Relationship between different types of evaporitic deposits, and the occurrence of organic-rich layers (potential source-rocks)

Abstract

Close relationships between evaporites and hydrocarbons are well known. Three major types of evaporitic deposits can be distinguished on a basin scale. The hydrocarbon potential of each of them is examined. I) In shelf (epeoric seas can be deep basins) evaporites, where salinity and depositional segregation are synchronous and lateral, the water depth must have been shallow and hence unsuitable for the formation of stratified water bodies. Such deposits generally have a low organic content, with very few associated reefs. II) In basin-center evaporitestats deposition results from successive stages of salinity increasing through time. Limestones form on high areas where the total formation accumulation is comparatively low; simultaneously, within the basin thin organic-rich layers form. Calcium sulfate forms in areas where the depositional sequence has a intermediate thickness and also may be contemporaneous with organic-rich beds in low-lying zones. Finally, halite fills the relatively deep and low-lying central areas, but may then overstep the basin margins. In these successive stages vertical salinity gradients are superposed (stratified systems)-at least during limestone and sulfate deposition. On a map, these facies show all apparent centripetally increasing salinity gradient. When located in a carbonate setting, basin-center evaporites are commonly associated with reefs which may have thrived in a ring-like marginal belt (eg. Silurian Salina, Michigan). Because of the occurrence of thin organic-rich layers, in low areas, and the occurrence of reefs with a salt seal, these basins are often rich in hydrocarbons. III) The last type of evaporitic deposits is represented by basin-margin evaporites. In these, the basin center remains “starved” — even “insatiable” — for a long time. Sedimentation remains marine, commonly characterized by the exclusive occurrence of planktonin and nektonic biota and by a more or less constant elevated organic content. Around the basin, carbonate banks or reefs develop and grade landward through lagoonal facies to terrestrial clastics on the marginal platforms, which are sometimes very extensive. Such deposits show an centrifugal salinity gradient, in contrast to the gradient of basin-center evaporite deposits. The paucity of the most soluble salts is significant and involves a deep brine outflow from evaporitic “annexes” (small peripheral shallow pans) located on back-reef shelves. The flow of water towards these back-reef evaporitic sub-basins results in a permanent nutrient input to the entire basin. The brine outfow enhances. the perenniality of the stratified system. Because of this balance and the system’s perenniality, the organic productivity of such basins can be very high. The deposition of basin margin evaporites with syncheonous organic-rich deposits in the basin center takes place over a long period of time, while the infilling of the central trough by basin-center evaporites can be very rapid and can result in the disappearance of the basin. Shelf evaporites overlapping previous basin and marginal shelves can represent the beginning of a new and independent or separate sedimentary cycle.