HYDROCARBON SOURCE ROCKS IN EVAPORITIC SYSTEMS: EXPLANATION AND CLASSIFICATION

There are 4 major mesohaline settings where organic-rich laminites (source rocks) accumulate in saline settings.

Style 1: Basin-centre or basinwide mesohaline source rocks; A) Basin-centre drawdown laminites, typically sealed by basinwide salts. Pervasive evaporite plugging means accumulations sourced from this style are relatively small scale (Silurian-Michigan Basin). B) Basin-centre laminites where the basin margin is a shallow, restricted, at times evaporitic, platform, which surrounds a slightly mesohaline deepwater starved-basin centre. Deep-basin bottom waters were never drawn down or concentrated to where basinwide salts precipitated. (Purbeck/Kimmeridge association)

Style 2:Local, sub-wavebase, density-stratified mesohaline intrashelf depressions. This is the dominant style in the prolific Jurassic source rocks of the Middle East. Another variation are the phosphate and organic-rich sediments deposited in salinity-stratified shelf edge depressions of epeiric sea platform-edge areas of marine upwelling.

Style 3: Lacustrine source rocks in sediment-starved or underfilled saline lakes. Most oil-prone style 3 lacustrine source rocks were deposited at the bottom of perennial salinity-stratified hydrologies, with water column depths measured in metres to tens of metres, not as subaerially exposed algal mudflats (Eocene Green River; Oligocene mudstones China).

Style 4) As yet poorly documented open-marine salt allochthon setting where density-stratified brine lakes accumulate organic-enriched laminites in anoxic depressions atop and adjacent to shallow and dissolving salt (Triassic-Cretaceous Northern Africa; Tertiary of Gulf of Mexico)

Basin styles 1a, 2 and 3) are typified by mm-scale oil-prone organic-rich laminites, typically with a micrite matrix. Style 1b typically retains higher levels of siliciclastic fines (basinal marl or a laminated siliciclastic shale). Matrix of style 4 laminites tends to be composed of whatever detrital sediment is accumulating atop the allochthon. In the Gulf of Mexico and the Mediterranean salt allochthon terranes, matrix is a siliciclastic mud/clay. In the Jurassic and Cretaceous of the Arabian Gulf and the Triassic of Tunisia, the matrix is hemipelagic micrite.