Evaporite-Hydrocarbon Relationships: The Case of the Laminite-Reef-Evaporite System in the Messinian of the Mediterranean Area

J. M. Rouchy

The peripheral Messinian evaporitic basins of the Mediterranean frequently show the association between pre-evaporitic laminites, carbonate complexes (red algaes, corals, and stromatolites), and evaporites which constitute a sedimentary system, including potential source rocks, reservoirs, and a cover.

The pre-evaporitic laminites (mainly diatomites and carbonates) characterize two different highly productive hydrodynamic systems which follow one another: (1) coastal upwellings during lower Messinian high sea level (transgressive phase), and (2) restriction (semiclosed or land-locked basins) related to closure from the Atlantic and subsequent lower sea level (regressive phase). Preservation of the organic matter results from periodic water stratification: local O₂ minimum intermediary layer and anoxic bottom conditions mainly related to hypersalinity.

Extensive carbonate complexes overlie the shores and the high areas--biogenic buildups (mainly red algaes and poorly diversified corals) and bioclastic accumulations locally overlapped by a stromatolitic blanket. Diagenesis and dissolution-karstification during drawdown periods (hypersaline episodes) increase the primary porosity.

The evaporites constitute an efficient cover. One major property of hypersaline environments is their high primary production whose traces are identified in the Messinian series as organic-rich layers (marine or hypersaline-adapted flora and fauna) and calcareous or gypsified stromatolites. Stratified waters or reducing conditions in the pore waters favor preservation of organic matter. The evaporitic conditions greatly influence the reservoir properties, increasing porosity or destroying it by interstitial precipitation.

Generally, the evaporites lie topographically lower than the earlier reef complexes, but they can overlap the whole sedimentary system, which could lead to interesting oil prospects.

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