Journal of African Earth Sciences最新文献

The application of rock physics modeling has been a breakthrough in oil and gas industry and has greatly managed exploration and interpretation risk. In the Rio del Rey Basin, the rock physics method was used to ascertain the reservoir's characteristics and relate them to the elastic characteristics. Using conventional techniques to connect to the elastic properties and assess various rock physics diagnostics, the estimated petrophysical parameters were water saturations, porosity, and shale volume. By connecting the saturated bulk modulus to its porosity, the bulk modulus of the porous frame, the bulk modulus of the mineral matrix, and the bulk modulus of the pore filling fluids, the Gassmann fluid replacement modeling was used to simulate various fluid scenarios. Finally, an evaluation was conducted on rock physics boundaries and cement models. The reservoirs are rated as extremely good based on the results. The rock physics model generated showed the friable sand model is a suitable model for the said basin and the rock physics bounds that were evaluated indicated that the sediments are deposited below the critical porosity and less compacted thereby depicting a soft sand model. Conventional petrophysical study would not be able to reveal the clustering of the gas sands from the brine sand and the shale zone in connection to the geologic trend as demonstrated by the rock physics template. The anticipated model will be useful to extrapolate the study to other sedimentary basins in Cameroon and estimate porosity from the impedance acquired from seismic data throughout the basin.

Within the northwestern Desert of Egypt, the Beni Suef Basin is one of the most significant hydrocarbon basins. After over 50 years of exploration and production, the main challenge today is to locate new areas that are prolific to oil and gas production. The study aims to identify potential drilling locations by integrating geochemical methods, 1D and 2D basin modelling, well log analysis, seismic interpretation, and hydrocarbon migration pathways. Results show that the Lower Kharita shale contain both oil and gas generating kerogen with generating potential that ranges from fair to very good. The source rocks within the Lower Kharita are mature (main oil window) and have a transformation ratio of up to 60% based on 1D basin modeling. The modelled 2D vitrinite reflectance map showed that hydrocarbon generation occurred at depths greater than 10850 ft (3306 m) with vitrinite reflectance values greater than 0.65% in both the northern and southern parts of the Beni Suef Field. Two main reservoirs were studied. The lower Bahariya and upper Kharita reservoirs are composed of well to moderately sorted sandstone. The subsurface structural maps, reservoirs, source, and charge modeling maps have been constructed to show the migration pathways and accumulation areas. There are three accumulation areas identified. The first accumulation area is located in the high structure up-dip section of the major normal fault, and migration direction is from northwest. The second and third accumulation areas are located in the low structure and down-dip from the normal fault, with migration of hydrocarbons from southeast. The latter two areas should be tested by drilling in the future.

Muscovite ⁴⁰Ar/³⁹Ar geochronology on ten samples from complex-type, rare-metal (e.g., Li, Cs, Ta, Rb, Be, Nb, Sn and W) pegmatites from the Bikita and Mweza pegmatite fields in Zimbabwe yield ages that range from ca. 2622–1940 Ma. One pegmatite from the Bikita field yield a relict age of ca. 2622 Ma which is similar to the ca. 2.62 Ga emplacement age of the Main Bikita Pegmatite previously determined by laser ablation analyses of Ta–Nb–Sn oxides. Younger muscovite ⁴⁰Ar/³⁹Ar ages of ca. 2580–2539 Ma in the Bikita field may reflect post-pegmatite emplacement tectono-thermal events under fluid-mediated conditions. Data from the Mweza pegmatite district yield dates of ca. 2027–1940 Ma which reflect a Palaeoproterozoic overprint. Palaeoproterozoic ⁴⁰Ar/³⁹Ar ages are spatially restricted to near the Northern Marginal Zone–Zimbabwe Craton boundary and are absent towards the interior of the craton in the Bikita pegmatite field.

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

The Western High Atlas is one of the most important sectors of the Variscan foldbelt in Morocco and also of its Alpine intracontinental orogen. Its tectonic evolution is thus essential for the understanding, not only of the Moroccan geology, but also of the interaction between successive orogenic cycles which is one of the focus of this article. Furthermore, as the Moroccan Variscides must be seen in the context of the complex dextral collision between Laurasia and Gondwana, this is also a contribution to the comprehension of the complexity of transpressive regimes and strain partitioning processes.

New data show that WNW-ESE sinistral Variscan shear zones (mainly the Addouz-Adassil-Anamrou one - AAAsz), although secondary to the main orogenic scale ENE-WSW dextral shear zones, must be considered in order to understand the Late Paleozoic deformation and its Alpine reactivation. In fact, the concentration of the last stages of the Variscan ductile deformation in the steep and irregular AAAsz played an important role in the initiation and localisation of the minor magmatic events and influenced the geometry and kinematics of the Late Variscan WNW-ESE sinistral shear zones. Furthermore, during the Alpine inversion, the orientation of the AAAsz and related major anisotropies tend to be reactivated, with a major reverse component, giving rise to the irregular Adassil-Medinet fault zone, one of the most important structures of the Western High Atlas.

Detrital quartz grains extracted from sebkha sediments in southeastern Tunisia underwent scanning electron microscopy analysis to identify sediment sources and assess the influence of the saline environment on the grains. Core sediments collected from Sebkha el Melah and Sebkha Mhabeul cover the last 5000 and 2000 BP, respectively. The uppermost Unit III, present in both cores, exhibits two distinct facies based on the mechanical microtextures of its quartz grains. The aeolian facies sediments are characterized by quartz grains with rounded outlines, upturned plates, and crescentic percussion marks. In contrast, the fluvial facies sediments are associated with quartz grains featuring subangular outlines, v-shaped percussion cracks, conchoidal fractures. Observations on the quartz grains of the sebkhas suggest multiple transportation and processing events, indicating long-distance transport and rapid deposition rates. The majority of quartz grains appear to originate from the surrounding terrain, reflecting the dynamic geological history of the region. This study delves into the connection between microtexture variations on quartz grain surfaces and specific historical climatic conditions in the sebkhas. By examining geochemical variations along the cores, facies with elevated salt concentrations corresponding to warmer periods reveal extensively weathered quartz grains. This substantial chemical alteration is evident through microtextures such as oriented etch pits, anastomosed dissolution networks and solution crevasses. The profound dissolution has significantly impacted the quartz lattice, resulting in the decomposition of the grains and the formation of "cauliflower" or "spongy" shapes, erasing prior microtextures. Conversely, during less warm periods, quartz dissolution was less severe, thereby preserving microtextures.

Sand grain surfaces are notably sensitive to both transport processes and variations in the physicochemical environment. In the hypersaline and confined environments of sebkhas in southeastern Tunisia, potent post-sedimentary processes can obliterate and obscure the original microtextures recorded on grains from previous environments due to highly fluctuating physicochemical conditions.

This study investigates the geophysical properties of the Abu Roash F Member (AR/F) as a potential unconventional oil reservoir in Heba Field, located in the eastern region of the Abu Gharadig Basin (Egypt). Analysis of seismic data reveals the influence of Late Cretaceous dextral wrenching tectonic activity, evidenced by the presence of ENE-WSW anticlinal folds and intersecting NW-SE as well as WNW-ESE extensional faults throughout the study area. This tectonic regime is believed to have played a significant role in the formation of micro-fractures within the AR/F carbonate, which can enhance the reservoir's potential by improving its permeability and porosity. Petrophysical assessment identifies favorable reservoir zones within specific depth intervals across multiple wells. These zones exhibit promising reservoir quality, characterized by low shale volume (below 0.20), which indicates a cleaner reservoir rock with less clay content. Additionally, the high total porosity (ranging from 0.20 to 0.25) and moderate effective porosity (ranging from 0.15 to 0.20) suggest that a substantial portion of the rock's pore space is available for storing hydrocarbons. The low water saturation (between 0.40 and 0.50) further supports the presence of hydrocarbons, as it indicates that the pores are not predominantly filled with water. Furthermore, the high hydrocarbon saturation (ranging from 0.50 to 0.60) confirms the significant presence of hydrocarbons within the reservoir. The quantitative assessment of these petrophysical properties across all analyzed wells further confirms the good reservoir quality and potential of the AR/F Member. The integration of seismic and petrophysical data provides a comprehensive understanding of the reservoir characteristics, highlighting the AR/F Member as a significant oil reservoir in the eastern border of the Abu Gharadig Basin, particularly in Heba Field. This study's findings complement the existing knowledge of oil-bearing formations in the region, suggesting that the AR/F Member could be a valuable addition to the basin's hydrocarbon resources. The results underscore the importance of further exploration and development of the AR/F Member to fully realize its potential as a major contributor to the area's oil production.

We present the first case of reservoir induced seismicity (RIS) ever reported in the Angola Craton, Laúca reservoir. Impoundment of the reservoir began in late 2017 and seismicity started in March 2018 shortly after the water level reached 86 m. Earthquakes continued to be recorded in the following years, with 287 events detected between March/2018 and April/2024. The largest event had magnitude 3.0 ML. The dam is 156 m tall, and the total reservoir volume is 5,044.85 Hm³. The reservoir area is monitored by two stations, LAUC and ZERO. Seismic data is of high quality, with sharp P- and S-wave arrivals at both stations. We derive a half-space velocity model for the lake area using a composite Wadati diagram and minimization of travel time residuals and locate 90 events with arrivals at both stations. S-to-P converted phases at the surface were used to help constrain hypocentral depths and epicentral locations. We use waveform modeling to determine focal mechanisms for ten events using the program FMNEAR, and invert for the local stress field. We find the faulting regime in the area to be strike-slip and SH_Max oriented roughly NE-SW. We compare our results to the broader stress field in western Central Africa and find them to be in agreement with nearby stress determinations from focal mechanisms in the Congo Basin. Our results make an important contribution to the state of knowledge of the stress field in western Central Africa, which can be taken into consideration in future geodynamic models of the Nubian plate as more data is gathered and help further our understanding of stress sources in this region.

A hydro-geophysical investigation was carried out in the northeastern parts of Botswana, in a wellfield called Dukwi, with the aim of estimating the aquifer hydraulic parameters employing the geo-electrical method. A Schlumberger electrode configuration of 16 vertical electrical soundings (VES) was carried out in the Dukwi Wellfield. The data were recorded with a maximum AB/2 spacing of 500 m and processed using the InterpexRESIXIP^tm software. In addition, physicochemical parameter measurements were carried out for 19 groundwater samples in the field. Aquifer hydraulic parameters, like hydraulic conductivity (K) and transmissivity (T), were computed from the pumping test data. An interpretation was made using statistics, including linear regression analysis. The major aquifer is the Mea Arkose Formation, and its estimated hydraulic conductivity and transmissivity ranges from 15.4 to 47.3 m/day and 539.9–2113.9 m²/day, respectively. The aquifer transverse resistance (Tr) and longitudinal conductance (S) of the rocks overlying the aquifer ranges from 945 to 2440 Ωm² and 1.5 to 9.7 Ω^-1, respectively. The R² value for the geo-electrical and pumping test K and T values are 0.689 and 0.896, respectively. The estimated K has a positive and strong correlation (R² = 0.946) with the resistivity. A strong positive correlation (R² = 0.892) is obtained between the estimated T and Tr of the aquifer. On the basis of these correlations, equations that relate the resistivity of the aquifer to its hydraulic parameters were formulated. These equations enable the evaluation of aquifer hydraulic parameters where there is a lack of pumping test data. Furthermore, these equations can be used in other parts of the country and in regions elsewhere that have similar geological and hydrogeological setups.

A total of 208 paleomagnetic core samples were collected from twenty-seven sites within the Butajira volcano chains of the central Main Ethiopian rift (MER), providing insights into tectonic rotation. Core samples were taken from ignimbrite (∼1.7–2.54Ma), basalt, and trachyte rock units (<1.6Ma) along the western margin of the central MER. Each core sample underwent analysis using twin specimens subjected to stepwise alternate field (AF) and thermal (Th) demagnetization techniques. Furthermore, a single specimen from every site was employed for rock-magnetic experiments. Detailed rock magnetic experiments identified titano-magnetite, magnetite, and hematite as the remanence carrier magnetic minerals. The natural remanent magnetization (NRM) directional analysis revealed two components:1) a low-stability (LS) component removed by low-temperature (<300 °C) and low-field (<20 mT) and 2) a high-stability (HS) with heating >300 °C or 20 mT increments showed vectors pointing to the plot's origin, indicative of the characteristic remanent magnetization (ChRM). All sites, except one, displayed reverse polarity NRM. The tilt-corrected mean direction calculated from 19 sites, yielded D_o = 355.7°, I_o = 9.9° (N = 19, K = 22.9, α₉₅ = 7.3°), indicates a 6.2° ± 6.2° counterclockwise (CCW) tectonic rotation relative to the reference dipole geomagnetic field direction of 2Ma D_x = 1.9°, I_x = 15.5° (N = 32, K = 105.6, α₉₅ = 2.5°) for stable Africa (Besse and Courtillot, 2003). The oblique rifting and pure-dip slip kinematics that characterize the border faults in the study area could have caused this minor counterclockwise rotation and agrees with analogue model prediction of Corti et al.,2013.