Sabatayn Basin Research Articles

Oil source rock characteristics of the pelagic carbonates in the Shabwah depression, southeastern Sabatayn Basin, Yemen

Bituminous lime mudstone from the Late Tithonian--Early Barremian Naifa Formation was collected from five wells in the Shabwah depression of the southeastern Sabatayn Basin for basic organic geochemical analyses. The collected samples’ nannofossil contents were also studied. The current study preliminarily evaluated the nature of the organic matter regarding the richness and type of organic facies and their relation to oil generation potential. Most of the lime mudstone samples are organically rich and include favorable source rocks, as indicated by the high TOC content of up to 1 wt%. The presence of the warm-water index nannofossil taxa Watznaueria barnesae suggests warm climate conditions, thereby contributing to high bioproductivity that consequently enhanced organic richness. The samples are also hydrogen rich, with HI values between 302 and 740 mg HC/g TOC, resulting in two dominant organic facies of kerogen Types I and II, and are consistent with high bioproductivity of phytoplankton algae in warm-water conditions during deposition. Both geochemical maturity values Tmax and PI indicate that the bituminous lime mudstones are at different thermal maturity levels and range from immature to early mature stages of the oil generation window. This implies that the bituminous lime mudstones were not buried to a sufficient depth for commercial oil generation. Therefore, further exploration efforts in deeper parts of the Shabwah depression, particularly in the area around well Misah-01, are recommended.

Organic geochemistry characterization of Late Jurassic bituminous shales and their organofacies and oil generation potential in the Shabwah depression, southeast Sabatayn, Yemen

Bituminous shales of the Late Jurassic Madbi Formation from Shabwah depression in the south-eastern Sabatayn Basin has been collected and analyzed. The organofacies, paleo-sedimentary environmental conditions and oil generation potential are discussed based on combined geochemistry and petrology investigations. Biomarkers indicate that the bituminous-analyzed shales contain mainly marine phytoplankton algae and minor land plants and deposited under reducing environmental conditions. The rich in lipids from phytoplankton algae and land plants suggest high Type II to mixtures of Types II and Type III kerogen as the original organic facies during deposition. This is consistent with significant amounts of alginite and amorphous organic matter, with minor vitrinite land plants observed under microscope and hydrogen index (HI) values of 210–679 mg HC per g TOC and indicated good to excellent oil-source rocks. The presence of the reducing conditions during deposition consequently enhanced the preservation and subsequently gave rise to enrichment of organic matter in the analyzed bituminous shale as indicated by the relatively high TOC values between 1 and 14 wt%.The geochemical maturity indicators show that the analyzed bituminous shales have reached a low maturity stage, and commercial oils have not yet generated. Therefore, the results presented and discussed in this study suggest that the low maturity bituminous shales can be heated to crack the kerogens and subsequently significant amount of oil can be generated. This will lead to a huge alternative potential of unconventional resources and provide a sense of extending the exploration activities for both unconventional and conventional petroleum resources in the whole Basin.

Source rock evaluation and hydrocarbon generation potential of Mid-Late Cretaceous sediments from Mintaq-01 well in the Wadi Hajar of Southern Sabatayn Basin, Yemen

Twenty-five cutting shale and coal samples from the Mid-Late Cretaceous Mukalla and Harshiyat formations in the Mintaq-01 well, Wadi Hajar of Southern Sabatayn Basin were investigated using organic geochemical methods. The geochemical results indicate that the analyzed shale and coal samples are fair to excellent source rocks, with TOC values between 0.50 and 53.13%. The analyzed samples also have Rock-Eval HI between 99 and 676 mg HC/g TOC, indicating types I/II and II/III kerogen and thus are oil and gas-prone source rocks. However, these Mid-Late Cretaceous sediments of the Wadi Hajar are still in an immature to very early mature stage, and they have not yet generated oil or gas.

Geochemical and geological modeling of the Late Jurassic Meem Shale Member in the Al-Jawf sub-basin, Yemen: Implications for regional oil and gas exploration

Abstract The Late Jurassic Meem Shale Member is widely exposed within the Sabatayn Basin, including in the Al-Jawf sub-basin. In this study, a total of 49 shale samples of the Meem Shale Member from two exploratory wells; Dahmar Ali−01 (28 samples in the depth range 1441–2351 m) and Kamaran-01 (21 samples in the depth range 2275–3011 m) in the south-eastern portion of the Al-Jawf sub-basin were studied as possible source rocks. Evaluation of source rock geochemistry including total organic matter (TOC) and Rock-Eval pyrolysis analyses was performed. The results of this study provided information regarding significant exploration of oil and gas resources from the Meem Shale Member in the Al-Jawf sub-basin. Both the incident light microscopic results and the Rock-Eval geochemical study have indicated that the analyzed shale samples contain predominantly Type III kerogen with minor contributions of Type I/II and IV, so as to they can generate both oil and gas. The values of hydrogen index (HI) are decreased with increasing thermal maturity and varied between 34 and 803 mg HC/g TOC. Based on the measured VRo% range of 0.63 and 2.00, the Meem shales are currently within the oil- and gas-generation window, thus, they are considered as shale-oil and -gas resource possible plays. The geochemical results were integrated with geological data of the Al-Jawf sub-basin and incorporated into a basin model to simulate petroleum generation and/or retention in the Meem Shale Member. The constructed basin models illustrated that the Meem Shale Member displayed a peak oil generation window during the Late Jurassic to Early Miocene, where large amounts of oil were generated, with a conversion ratio of 10–75 TR%. Further, the high heating rates from local volcanic rocks from the Early Miocene till date have caused gas generation from both of the original Type III kerogen and from secondary cracking of retained oil in the Meem Shale Member. However, the main contribution of gas generation within the Meem Shale Member came from secondary cracking processes through geologic time. The good thickness (736–910 m) of the organically-rich, high genetic potential and matured Meem Shale Member is expected to provide large amounts of oil and gas within the south-eastern portion of Al-Jawf sub-basin. Therefore, the Al-Jawf sub-basin can be considered as a rich oil and gas exploration province.

Late Jurassic Safer Salt Member in the Al-Jawf sub-basin of NW Sabatayn Basin, Yemen: geochemical evaluation of organic-rich oil-source rock potential

Late Jurassic Safer shales in the Al-Jawf sub-basin are analyzed to evaluate the organic matter input, depositional conditions and petroleum generation potential. The shales have high organic matter, with TOC values of 1.0-13.5% and they contain predominantly Types II and III kerogen, referring to mainly oil- and gas-prone. These kerogens are indicative for dominate marine algal component and some terrestrial organic matter input as indicated from biomarkers. Moreover, the presence of the gammacerane also confirmed a high salinity stratification condition and suggests that the dominate Type II kerogen is rich-sulphur kerogen (Type II-S). The vitrinite reflectance (VRo%) and Rock-Eval pyrolysis Tmax data indicate that the analyzed shales are immature.

Late Jurassic Arwa Member in south-eastern Al-Jawf sub-basin, NW Sabatayn Basin of Yemen: Geochemistry and basin modeling reveal shale-gas potential

The current study evaluated the source rock characteristics, thickness, and lithology of the Late Jurassic Arwa Member to provide information about both conventional and unconventional resource systems in the Al-Jawf sub-basin. Three exploratory wells in the south-eastern portion of the Al-Jawf sub-basin were used for the source rock geochemistry and basin modeling study.Results from the geochemical and basin modeling indicate that the Arwa Member is a self contained source-reservoir whose shales are considered as gas-prone source rocks, and have generated large amounts of thermogenic gas through secondary cracking of oils at high thermal maturity levels. The geochemical results reveal that the Arwa shales currently contain Type III and IV kerogen in a gas-window maturity stage and are shale-gas resources.The basin models illustrate that the late Jurassic to early Miocene age was the peak-oil generation window of the Arwa shale source rock, with a transformation ratio (TR) of 10–85%. Most of the oil was expelled along micro-fractures caused by the pressure of oil generated within the Arwa shales, which was then trapped in the carbonate reservoir rocks within the Arwa Member itself. The oil retained in the Arwa carbonate rocks was partially and/or completely cracked into gas between the early Miocene and the present-day. This thermogenic gas was generated due to high thermal maturity caused by Tertiary volcanic rocks. Assuming a thickness of 360–1590 m and partial and/or complete cracking of retained oil to gas, the Arwa Shale Member could have a huge gas-generation potential.

Petroleum that has migrated into an outcropping of the Jurassic Ayad Salt Dome of Shabwah depression, Yemen

Two oil seep samples were collected from outcropping Jurassic Ayad Salt Dome of Shabwah depression. The results of this study have been used to provide information of the origin of the oil seeps and the type of organic matter and maturity of their potential source rock in the basin. Although these are surface oil seeps, their high saturated hydrocarbon content indicates that they are non-degraded oils, probably due to an arid environment of the thick salt sediments. The analyzed oil seep samples are characterized by full complement of n-alkanes, a very high component of phytane, relatively low CPIs of less than unity, and an unusually high content of aliphatic HCs. These features suggest that the analyzed oil seeps are generated from an algal marine organic matter in the source rock that deposited under highly anoxic hypersaline conditions and indicate a moderately low level of maturity. The geochemical characteristics of the analyzed oil seeps in this study are similar and consistent with the source rock characteristics of the Safer Member in the Sabatayn Basin.

Late Jurassic bituminous shales from Marib oilfields in the Sabatayn Basin (NW Yemen): Geochemical and petrological analyses reveal oil-shale resource

In this study, bituminous shale samples were collected from Late Jurassic sedimentary section in the Marib oilfields to study their petrologic and organic geochemical properties. The results of this study indicate that the Late Jurassic bituminous shales can be considered as an oil-source rocks in the Sabatayn Basin (NW Yemen). The analyzed Late Jurassic shales have high organic matter (TOC up to 10%) content of kerogen Types I/II. These kerogen types in the analyzed samples are consistent with the high dominance of alginite and amorphous organic matter. Open pyrolysis–gas chromatography results are generally consistent with the Rock-Eval results and further indicate that the analyzed shale samples contain Types I/II and II-S kerogen and generally can produce paraffinic oils with low to high wax content. Type II-S kerogen can produce oil with high sulphur content at low maturity level. The maturity data reflect immature and very early maturity stages of petroleum formation of the Late Jurassic bituminous shales. Therefore, the kerogen types in the Late Jurassic bituminous shales has not been altered by thermal maturity for oil generation, thus, artificial heating is required of kerogen cracking generate significant oil.

Reservoir characteristics of the Kuhlan sandstones from Habban oilfield in the Sabatayn Basin, Yemen and their relevance to reservoir rock quality and petroleum accumulation

Integrated petrophysical and petrographic analyses were performed on the Kuhlan clastic rocks to evaluate the petrophysical reservoir properties, identify grain sorting along with the clay type and mineral volumes using well logs coupled with core data. Core samples were collected from two key wells and their results were used to determine the reservoir properties and bulk mineralogical composition and, additionally, to calibrate the well log results from three un-cored to the two cored wells.The Kuhlan clastic reservoir rocks have porosity values between 6% and 12%. These porosity values are mainly intergranular primary and secondary porosity. Permeability is likewise variable with values in the range of 78–304 mD. The Kuhlan reservoir rocks also have high hydrocarbon saturation exceeding 70%, with relatively high movable oil. These reservoir properties are achieved from log-derived and core porosity and permeability results.The lithology and mineral composition of the well logs and petrographic studies in this study indicate that the Kuhlan sandstone can be considered as an alternative reservoir rock in the basin. The Kuhlan Formation is primarily composed of sandstone that contains quartz, feldspar (i.e., orthoclase, plagioclase (albite) minerals and illite, and calcite cements. The quartz mineral is dominant followed by feldspar as the second most abundant mineral in this formation. The Kuhlan Formation sandstones are classified (based on standard rock classification schemes) into arkose, sub-arkose and feldspathic lithofacies. The results of petrographic analysis also indicate a continental block transitional source of the Kuhlan Formation sandstones. Petrographically, the Kuhlan sandstones are poorly sorted and range from fine to coarse-grained and contain high percentages of detrital quartz and feldspar-rich grains with a significant amount of rock fragments. The Kuhlan sandstones also contain a variable range of matrix and cement contents.Based on the results of this study, the reservoir quality of the Kuhlan sandstone ranges between good to poor reservoir quality. The Kuhlan Formation has arkosic and sub-arkosic sandstones that result in a good reservoir quality with a significant increase of the permeability and intergranular porosity. This is attributed to the coarse-grained texture, high percentages of detrital quartz, slightly well sorted texture and the low degree of matrix and cements. In contrast, the Kuhlan Formation feldspathic sandstone has poor reservoir quality due to the poor sorting texture, pore space occluding clayey groundmass, and carbonate cement.

Preliminary source rock evaluation and hydrocarbon generation potential of the early Cretaceous subsurface shales from Shabwah sub-basin in the Sabatayn Basin, Western Yemen

A conventional organic geochemical study has been performed on the shale samples collected from the early Cretaceous Saar Formation from the Shabwah oilfields in the Sabatayn Basin, Western Yemen. The results of this study were used to preliminary evaluate the potential source-rock of the shales in the Saar Formation. Organic matter richness, type, and petroleum generation potential of the analysed shales were assessed. Total organic carbon content and Rock- Eval pyrolysis results indicate that the shale intervals within the early Cretaceous Saar Formation have a wide variation in source rock generative potential and quality. The analysed shale samples have TOC content in the range of 0.50 and 5.12 wt% and generally can be considered as fair to good source rocks. The geochemical results of this study also indicate that the analysed shales in the Saar Formation are both oil- and gas-prone source rocks, containing Type II kerogen and mixed Types II-III gradient to Type III kerogen. This is consistent with Hydrogen Index (HI) values between 66 and 552 mg HC/g TOC. The temperature-sensitive parameters such as vitrinite reflectance (%VRo), Rock-Eval pyrolysis Tmax and PI reveal that the analysed shale samples are generally immature to early-mature for oil-window. Therefore, the organic matter has not been altered by thermal maturity thus petroleum has not yet generated. Therefore, exploration strategies should focus on the known deeper location of the Saar Formation in the Shabwah-sub-basin for predicting the kitchen area.

Petroleum source rocks characterization and hydrocarbon generation of the Upper Jurassic succession in Jabal Ayban field, Sabatayn Basin, Yemen

The Upper Jurassic samples of Jabal Ayban field, Sabatayn Basin, western Yemen were used to determine the source rock characteristics and petroleum generative potentials. Based on organic geochemical analysis, the Upper Jurassic source rocks have variable TOC content in the range of 0.82–13 wt%, indicating a fair to very good source rock generative potential. The bulk geochemical results showed that Safer Member contains Type III kerogen grading to mixed Type II–III, while the Lam and Meem members displaying generally Type II, mixed Type II–III and Type III kerogens, which have the ability to generate mixed oil and gas accumulations under thermal maturation level. Vitrinite reflectance in the range (0.35–0.78% Ro) and Tmax in the range (411–445 °C), generally proving that the Meem source rocks have sufficient thermal maturity for hydrocarbon-generation, whereas the Lam and Safer are immature source rocks. 1D basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the study area. Calibration of the model with measured VR (% Ro) and (BHT) data shows that the paleo-heat-flow was high at Late Jurassic. The burial/thermal history models indicate that the source rocks in the Lam and Safer members are immature source rocks and the peak oil generation has not been reached yet. The models also indicate that the early hydrocarbon generation (0.64% Ro) in the Meem source rock occurred during late Jurassic at 150 Ma and the main hydrocarbon generation (0.79% Ro) has been reached approximately at 146 Ma. Therefore, the Meem Member can be consider as generative potentials of prospective source rock horizons in Jabal Ayban field, Sabatayn Basin.

Significance of Surface Lineaments for Gas and Oil Exploration in Part of Sabatayn Basin-Yemen

Sabatayn basin is one of the two mature basins producing oil and gas in Yemen. Due to the recent decline in oilproduction, the exploration activities have been extensively concentrated mainly on these two basins (Sabataynand Sayun-Masilah). So, this present study has been conducted to evaluate the exploration potential of theremaining trap areas within these mature basins. Fracture basement of reservoir becomes very attractiveexploration target in Yemen recently, after the recent discoveries of basement fracture oil fields. As theevaluation of basement traps needs time and fund, to reduce the same, this research study uses a new techniquewhich was devised with the help of Geoinformatics by analyzing the surface and subsurface lineamentseffectively. In this study, a satellite image Enhanced Thematic Mapper (ETM+7) based analysis was conductedfor extracting surface lineaments, and for the subsurface lineaments, the basement lineaments were extractedfrom seismic, magnetic and gravity data. An orientation analysis of these surface and subsurface linear featureswas performed to detect the basic structural grains of the study area. The correlation between these structuralgrains and subsurface oil and gas traps was performed to understand the connectivity to the reservoirs. Thispaper discusses in detail about the same and the importance of using surface and subsurface lineament analysisfor delineating hydrocarbon reservoirs in the study area.