Oil families and petroleum geochemistry of the western part of the Sirt Basin Libya

Abstract

This thesis describes a detailed geochemical evaluation of the hydrocarbon potential of source rocks and the origins of the crude oils in the western and central parts of the Sirt Basin in Libya. The Sirt Basin is one of North Africa's richest and most prolific oil-bearing basins, with most of the oil being considered to be derived from the Campanian Sirte Shale and other local source rocks such as Rachmat, Etel and Hagfa Shale formations. The primary aims of this research were to determine the main source rocks that generated petroleum, determine the number of genetically distinct oil families in the basin and compare them with their parent source rocks, and to assess the regional migration and the filling directions of the reservoirs, since this information can exert a profound influence on current and future exploration activities across the study area. A study was undertaken on these source rocks and crude oils using 269 rock cuttings and 51 crude oil samples from several boreholes and oilfields in the Sirt Basin. Routine geochemical analysis in addition to biomarker analysis by gas chromatography-mass spectrometry, compound specific carbon isotopic analysis on n-alkanes and diamondoid analyse were carried out on selected source rock samples and on all of the crude oil samples. The geochemical results demonstrated the presence of various organic-rich zones within the Upper Cretaceous Sirte Shale and Rachmat source rocks. The Sirte Shale Formation is considered to have variable fair, good to very good source potential, and has good hydrocarbon generation in the study area. The Rachmat Formation shale is considered as the second potential source rock in the basin. Vitrinite reflectance, Spore Colour Index, and pyrolysis Tmax data indicate that the Upper Cretaceous shale samples are early to mid-mature in the west of the basin, and middle to late mature in the north central of the basin. Optical analysis of palynofacies slides showed that structureless, amorphous organic matter is dominant, along with the presence of some phytoclasts and reveals moderate to well preserved, fluorescent Type II marine kerogen and Type II-III kerogen. A number of biomarker and other organic facies and maturity indicating molecular marker parameters, as well as isotopic data, show that the crude oils in the western and v central parts of the Sirt Basin are genetically related and only minor variations are present between them, likely due to minor organic facies variations in the Sirt Shale and Rachmat source rocks. The biomarker parameters show dissimilarities between the crude oils in eastern part relative to the western and central part of the basin, due to variations in the organic facies and depositional environments setting of the source rocks or due to higher maturity. Based on molecular marker characteristics, oil-oil correlation identified nine oil families, plus two subfamilies in the study area: Oils from families 1A, 1B, 2, 3 and 4 are situated in the western and central parts of the Sirt Basin, while oil families 5, 6, 7, 8 and 9 are located in the eastern part of the basin. Crude oils of families 1A, 1B, 2, and 3 were interpreted as having been generated from a suboxic to anoxic marine, clay-rich and early to middle maturity source rock. Molecular and other compositional variation between oil families were attributed to organic facies and subtle maturation variations. Age-related biomarker parameters in the oils suggested that their source was Upper Cretaceous. Migration of the generated and expelled oil and gas from the Sirte Shale and Rachmat source rocks to the reservoirs of the Upper Cretaceous-Tertiary petroleum system was interpreted to have occurred along both vertical and lateral pathways along the faults, in the Oligocene to Miocene, while oil carbazole data indicated that this migration was generally likely to have been over relatively short distances.