Journal of African Earth Sciences最新文献

The abundances and distribution diamondoids were studied with a series of crude oil samples from Block 4 of the Mugald Basin in Sudan, to investigate the validity of various diamondoid parameters on the oil source, thermal maturity and biodegradation information. All crude oils of Block 4 were generated by lacustrine source in lower part of the Abu Gabra Formation as a whole, and can be further divided into two crude oil groups: oil group of the Neem-Azraq Uplift and oil group of K-3 well and B-1 well. They are all mature oils within oil window and four crude oils from the Neem-Azraq Uplift were slightly biodegraded. The concentrations of diamondoids in oils of K-3 well and B-1 well, especially the concentration of diamantanes, are higher than those of oils from the Neem-Azraq Uplift. The concentrations of diamondoids are mainly controlled by source of oil, while the influence of thermal maturity and biodegradation may be limited. The isomerization ratios as indicators of thermal maturity are inapplicable at the mature stage, but DMDI-2, 4,8/3,4-DMD and 1-EA/3-EA can clearly distinguish the two groups of crude oils. The concentration ratios of oils from K-3 and B-1 wells are significantly lower than these of oils from Neem-Azraq Uplift, and these concentration ratios including newly proposed parameters such as MAs/DMAs, DMAs/TMAs, D/MDs, MDs/DMDs and so on, are effective indicators on organofacies and oil group division within oil window.

The Shiranish Formation represents one of the most important fractured reservoirs in northern Iraq. In this work, the petrophysical properties of the formation have been fully characterised using microscopy, core analysis, and well log analysis using conventional methods as well as new quantitative diagenetic approaches. During this work we have developed methods to quantify a petrophysical heterogeneity index (χ), reservoir quality indicator (RQI), and fracture effect index (FEI) for each of the stratigraphic units of the formation. The FEI was calculated by dividing the difference between the mean permeability of the wireline log data and the mean permeability of the unfractured core plug samples by the difference between the mean porosity of the wireline log data and the mean porosity of the unfractured core plug samples. This study shows that the Shiranish Formation has a fracturing pore system in all the characterised units, but it is particularly well developed in U.4, which shows the best reservoir quality (A and B). The new methods developed in this study can be applied to any carbonate formation to provide a trustworthy way to obtain a reservoir quality indicator linked to the petrophysical heterogeneity of the studied formation.

This study presents the evaluation of the potential reservoir intervals in the early Cretaceous Alam El Bueib Formation of the Shushan Basin, Western Desert. Seismic 2D lines, and wireline logs (including image logs) were assessed to characterize the potential intervals. The study area is characterized by E-W to ENE-WSW striking parallel sets of steeply dipping normal faults. Based on the breakouts on image log, the regional maximum horizontal stress orientation is inferred as NE-SW. The AEB Formation, as observed on the image log, consists of massive sandstones, planar laminated siltstones, sandstone-siltstone heteroliths and laminated shales, deposited in a fluvial depositional environment. The bedding planes are WNW-ESE striking with a mean true dip of NNE (around 10°). Wireline log based quantitative petrophysical assessment identified multiple promising hydrocarbon-bearing reservoir intervals within the AEB Formation. The potential reservoir intervals are clean with shale content <10% with water saturation <50%. However, all these intervals are tight with effective porosity between 4 and 12%, dominantly ∼5%. Such tight effective porosity can be contributed by extensive silica cementation in the AEB Formation, as seen from the nearby fields in Western Desert. High porosity zones are observed to be water-bearing. The wells drilled in the north and northeastern area exhibit a cumulative net pay thickness between 70 and 150 ft, while south-southeastern region exhibits a very low cumulative net pay of 10–30 ft. Based on the breakout son image log, the regional minimum horizontal stress orientation is inferred as NW-SE, which can be preferred azimuth for placing highly deviated or horizontal wells to exploit such tight clastic reservoirs by optimizing hydraulic fracture propagation. The formation evaluation presented in this work shed critical insights into the tight hydrocarbon reservoir potential of the early Cretaceous AEB.

This study aims at reconstructing the paleostress history of the Outer (offshore) Kwanza basin (West African passive margin) and at comparing it to stress results acquired further north in the Congo basin. Three oriented borehole cores provided by TotalEnergies and reaching the syn-rift, Barremian-Aptian pre-salt carbonates offshore Angola were investigated. Paleopiezometry based on the Stylolite Roughness Inversion Technique (SRIT) and Calcite Twin Inversion Technique (CSIT) was combined with fracture analysis, U–Pb geochronology of carbonates and burial modelling to unravel the orientations and magnitudes of horizontal and vertical stresses affecting the pre-salt carbonates over time. Calcite twins were measured from a primary sparite matrix, and the inversion process unravelled a polyphase stress history, comprising ∼ E-W and NE-SW extensional trends that we associate to the rifting (130-112 Ma) that led to the opening of the South Atlantic ocean. The ∼ E-W extension is consistent with the early occurrence of N-S striking normal faults which developed in relation to the reactivation of inherited basement structures. This ∼ E-W extension evolved during the Barremian-Aptian (?) into the dominant regional NE-SW extension marked by large-scale NW-SE striking normal faults. The stress history also comprises compressional and strike-slip stress regimes associated with a ∼N-S trending σ₁ which can be related to the transfer of orogenic stresses from the distant Africa-Eurasia plate boundary at ∼67-60 Ma. Finally, compressional and strike-slip stress regimes associated with a ENE-WSW to ∼E-W trending σ₁ dominated since at least ∼17-15 Ma (possibly ∼34 Ma); they are interpreted as the expression of the mid-Atlantic ridge push. These (paleo)stress results are compared and combined with earlier paleostress reconstructions in the northern offshore Lower Congo basin (also belonging to the Central segment of the margin) and in the onshore Congo basin in order to refine the stress record and the timing of tectonic events since the early Cretaceous, thus providing unprecedented constraints on the tectonic history of the West Africa passive margin. This tectonic history includes both extensional and compressional events, and was driven mainly by far-field stresses, either gravitational or tectonic in origin, which are related to interactions between the African plate and surrounding plates.

Sediment mass movements and their associated Mass Transport Deposits (MTDs) have been widely studied due to their economic and geohazard potentials. This study combines 2D and 3D seismic reflection data with a seismic facies approach and attribute analysis to reveal the presence and distribution of different MTDs in the southernmost region offshore Tanzania. Results of seismic facies analysis show that the study area contains different Late Cenozoic MTDs. The MTDs have limited petroleum reservoir potential and include slides, slumps, debris flow deposits and occasional turbidites. The formation of these MTDs was caused by tectonic events associated with the development of the East African Rift System. Seismic attribute maps have shown the locations of channels, remnant blocks, headwall scars, and grooves confirming downslope sediment mass mobilization. The seabed attribute maps have shown areas where recent mass mobilizations were initiated. Some of these areas coincide with faults which have dissected the seabed, forming potential future gravity failure sites. Other future gravity failure sites include channel banks and slope edges, which may be present over the whole Tanzania continental margin. Sediment mass movements may be catastrophic, and therefore future infrastructure installations in the area must involve detailed mapping of the seabed to assess geohazard risks and act accordingly.

Detailed field and stratigraphic (lithostratigraphy & biostratigraphy) studies were carried out on the Upper Cretaceous–lower Paleogene successions at Esh-ElMellaha half-graben, Gulf of Suez, Egypt. Four stratigraphic sections were investigated and arranged in a geologic profile extends from south to north as follow: Gabal El-Mellaha, Wadi Abu Had, Wadi Dib and Gabal Tarbul. The field work led to recognize four lithostratigraphic units (formations): Sudr (upper part), Dib, Esna and Thebes (top). The distinctive Dababiya Quarry Member (DQM) which characterizes the Paleocene Eocene Thermal Maximum (PETM) was initially recorded at Esh-ElMellaha region. Sudr Formation is stratigraphically differentiated into two distinctive informal rock units, argillaceous bedded limestone unit and calcareous shale unit. The Dib Formation is here reviewed and correlates with the Dakhla Formation (upper part) in the different geographic neighborhoods of Egypt. It consists of glauconitic carbonate rocks (bioclastic limestone) embracing reworked gravelly and pebbly extra-clasts and broken exhumed mega-fossils (e.g. cephalopods, gastropods and bivalves, corals). Dib Formation is assigned to thelower Paleocene (Danian Stage) according to the occurrence of praemuricids taxa (e.g. Praemurica inconstans and P. uncinata) index fossils. Esna and Thebes formations are assigned to the Ypresian Stage. Dib Formation is bounded by two regional unconformity surfaces (erosional surfaces) as a result of two tectonic events (I and II) linked to Syrian Tectonic Event.

Clay deposits across the globe have variations in mineralogical compositions that elicit their use in numerous applications. In this study, two clay deposits were identified and blended with feldspar and quartz in various amounts to produce different samples of composites that could be used for applications in building services and refractories. Chemical analysis, mineralogical composition, microstructure, physical properties, and thermal behaviours were characterized. The X-ray fluorescence (XRF) analysis results show that the clays belong to the aluminosilicate group containing traces of other oxides which enhance strength and the crystallization of thermally stable phases. The XRD patterns confirmed the presence of dominant quartz with moderate to minor presence of kaolin, orthoclase, and albite. The texture of the clays showed particles of different sizes and shapes with uneven distribution. The EDX characteristic spectrum showed x-ray characteristic peaks of Si, Al, Fe, K, and O which affirmed their major oxide compositions. Physical properties results show that firing at various temperatures and blending resulted to increase in Bulk density and flexural strength while apparent porosity and water absorption decreased. The TGA, DTA, and DSC results show that the two clay minerals are thermally stable. This can be attributed to the nucleation and crystallization of refractory phases like cristobalite and mullite. Addition of feldspar and quartz was found to contribute significantly to improve the overall properties of the investigated clays. A comparison of the various results from the two clay deposits suggests that they could be suitable for building services and refractory applications.

This paper presents the first integrated regional outcrop-based sedimentological study of the northern Aaiun-Tarfaya Basin located in Morocco (NW Africa). The Lower Cretaceous Tan-Tan Formation has been subdivided into six new members and placed within a sequence stratigraphic framework that includes two incomplete depositional sequences. Strong thickness variations of individual lithostratigraphic units from north to south suggest differential subsidence during sedimentation and/or the existence of major topography on the basal unconformity that the succession onlaps. The results provide valuable insights into the timing of local tectonics in the Western Anti-Atlas and the control on the evolution of the sedimentary system. Deposition of each of these six units is interpreted to be the result of a complex interplay between an overall eustatic sea-level rise during the early Cretaceous, sediment delivery controlled by tectonic movements in the Western Anti-Atlas and Reguibat Shield and periods of differential subsidence in the basin. The results document the style of evolution of a back-stepping wave-dominated system feeding into the Central Atlantic during the passive margin phase. The improved facies and depositional models together with improved understanding of the evolution of the delta have significant implication for exploring the deep-water equivalents offshore.