Organic facies variability in the Posidonia Black Shale from Luxembourg: Implications for thermal maturation and depositional environment

Abstract

Posidonia Shale (Lias ε, Lower Toarcian, “Schiste bitumineux”) samples were obtained from a well in southern Luxembourg (Esch-sur-Alzette). Elemental composition and organic facies were studied along the cores, and accordingly the thermal maturity and depositional environment were evaluated. The organic matter content is high with an average of 7.2%, ranging between 2.8% and 13.5%. The average vitrinite reflectance value is ca. 0.55%, indicating an early stage of oil generation. Several biomarker based maturity parameters support the low thermal maturity including OEP values and C29 5α, 14α, 17α (H)-steranes 20S/(20S+20R).Rock-Eval data and microscopy observations reveal that the organic matter is composed of hydrogen-rich type II kerogen derived from alginite. Small algal bodies derived from nannoplankton predominate, but large telalginite (Tasmanales) also occurs. Pyrites are abundant and fish bones are frequently observed, characterizing a marine depositional environment where strong sulfate reduction occurred. Small amount of autochthonous vitrinite observed microscopically, combined with predominance of short-chain n-alkanes over long-chain n-alkanes, as well as a low terrigenous/aquatic ratio (TAR), indicate a low input of terrigenous organic matter. Sterane distribution is in good agreement with an origin from a marine carbonate depositional environment which corresponds to the general marlstone lithology. Additionally, C30-24-propyl-14α(H), 17α(H)-cholestane (20R) was detected, supporting the marine origin.The depositional environment was strongly oxygen-depleted in bottom water, as indicated by low concentrations of C35 homohopane, fair to low Pr/Ph ratios (slightly less than 1.0) and high sulfur content (ca. 3wt.% on average) and sulfur/organic carbon ratios, with only few interruptions of dysoxic to suboxic conditions during deposition of the Posidonia Shale in Luxembourg. Gammacerane detected in low abundance reveals that a stratified water column existed and water salinity was slightly enhanced in the paleoenvironment. The highest C27/C29 sterane ratio due to more algal input indicates a maximum flooding during the upper falciferum zone of the Lower Toarcian in Luxembourg, which fits well to the global sea level curve and might correspond to a productivity-oceanic anoxic event (P-OAE). Furthermore, Toarcian greenhouse climate led to generally low oxygen concentration in bottom water of the shallow sea.