Miocene Synrift Evaporites of Red Sea--Their Deposition and Hydrocarbon Source Potential

Mark Richardson, Michael A. Arthur, Barry J. Katz

The Red Sea rift basin and its northern continuation, the Gulf of Suez, underwent almost continuous deposition of marine evaporites during a period of rapid subsidence 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.

Evaporite textures from coastal outcrops and subsurface cores indicate deposition predominantly in a shallow subaqueous hypersaline environment. The evaporites are commonly interfingered 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 contents, particularly in thin intrahalite shale beds. Preliminary geochemical screening shows that these organic carbon-rich deposits are oil-prone. Preservation of organic matter is enhanced in hypersaline environments by the lack of grazing and predation and decreased rates of decomposition in low-oxygen brines. We suggest that the organic matter in intrahalite shales was introduced during episodes of high freshwater runoff or produced in the less saline surface layer that developed during these periods. The low density organic matter would then be transported with clay flocculates, introduced with surface runoff, to the basin floor without significant mixing or "freshening up" f the main stratified brine body. Burial of these organic carbon-rich evaporite sequences within the deeper parts of the graben system and high paleogeothermal gradients may have allowed hydrocarbon generation from these strata.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.