Carbonate Depositional Environments and Reservoir Properties of the Miocene Rocks, East Gulf of Suez, Egypt

Mohamed A. K. Barakat, M. M. Kholief

Miocene carbonate rocks in six surface sections on the eastern side of the Gulf of Suez were carefully studied in the field, petrographically examined, and mineralogically analyzed using SEM and x-ray micro-analysis for all elements.

The limestones and marls are very fossiliferous (e.g., algae, foraminifera, ostracodes, echinoderms, and mollusks). These fossils were composed of aragonite and were diagenetically transformed into calcite and/or dolomite. The meteoric waters, during subaerial exposure, were responsible for that transformation after lithification. Sometimes calcite was replaced by celestite or chalcedony. These muddy limestones have textural, lithological, and fossil content characteristic of shallow-water origin. The carbonates were deposited in reef facies as corals, algal reefs, and muddy (micritic) carbonates. The association of shallow marine limestones with the high-energy marine limestones in the studied sections strongly indicates fluctuation between lagoon and beach environments. The presence of saline indicators (dolomite, anhydrite, gypsum, and celestite) with these carbonates is good evidence for lagoonal environment in humid or semiarid climate (i.e., supratidal flat environment).

In general, reservoir quality of the Miocene carbonates was poor because of original composition and texture. Three different types of porosity were distinguished in the studied Miocene carbonates: primary (intergranular and moldic), secondary (leached), and fracture. Much of the porosity in the Miocene reservoirs is secondary; however, sometimes this secondary porosity may be reduced by compaction and/or precipitation of evaporites in fractures and pores.

We conclude that the primary porosity of the carbonate rocks in the reef complex was eliminated by lithification and cementation; only secondary porosity remained. This secondary porosity developed in all environments (supratidal, intertidal, and subtidal), but the best porosity developed in the subtidal facies. This high porosity occurs because the dolomites in the subtidal facies are coarser and free from anhydrite cement, whereas in supratidal and intertidal facies the dolomites are finer and their porosity is plugged by secondary anhydrite cement.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.