Shallow-water evaporitic environments and their source rock potential

Abstract

In the major evaporitic environments on the world's surface today, most organic matter accumulates in shallow, subaqueous to seasonally, subaerially exposed algal-mat sediments. Given the present depositional setting and the most likely flushing by oxidizing water, it is unlikely this organic matter could be preserved to form source rocks. However, if we place such evaporite deposition into a geologic context, then there are times in the past when source rocks could have formed in shallow-water settings. Such settings were characterized by hydrological conditions which allowed the retention of hypersaline, anoxic pore waters to depths where the organic material was buried deeply enough to generate hydrocarbons. When deep-basin, shallow-water evaporite successions were laid down in basins such as the Mediterranean in the Late Miocene, the Michigan Basin in the Silurian, and other large saline giants, then conditions were right for the formation of source rocks within shallow-water and salt-flat evaporitic environments. The evaporites in these saline giants were deposited under conditions of relatively shallow water (< 50 m); the basin never appears to have dried out, but water levels changed quickly ( nearly equal 10,000 years) from shallow to deep. Continual water saturation coupled with saline pore fluids prevented the inflow of fresh, oxidizing groundwater into the basin center of shallow-water, organic-rich evaporites. Immature hydrocarbons derived from such rocks today drip from the 5.5 m.y. evaporites of Sicily in active salt and sulfur mines. Organic-rich sediments could also be preserved to generate hydrocarbons in rapidly subsiding rift basins. In such basins, rapid burial has prevented the entrance of fresher oxygenated waters and the associated degradation and destruction of the organic matter. The early continental riff stage generates the source rocks; the ephemeral streams, wadis, and dune fields generate the reservoirs; and the subsequent evaporite stage seals the reservoir.