DEPOSITIONAL ENVIRONMENT AND SOURCE-ROCK CHARACTERISATION OF ORGANIC-MATTER RICH UPPER SANTONIAN - UPPER CAMPANIAN CARBONATES, NORTHERN LEBANON

Samples of Turonian - upper Campanian fine-grained carbonates (marls, mud- to wackestones; n = 212) from four boreholes near Chekka, northern Lebanon, were analysed to assess their organic matter quantity and quality, and to interpret their depositional environment. Total organic carbon (TOC), total inorganic carbon and total sulphur contents were measured in all samples. A selection of samples were then analysed in more detail using Rock-Eval pyrolysis, maceral analyses, gas chromatography - flame ionization detection (GC-FID), and gas chromatography - mass spectrometry (GC-MS) on aliphatic hydrocarbon extracts. TOC measurements and Rock-Eval pyrolysis indicated the very good source rock potential of a ca. 150 m thick interval within the upper Santonian - upper Campanian succession intercepted by the investigated boreholes, in which samples had average TOC values of 2 wt % and Hydrogen Index values of 510 mgHC/gTOC. The dominance of alginite macerals relative to terrestrial macerals, the composition of C-27-C-29 regular steranes, the elevated C-31 22R homohopane / C-30 hopane ratio (> 0.25), the low terrigenous / aquatic ratio of n-alkanes, as well as C-13(org) values between -29 and -27 parts per thousand together suggest a marine depositional environment and a mainly algal / phytoplanktonic source of organic matter. Redox sensitive geochemical parameters indicate mainly dysoxic depositional conditions. The samples have high Hydrogen Index values (413-610 mg/g TOC) which indicate oil-prone Type II kerogen. T-max values (414 - 432 degrees C) are consistent with other maturity parameters such as vitrinite reflectance (0.25-0.4% VRr) as well as sterane and hopane isomerisation ratios, and indicate that the organic matter is thermally immature and has not reached the oil window. This study contributes to the relatively scarce geochemical information for the eastern margin of the Levant Basin, but extrapolation of the data to offshore areas remains uncertain.