Basin modelling of the Gabes-Tripoli basin and geology of the Farwah Group reservoirs, Western Offshore, Libya

• Main Author: Mriheel, Ibrahim Youssef
• Published: University of Manchester 2000
• Subjects: 556.12
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496686

The Gabes-Tripoli Basin (G-T Basin) is a Mesozoic-Cenozoic basin which was initiated as a result of widespread, late Triassic-Middle Jurassic extensional movements that developed over a broad zone of strain between the African and European plates. The basin formation is probably related to strike slip movement between the western Libyan Coastal Fault System and the north margin of the basin. The basin rifting phase of lithospheric extension lasted as much as 40my. The dominant driving mechanism of subsidence seems clearly to have been subsidence due to cooling following lithospheric thinning and the tectonic subsidence history shows that a simple stretching model successfully predicts the overall characteristics of the long-term patterns of the tectonic subsidence of the basin. The sedimentary succession in the G-T Basin ranges in age from Triassic-Recent. It comprises a 10 km-thick succession of pre-rift Early-Middle Triassic, nonmarine and marine clastics, syn-rift late Triassic-Middle Jurassic, predominantly shallow marine carbonates and evaporites and Middle Jurassic-Recent post-rift marine carbonates and clastics. The tectono-stratigraphic units comprise 19 sequences on the time scale of second order sequences. For most sequences and sequence boundaries, either an eustatic or tectonically enhanced origin can be established. The analysis of the basin-fill history of the G-T Basin from the Triassic until the Holocene reveals that the basin underwent development from a continental sedimentary basin located on Gondwana to an epicratonic rift basin. When extensional movement ceased (middle Middle Jurassic), the basin subsided thermally and developed as part of a passive continental margin on the north African plate margin. The variation of heat flow over the G-T Basin from higher values at the basin centre to lower values toward the southern and northern margins is consistent with the calculated amount of crustal attenuation. Hence, the proposed stretching model is considered as paramount to the understanding of basin evolution and hydrocarbon accumulation. The G-T Basin is a passive groundwater basin and thermal conduction is the dominant mechanism of heat transport. Heat flow values range from 50-65 mW/m , typical values for granitic basement and are characteristic of the passive margin nature or thermally subsided postrift basins. The geothermal gradient of the G-T Basin ranges from 32.5 0C/km. to 45 OC/km. The geothermal gradient map shows relatively high average regional gradient at the centre of the basin and a progressive decrease from the depocentre to the basin margins. The high geothermal gradient zone at the centre of the basin coincides roughly with the zones of maximum crustal thinning in the basin. The observed organic thermal maturity measurements in the G-T Basin indicate that the pre Middle Eocene sequences are at a mature to over mature stage. The Middle-Upper Eocene Tellil sequence has attained an early mature stage while locally the Oligocene-early Miocene is in maturation stage. The rest of the Tertiary sequence is immature. The depth variation to the modelled top of the oil maturity 0.7 Ro ranges from 2000-2400m in the G-T Basin. The calculated organic maturities indicate that the early Eocene-late Cretaceous sequences are either mature or over mature with respect to the oil window. The current depth to the top of the oil window ranges from 2000 to 2400m, and the base of the oil window ranges from 3000 to 3650m. Combined geohistory and basin modelling indicates that the main phase of hydrocarbon generation from the Farwah Group source began 22.5 to 5.0 Ma. and continues until present. A relatively late generation (approximately 30-50 my after deposition) is ascribed to the lack of high palaeoheat flow and moderate burial, which is consistent with passive margin nature and post-rift thermally subsiding basin. Time of oil generation varies widely from the depocentre to the northern and southern margins of the basin depending on the heat flow variations. Geochemical analysis and basin modelling have confirmed the source potentiality of the early Eocene-late Cretaceous sequences to generate and expel hydrocarbon in the study area. Hydrocarbon has been generated in a wide span of time from several proven late Cretaceous- Palaeogene organic rich sources. But none of the sequences younger than the early Eocene had the capability to expel out hydrocarbon in the basin. Hydrocarbon was generated from the Al Jurf and Farwah sources in the basin centre first and earlier than the basin margins by about 15my. Hydrocarbon generation commenced about 30my from the Al Jurf Formation and at 22.5my from the Farwah source in the basin centre. At the basin margins, however, it began to generate 15my later from both principal sources. Thus, the earlier generated hydrocarbon at the basin centre has been subjected to secondary thermal cracking and as a result, huge gas accumulations have been discovered at the central parts of the G-T Basin. The Jirani Dolomite was formed during relative fall in sea-level in middle Ypresian (early Eocene) times, in shallow, hypersaline lagoons on a restricted shallow shelf. Petrographic studies , show that dolornitization proceeded in three stages. Stage I involved penecontemporaneous/early diagenetic dolornitization under hypersaline seepage reflux conditions. Stage II dolornitization, mainly confined to the non-anhydritic facies, was probably formed in the mixing zone between meteoric and seawater, probably at shallow depths of burial. Stage III dolornitization, which is volumetrically unimportant occurred at depth in the late stage of basin evolution, causing some filling of mouldic and vuggy porosity by medium crystalline, saddle dolomite. Evidence of mixing zone dolornitization indicates that the Jirani Dolomite was exposed subaerially. Exposure of the deposits to flushing by meteoric waters explains the dissolution of large portions of the anhydrite from the non-anhydritic dolomite facies, and the development of excellent reservoir characters in this facies, which make it one of the most important reservoir rocks in the offshore region. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity are of both predolornitization and syndolornitization origins but the later appears to be the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics.