Tanezzuft Formation Research Articles

Mineralogical and geochemical changes in subsurface shales straddling the Ordovician–Silurian boundary in the eastern Kufra Basin, Libya

This paper reports on the elemental geochemistry of 47 shale samples from the uppermost Hirnantian–lowermost Rhuddanian Tanezzuft Formation, collected from drill core (depth interval 20.8–73.2 m) from borehole JA-2 at the eastern Kufra Basin, SE Libya. Eighteen of these samples were also analysed by X-ray diffraction. This study was carried out (i) to search for shales with high total organic carbon (TOC >3 wt%) and associated high uranium (U > 30 ppm) concentrations, commonly referred to as ‘hot’ shales, (ii) to test whether TOC and U correlate, (iii) to study the effects of surface weathering on the behaviour and mobility of major oxides, trace elements and rare earth elements (REE), and (iv) to examine the relationship of the elemental composition obtained in this study and Rock-Eval pyrolysis and kerogen data obtained in previous studies on the same core samples. The studied core can be divided into three intervals: upper weathered section (20.8–46.5 m, influenced by Quaternary weathering), unweathered section (46.5–68.5 m), and lower weathered section (68.5–73.9 m, influenced by latest Ordovician weathering). Overall, the shales have low TOC values (<1 wt%) and low U concentrations (<12 ppm). TOC and U do not show any correlation likely due to their low values. Hence, ‘hot’ shale has not yet been proven in this part of the Kufra Basin. Trace elements such as Mn and Sc show significant changes from the weathered into the unweathered core section. In contrast, the majority of the major oxides, trace elements and REE seem to be unaffected or at least largely unaffected by weathering processes. The exception is the transition from the unweathered section into the lower weathered section of the core. Here, most of the major oxides, trace elements and REE show either abrupt or gradual shifts towards lower or higher element concentrations. The observed whole-rock geochemical (e.g., K2O, Rb, Sr, Th/K) and mineralogical (e.g., kaolinite/illite, K-feldspar/quartz) changes suggest either a switch from one source area to another or, more likely, climatic influence. The deepest part of the core may have been influenced by a more humid and warm climate that has led to intensive chemical weathering and dissolution of K-feldspar. During the latest Hirnantian–earliest Rhuddanian a temporary emersion event may have occurred due to post-glacial rebound, which has led to the weathering of older strata.

Estimating ancient seawater isotope compositions and global ocean redox conditions by coupling the molybdenum and uranium isotope systems of euxinic organic-rich mudrocks

The sedimentary Mo and U isotope systems have been commonly used as novel global ocean redox tracers due to their long oceanic residence times and redox-sensitive behavior. However, local sedimentary environments and global ocean redox conditions both influence the Mo and U isotope compositions of euxinic organic-rich mudrocks (ORM). Here, we further develop the coupled use of Mo and U isotope data from euxinic ORM to more robustly infer coeval global ocean redox conditions. We measured δ238U from eight late Neoproterozoic to middle Paleozoic ORM units that have previously reported Mo isotope and Fe speciation data. Integration of our new data with previously published Proterozoic and Phanerozoic Mo and U isotope data reveals that there is no overall correlation between the Mo and U isotope compositions of euxinic ORM. This observation confirms that the extent to which local versus global environments influenced the preserved Mo and U isotope compositions in ORM was variable. Individual ORM units can have negative, positive, or no correlation between δ98Mo and δ238U. A negative correlation between δ98Mo and δ238U in the Upper Devonian Kettle Point Formation is similar to the observations from modern euxinic basins, reflecting a major control on the Mo-U isotope systematics by changes in the local depositional environment, such as bottom-water sulfide concentrations. A positive correlation between δ98Mo and δ238U observed in the Upper Ordovician Fjäcka Shale is best explained by changes in global ocean redox conditions that simultaneously shifted the Mo and U isotope compositions of the global seawater and the Fjäcka Shale ORMs in the same direction. No correlations between δ98Mo and δ238U for euxinic ORM may be caused by specific local depositional changes, a lack of or a combination of local and global environmental changes, and/or is an artifact of limited data. For example, a vertical trend (variable δ98Mo but similar δ238U) is shown by most samples from Member IV of the Ediacaran Doushantuo Formation, implying a strong influence on the Mo isotope data by an Fe-Mn particulate shuttle. A horizontal trend (similar δ98Mo but variable δ238U) is observed from the Paleoproterozoic Zaonega Formation, implying that relatively constant bottom water sulfide concentrations caused similar magnitudes of Mo isotope fractionations whereas other factors (e.g., U reduction pathways, aqueous U species, productivity) were responsible for variable U isotope fractionations. Relatively constant elemental concentrations and isotope compositions from the Tanezzuft Formation are indicative of stable conditions at local and global scales. We further propose a method to estimate the coeval seawater Mo and U isotope compositions based on a coupled Mo-U isotope mass balance model and the observations from modern euxinic basins. The coupled Mo-U isotope data from euxinic ORMs provide more insights on the local and global environmental controls on the preservation of both isotope systems than previously realized. Our study highlights the importance of examining the local depositional environment and using large datasets of coupled Mo-U isotope compositions from euxinic ORM intervals to reconstruct paleocean redox conditions.

Source Rock Characterization of Silurian Tanezzuft and Devonian Awaynat Wanin Formations the Northern Edge of the Murzuq Basin, South West Libya

Drill cutting samples (n = 92) from the Devonian Awaynat Wanin Formation and Silurian Tanezzuft Formation, sampled from three wells F1, G1 and H1, locate in the northern edge of the Murzuq basin (approximately 700 kilometers south of Tripoli). The studied samples were analyzed in the objective of their organic geochemical assessment such as the type of organic matter, depositional conditions and thermal maturity level. A bulk geochemical parameters and precise biomarkers were estimated, using chromatography-mass spectrometry (GC-MS) to reveal a diversity of their geochemical characterizations. The rock formations are having varied organic matter contents, ranged from fair to excellent. The total organic carbon (TOC) reached about 9.1 wt%, ranging from 0.6 to 2.93 wt% (Awaynat Wanin), 0.5 to 2.54 wt% (Tanezzuft) and 0.52 to 9.1 wt% (Hot Shale). The cutting samples are ranged oil-prone organic matter (OM) of hydrogen index (HI) ranged between 98 –396 mg HC/g TOC, related kerogen types are type II and II/III, with oxygen index (OI): 6 - 190 with one sample have value of 366 mg CO2/g. Thermal maturity of these source rocks is different, ranging from immature to mature and oil window in the most of Tanezzuft Formation and Hot Shale samples, as reflected from the production index data (PI: 0.08 - 034). Tmax and vitrinite reflectance Ro% data (Tmax: 435 – 454 &amp; Ro%: 0.46 - 1.38) for the Awaynat Wanin. Biomarker ratios of specific hydrocarbons extracted from represented samples (n = 9), were moreover used to study thermal maturity level and depositional environments. Pristine/Phytane (Pr/Ph) ratios of 1.65 - 2.23 indicated anoxic to suboxic conditions of depositional marine shale and lacustrine source rock.

Petroleum Geochemistry Regional Study of Murzuq Basin: Insights from Biomarkers Characteristic, Stable Carbon Isotope and Environmental Characterization

This search aims to apply developed geochemical methods to a number of oils and source rock extracts to better establish the features of ancient environments that occurred in the Murzuq basin. Geochemical and geophysical approaches were used to confirm further a source contribution from other Paleozoic formations to hydrocarbon accumulations in the basin. One hundred and forty rock units were collected from B1-NC151, D1-NC174, A1-NC 76, D1-NC 151, F1-NC58, A1-NC 186, P1-NC 101, D1-NC 58, H1-NC58 and A1-NC58 wells. Seven crude oils were collocated A1-NC186, B1-NC186, E2-NC101, F3-NC174, A10-NC115, B10-NC115 and H10-NC115 wells. A geochemical assessment of the studied rocks and oils was done by means of geochemical parameters of total organic carbon (TOC), Rock-Eval analysis, detailed-various biomarkers and stable carbon isotope. The TOC values from B1-NC151 range 0.40% to 8.5%, A1-NC186 0.3% and 1.45, A1-NC76 0.39% to 0.74%, D1-NC151 0.40% to 2.00% to F1-NC58 0.40% to 1.12%. D1_NC174 0.30% to 10 %, P1-NC101 0.80% to 1.35%, D1-NC58 0.5% to 1.10%, H1-NC58 0.20% to 3.50%, A1-NC58 0.40% to 1.60%. The categories of organic matter from rock-eval pyrolysis statistics point to that type II kerogen is the main type, in association with type III, and no of type I kerogen recognized. Vitrinite reflectance (%Ro), Tmax and Spore colour index (SCI) as thermal maturity parameters reflect that the measured rock units are have different maturation levels, ranging from immature to mature sources. acritarchs distribution for most samples could be recognized and Palynomorphs are uncommon. Pristane to phytane ratios (&gt; 1) revealed marine shale to lacustrine of environmental deposition. The Stable carbon isotope ( δ 13 C) values of seven rock-extract samples are -30.98‰ and -29.14‰ of saturates and -29.86‰ to -28.37‰ aromatic fractions. The oil saturate hydrocarbon fractions range between -29.36‰ to -28.67‰ and aromatic are among -29.98 ‰ to -29.55 ‰. The δ 13 C data in both rock extractions and crude oils are closer to each other, typical in sign of Paleozoic age. It is clear that the base of Tanezzuft Formation (Hot shale) is considered the main source rocks. The Devonian Awaynat Wanin Formation as well locally holds sufficient oil prone kerogen to consider as potential source rocks. Ordovician Mamuniyat Formation shales may poorly contain oil prone kerogen to be addressed in future studies. An assessment of the correlations between the oils and potential source rocks and between the oils themselves indicated that most of the rocks extracts were broadly similar to most of the oils and supported by carbon stable isotope analysis results.

Shale oil and gas exploration potential in the Tanezzuft Formation, Ghadames Basin, North Africa

The Silurian Tanezzuft Formation ‘hot shale’ in North Africa is a high-quality source rock and a major contributor to the oil and gas reserves of the Paleozoic Ghadames Basin. This hot shale has similar sedimentary characteristics to those of the Silurian Longmaxi Formation shale in the Sichuan Basin in China, which is a proven prolific source of shale gas. In this study, the oil and gas accumulation conditions and sedimentary characteristics of the Tanezzuft shale are compared with those of the commercially exploited Longmaxi shale and the Marcellus and Barnett shales (North America), and criteria for shale oil and gas potential in the Tanezzuft shale are established from these commercially exploited shales. For the Tanezzuft shale, the net pay thickness of effective source rock, total organic carbon, thermal maturity and burial depth, and the locations of faults in the Ghadames Basin are analyzed and mapped. The distributions of these variables are then used to predict the distribution of shale oil and gas with respect to the established criteria. It is shown that the Tanezzuft shale has significant potential for the production of shale oil and gas; in particular, the northeastern and southwestern slopes of the Ghadames Basin are the most favorable in terms of prospective shale oil and gas exploration.

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

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.

Palynostratigraphic, palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).

Methods for source rock identification on seismic data: An example from the Tanezzuft Formation (Tunisia)

Source rocks occurrence, extension and quality in the subsurface are usually characterized directly from well data. Seismic characterization is however becoming an emerging technique for extending source rock properties beyond well locations and an effective tool for mapping their lateral and vertical heterogeneities. The Tanezzuft Formation in Tunisia, an extremely organic-rich source rock, was used as a test for applying and comparing different seismic methods aimed at organic-rich sequences characterization. The data used consist of a 2D seismic line shot in the northern flank of the Ghadames Basin (Tunisia), pre-stack gathers extracted from an inline of a 3D survey and four wells with a complete set of wireline and petrophysical logs. Impedance inversion, AVO and seismic attributes were computed on the seismic lines and compared to well data. Impedance inversion together with a rock physics model of the formation generated a quantitative estimate of total organic carbon (TOC) content in the studied 2D seismic line. AVO analysis showed a Class IV and a Base Class IV response for the upper and lower reflectors delimiting the organic-rich interval respectively. Seismic attributes analyzed the formation by exploiting its high impedance contrast and intrinsic anelastic attenuation. The three methods here discussed showed coherent results both when compared each other and when compared with the well data and the regional geological model. In particular, AVO and seismic attribute analysis are methods that can be applied without the need of well information, and are then feasible methods for the investigation of source rock properties in undrilled basins.

ORGANIC GEOCHEMISTRY OF THE LOWER SILURIAN TANEZZUFT FORMATION AND BIOMARKER CHARACTERISTICS OF CRUDE OILS FROM THE GHADAMES BASIN, LIBYA

The Ghadames Basin of NW Libya contains more than 10 B brls oil‐equivalent in Palaeozoic siliciclastic reservoirs which are charged by organic‐rich “hot shales” in the Lower Silurian (Rhuddanian) Tanezzuft Formation. Geochemical analysis of 85 shale samples and ten oils from three fields (NC2, NC4 and NC7) in the central and northern part of the basin provides a robust description of the Tanezzuft – Mamuniyat/Acacus petroleum system in this region, and of the associated source facies and oil families. The shale samples underwent total organic carbon (TOC) analysis and Rock‐Eval pyrolysis, and the ten crude oil samples were analysed by gas chromatography – flame ionization detection (GC‐FID) and GC‐mass spectrometry (GC‐MS) of the saturated and aromatic hydrocarbon fractions, accompanied by stable carbon isotopic analysis. Organic matter ranges from Type II to mixed Type II/III kerogen with varying oil and gas generation potential in the early to main stages of the oil generation window.The analysed oils are characterised by low sulphur, nickel and vanadium contents, and relatively high API gravity (34.9–46.8oAPI). Biomarkers suggest that they were generated from marine shales containing abundant Type II to mixed Type II/III kerogen deposited in mildly anoxic – suboxic conditions with a dominance of C29 over C27 or C28 steranes, indicative of a source rich in brown algae and cyanobacteria. Their close stratigraphic association and biomarker characteristics support a Tanezzuft “hot shales” source for the Mamuniyat and Acacus oils in the three fields. Maturity‐related parameters based on terpanes, steranes, aromatic hydrocarbons and low molecular‐weight hydrocarbons, suggest generation from marine shales in the early to peak oil‐generation window. The ZE3‐NC7, A19‐NC7 and A20 NC7 oils from the NC7 oil field are significantly more mature than those from the NC2 and NC4 fields. This indicates a difference in expulsion timing and may be related to the presence of two “hot shales” or to fault‐triggered vertical migration preceding and during the Neogene.

Organic geochemistry of the Silurian Tanezzuft Formation and crude oils, NC115 Concession, Murzuq Basin, southwest Libya

Thirty-six Silurian core and cuttings samples and 10 crude oil samples from Ordovician reservoirs in the NC115 Concession, Murzuq Basin, southwest Libya were studied by organic geochemical methods to determine source rock organic facies, conditions of deposition, thermal maturity and genetic relationships. The Lower Silurian Hot Shale at the base of the Tanezzuft Formation is a high-quality oil/gas-prone source rock that is currently within the early oil maturity window. The overall average TOC content of the Hot Shale is 7.2 wt% with a maximum recorded value of 20.9 wt%. By contrast, the overlying deposits of the Tanezzuft Formation have an average TOC of 0.6 wt% and a maximum value of 1.1 wt%. The organic matter in the Hot Shale consists predominantly of mixed algal and terrigenous Type-II/III kerogen, whereas the rest of the formation is dominated by terrigenous Type-III organic matter with some Type II/III kerogen. Oils from the A-, B- and H-oil fields in the NC115 Concession were almost certainly derived from marine shale source rocks that contained mixed algal and terrigenous organic input reflecting deposition under suboxic to anoxic conditions. The oils are light and sweet, and despite being similar, were almost certainly derived from different facies and maturation levels within mature source rocks. The B-oils were generated from slightly less mature source rocks than the others. Based on hierarchical cluster analysis (HCA), principal component analysis (PCA), selected source-related biomarkers and stable carbon isotope ratios, the NC115 oils can be divided into two genetic families: Family-I oils from Ordovician Mamuniyat reservoirs were probably derived from older Palaeozoic source rocks, whereas Family-II oils from Ordovician Mamuniyat–Hawaz reservoirs were probably charged from a younger Palaeozoic source of relatively high maturity. A third family appears to be a mixture of the two, but is most similar to Family-II oils. These oil families were derived from one proven mature source rock, the Early Silurian, Rhuddanian Hot Shale. There is a good correlation between the Family-II and -III oils and the Hot Shale based on carbon isotope compositions. Saturated and aromatic maturity parameters indicate that these oils were generated from a source rock of considerably higher maturity than the examined rock samples. The results imply that the oils originated from more mature source rocks outside the NC115 Concession and migrated to their current positions after generation.

Hydrocarbon source potential of the Tanezzuft Formation, Murzuq Basin, south-west Libya: An organic geochemical approach

A detailed organic geochemical study of 20 core and cuttings samples collected from the Silurian Tanezzuft Formation, Murzuq Basin, in the south-western part of Libya has demonstrated the advantages of pyrolysis geochemical methods for evaluating the source-rock potential of this geological unit. Rock-Eval pyrolysis results indicate a wide variation in source richness and quality. The basal Hot Shale samples proved to contain abundant immature to early mature kerogen type II/III (oil–gas prone) that had been deposited in a marine environment under terrigenous influence, implying good to excellent source rocks. Strata above the Hot Shale yielded a mixture of terrigenous and marine type III/II kerogen (gas–oil prone) at the same maturity level as the Hot Shale, indicating the presence of only poor to fair source rocks.

Hot shale in an ice world: Paleoceanographic evolution of the northern Gondwana margin during the early Paleozoic (Tanezzuft Formation, Tunisia)

The Tanezzuft Formation deposited in marine periglacial conditions on the northern Gondwana margin during the end of the “ice-house” climate that characterized the lowermost Silurian. The basal part of this sedimentary sequence is characterized by organic-rich facies with locally very high measured Total Organic Carbon (TOC) content up to values greater than 20%. While deposition of organic-rich sediments during greenhouse time interval is well known, deposition of black shales during ice-house conditions is poorly documented. The extraordinary paleoceanographic conditions that led to the accumulation and preservation of enormous amounts of organic matter in periglacial settings, makes this formation an atypical example of black shales deposition.The study area is situated on the North African Platform in southern Tunisia, on the northern flank of the Ghadames Basin. Petrophysical logs, biostratigraphic, organic- and inorganic-geochemical data from nine wells, are here integrated with the aim of reconstructing the depositional history of the Tanezzuft Formation and the role played by organic matter production, preservation and dilution through time. Data indicate that dilution – that is, depositional style and framework – had an important control on lateral and vertical distribution of the C organic-rich facies. TOC and Hydrogen Index (HI) maxima are found in correspondence of the main transgressive phases, with repetitive stacking patterns strongly associated with source rock properties. Organic matter production, as observed by detailed palynofacies analysis, was mainly marine in nature, with important contribution by Amorphous Organic Matter, Leiosphaeridia/Tasmanaceae and minor amounts of graptolites fragments. Geochemical data indicate that the organic-richest interval (“Hot Shale”) of the Tanezzuft Formation deposited under severe anoxic conditions that resulted in enhanced organic matter preservation at the sea-floor. OF-Mod 3D, an organic facies modelling software tool (by SINTEF), is used in order to reconstruct and quantify the peculiar processes that controlled the exceptional accumulation of organic matter at the time of deposition. Modelling results indicate that water stratification was the most plausible process that drove organic-matter sedimentation under mesotrophic conditions. Density stratification was probably controlled by the deglaciation phase after the Hirnantian glaciation and/or by the flooded complex basin physiography that induced an inefficient circulation of the water masses. The complete recovery from these extreme paleoceanographic conditions implied progressive processes that took several hundred thousand years.

The search for ‘hot shales’ in the western Kufra Basin, Libya: geochemical and mineralogical characterisation of outcrops, and insights into latest Ordovician climate

Across the Saharan platform, mudrocks of latest Ordovician–Silurian age (the Tanezzuft Formation) are a major source rock interval for Palaeozoic petroleum systems, but source rock quality is variable and difficult to predict. In the Kufra Basin of southern Libya, evidence for organic enrichment in this formation is scarce. This paper presents the results of a spectral gamma-ray study of siliciclastic sedimentary rocks in Jabal Eghei at the western margin of the basin. The study spans the Ordovician–Silurian interval together with overlying Mesozoic strata and was conducted at outcrop using a hand-held gamma-ray spectrometer. Element concentrations of potassium (K), uranium (U) and thorium (Th) were collected from mudrock and sandstone successions to identify natural background values of these elements for different formations. Uranium contents were used to identify possible ‘hot shales’ with source rock potential. This study shows that sandstones and mudrocks are clearly discriminated by their K, U and Th contents. Most sandstones have 0.3 % of K, >4 ppm of U and >10 ppm of Th. Based on gamma-ray spectrometry, the Tanezzuft Formation is divisible into lower, middle and upper members. There is a significant difference in K content between the three members. Exposures of the lower member show K contents less than 0.5 %; those from the middle member are between 1.5 and 3.6 %; and the upper member has K contents between 0.5 and 1.5 %. Notably, U values >10 ppm are restricted to the upper member, indicating the presence of ‘warm shales’ in Jabal Eghei. There is a progressive increase of Th/K ratios from the basal part of the Tanezzuft Formation towards the upper member. The decrease in K content and increase in Th/K ratios between the middle and the upper members of the Tanezzuft Formation can be explained by a mineralogical change, confirmed by x-ray diffraction analyses on representative samples. The total volume of clay minerals is 64 % higher in the upper member than in the middle member, with kaolinite predominant. These changes in composition suggest a change towards a more weathered sediment source, which was probably caused by increased continental weathering due to climate change from icehouse to greenhouse conditions at the Ordovician–Silurian transition.

New insight on petroleum system modeling of Ghadames basin, Libya

Underdown and Redfern (2008) performed a detailed petroleum system modeling of the Ghadames basin along an E–W section. However, hydrocarbon generation, migration and accumulation changes significantly across the basin due to complex geological history. Therefore, a single section can't be considered representative for the whole basin. This study aims at bridging this gap by performing petroleum system modeling along a N–S section and provides new insights on source rock maturation, generation and migration of the hydrocarbons using 2D basin modeling. This study in conjunction with earlier work provides a 3D context of petroleum system modeling in the Ghadames basin.Hydrocarbon generation from the lower Silurian Tanezzuft formation and the Upper Devonian Aouinet Ouenine started during the late Carboniferous. However, high subsidence rate during middle to late Cretaceous and elevated heat flow in Cenozoic had maximum impact on source rock transformation and hydrocarbon generation whereas large-scale uplift and erosion during Alpine orogeny has significant impact on migration and accumulation. Visible migration observed along faults, which reactivated during Austrian unconformity. Peak hydrocarbon expulsion reached during Oligocene for both the Tanezzuft and the Aouinet Ouenine source rocks. Based on modeling results, capillary entry pressure driven downward expulsion of hydrocarbons from the lower Silurian Tanezzuft formation to the underlying Bir Tlacsin formation observed during middle Cretaceous.Kinetic modeling has helped to model hydrocarbon composition and distribution of generated hydrocarbons from both the source rocks. Application of source to reservoir tracking technology suggest some accumulations at shallow stratigraphic level has received hydrocarbons from both the Tanezzuft and Aouinet Ouenine source rocks, implying charge mixing. Five petroleum systems identified based on source to reservoir correlation technology in Petromod*. This Study builds upon the original work of Underdown and Redfern, 2008 and offers new insights and interpretation of the data.

BIOMARKER CHARACTERISTICS OF POTENTIAL SOURCE ROCKS IN THE JABAL NAFUSAH AREA, NW LIBYA: PETROLEUM SYSTEMS SIGNIFICANCE

The Jifarah Arch of NW Libya is a structurally prominent feature at the eastern end of the regional Talemzane Arch, separating the Ghadamis hydrocarbon province to the south from the offshore Pelagian province to the north. The Arch has experienced a complex structural history with repeated episodes of uplift, exhumation and burial. This paper provides a provisional assessment of its hydrocarbon habitat based on detailed geochemical analyses of potential Triassic, Silurian and Ordovician source rocks encountered by wells drilled in the area.Twenty‐seven core and cuttings samples of marine shales were collected from eight widely‐ dispersed wells and analyzed using standard Rock‐Eval pyrolysis techniques. Kerogen types II‐III were identified in the majority of Triassic samples analysed, indicating a low hydrocarbon generation potential, but oil‐prone Type II kerogen was found in the basal Silurian Tanezzuft Formation and Ordovician Memouniat Formation. The presence of steranes and acyclic isoprenoids suggested variable inputs of algal, bacterial and terrestrial organic matter, while biomarkers including C30‐gammacerane and β–carotene and selected biomarker ratios (Pr/Ph ratio and homohopane index) were used to assess their depositional environment. Results indicate that extended zones with periodic (if not continuous) oxygen‐deficient conditions existed throughout the basin during Late Ordovician and Early Silurian time, favouring the preservation of organic matter.The thermal maturity of the samples was assessed by Rock‐Eval pyrolysis, zooclast reflectance, molecular ratios including C32‐22S/(22S+22R)‐homohopanes, Ts/(Ts+Tm), C29‐steranes and parameters based on the relative abundance of methylphenanthrene, methyldibenzothiophene and methylnaphthalene isomers. The results indicate significant variability in thermal maturity, with Ordovician and Silurian source rocks ranging from 0.6% to 0.7% VRo equivalent increasing to 1.0% locally. These values represent palaeo‐maturities achieved at different times in the past and are considered too low to have generated significant volumes of hydrocarbons directly. However the downdip equivalents of these source rocks in the adjacent Ghadamis Basin contributed to prolific petroleum systems. The absence of large petroleum accumulations on the Jifarah Arch contrasts with the western part of the geologically similar Talemzane Arch, which harbours several giant and supergiant oil and gas fields. This difference is attributed both to the complex structural history of the Jifarah Arch, which permitted post‐charge leakage of palaeo‐accumulations, and stratigraphic migration barriers which restricted migration between Tanezzuft source rocks and Ordovician and Triassic reservoirs.

Thermal history of the Murzuq Basin, Libya, and generation of hydrocarbons in its source rocks

The history of burial, temperature variations, and organic maturation in the sedimentary rocks of the Murzuq Basin in southwestern Libya was numerically reconstructed for eight wells and one pseudowell along northeast-southwest and northwest-southeast profiles across the basin. The reconstruction was performed using the GALO system for basin modeling taking into account that the basin lithosphere underwent repeated tectonic and thermal activation. The modeling allowed us to refine the reconstructions of the thermal history of the basin and assessment of its hydrocarbon potential obtained by previous models, which assumed a constant temperature gradient during the whole period of basin development. The Murzuq Basin is characterized by moderate basement subsidence (2200–2800 m), which would have corresponded in an ordinary basin to immature or early mature organic matter. However, the history of the basin included several periods of extensive uplift and subsidence, which were accompanied by the erosion of the sedimentary cover and thermal activation of the lithosphere. This resulted in variations in organic matter maturity reached in different segments of the basin and a peculiar distribution of the degree of maturation, which is higher at the flanks of the Murzuq Basin compared with the same rocks from deeper buried zones. Our modeling indicated that the rocks of the Tanezzuft Formation could generate significant volumes of liquid hydrocarbons at some areas of the basin, but the situation is strongly dependent on the depth of rock burial and amplitude of erosion of the sedimentary cover at various areas of the basin.

Early Palaeozoic evolution of Libya: perspectives from Jabal Eghei with implications for hydrocarbon exploration in Al Kufrah Basin

AbstractThis paper presents new stratigraphic and sedimentological data of the Ordovician, Silurian, and Mesozoic succession exposed on the western flank of Al Kufrah Basin. Field data (logged sections, photographs, palaeocurrent analyses) are presented from the Jabal Eghei region. This region lies ca. 200 km E of the closest stratigraphic tie point at Mourizidie on the eastern flank of the Murzuq Basin. The succession starts with the Hawaz Formation (Middle Ordovician) comprising &gt;100 m of cross‐bedded and bioturbated sandstones that are interpreted as deposits of tidal currents in an open shelf setting. The contact between the Hawaz and Mamuniyat formations is an erosional unconformity, incised during advance of Late Ordovician ice sheets towards the NE. The Mamuniyat Formation comprises &gt;150 m of massive and graded sandstones tentatively assigned to the Hirnantian, and contains an intraformational, soft‐sediment striated surface that is interpreted to record re‐advance of ice sheets over Jabal Eghei. The outcrop section suggests the sandstone would form an excellent reservoir in the subsurface. The Mamuniyat Formation is overlain by the Tanezzuft Formation (uppermost Ordovician–lowermost Silurian). This includes sandy limestone/calcareous sandstone, a Planolites horizon, and then 50 m of interbedded shale, silt and fine‐grained, graded and hummocky cross‐stratified sandstone recording deposition from both shallow marine turbidity currents and storm flows. A striated pavement in the lower part of this sequence is overlain by calcareous lonestone‐bearing intervals (interpreted as ice‐rafted debris). These features testify to late phases of glacial advance probably post‐dating the regional Hirnantian glacial maximum. The basal Silurian ‘hot shale’ facies is not developed in this area, probably because late glacial advance suppressed the preservation of organic matter. The upper part of the Tanezzuft Formation is truncated by an unconformity above which palaeosol‐bearing fluvial deposits (undifferentiated Mesozoic) occur.

Hydrocarbon source rock potential and elemental composition of lower Silurian subsurface shales of the eastern Murzuq Basin, southern Libya

A shallow borehole was drilled in lower Silurian (Rhuddanian–lower Aeronian) Tanezzuft Formation siliciclastic sediments at the eastern margin of the Murzuq Basin. Shale samples were retrieved from the borehole to study their source rock potential and trace element geochemical composition. Thirty shale samples analysed from borehole CDEG-2a reveal three geochemically distinct intervals. Shales from the top (23–38 m) interval are weathered and have negligible organic content. Shales from the middle (38–42 m) interval have moderate to good organic richness (0.6–1.7 wt.% TOC) with Hydrogen Index (HI) values up to 443 mg S2/g TOC. Shales from the bottom (42–51 m) interval have poor to moderate organic richness (0.07–0.8 wt.% TOC) with HI values up to 277 mg S2/g TOC. All samples have limited source rock potential and are immature to early mature for hydrocarbon generation. It is unlikely that they have ever been buried deeper than ∼2 km. Low TOC values (<2 wt.%) together with low concentrations of uranium (<10 ppm) indicate the absence of organic-rich black shale (‘hot shale’) source rocks in the studied interval.Inorganic geochemical redox proxies (Ce-anomaly, authigenic U, Th/U, V/Cr, Ni/Co, and V/Sc) indicate that the lower Silurian Tanezzuft Formation shales from borehole CDEG-2a were deposited under oxic bottom waters, explaining their relatively low TOC values. Deposition took place in a shallow marine environment, in a proximal position with regard to an early Silurian palaeo-shoreline. This combined Rock-Eval pyrolysis and whole-rock geochemical study provides new insights into the Silurian source rock distribution and hydrocarbon prospectivity in central North Africa.

Latest Ordovician–earliest Silurian acritarchs and chitinozoans from subsurface samples in Jebel Asba, Kufra Basin, SE Libya

Latest Ordovician–earliest Silurian Tanezzuft Formation shales recovered from core material of the shallow borehole JA-2 drilled in Jebel Asba at the eastern margin of the Kufra Basin, southeastern Libya, yielded well-diversified palynomorph assemblages with transparent and brownish to yellowish vesicles and organic matter (visual kerogen Types 1 and 2) from depth interval 46.20 to 67.82m. In addition, miospores including cryptospores, and Tasmanites sp. (“Tasmanites with nodules”), scolecodonts, and a stratigraphically significant palaeo-marker, the enigmatic, tubular organic structure Tortotubus protuberans, were also recorded frequently in most samples. Kerogen colour based on miospores (TAI <3) and chitinozoan reflectance indicate an immature facies for oil generation. The two uppermost samples (from 33.33m and 46.20m depths) and the lowermost ones (from 67.92 to 73.21m depth) contain rare palynomorphs and other organic remains and have been partially affected by oxidation.Furthermore, palynological and palynofacies analyses were carried out on cuttings from an old well (UN-REMSA well), ca. 530m towards the NNE from well JA-2. The composition of the organic residue is similar in both wells. However, the UN-REMSA well yields fairly numerous chitinozoans, scolecodonts and biofilms but lacks the “thread-like structures” and “Tasmanites with nodules” observed in well JA-2.All the investigated samples in well JA-2 are dominated by a single chitinozoan species, Euconochitina moussegoudaensis Paris (in Le Hérissé et al., 2013). Based on correlation with chitinozoan-bearing strata around the Ordovician–Silurian boundary, the analysed samples from well JA-2 and from the UN-REMSA well are regarded as post-glacial, but still of either latest Hirnantian age, or at least no younger than earliest Rhuddanian. A well-diversified acritarch, miospore and cryptospore assemblage recorded in well JA-2 supports a marginal marine (nearshore) depositional environment. This assemblage is no older than earliest Rhuddanian yet the latest Hirnantian age of the assemblage cannot be completely ruled out as our current knowledge on the post-glacial, latest Hirnantian acritarch and miospore assemblages is poorly documented in North Africa.

HYDROCARBON SOURCE ROCK POTENTIAL OF LATEST ORDOVICIAN – EARLIEST SILURIAN TANEZZUFT FORMATION SHALES FROM THE EASTERN KUFRA BASIN, SE LIBYA

This paper summarizes the results of Rock‐Eval pyrolysis data of 43 shale samples collected from the latest Ordovician – earliest Silurian (Tanezzuft Formation) interval in the CASP JA‐2 well at Jebel Asba on the eastern margin of the Kufra Basin, SE Libya. The results are supported by analysis of cuttings samples from an earlier well of uncertain origin nearby, referred to here as the UN‐REMSA well. The Tanezzuft Formation succession encountered in the JA‐2 well can be divided into three intervals based on Rock‐Eval pyrolysis data. Shales in the shallowest interval (20 – 46.5 m depth) are altered probably by weathering and lack significant amounts of organic matter. Total organic carbon (TOC) contents of shales from the intermediate interval (46.5 – 68.5 m depth) vary between 0.19 and 0.75 wt%. Most samples in this interval have very limited source rock potential although a few have Hydrogen Index (HI) values up to 378 mg S2/g TOC. Tmax values of 422 – 426°C indicate the organic matter is immature. Shales from the deepest interval (68.5 – 73.9 m depth) are diagenetically altered, perhaps by fluids flowing along a nearby fault or along the contact between the Tanezzuft Formation and the underlying Mamuniyat Formation and apparently lack any organic matter. Cuttings samples from the UN‐REMSA well have TOC contents of 0.48–0.87 wt%, HI values of 242–252 mg S2/g TOC, and Tmax values of 421–425°C. These results offer little support for the presence of the basal Silurian (Tanezzuft Formation) source rock which is prolific elsewhere in SW Libya and eastern Algeria and, together with the overall immaturity of the equivalent section, reduces the probability of finding major oil reserves in the eastern part of the Kufra Basin.