Palynofacies and geochemical analyses of the Silurian Tanezzuft Formation, NC115 concession, Murzuq Basin, south-west Libya: Implications for source rock potential and palaeoenvironment

Abstract

A palynofacies and organic geochemical investigation has been undertaken on 46 core and cuttings samples taken from seven wells drilled in the NC115 Concession, Murzuq Basin, south-west Libya. Most of the samples yielded marine and non-marine palynomorphs in a fair to good state of preservation, which together with palynological data have been used to confirm the age, determine the environment of deposition, and evaluate the petroleum source potential of the Tanezzuft Formation. The palynological assemblage recovered from the basal Hot Shale contains an abundance of acritarchs, chitinozoans and cryptospores, with some taxa, such as Plectochitina sp., Evittia remota, and Tetrahedraletes medinensis, indicating an early–mid Rhuddanian (early Llandovery) age for this unit. The deposits overlying the Hot Shale yielded diverse assemblages of palynomorphs, the occurrence of Angochitina qusaibaensis and Leiofusa banderillae in particular indicating a late Rhuddanian–Telychian (Llandovery) age. Two palynofacies associations have been identified. The Hot Shale palynofacies (PF-A) contains Type-II ≥ I kerogen, which is highly oil-prone and reflects a distal suboxic–anoxic sedimentary environment. The palynofacies of the overlying deposits of the Tanezzuft Formation (PF-B) is dominated by Type-II/III kerogen, which is both oil- and gas-prone, and indicates depositional conditions that range from distal dysoxic–oxic shelf to mud-dominated oxic shelf (distal shelf). Rock-Eval pyrolysis results indicate that the Hot Shale is organically-rich with an average total organic carbon (TOC) content 7.23 wt%, whereas the TOC of the overlying deposits averages 0.62 wt%. The organic matter in the Hot Shale has the capability of generating more liquid hydrocarbons than the rest of the formation, which lies within the early oil-maturity window. A simulated burial history for the NC115 Concession allowed the time and magnitude of hydrocarbon generation from the main Silurian Tanezzuft Formation source rock to be determined. Oil generation from this source rock took place during the Carboniferous/mid-Cretaceous period, whereas the maturation level for gas had not been reached at this stage.