonate is a biogenic-epigenetic carbonate that formed under low temperatures by biochemical reactions between seeping hydrocarbons and the overlying Miocene evaporites. The carbonate is vuggy, is included within or caps the evaporites, and usually shows hydrocarbon stains or emits a hydrocarbon smell. The sulfur is usually yellow, but may be brown to black due to bitumen staining. Carbonates replace gypsum or anhydrite and occur as aragonite, calcite, or dolomite. The delta ¹³ C mean values for carbonate minerals range from -7.8 to -27.2 per mil PDB, being relatively heavier than the delta ¹³ C values of the nearby crude oil (-28.1 to -29.3 per mil PDB). Carbon isotope values indicate that these carbonates were formed from the heavier organic carbon dioxide under reducing conditions, where the lighter methane is stable. The occurrence of biogenic-epigenetic carbonates and sulfur in the evaporite deposits of the Gulf of Suez petroliferous basin is similar to that recorded in other basins such as the Gulf Coast basins of Texas, Louisiana, and southern Mexico; the Fergana and Amudarya depression of central Asia; and the Mesopotamian depression in Iraq. The biogenic-epigenetic carbonates or sulfur in some regions is considered as a criterion for petroleum microseepage that may indicate a potential hydrocarbon-bearing reservoir at depth.

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