Depositional Setting and Hydrocarbon Source Potential of the Miocene Gulf of Suez Syn-Rift Evaporites

Mark Richardson, Michael A. Arthur, James S. Quinn, Jean K. Whelan, Barry J. Katz

The Red Sea rift basin and its northern continuation, the Gulf of Suez, has experienced continuous deposition of marine evaporites throughout much of its development from the early Miocene to the Pliocene (15 m.y.), resulting in the accumulation of up to 5 km of evaporite strata in the rift. Timing of evaporite deposition within the rift is controlled by conventional biostratigraphy where possible. Preliminary investigations indicate that anhydrites from this evaporite sequence preserve original Miocene seawater Sr⁸⁷/Sr⁸⁶ values, which can be compared to Neogene strontium isotope-vs.-time curves to further constrain the age of the nonfossiliferous evaporite group.

Evaporite textures from coastal outcrops and subsurface cores indicate deposition predominantly in a shallow, subaqueous, hypersaline environment. Lithofacies studies of the evaporites show they commonly interfinger with carbonates and clastics along the basin margins. Periodic "freshening" occurred within the basin during deposition of the evaporite series. This freshening is demonstrated by intercalated reef or carbonate-bank deposits.

Certain zones within the evaporite sequence have been found to have very high (up to 30%) organic carbon (OC) content, particularly in thin intrahalite beds consisting of finely laminated clays, magnesite, and anhydrite. High organic contents have also been found in celestite and dolomite-bearing evaporitic carbonates. Elemental and pyrolysis analysis shows that at least some of the relatively OC-rich intervals are composed of well-preserved, oil-prone algal material. Gas chromatograph (GC) and gas chromatograph-mass spectrometer (GCMS) studies indicate that some OC-rich horizons have a subsidiary terrestrial OC source and that deposition occurred in a strongly reducing environment. Evaporite facies deposited in shallow subaqueous environments have much better OC preservation than the laterally and vertically associated evaporites of the more oxygenated sabkha-type environment.

We hypothesize that rapid deposition of organic matter occurred during episodic storms and "freshening events" in which a less saline surface layer developed. Organic matter from both runoff and subsequent biotic "blooms" in this upper, fresher layer was transported with evaporite minerals and clay flocculates to the basin floor without significant mixing or "freshening up" of the main stratified brine body. Subsequent evaporation and sedimentation in the subsiding rift basin aid in preserving the organic matter. By using GC and GCMS techniques in current research, we have "fingerprinted" the potential source rocks in the evaporites by identifying biomarkers specific to hypersaline environments; these results will be compared to similar analyses of crude oil and tar samples.

In the southern Gulf of Suez and northern Red Sea, burial depths and present and paleogeothermal gradients (on the basis of thermal and subsidence modeling studies) are sufficient to place potential evaporite source rocks within the oil windows. Suitable reservoir rocks range from the marginal evaporite equivalent carbonates and clastics to fault-juxtaposed pre-evaporite clastics and basement rocks.

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