Results in Geophysical Sciences最新文献

This study mapped deep crustal network of the Equatorial Atlantic Fracture Zones in southern Nigeria using regional scale magnetic and gravity data in an attempt to link the Equatorial Atlantic Fracture Zones to structures of similar trend in the continental margin of southern Nigeria. The data were analysed by removing the regional trend of the data and isolating deep-seated anomalies of interest. Thereafter, the theta potential gradient tensor method was applied to delineate the network of the Equatorial Atlantic Fracture Zones and their linkage to other associated deep-seated lineaments in southern Nigeria. Within the limit of the study area, the results obtained revealed the network of the Romanche, Chain and Charcot Fractures Zones, and their extent, several hundreds of kilometers within the landmass of southern Nigeria. The build-up and transformation of the mapped lineaments further supported evidence of extensional crustal deformation within the Niger Delta Basin and Benue Trough axis which may have influenced rifting in the early Cretaceous.

The Rough Creek Graben (RCG) within the U.S. Midcontinent craton possesses some of the world's longest cave and karst landforms such as the Mammoth Cave. Despite the graben's position at the junction of two active seismic zones, the New Madrid Seismic Zone (NMSZ) and the Wabash Valley Seismic Zone (WVSZ), the RCG is seismically inactive. Here, we map mesoscale geologic structures within the RCG for understanding structural deformation and styles. We integrated DEM (Digital Elevation Model)-based structural analysis, field-based structural documentation, 3-d models, and GIS-based spatial analysis. Results of this integrated study reveal the presence of dominantly E-W-striking mesoscale structures with secondary NW-SE- and NE-SW-strikes. Many of these structures are documented in the field as high-angle normal faults with distinct fault cores, displacements, and damage zones. Most of the mapped structures are likely originated from extensional stresses associated with the development of the RCG. The presence of less frequent contractional structures (reverse and thrust faults, and folds) suggests subsequent strain localization within this graben. Although most of these structures are seismically inactive, understanding their deformation and kinematics is crucial for synthesizing any reactivation in response to a large magnitude earthquake originating from either the NMSZ or the WVSZ. The findings of this study not only advance our knowledge of the mesoscale structures in a midcontinent cratonic rift but also help evaluate potential earthquake hazards and risks. Moreover, positive correlations between these mesoscale structures and the karst systems suggest a relationship between these structures and the ultimate morphology of conduit systems that assisted in developing these karst landforms.

P-wave velocity and gas content of major coal seams are predicted from three wells from the South Karanpura coalfield, India. Multilayered feed-forward neural network (MLFN) model is developed for prediction P-wave velocity from two wells using three input log parameters: gamma ray, resistivity and bulk density. The model is tested on coal seams: Sayal, Balkudra, Banasgarah and Argada of three wells namely; A, B and C with satisfactory goodness of fit (R² = 0.77). The ratio of P-wave and S-wave velocities (Vp/Vs) ranges from 1.68 to 3.10 in the coal seams of this field. Cleat density using image log is obtained from four major coal seams namely; Balkudra, Kurse, Hathidari, Banasgarah varying from 1/m to 11/m. Ash content varies from 15 to 51% whereas gas content of major coal seams varies from 2.30 to 4.4 10⁻3 kg/m³. There is a good correlation between ash content and Vp/Vs of coal seams under the study area. MLFN model for prediction of gas content of the above-mentioned coal seams from two well is trained with input parameters such as: Vp/Vs, bulk density, cleat density and ash content. The model estimates gas content of other coal seams: Argada from well A, Hathidari from well B and Balkudra, Kurse, Banasgarah from well C with satisfactory R² = 0.77. Maximum horizontal stress orientation is observed from azimuthal shear wave anisotropy from cross multipole array acoustic (XMAC) log for a well A. It varies from N110° to N115° in these coal seams. Permeability of coal seams are computed from X-tended Range Microresistivity Imager (XRMI) tool from a well C. Permeability of coal is varying from 0.5 md in Saunda seam to 17.29 md in Banasgarah seam. The estimated permeability matches well with the pre-fracture permeability of these seams. It is observed that increase in cleat density enhances coal seam fracture permeability in coal seam.

The characterization of the Pouma-Cameroon quartzite deposit with the aim of determining its lateral extension along the Douala-Yaoundé axis required, among other things, the examination of the petrographic and morpho-structural domain as well as the determination of the electrical properties of the formations in the study area. 24 vertical electrical sounding points were carried out using a Schlumberger type electrode configuration; also, 7 manual auger boreholes were carried out, these resulted on one hand in the following types of curves: KH, QH, AKH, H, HK, AK, HA, A; and on the other hand to lithological logs with a clayey sandy texture, gravelly clayey sand to lateritic. The calculation of the densities as well as the petro-structural studies made it possible to confirm the geological context of the study area. The various results obtained permitted us to locate the fresh quartzite formation at the center of the study area. It stretches along the Douala-Yaoundé axis over 1871.4 m for the fresh formation and 1991 m for the slightly altered formation.

Adequate reservoir information is needed for accurate reservoir characterization towards enhancement of hydrocarbon recovery. The reservoir petrophysical parameters and elastic (seismic) properties are related to the quantity of reservoir rock fluids as well as the interaction of fluid types in the reservoirs’ pore space. Apart from the usually study of the petrophysical properties, the elastic properties are also vital because they are affected by the pore fluids interchange during hydrocarbon production or extraction. This study is aimed at examining and predicting those elastic properties that are most responsive and sensitive to changes during the fluid substitution. Information from three well logs from the ‘Royal G’ field’ situated in Niger Delta (onshore) were used for the petrophysical analysis and rock physics interpretation. The 3D crossplot outcomes were established by utilising Gassmann's fluid replacement or substitution modelling evident considering 0% oil and 80% brine (full) saturations in the reservoirs. Reservoir A and B have porosity, permeability and hydrocarbon saturation of 10–29%, 285–670 mD and 62–90% respectively. The 3D crossplot of the pseudo elastic logs give descriptive and distinct lithology and definite fluid content separation. Rock properties identified and noted to be extremely sensitive/reactive for lithology as well as fluid differentiation in the probed reservoirs are Acoustic Impedance, Density, Poisson ratio and Lambda-Rho. The outcome will generally enhance production and recovery of hydrocarbon.