Sedimentary facies, organic facies, and hydrocarbon generation in evaporite sediments of the Mulhouse Basin, France

The sediments of the S unit (lower Oligocene) of the Mulhouse Basin, France, display lithofacies characteristic for the deposition in a perennial evaporitic lake that received frequent marine influx. The sediments of the S unit consist of marls, anhydrites (deposited as gypsum), and halite.;The organic content of these sediments stems from algal and most likely bacterial sources. Terrigenous, plant-derived organic matter comprises on average less than 10% of the total organic matter. The mass of the organic matter is concentrated in the marl lithofacies (up to 4.5% TOC), which display a varve-like lamination. The anhydrite and halite lithofacies contain only minor amounts of organic matter (on average less than 0.5% TOC). Based on the maceral content and the molecular composition of the extractable organic matter, two organic facies have been identified (A and B). The organic facies correlate with distinct lithofacies. It therefore appears that the deposition of sediments and their organic content was governed by the physical conditions of the lake.;The accumulation of organic matter-rich sediments in the S unit of the Mulhouse Basin is thought to have been favored by a high paleoproductivity and good to excellent preservation conditions. Low sedimentation rates in conjunction with elevated salinities led to the accumulation of marls rich in organic matter.;The kerogens of the S unit can be classified as type II. The organic matter from the sites Amelie II and Berrwiller is immature and corresponds to a maturity level of 0.35 and 0.45% vitrinite reflectance, respectively. Petroleum formation resulted in significantly higher amounts of bitumen as expected from shale source rocks of this maturity level. The bitumens are dominated by asphaltenes and NSO-compounds and contain less than 50% hydrocarbons.;In the maturity interval from 0.35-0.45% R;The massive anhydrites of the S unit have already expelled hydrocarbons. The naphthalene patterns of this lithofacies show fractionation effects that resulted from water washing during the phase transition from gypsum to anhydrite.