Geophysics

Sixty-seven (67) groundwater samples from different land use areas such as dumpsites, industrial, markets and residential all within Lagos, southwest Nigeria, were sampled and sent to ACME Laboratories, Ontario, Canada, for geochemical analysis to determine some essential and toxic elements using inductively coupled plasma-mass spectrometer method. The purpose was to establish baseline values for the geochemical composition of groundwater in some parts of Lagos as well as the health risk assessment. Based on essential elements, 83%, 80% and 100% of groundwater samples were deficient in Ca, Mg and Se, respectively, while 75%, 10%, 7% and 8% of samples can be categorised as soft, moderately hard, hard and very hard water, respectively. The soft water, as well as different deficiencies in water, had been linked to heart problems, impeded growth, fertility disorders and liver diseases. Concentrations of As, Cd, Cr, Pb, Ni and Fe were higher than recommended standards in 2.94%, 1.5%, 1.5%, 29.4%, 4.4% and 4.7% of groundwater samples, respectively. Groundwater in the study area can be classified between not contaminated (< 1) to very highly contaminated (CI > 5) based on the contamination index. Highly contaminated samples were within dumpsites and industrial areas. The land-use activities have a direct or indirect impact on groundwater quality. The apparent sources of groundwater pollutants include industrial discharge, effluents, leachates and municipal wastes. The result of total health risk index for both ingestion and dermal exposure to water showed values far > 1 for all the toxic elements and generally pose non-carcinogenic health risk hazards for child and adult population in the study area. THI for samples from residential, market, industrial and dumpsites showed mean values of (9.60 and 3.75), (14.17 and 5.37), (9.8 and 3.4) and 95.44 and 37.48) for child and adult population, respectively. The degree of contamination in groundwater showed the following trends: dumpsite > industrial > markets > residential in the study area.

Saline water intrusion has been reported within Badore–Ajah area of Lagos coastal environment
which necessitated electrical resistivity survey involving the use of Vertical Electrical Sounding
(VES) and 2-D imaging with the aim of delineating fresh water aquifer in the area. Twenty-eight
(28) VES and 2-D data along four (4) traverses were acquired within the study area. The
interpretation of the 2-D data was carried out using the DIPPRO software, while the results of the
inverted VES data using partial curve matching technique was improved with the aid of
WINRESIST software. Results obtained were further refined with output data constrain using
wireline log and lithology log. The results of the sounding curves revealed four geoelectric layers
comprising of topsoil (mixture of sand and clayey sand) with resistivity values ranging from 61 to
920 Ωm, clayey sand (61-98 Ωm), sandy clay (35-56 Ωm), clay (5-19 Ωm) and sand (123-495
Ωm) respectively. The topsoil has layer thickness values that vary between 0.9 m and 1.4 m. The
second stratum consists of sand with some lenses of clayey sand, having layer thickness values
that ranges from 2.3 and 20.6 m. The third layer is predominantly sandy clay with some
intercalation of sand and clay with layer thickness values ranging from 17.0 to 56.2 m and depth
extending from 26.0 to 65.6 m. The fourth horizon is predominantly sand on traverses 4 and 5, and
mostly clay on traverse 2, which falls within the swamping area. It was established that sand
horizons on traverses 1, 4 and 5 were potential fresh water aquifers from depth of 50 meters and
above. The correlation of the geo-sections with the wireline and borehole lithology logs have
established the distribution of freshwater aquifer within the study area to vary from 50 to about
130 meters. From this study, VES 1, 3, 23, 27 and 28 locations were recognized to be favourable
for groundwater development with fresh water aquifers at depth equal or greater than 50 m.

This study focuses on the effective integration of interpreted 3D seismic and well log data to build static geological models which were used to estimate the Stock Tank Oil initially In Place (STOIIP) volumes of two sandstones reservoirs in the Roan Field, Niger Delta, Nigeria. The data available for this study included two (2) 3D interpreted horizons for Reservoirs X and Y, interpreted fault data and well logs. The method adopted for the study involved the determination of log derived petrophysical parameters (Vshale, porosity and water saturation). This was later followed by fault modeling, pillar gridding and upscaling to quantify the spatial heterogeneity in the reservoirs prior to the distribution of petrophysical properties across the 3D grid. The reservoir structural model showed a common major growth fault defining the boundary of the structure. A rollover anticline formed as a result of deformation of sediments deposited on the downthrown block of the major growth fault in the field. Hence, the trapping mechanism is a fault assisted anticlinal closure. The STOIIP volumes of 2.6 MMSTB and 20.45 MMSTB were estimated for Reservoirs X and Y respectively, as against 2.4 MMSTB and 26.87 MMSTB that were initially estimated using 2D seismic data. These discrepancies can be attributed to overestimation and underestimation of the Gross Rock Volumes (GRVs) of the respective reservoirs from 2D seismic data. Smoothed and well-corrected top reservoir surface for Reservoir X increased GRV and therefore STOIIP while lower range in reservoir thickness for Reservoir Y reduced GRV and STOIIP estimate. Thus reliance on 2D seismic data for accurate estimation of STOIIP volumes comes with lots of uncertainties and consequently, considerable care needs to be taken.