Sirte Basin Research Articles

Progradation of a middle Eocene carbonate slope system, Assamoud Field, Sirte Basin, north central Libya – Implications on the dynamics of lateral growth of isolated carbonate platforms

This study investigates the internal configuration and stacking patterns of well-defined prograding clinothems within the middle Eocene Gialo Formation in the Assamoud Oilfield of the Sirte Basin through an integrated analysis of wire-line log and three-dimensional seismic data. The clinothem interval forms a highstand systems tract that is bounded by a downlap surface at the base and a sequence boundary at the top. The prograding clinothems occur over an area of at least 8 km wide along strike and at least 9.5 km along depositional dip. They range in height from 240 to 330 m and have a N progradation direction. Nine clinothems were identified on the basis of stacking patterns of log facies. Individual clinothems are 47–139 m thick and consist of a thin shale unit at the base overlain by thick limestone units. The clinothem interval is interpreted as a slope system and can be divided into two packages on the basis of their log facies, seismic characteristics, and position along the platform: a proximal package composed of high-angle sigmoidal clinothems and a distal package with low-angle sigmoidal pattern. The maximum slope angle of the proximal package is 24° and that of the distal package is 16°. The decrease of slope angle may be caused by an increased amount of siliciclastic mud. The progradation direction indicates that the major sediment source, which is most likely the platform top and margin, was located to the south of the study area. The sediments entered the slope from the platform margin and were redistributed downslope by surface currents driven by southerly probably Sirocco-like winds. The evolution of the platform was controlled by several factors. A pre-existing isolated platform to the south generated the topographic relief between the platform top and slope. Sea-level rise generated accommodation space, whereas sea-level fall caused subaerial exposure and terminated progradation. Subsidence caused a sea-level rise and enhanced accommodation space for platform expansion. Prevailing currents determined the direction of platform lateral growth. The results provide insights on the dynamics of carbonate platform lateral growth and may serve as an analog for future studies on carbonate slope systems.

Pore modification mechanisms in a deeply buried non-marine sandstone: The Early Cretaceous Upper Sarir Sandstone Formation, Sirte Basin, Libya

Critical understanding of the reservoir pore-system is crucial for effective fluid-flow. Core-sedimentological description (40 core plugs), petrography (thin section and scanning electron microscopy with energy dispersive spectrometry), x-ray diffractometry, and high-resolution imaging were integrated to characterize pore-modification processes in a deeply buried (ca. 12470–12558 Ft.) non-marine sandstone. The reservoir is dominantly subrounded to subangular, well-sorted to moderately well-sorted, and medium-to fine-grained quartz arenite and lithearenite. Framework ranges from matrix-supported to matrix-devoided types. With a mean point-counted composition of Q95F0L5, quartz is the dominant mineral in all samples. Core-derived porosity and permeability range from 0.88 to 16.89% (Average 9.99%) and ~0.00–431.06 mD (Average 66.42 mD), respectively. Intergranular (ɸI), micro (ɸμ), fracture (CF), and dissolution (Diss) porosities constitute an estimated 60%, 35%, 3%, and 2% of the total porosity. The pore-spaces are randomly distributed and largely disconnected from one another. Connected pores are linked by necking, tubular, and lamellar pore throats. Pore-destructive diagenetic processes occurred mainly in the early-intermediate diagenetic phase, and to lesser extent in the late diagenetic phase. Pore-enhancement processes were of sparse occurrences. Mechanical compaction, quartz overgrowth development, kaolinitization, sideritization, and calcite cementation are the main pore-occluding processes identified. Quartz overgrowth increased with depth and it shows an inverse relationship with poro-perm. The formation of quartz overgrowth haloes in the Upper Sarir Sandstone Formation were most likely formed from non-insitu silica mobilized into the pore system during post-rift cooling. This study confirms the need for careful heterogeneity modelling even in non-marine deposited reservoir sandstone, notwithstanding their known clean nature in contrast to mud-rich marine reservoirs.

SEISMIC ATTRIBUTES AIDED DETECTION OF NW-SE TRENDING FAULTS DEVELOPED ON AN ISOLATED CARBONATE PLATFORM IN THE NW SIRTE BASIN, NORTH CENTRAL LIBYA

The lower and upper Paleocene reservoir formations, the primary producing formations in the northwest Sirte Basin, north-central Libya have complex structures which have an impact on the performance of the reservoirs. It is extremely crucial to understand the complex relationships between the fault networks and stratigraphy of the area for future field development. However, delineating faults particularly subtle faults is not an easy process due to the low signal-to-noise ratio in the post stack seismic data despite the effort and careful process of the pre-stack data. Seismic attributes are critical tools in detecting and enhancing major and minor fault interpretation beyond the seismic resolution of the conventional seismic dataset. This study utilizes variance, root mean square, and curvature attributes computed from the post-stack 3D seismic data acquired in the northwest Sirte Basin to detect major and minor faults along an isolated carbonate platform. A spectral whitening and median filter were applied to improve the quality of the data and remove random noise resulted from data acquisition and processing steps. Those methods were utilized to provide high-resolution seismic data and better show edges and structural features. Numerous faults have been detected in the study area. Most major faults in the lower and upper Paleocene reservoir formations are located along the margins of the isolated carbonate platform and have a NW-SE trend. Data conditioning and seismic attribute analyses applied on the 3-D seismic dataset effectively enhanced our understanding of the reservoir complexity and improve the detection of the major and minor faults and fracture zones in the study area.

A new parapithecine (Primates: Anthropoidea) from the early Oligocene of Libya supports parallel evolution of large body size among parapithecids

Parapithecines are an extinct subfamily of stem anthropoid primates previously known only from the Jebel Qatrani Formation in Egypt. Here, we describe isolated teeth pertaining to Simonsius harujensis sp. nov., a relatively small-bodied parapithecine from strata near Zallah Oasis in the Sirt Basin of central Libya that is estimated to date to ∼31 Ma on the basis of mammalian biostratigraphy. The dental morphology of S. harujensis sp. nov. is generally intermediate between that of the closely related parapithecines Parapithecus fraasi and Simonsius grangeri, highlighting some of the anatomical features distinguishing the latter taxa and providing further support for their generic separation. A phylogenetic analysis using parsimony methods was performed on a character-taxon matrix incorporating data from the new Libyan parapithecine, virtually all other parapithecids and the proteopithecid Proteopithecus sylviae. Results of this analysis suggest that parapithecids comprise a basal clade consisting of three species of Biretia and a more derived clade including Parapithecinae (Parapithecus and Simonsius) and Qatraniinae (Qatrania, Ucayalipithecus, and Apidium). Body mass estimates for parapithecids were calculated on the basis of regression equations generated to predict body mass from the occlusal area of upper and lower cheek teeth in extant anthropoids. The relatively small body mass of S. harujensis sp. nov. and its reconstructed phylogenetic position as the sister group of S. grangeri, which is the largest known parapithecid, support the convergent acquisition of body mass larger than 500 g among multiple clades of early Oligocene African anthropoids. The new Libyan parapithecine augments previously reported evidence supporting a substantial degree of faunal provincialism across northern Africa/Arabia during the early Oligocene.

Middle to Late Paleocene larger benthic foraminifera from the Sirt Basin (Libya)

The Sirt Basin of Libya hosts significant hydrocarbon reservoirs in Paleocene shallow-marine carbonates (Upper Sabil Formation) with abundant larger benthic foraminifera. Benthic foraminifera proved to be an essential biostratigraphic tool for precise biostratigraphic dating of these carbonate platform/ramp deposits. A detailed systematic description of the Middle to Late Paleocene larger benthic foraminifera from the Sirt Basin is presented, which refines the biostratigraphic age of the Paleogene shallow-marine carbonates. Eight taxa are identified at a species level. Among them, Ranikothalia solimani is the most important taxon for the correlation of the Upper Sabil Formation. The taxa Miscellanea yvettae, Miscellanites primitivus, and Glomalveolina primaeva with co-occurrence of orthophragminid foraminifera constrain the age of the Upper Sabil Formation as Middle to Late Paleocene (Selandian-Early Thanetian).

Source rock maturity evaluation and its implications for regional petroleum charge, Sirt Basin, Libya: Insights from regional geochemical assessments and modelling study

Six marine and lacustrine source rocks were studied to define their thermal maturation levels in the largest, youngest, and most productive petroliferous sedimentary basin in Libya (Sirt Basin). From the oldest to the youngest, the studied source rock formations were: 1) Middle Nubian; 2) Etel; 3) Rachmat; 4) Sirte; 5) Kalash; and 6) Hagfa. The calibrated basin modelling studies (1D, 2D, and 3D) were used to generate regional maturation maps for each source rock in the basin. Thermal maturity calibration was carried out at different levels utilising measured and simulated vitrinite reflectance (VRo), pyrolysis Tmax data, and 39 typical wells for burial history modelling and geochemical screening. Corrected bottom hole temperature data were used to obtain the geothermal gradient and heat flow throughout the basin. The thermal gradient and heat flow analyses results indicated that the Sirt Basin has an average geothermal gradient of approximately 1.58 °F/100 ft and an average heat flow of 72 m.W.m−2 with a systematic heat flow increase toward the basin platforms and a decrease toward the basin troughs. The conducted assessments in the Sirt Basin presented different levels of source rocks thermal maturity due to variable burial depths, with an average VRo range of 0.35%–2%. The lower Cretaceous source rocks were subjected to higher degrees of diagenesis of organic matter (i.e. mid-to over-mature rocks), with an average VRo of 0.9%–2%. In contrast, the Upper Cretaceous source rocks demonstrated a wide range of thermal maturity (from marginally to late mature) in the deepest part of the Sirt Basin (Ajdabiya trough), whereas the late Palaeocene source rocks entered the mid-to late-maturity levels in the Wadayet trough (North Ajdabiya trough).

Analysis of gravity and aeromagnetic data to determine structural trend and basement depth beneath the Ajdabiya Trough in northeastern Libya

In this study, we analyse both ground gravity and aeromagnetic data in order to delineate structural trends, fault systems and deduce sedimentary thicknesses within the Ajdabiya Trough in Libya’s northeast. A high-pass filter and a reduced-to-the-pole (RTP) transformation are applied to the gravity and aeromagnetic data respectively. Different filters are used to enhance the structural signatures and fault trends within the study area. The Werner deconvolution and source parameter imaging (SPI) techniques are applied to the RTP magnetic data for source depth estimation. Four well-data within the area are used as constraints in the two-dimensional forward modelling process. The results show that the Ajdabiya Trough is characterised by gravity anomaly highs and magnetic anomaly lows. The analysis of gravity data shows predominant Northeast–Southwest structural trends, whereas the analysis of magnetic data shows predominant North–South magnetic lineaments within the Ajdabiya Trough. The Euler deconvolution depth estimates of faults depths range between 1500 and 9500 m. The SPI estimates of the magnetic basement range between 2500 and 11,500 m beneath the study area (deepest beneath the Ajdabiya Trough). Constrained by the well-data, six major layers characterize the four profiles that are taken within the area. One of the profiles shows a high-density intrusion (about 4 km from the surface) within the sedimentary sequence. The intrusion may be the result of the rifting Sirt Basin which caused a weakening of the crust to allow for mantle intrusion.

Miocene Rocks Around the Marádah Oasis, Central Sirt Basin, Libya: Facies Development and Implication on Stratigraphy

The Miocene rocks of the Marádah Formation have been stratigraphically investigated from four stratigraphical sections around the Marádah Oasis in the Central Sirt Basin of Libya. The field investigations led to the identification of two members, the lower Qarat Jahannam Member and the upper Ar Ráhlah Member. Fourteen sedimentary facies at the outcrop-scale representing a gradual development of sedimentation from a continental clastic witness in the southwestern outcrops to transitional estuarine, lagoonal, and beaches to the proximal offshore in the northern outcrops, were recognized. The results indicates that the accumulation of the Marádah Formation is transgressive in nature and corresponding to two phases of deposition which have been mentioned in the earlier studies.
 
 The first phase is continental-dominated facies in which cross-bedded sandstones and calcareous sands comprise most of the depositional sequence of the lower Qarat Jahannam Member at the southwestern outcrops. This phase, however, is characterized by extremely bioturbated laminated-shale conquered by Skolithos ichnofacies in the lower part of the upper Ar Ráhlah Member at the northern outcrops. This phase is providing further evidence that the contact between the two members is diachronous everywhere in the study area. The clastic-phase has thought to be deposited in the Lower Miocene (Aquitanian-Burdigalian) since the lower Qarat Jahannam Member rests on an erosional surface of submarine origin in the southwestern outcrops above a 0.5 m. thick of a nummulitic unit of the Oligocene Bu Hashish Formation.
 
 The second phase is marine-dominated facies in which a bioclastic limestone unit rich in thick and disarticulated oysters, including Crassostrea gryphoides (Schlottheim), characterizes the sediments of the Ar Ráhlah Member at the southwestern outcrops. This phase also includes the upper part of the latter member at the northern outcrops in which a detrital limestone unit rich in turritelline gastropods is overlying by thick-bedded calcarenites rich in disarticulated oysters, gastropods, irregular echinoids (notably, Clypeaster and Echinolampas), bryozoans, and celestite corals. The upper part of the Ar Ráhlah Member at the northern outcrops, nevertheless, is terminated by a quite hard dolomitic limestone and by a pretty soft dolomitic marly limestone. Both lithologies, however, are combined with medium-sized oysters, including Ostrea digitalina Fuchs, and pectinid bivalves. The second phase, however, is interpreted to be deposited in the Middle Miocene (Langhian and Serravallian) based on the total-stratigraphic range of the larger benthic foraminifera Borelis melo melo (Fichtel & Moll), which recovered from the studied washed residues, and the associated microfacies.

Verification of Gardner's equation and derivation of an empirical equation for anhydrite rocks in Sirte basin, Libya: case study

Bulk density is a physical property of rocks measured in the laboratory on rock samples or obtained from oil field logging tools. When bulk density is not measured, a synthetic bulk density log can be calculated, for which Gardner's equation is the most widely used. However, Gardner's equation might not be appropriate for regions in which the density–velocity relationship does not conform to Gardner's curves. Here, we verified the applicability of Gardner's equation to calculation of synthetic bulk density of anhydrite rocks in the Sirte Basin (Libya) and compared the results to those obtained from an equation derived from the available measured bulk density and sonic logs. We used fifteen wells to calibrate Gardner's equation and three wells to derive an equation for the anhydrite rocks. The anhydrite rocks were 10–510 feet thick. The bulk density calculated by Gardner's equation differed only slightly from the measured log values, with the exception of the eastern part of the Sirte Basin. The average of the differences in bulk density between the measured values and Gardner's equation results were 0.022–0.040 g/cm3, and between the measured values and the derived equation results 0.002–0.045 g/cm3, both with a standard error of about 0.01 of the bulk density estimated results. We conclude that while Gardner's equation is more appropriate for estimating the bulk density of anhydrite rocks in the eastern part of the basin, the derived equation could be more appropriate for the western region.

Basin architecture and tectonic controls on the Early Cretaceous Sarir Sandstone reservoir, eastern Sirt Basin, Libya

An integrated study using gravity, 2D seismic, well logs, and geological data in the eastern Sirt Basin is presented in this research focussing on the Lower Cretaceous Sarir Sandstone which is the most important oil reservoir in the study area. Interpretation of basement architecture shows individual blocks are named in the present paper A, B and C, whilst depocentre represents the most pronounced topographic low within the study area and named D-Graben in this paper.The observed onset of rifting phase in the study area took place in the Early Cretaceous and was divided into three active and inactive phases contemporaneous with deposition of the continental clastic Sarir Sandstone. The basin-wide NW-SE syndepositional fault segments played consequently a significant role in the evolution of the irregular paleotopography, sediment infill and the created accommodation space resulting in controlling sedimentary thickness and facies distributions of the Sarir Sandstone. Some faults were reactivated during the Late Cretaceous-Palaeocene with no abrupt changes in stratigraphic thickness in the post Early Cretaceous section, which suggests non-tectonic post-rift (sagging) phase.The Sarir Sandstone reservoir unconformably overlies the basement (pre-rift sequence), and is therefore unconformably overlain by post-rift marine shales and carbonates of Late Cretaceous. The detailed analyses of log signatures variations, constrained by some core and cuttings information revealed that the deposition of Sarir Sandstone resulted essentially from alterative episodes of braided and meandering fluvial rivers. As a result, the Sarir Sandstone has been divided in general into lower and upper units, separated by a Red Shale bed. In turn the lower Sarir unit has been subdivided into two layers, whilst the Upper Sarir unit, which represents the main reservoir in the study area, has been subdivided into nine layers.

A new carnivoraform from the early Oligocene of Libya: Oldest known record of Carnivoramorpha in Africa

Carnivoraformes is a clade comprised of members of the Carnivora, as well as the paraphyletic assemblage of stem taxa previously united under the “Miacidae”. Here we describe the earliest known African occurrence of a carnivoraform from the early Oligocene Zallah Incision locality in the Sirt Basin of central Libya. The new taxon is represented by three teeth (m1, P4, and dP3), all of which are carnassial loci diagnostic of the Carnivoraformes. Despite its meager anatomical representation, the new Libyan carnivoraform possesses numerous characters that distinguish it from contemporaneous African hyaenodonts, including the retention of a large m1 metaconid, an enlarged and transversely oriented P4 metastyle, and the absence of a metacone on dP3, among other features. The generally primitive nature of known tooth loci and the absence of clear synapomorphies with crown carnivorans suggest that the new carnivoraform may be a relatively basal member of the clade. If so, the new Libyan taxon would be the only known African occurrence of a basal carnivoraform.

Organic geochemical characterization of crude oils and source rocks from Concession 6, central Sirt Basin, Libya

The Sirt Basin contains most of Libya’s producing oil fields and is in northern Libya, south of the Gulf of Sirt in the Mediterranean Sea. The main aims of this study were to characterize the type of organic matter and depositional conditions of the source rocks and thermal maturity of the crude oils in this region using a variety of geochemical techniques including gas chromatography (GC), gas chromatography/mass spectrometry (GCMS), stable carbon and hydrogen isotopes, total organic carbon (TOC), and Rock Eval analysis. Twelve crude oil samples from Cambrian–Ordovician Gargaf, Upper Paleocene Zaltan, and Middle Eocene Gialo reservoirs and seventy-nine source rock samples representing Sirt Shale samples from nine wells were collected from the Zaltan Platform and surrounding troughs in the central Sirt Basin. Organic geochemical parameters indicate that these oils were sourced from marine organic matter deposited under oxic to suboxic conditions. The oils were divided into three groups based on their whole oil carbon and hydrogen isotopic composition and thermal maturity as manifested by several saturate and aromatic hydrocarbon maturity parameters. Based on the geochemical data, it is proposed that there are at least two sources for the crude oils in the Zaltan Platform area of the central Sirt Basin. The study also indicates that the Sirt Shale source rocks from Maradah Trough are dominated by marine organic matter and have reached the main phase of oil generation.

The power of the physical fabric of mudstone in creating trace fossil like redoximorphic features, case study from the late Eocene, Sirt Basin

This article presents complex paleo-pedoturbation features, preserved in a massive mudstone stratum belonging to a siliciclastic sequence in the upper Eocene. The outcrop of these turbations presents an intricate network of iron oxide in semi tubular forms. The mudstone to which these structures are restricted is lithologically similar to other mudstone beds, below and above it in the sequence. In contrast, it presents more paleo-pedogenic features.Attributing these tubular structures to a specific burrow is difficult for two reasons: the apparent similarities in morphology to numerous trace fossils coupled with the absence of diagnostic excavation-locomotion proxies, as well as the lack of any faunal or floral body fossils in the parent mudstone. Moreover, these structures do not convincingly conform to the simplified image documented in numerous publications for fossil roots.Taking into account the exclusive, physicochemical properties of the clay rich rocks (shrink-swell behaviour, erratic permeability and cation-exchange capacity), this article provides potential evidence that such trace fossil-like features could, alternatively, be produced by abiotic redox processes without pre-existing faunal or floral predecessors. Regardless of the extent to which the provided justifications are acceptable, they might increase awareness as to just how treacherous trace fossil identification can be. The studied case draws attention to the fact that, in mudstones, the dividing line between valid and spurious trace fossils can be very subtle. It also assists in better recognition of paleosol features in mudstones.

HYDROCARBON POTENTIALITY OF FACHA RESERVOIR IN ASWAD OIL FIELD, SIRT BASIN, LIBYA

This paper is to study the structural framework, stratigraphy, and the petro-physical characteristics of Facha reservoir of Gir Formation in Aswad oil field, which is located in Block NC74B at the Zella Trough, south-west of Sirt basin, Libya. The data used have been got from well-logging records of nine exploratory wells distributed in Aswad oil field. These data have been analyzed and interpreted through using analytical cross-plots in order to calculate the petro-physical parameters. The results revealed that the lithological facies consists mainly of dolomite. Moreover, they revealed that the lateral distribution of the petro-physical parameters of Facha reservoir indicates that average porosity is 10-23%, average water saturation is 52- 93%, and net pay is of 62.44 ft. This shows that Facha member is a good reservoir rock. The variations in values between wells have been affected by the trend of faults; this indicates that the area is structurally controlled.

PANORAMIC OF PALEOHABITATS OF PLIOCENE SAHABI FORMATION AND RECENT AS-SAHABI AREA, NORTHERN LIBYA

As-Sahabi area is located to the south of Ajdabiyah city in Sirt Basin, Libya. This area considered as a spectacular open Museum of vertebrate remains (i.e. teeth, bones, skulls and skeletons) of mammals, reptiles, aves and fish. This paper presents the methods used during the excavation and lists most of the documented taxa and finally compare these paleo-habitats with the corresponding present day habitat of As-Sahabi.The As-Sahabi fossils played important role in delineating the paleoenvironment and in understanding the migration routes as well as in tracing their evolutionary trends. These remains are preserved in continental deposits (mainly sands and clays) since 5 million years ago. Six paleo-habitats have been suggested “Salt neritic water paleo-habitat, Eo-Sahabi River paleo-habitat, Riverine forest and river edge: paleo-habitats, Water's edge and Eo-Sahabi estuary Paleo-habitats, Savana Paleo-habitat, and Arid Desert-like paleo-habitat. As-Sahabi vertebrates (e.g. crocodiles, hippos, hyena, etc..) are considered to be the ancestral taxa of the descendant vertebrates exist today in other parts of the world. However, some are found to be indigenous taxa due to migration failure such as the gigantic size, Mastodon (Stegotetrabeledon lybicus) and shovel-tusker proposcidean (Amebelodon cyrenaicus) or medium-sized with short-legged Anthracothere (Libycosaurus petrocchii). &nbsp

Coprolites from the Neogene Sahabi Formation, northeastern Sirt Basin of Libya

A large number of coprolites are newly documented from the As Sahabi area in the Sirt Basin of Libya. They are collected from the fossiliferous horizon within the Late Miocene sediments belonging primarily to the lower member (member “U1”) of the Sahabi Formation. Some of the coprolite specimens were subjected to microscopic examination and geochemical analysis to determine the possible producer and food type. The study proved that carnivores are the major producers as understood from both the observed undigested material such as bone and tooth fragments by reference to the known carnivore taxa present. The collected coprolite specimens are classified into a number of ichnomorphotypes in order to facilitate their study.

锡尔特盆地石油地质条件分析

锡尔特盆地石油地质条件分析

Assessment of Geological Structure in Nafoora Oil Field, Sirt Basin, Libya

The Nafoora oilfield is situated on a major tectonic uplift in the Eastern Sirte Basin, the Amal-Nafoora High. Four oilfields have been discovered on this uplift – Amal, Rakb, Augila, and Naffora. The Nafoora oilfield was surveyed by AGOCO company as a 2D seismic Survey in directional groups. this study used 35 seismic lines in different directions with 6 wells. The methodology used in this study aimed at the integration of geophysical and geological aspects. The seismic reflection technique is used to provide the basic structural control of the subsurface geology integrated with available exploration wells to control the interpretation. The resulted structure gives us an expecting to the new future drilling wells where it should be between the two WNW-ESE major strike-slip faults in the south, and on the right side (up throw) of the N-S minor faults which found in the north of the Nafoora oil field.

Subsurface StructuralModeling in Assumoud and Sahl Gas Fields, SirteBasin, Libya

The area of study Assumoud and Sahl fields are situated in the southeastern Sirte Basin within Wadayat trough. Using seismic imaging and interpretation of the basin structures, as well as the interpretation and analysis of other geological and geophysical data, and modeling over the areas of interest, the study aims to delineate the major and minor subsurface structures that dominated the area. A structure maps were created for top Mulallah, top Etel and top Sheghega and construct a structural model for the area of study then many cross-sections were taken to the area model. The analysis of these maps show that the area uplifted in the late Cretaceous.

Prospect evaluation and hydrocarbon potential assessment: the Lower Eocene Facha non-clastic reservoirs, Hakim Oil Field (NC74A), Sirte basin, Libya\u2014a case study

Petrophysical assessment of Facha Formation based on log data of six wells A1, A3, A4, A5, A8 and A13 recorded over the entire reservoir interval was established. Hakim Oil Field produces from the Lower Eocene Facha reservoir, which is located at the western side of Sirte basin. Limestone, dolostone and dolomitic limestone are the main lithologies of the Facha reservoir. This lithology is defined by neutron porosity—density cross-plot. Noteworthily, limestone increases in the lowermost intervals of the reservoir. Structurally, the field is traversed by three northwest–southeast faults. The shale of the Upper Cretaceous Sirte Formation is thought to be the source rock of the Facha Formation, whereas the seals are the limestone and anhydrite of the Lower Eocene Gir Formation. In this study, the Facha reservoir’s cutoff values were obtained from the cross-plots of the calculated shale volume, porosity and water saturation values accompanied with gamma ray log data and were set as 20%, 10% and 70%, respectively. Isoparametric maps for the thickness variation of net pay, average porosity, shale volume and water saturation were prepared, and the authors found out that the Facha Formation has promising reservoir characteristics in the area of study; a prospective region for oil accumulation trends is in the north and south of the study area.