GEO-ELECTRIC INVESTIGATION OF GROUNDWATER POTENTIAL IN LADERIN HOUSING ESTATE, ABEOKUTA, SOUTHWESTERN, NIGERIA

Vertical Electrical Sounding (VES) and 2D Electrical Resistivity Tomography (ERT), with Schlumberger and Wenner electrode array configurations respectively were employed to investigate the groundwater potential of Laderin Housing Estate located at Oke-mosan, Abeokuta, Ogun state, southwestern Nigeria. The area is underlain by the basement complex of the southwestern Nigeria. The research aimed at determining the aquifer/groundwater zone and characterizes the lithology of the study area. The geophysical survey involving nine VES and four profile of 2-D ERT lines with lengths varying from 100 m to 150 m were carried out. The field data from both the ERT and VES were processed and interpreted using RES2DINV and WINRESIST software respectively. Geometrical effects from the pseudo-section were removed and an image of true depth and true formation resistivity were produced. Three to four geo-electric layers were revealed in all, which are; topsoil, weathered layer, fractured and fresh basement rock. The result of 2 D inversion provide lithologic unit, weathering profile and geological structure favourable for groundwater potential. The results show that the basement rock was delineated with resistivity values that range between 701.3 Ωm and 9459.3 Ωm. The bedrock topography has a variable thickness of overburden between 3 m and above 16.4 m, which is fairly shallow. The geophysical survey show that VES 2, VES 3, VES 4, VES 5, and VES 6 are possible location for groundwater extraction. The difficult terrain, where thick overburden are located are also promising target for groundwater development.

Oil exploration and discovery in the basement rocks of Bach Ho field: Documentation, facts and lessons

During the 40 years of operation and development (1981 - 2021), the Vietnam - Russia Joint Venture "Vietsovpetro" has witnessed many historical milestones, but the discovery of industrial oil for the first time from the fractured and weathered granite basement at exploration well BH-6 on 11 May 1987 is the most important one. From the first oil in the basement rock of Bach Ho field, Vietsovpetro consecutively discovered industrial oil in other fields in its area of operation, such as Dong Bac Rong (1991), Dong Nam Rong (1995), Nam Rong (2005), Nam Trung Tam Rong (2006), etc. At the beginning of 2018, the 2P (P1+P2) oil reserve from the basement rock accounted for 74% of Vietsovpetro's total balanced reserves at that time. As of 1 October 2021, the total oil produced from the basement reached 235 million m3 (195 million tons), accounting for 86% of Vietsovpetro's total oil output. Being encouraged by the success and experience of Vietsovpetro, other domestic and foreign oil and gas companies (PVEP, JVPC, Talisman, and Petronas, etc.) have explored and discovered oil and gas from the granite basement and put the fields of Rang Dong, Su Tu Den, Hong Ngoc, and Hai Su Den, etc. into operation. This fact has, at the same time, created a strong attraction for domestic and foreign investors, making important contributions to the rapid development of Vietnam's oil and gas industry which was still very young at the time. The above-mentioned shows that it is time to study data and documents, draw lessons from success and failure gained during the 40 years of basement exploration. The outcomes should be used as a basis to formulate an appropriate exploration strategy for Vietsovpetro in the coming decades with strong fluctuations in the oil and gas market expected, and the inevitable depletion of non-renewable resources worldwide.

Gravity Analysis for Subsurface Characterization and Depth Estimation of Muda River Basin, Kedah, Peninsular Malaysia

Gravity survey is one of the passive geophysical techniques commonly used to delineate geological formations, especially in determining basement rock and the overlying deposit. Geologically, the study area is made up of thick quaternary alluvium deposited on top of the older basement rock. The Muda River basin constitutes, approximately, of more than 300 m of thick quaternary alluvium overlying the unknown basement rock type. Previous studies, including drilling and geo-electrical resistivity surveys, were conducted in the area but none of them managed to conclusively determine the basement rock type and depth precisely. Hence, a regional gravity survey was conducted to determine the thickness of the quaternary sediments prior to assessing the sustainability of the Muda River basin. Gravity readings were made at 347 gravity stations spaced at 3–5 km intervals using Scintrex CG-3 covering an area and a perimeter of 9000 km2 and 730 km, respectively. The gravity data were then conventionally reduced for drift, free air, latitude, Bouguer, and terrain corrections. These data were then consequently analyzed to generate Bouguer, regional and total horizontal derivative (THD) anomaly maps for qualitative and quantitative interpretations. The Bouguer gravity anomaly map shows low gravity values in the north-eastern part of the study area interpreted as representing the Main Range granitic body, while relatively higher gravity values observed in the south-western part are interpreted as representing sedimentary rocks of Semanggol and Mahang formations. Patterns observed in the THD anomaly and Euler deconvolution maps closely resembled the presence of structural features such as fault lineaments dominantly trending along NW-SE and NE-SW like the trends of topographic lineaments in the study area. Based on power spectral analysis of the gravity data, the average depth of shallow body, representing alluvium, and deep body, representing underlying rock formations, are 0.5 km and 1.2 km, respectively. The thickness of Quaternary sediment and the depth of sedimentary formation can be more precisely estimated by other geophysical techniques such as the seismic reflection survey.

Elevation of underlying basement rock, Ogdensburg Harbor, NY

Over six linear miles of shallow acoustic reflection geophysical data were collected in an 800 ft by 300 ft survey region at Ogdensburg Harbor, Ogdensburg, NY. To better accommodate modern commercial vessels and expand the harbor’s capacity, the current navigable depth of -19 ft Low Water Depth (LWD) needs to be increased to -28 ft LWD, and an accurate map of the nature of the riverbed material (e.g., unconsolidated sediment, partially indurated glacial till, or bedrock) is required to effectively plan for removal. A total of 28 boreholes were previously collected to map the stratigraphy, and the effort revealed significant spatial variability in unit thickness and elevation between adjacent boreholes. To accurately map this variable stratigraphy, chirp sub-bottom profiles were collected throughout the region, with an average line spacing of 13 ft. These sub-bottom data, validated and augmented by the borehole data, resulted in high-resolution spatial maps of stratigraphic elevation and thickness for the study area. The data will allow for more accurate assessment of the type and extent of different dredging efforts required to achieve a future uniform depth of -28 ft LWD for the navigable region.

Biostratigraphic and petrological characteristics of Cretaceous–Paleogene sediments in the eastern Cuu Long delta

The basic structure of reservoir models is strongly affected by the stratigraphic interpretation and the properties of reservoir rock. The estimation of the volume of hydrocarbon accumulation will depend on the accuracy of predicting and distributing reservoir quality. Biostratigraphic and sedimentary petrologic results provide geological information to clarify the stratigraphy and properties of sedimentary rocks on the eastern margin of the Cuu Long delta plain trough. On the basis of stratigraphic correlation of the wells and studying the structures of the area, the deposition, thickness of sediments and the ability to correlate with prospect rocks on the southeastern Vietnam continental shelf were determined. The results have identified the basement rock of wells TC-1 and TC-2, which are Cretaceous metasandstones and were deposited in freshwater fluvial environments with high-energy conditions. The upper Paleogene sediments were overlaid directly on the Cretaceous basement rock in the TC-2 well but were completely absent in the TC-1 well. The lithologic composition is mainly feldspathic litharenite sandstone, which is formed in freshwater fluvial and freshwater lacustrine environments. This study provides new data on stratigraphic column of the eastern Cuu Long delta plain. The data show that the age of the sedimentary basement rock is Cretaceous which has not been determined in previous studies. The findings of this study can help for better understanding of the geological development history and the completely stratigraphic column of the Cuu Long delta to correlate with objects that are hydrocarbon accumulation in Cuu Long sedimentary basin in order to improve efficiency in petroleum exploration activities.

Improvement of Groundwater Recharge Mechanisms Knowledge in Rural Sahelian Zones: Case Study of Tougou Catchment, Burkina Faso

<p>The lack of adequate information on groundwater recharge mechanisms in the basement rock area of Sahelian regions does not allow to estimate recharge rates. Thus, this study, which aims to improve the knowledge of the groundwater recharge mechanisms of the Tougou (catchment of 37 km<sup>2 </sup>representing a basement rock in Sahel of West Africa) aquifers was initiated. The first step was to characterize the geology in terms of geometry and structure. The ERT profile (1.2 km length) crossing perpendicularly the river and lithologs from 10 observation wells (Average depth: 25m) and 1 borehole (Depth: 60 m) were used to make the correspondence between geological and geophysical data. The second step was to characterize vertically and laterally aquifers recharge mechanisms under the ephemeral river and two river banks. Hence, hourly to daily groundwater levels, electrical conductivity, and temperature of groundwater have been measured in those 10 observation wells and 1 borehole (Period: 2016-2020). The river water levels and the rainfall were also collected. The cross-correlation function was used between the rainfall or river water levels and the hydraulic heads time series. The geological characterization showed from top to bottom:</p><ul><li><strong>Residual soils: </strong>1 m to 2 m thick, present in the riverbed and on the right bank;</li> <li><strong>Laterite </strong>(lateritic clays and lateritic cuirass): 2 m to 14 m thick, absent in the riverbed and present on the two banks;</li> <li><strong>Laterally continuous clayey saprolite</strong>: 10 m to 22 m thick;</li> <li><strong>Weathered schist:</strong> 32 m thick in the river. A bedrock was found at a depth of 55 m.</li> </ul><p>This geological conceptual model was a grounding for interpreting the results incurred from other data collected. It was ascertained that the weathered schist aquifer below the river is semi-confined (Average water depth: 9.5 m < top: 25 m) and vertically recharged by the saprolite aquifer. Laterally, the clayey saprolite aquifer is recharged by two main flows from:</p><ul><li><strong>The river:</strong> the electrical conductivity and temperature of the groundwater in the clayey saprolite aquifer below the river vary at the same time as the water table increases during the rainy season. In addition, mean hydraulic head differences of +0.3 m and +2 m have been observed between the piezometer located in the river and respectively, the piezometer located at 20 m from the river on the left bank and other piezometers located on the right bank (up to 600 m from the river). A maximum good cross-correlation between hydraulic heads and river water levels rather than with rain was found in all piezometers, but mostly in the one located in the river (cross-correlation = 0.56). These indicate an indirect recharge process.</li> <li><strong>The left bank:</strong> An mean hydraulic head difference (+3 m) which is related to a transfer of hydraulic pressure from probably a nearby recharge area was noted between the piezometers located at 300 m and the riverbed.</li> </ul><p>For further studies, the information obtained will be used to estimate the recharge through different methods including numerical modeling.</p>