Exploration Boreholes Research Articles

Optimal experimental design for reservoir property estimates in geothermal exploration

During geothermal reservoir development, drilling deep boreholes turns out to be extremely expensive and risky. Thus, it is of great importance to work out the details of suitable borehole locations in advance. Here, given a set of existing boreholes, we demonstrate how a sophisticated numerical technique called optimal experimental design helps to find a location of an additional exploratory borehole that reduces risk and, ultimately, saves cost. More precisely, the approach minimizes the uncertainty when deducing the effective permeability of a buried reservoir layer from a temperature profile measured in this exploratory borehole. In this paper, we (1) outline the mathematical formulation in terms of an optimization problem, (2) describe the numerical implementation involving various software components, and (3) apply the method to a 3D numerical simulation model representing a real geothermal reservoir in northern Italy. Our results show that optimal experimental design is conceptually and computationally feasible for industrial-scale applications. For the particular reservoir and the estimation of permeability from temperature, the optimal location of the additional borehole coincides with regions of high flow rates and large deviations from the mean temperature of the reservoir layer in question. Finally, the presentation shows that, methodologically, the optimization method can be generalized from estimating permeability to finding any other reservoir properties.

Deep Borehole Heat Exchangers — A Conceptual and Comparative Review

Borehole heat exchangers (BHEs) are used for transforming a rock mass into an underground heat storage. Usually, their depth does not exceed 200[Formula: see text]m, but some extend to a depth of almost 3000[Formula: see text]m. Underground heat storages can operate as part of heating and cooling systems, often economically. In winter they extract heat from the rock mass for space heating, while in summer the cooled rock mass is used for air conditioning. The heat extracted from buildings via air conditioning is transferred into the rock mass, thereby regenerating its condition for winter time. Deep borehole exchangers also may operate only in the heating mode. Then, the rock resource conditions are regenerated via heat transfer through neighboring rocks. If a groundwater flow is present, the heat can also be removed and the source conditions regenerated through convection.Here, an overview of the use and operation of deep BHEs around the world is provided. Special emphasis is placed on the Carpathians, where numerous analyses of geothermal heat use have been performed since 1999. Examples of calculations for old oil and gas wells as well as negative exploration boreholes are given. Such analyses have been performed for boreholes in Poland and the Ukraine. However, little research has been published on this subject to date, for reasons described herein.

Geological Engineering Survey and Geotechnical Analysis of Substrate Soil of Erbil Steel Fusion Company

<p>Construction projects steps among the vast and varied construction has remarkable significance range in civil<br />industry. Foundation as an important part of building the, is made of various systems in order to transfer the load<br />from the superstructure to basis. In order to subsurface location checks could be used of identifiers such as<br />boreholes, wells, trenches and in situ tests and laboratory tests. Project facilities are the most important factors to<br />identification the equipment and tools. In this study, by using the results of the exploratory boreholes, field<br />observations, in situ tests and laboratory studies have been consider to evaluate geotechnical condition of<br />Erbil Altun kupri project site. According to this by studying the Geological Engineering condition of site<br />project and Lefran tests, spt and cutting feathers standing on samples taken from boreholes from drilled and<br />comparing results together, which generally consists of sand and gravel with fine particles of silt and Clay, has<br />been determined geotechnical parameters of location. In the calculations after 6.5 meters excavation, location<br />soil considered pre-consolidation and for settlement calculation, under layer soils were divided into several<br />layers. The detailed results of calculations to verify foundation settlement before and after soil improving<br />operation that used DSM method and it’s analysis by using Plaxis 3D software, showed the elastic settlement<br />0.0105 mm in the center of foundation.</p>

The St. Gallen Fault Zone: a long-lived, multiphase structure in the North Alpine Foreland Basin revealed by 3D seismic data

The St. Gallen Fault Zone (SFZ) is a system of major NNE–SSW striking normal faults within the North Alpine Foreland Basin (NAFB), just west of the city of St. Gallen. It used to be only roughly known from 2D seismic data, locally displaying offsets of up to 300 m at the level of the Mesozoic strata. We present a detailed structural interpretation of a recently acquired 3D seismic dataset that reveals the occurrence of multiphase tectonic activity along the SFZ from at least the Late Paleozoic to the early Oligocene, and possibly even later. We can show that the SFZ roots in extensional basement structures that bound small Permo-Carboniferous grabens. Thickness changes in the younger sediments above these Paleozoic grabens indicate several phases of tectonic subsidence during the Triassic and the Jurassic. The Lower Cenozoic units in the northernmost part of the 3D seismic area are also offset by the SFZ. No offsets can be identified in the overlying, shallower part of the Cenozoic units. Most faults constituting the SFZ are favourably oriented in the present-day stress field (SHmax NNW–SSE) to be reactivated in strike-slip mode. The seismic events induced by testing operations at the geothermal exploration borehole “St. Gallen GT-1” (SG GT-1) in July 2013 revealed that, even though the seismicity of northeastern Switzerland is considered to be low and diffuse, parts of the SFZ have to be regarded as critically stressed. Combining the interpretation of geological and seismic data, we conclude that the SFZ represents a reactivated basement-rooted normal fault, which was active during several phases in Permo-Carboniferous and Mesozoic times and that is still active today in strike-slip mode.

Modern method for sealing deep boreholes

Deep, investigation boreholes for suitable location of radioactive repositories and holes with larger diameter for storing such waste at great depth must be effectively isolated from the biosphere. A method for sealing boreholes by use of dense smectite clay where the rock is low-permeable and has only very fine fractures, and by concrete cast where the holes intersect water-bearing fracture zones, has been used for sealing of up to 500m deep exploration boreholes in Sweden and Finland. The paper describes the practical conditions for construction of seals, and the processes that lead to the required tightness of the clay and concrete materials. Application of this technique to even deeper boreholes requires use of mud instead of water for stabilization and for moderating the rate of maturation. Placeability requires that the maturation of the initially unsaturated clay is neither too fast nor too slow and means of controlling the rate are described. The proposed concrete material has a very low cement content, and talc as fluidizer, which gives slow but ultimately very significant strengthening and low solubility of the cement reaction products. At a few kilometer depth the temperature can be more than 60°C, which has to be sustained by the sealing materials, and this makes saponite and mixed-layer smectite/illite clay possible alternatives to pure smectites. The concept is judged to be applicable also to sealing of abandoned boreholes used for fracking.

Proposal of Database Structure for Foundation Soil for Civil Engineering Purposes

Proposal soil characteristics represent the most important physical-mechanical parameters for foundation engineering purposes. At present, there are a number of databases produced by the Czech Geological Survey, which building companies use free of charge. Further to the existing data, the databases could be completed with the proposal soil characteristics that would provide free information on the benchmark characteristics of different foundation soils in a modern, fast and easy manner, and thus facilitated its preliminary assessment. In the future, such data could be interconnected with the database information on borehole exploration, slope instabilities, information on the area geology, etc., on the basis of which could be generated a complex preliminary assessment report whether a locality is suitable for foundation engineering purposes. This would render a fast and easy orientation in the given locality even before the survey itself and would be completely free of charge.

Comparison of Czech and German Information Systems Used for Exploration of Geological Situation in Civil Engineering Practice

The main aim of publication is to compare the information systems of the institutes of Czech Geological Survey and German Federal Institute for Geosciences and Natural Resources from a point of view of information which is needed for civil engineering purposes. The information systems are a base of the modern society. Thanks to them we are able to apply older exploratory works in order to evaluate currently solved projects. An information system becomes a strategic tool to evaluate exploratory works but also for land-use planning. The Czech Geological Survey information systems consists of the database of Borehole Explorations, the Final Reports Archives and the Inventory of Landslides. These three databases are fundamental for civil engineering purposes. The German Federal Institute for Geosciences and Natural Resources information system contains similar databases. It deals with the database of boreholes and the archives of scientific reports and publications. An Inventory of landslides is not available in the German Geological Survey information system even if these questions are discussed within Geohazards. As for the basic map documents which are available online, both the institutes include a geological map, a map of soils, a geophysical map and a map of underground waters or a hydrogeological map and a map of sources or raw materials information system. On the top of that, there is an engineering geological map available in Germany.

The Permian–Triassic transition in the Moatize–Minjova Basin, Karoo Supergroup, Mozambique: A palynological perspective

The Permian–Triassic transition has been identified for the first time in the Karoo Supergroup of the Moatize–Minjova Basin in Mozambique. This transition was identified in the subsurface in deep (ca. 500m) coal exploration boreholes that penetrated the Matinde Formation. Two palynomorph assemblages (assemblage 1 and assemblage 2) assigned to the latest Permian were defined for the Matinde Formation. These assemblages are both dominated by taeniate pollen, cavate trilete spores of Kraeuselisporites spp., associated with conifer (Lueckisporites virkkiae) and pteridosperm pollen (Guttulapollenites hannonicus and Weylandites lucifer). Assemblage 3 represents a microfloral turnover. For the first time specimens of Lunatisporites pellucidus appear in an assemblage dominated by lycopod spore taxa such as Lundbladisporites spp. This assemblage was assigned to the Early Triassic and occurs at the top of borehole DW 132 within the Matinde Formation. The palynostratigraphic signature obtained for the Permian–Triassic transition puts the Moatize–Minjova Basin in the central Gondwana palaeobiogeographic province with strong affinities with the Karoo basins of Madagascar and the Salt Range Basin in Pakistan. These new data indicate that coal deposits accumulated in the Moatize–Minjova Basin until the latest Permian and that these are possibly more extensive temporally than previously described, opening new perspectives for coal and unconventional hydrocarbon exploration in this basin.

A new approach provides opportunities for spectral gamma analysis in boreholes for mineral exploration

Wide-spread application of gamma ray logging system in mineral exploration has long been desired by the mining industry, but not achieved. New developments in borehole logging approaches the Deep Exploration and Technologies Commonwealth Research Centre (DET CRC) have recently created Logging-While-Drilling tools, named the Autosonde and Shuttle, to measure natural gamma ray activity during drilling process. The Shuttle in particular will allow the collection of high quality natural gamma spectra by placing a sensor on the core-barrel and logging whilst drilling; a much slower process than wireline logging. Thus, a new approach collecting gamma data provides opportunities to use natural gamma radiation data in ways not normally done with conventional wireline tools. We have used a prototype spectral gamma sensor using BGO crystal on a wireline to simulate the data that will be collected by a shuttle system to demonstrate the data quality and to test whether more sophisticated data analysis of spectral attributes such as the ratio of Photo-electric to Compton gamma bands, or minerals indicator, will further lithological information than the standard K, U, Th analysis. Our preliminary results indicate that spectral data collected by the Shuttle will allow the heavy mineral indicator to be used and opens the possibility of better lithology identification tools.

Analytical research of the relation between hydraulic mechanism of the axial pressure of the drilling rig and technological parameters of the drilling regimes of the geological exploration boreholes

Analytical research of the relation between hydraulic mechanism of the axial pressure of the drilling rig and technological parameters of the drilling regimes of the geological exploration boreholes

Lead and zinc geochemical behavior based on geological characteristics in Parkam Porphyry Copper System, Kerman, Iran

Parkam (Sarah) porphyry system is located on the metallogenic belt of Kerman, Iran. Due to existence of some copper-rich resources in this region, finding out the exact statistical characteristics such as distribution of data population, mean, variance and data population behavior of elements like Cu, Mo, Pb and Zn is necessary for interpreting their geological behavior. For this reason, precise calculation of statistical characteristics of Pb and Zn grade datasets was performed and results were interpreted geologically. The natures of Pb and Zn distributions were initially identified and their distributions were normalized through statistical treatment. Subsequently, the variograms were calculated for each exploration borehole and show that both Pb and Zn geochemical variates are spatially correlated. According to the similarity of the behavior of Pb and Zn in these calculations, it is decided to measure their exact behavior applying K-means clustering method. K-means clustering results show that the Zn grade varies linearly relative to that of Pb values and their behavior is similar. Based on the geochemical behavior similarity of Pb and Zn, throughout the pervasive secondary hydrothermal activity, they are remobilized in the similar manner, from the deep to the shallow levels of the mineralization zones. However, statistical analysis suggests that hydrothermal activity associated with secondary waters in Parkam is effective in remobilizing and enriching both Pb and Zn since they have similar geochemical characteristics. However, the process does not result in generation of economic concentrations.

Application of the fractal method to determine the membership function parameter for geoelectrical data (case study: Hamyj copper deposit, Iran)

The selection of a suitable membership function and its parameters plays a critical role in the integration of layer information by the fuzzy method. In this paper, parameters of membership function for induced polarization (IP) and resistivity (RS) data (in the Hamyj copper deposit) have been determined by the threshold parameter of IP and resistivity data, already determined by expert opinion or drilling data. The Hamyj deposit is located about 80 km west of Birjand city, South Khorasan province, Iran. In this area, resistivity and induced polarization data have been surveyed by dipole–dipole array. In this paper, outlier-induced polarization data have been corrected by the Doerffel method and then IP and resistivity data have been inversed by the Newton and Gauss–Newton methods. The threshold of the IP data is recognized by statistical (gap statistic) and fractal (concentration–area) methods. The determined threshold by the fractal method is higher than the gap statistic. These two thresholds have been used to determine the S-shape function for the IP data. The thresholds of the RS data are recognized by the fractal method. These two thresholds have been used to determine the Z-shape function for the RS data. The integration of geoelectrical layer information has been carried out by the Gama method. Finally, the best drilling points were proposed based on fuzzy modelling for the area. The results show that the optimum exploration borehole is located at a depth of 25 m.

Thermal history of the Acoculco geothermal system, eastern Mexico: Insights from numerical modeling and radiocarbon dating

Acoculco is a geothermal prospective area hosted by a volcanic caldera complex in the eastern Trans-Mexican Volcanic Belt. Surface manifestations are scarce and consist of gas discharges (CO2-rich) and acid-sulfate springs of low temperature, whereas hydrothermal explosive activity is profusely manifested by meter-scale craters and mounds of hydrothermal debris and breccias. Silicic alteration extends for several square kilometers around the zone with gas manifestations and explosive features, affecting surficial volcanic rocks, primarily tuffs and breccias. In the subsurface, an argillic alteration zone (ammonium illite) extends down to a depth of ∼600m, and underneath it a propylitic zone (epidote–calcite–chlorite) occurs down to ∼1000m.Thermal logs from an exploratory borehole (EAC-1, drilled in 1995 down to 1810m) showed a conductive heat transfer regime under high geothermal gradient (∼140°C/1000m). In contrast, the thermal profile established from temperatures of homogenization of fluid inclusions—measured on core samples from the same drill hole—suggests that convection occurred in the past through the upper ~1400m of the geothermal system. A drop in permeability due to the precipitation of alteration minerals would have triggered the cessation of the convective heat transfer regime to give place to a conductive one. With the purpose of determining when the transition of heat transfer regime occurred, we developed a 1D model that simulates the time-depth distribution of temperature. According to our numerical simulations, this transition happened ca. 7000years ago; this date is very recent compared to the lifespan of the geothermal system. In addition, radiocarbon chronology indicates that the hydrothermal explosive activity postdates the end of the convective heat transfer regime, having dated at least three explosive events, at 4867–5295, 1049–1417 and 543–709y cal. BP. Therefore, hydrothermal explosions arise from the self-sealing of the Acoculco geothermal system, involving a natural hazard that could affect future geothermal-power infrastructure.

The thermal history of the Karoo Moatize-Minjova Basin, Tete Province, Mozambique: An integrated vitrinite reflectance and apatite fission track thermochronology study

The Moatize-Minjova Basin is a Karoo-aged rift basin located in the Tete Province of central Mozambique along the present-day Zambezi River valley. In this basin the Permian Moatize and Matinde formations consist of interbedded carbonaceous mudstones and sandstones with coal seams. The thermal history has been determined using rock samples from two coal exploration boreholes (ca. 500 m depth) to constrain the burial and exhumation history of the basin. Organic maturation levels were determined using vitrinite reflectance and spore fluorescence/colour. Ages and rates of tectonic uplift and denudation have been assessed by apatite fission track analysis. The thermal history was modelled by inverse modelling of the fission track and vitrinite reflectance data. The Moatize Formation attained a coal rank of bituminous coals with low to medium volatiles (1.3–1.7%Rr). Organic maturation levels increase in a linear fashion downhole in the two boreholes, indicating that burial was the main process controlling peak temperature maturation. Calculated palaeogeothermal gradients range from 59 °C/km to 40 °C/km. According to the models, peak burial temperatures were attained shortly (3–10 Ma) after deposition. Apatite fission track ages [146 to 84 Ma (Cretaceous)] are younger than the stratigraphic age. Thermal modelling indicates two episodes of cooling and exhumation: a first period of rapid cooling between 240 and 230 Ma (Middle – Upper Triassic boundary) implying 2500–3000 m of denudation; and a second period, also of rapid cooling, from 6 Ma (late Miocene) onwards implying 1000–1500 m of denudation. The first episode is related to the main compressional deformation event within the Cape Fold Belt in South Africa, which transferred stress northwards on pre-existing transtensional fault systems within the Karoo rift basins, causing tectonic inversion and uplift. During the Mesozoic and most of the Cenozoic the basin is characterized by very slow cooling. The second period of fast cooling and denudation during the Pliocene was likely related to the southward propagation of the East African Rift System into Mozambique.

Petrological, mineralogical, and geochemical compositions of Early Jurassic coals in the Yining Coalfield, Xinjiang, China

The petrological, geochemical, and mineralogical compositions of 10 coal seams (Early Jurassic) from an exploratory borehole (Hole ZKJ502) in the Jieliangzi mining area of the Yining Coalfield, Xinjiang Uygur Autonomous Region, China, were investigated, using optical microscopy and field emission-scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometer (FE SEM-EDS), as well as X-ray powder diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The Yining coals are of subbituminous to highly volatile C bituminous in rank (vitrinite reflectance, average 0.49%) and are characterized by low sulfur content (average 0.45%), low ash yield (average 4.10%), high volatile matter (average 38.82%), high proportion of inertinite (average 49.5%), and low concentrations of most trace elements. Relative to the upper continental crust, the rare earth elements and yttrium in the Yining coals are mainly characterized by light and heavy REY enrichment. The minerals in the Yining coals are dominated by quartz, kaolinite, and siderite, followed by pyrite, bassanite, and hexahydrite, along with small proportions of illite, calcite, dolomite, gorceixite, goyazite, ankerite, marcasite, diaspore, florencite, hematite, and millerite. The similarity between the Yining coals and the magmatic rocks from the peripheral mountain area of the basin in terms of the Al2O3/TiO2 ratio, the REY enrichment type, the REY distribution patterns, and the Eu anomalous fractionation suggest that the sediment source region was mainly Carboniferous volcanic rocks (andesite and basalts) on peripheral mountains. The acid conditions and lowered water table during peat accumulation may have caused detrital minerals to be leached, leading to the relatively low ash yield. Another reason for the low ash yield may be the simple coalfield structure and poor groundwater migration channels in the rocks and coal seams in the central zone of the basin, which would limit the minerals carried by the groundwater into coals. A number of authigenic minerals (quartz, kaolinite, siderite, and gorceixite) occur as cell-fillings and elevated concentrations of Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Zr, Nb, Mo, Cs, Hf, W, Pb, Th, and REY occur in Nos. 26 and 27 coals, indicating that there was an injection of a small-scale solution.

Stability assessment of high-steep cut slope debris on a landslide (Gumushane, NE Turkey)

The objective of this study was to examine the effects on the stability of slope debris of planned slope excavations around the vicinity of Gumushane Imam-Hatip High School in NE Turkey. To assess the geotechnical properties of slope debris along the determined cross-sections, four exploration boreholes with a total length of 100 m were drilled and undisturbed soil samples were collected. Slope debris is composed of two soil zones and its depth varies between 15 m and 21 m. It was determined from the experimental results that the soil class of the upper zone is CL (low plasticity clay) and CH (high plasticity clay) whereas the soil class of the lower zone is SC (clayey sand) and GC (clayey gravel). Peak cohesions of the upper and lower zones were 26.3 kPa and 10.2 kPa, and their peak internal friction angles were 16° and 24°, respectively. Limit equilibrium (LE) analyses for the slopes were evaluated using Slide v5.0 software for the pre-excavation, post-excavation, and supported cases. The results of LE analyses showed the existence of a landslide along the SW direction; the landslide would extend to cover the whole region if the excavation were to be performed without applying engineering measures. Finally, the precautions to be taken were determined and stability analyses for the supported case were evaluated. These analyses showed that the active lateral forces applied by the proposed retaining walls to the cut slope should be 8325 and 3103 kN, respectively.

Defining a general hydrogeological model for Susak Island, Adriatic Sea, Croatia

The island of Susak differs from other Croatian islands in its geological and hydrogeological properties, because it is, for the most part, covered with fine-grained loess-type sands. At the outset of the research it was unclear whether the main aquifer lies within the superficial deposits or within the underlying carbonate rocks. This gave rise to two alternative conceptual models, depending on whether the interface between the superficial cover and the carbonates occurs above or below sea level. Hydrogeological and geophysical investigations were conducted. The primary objective of the geophysical exploration was to determine optimum positions for exploratory boreholes. The results showed that the main aquifer is contained within the carbonate rocks, and that this carbonate aquifer is overlain by a second, confined, aquifer within the superficial sandy deposits. Groundwater within the carbonate aquifer is hydraulically connected to the sea by fault zones. Measurement of electrical conductivity in boreholes established that the mixing zone of fresh and salt water is at depths from 22 to 30 m below ground level. It is necessary to ensure constant control of pumping quantities and control of the brackish water lens by monitoring of the groundwater salinity. For inclusion in public water supply, the groundwater should be treated by desalination.

Paleokarst crust of Ordovician limestone and its utilization in evaluating water inrushes in coalmines of North China

Detailed analysis of exploratory borehole data suggests presence of a paleokarst crust at the top of Middle Ordovician Fengfeng Formation in North China. Because of its mechanical strength and low permeability to water, the paleokarst crust can function as an additional water-resisting layer in evaluating water inrush risks in mining the Carboniferous-Permian coal seams in the area. The paleokarst crust is characterized by heterogeneity and may not be ubiquitous. This paper takes Sihe Mine of Shanxi Province as an example to study the geotechnical and hydrogeological characteristics of the paleokarst crust. Incorporation of this additional hydrological barrier into water inrush risk assessment led to more minable coal seams in the coalmine.

Diagenesis to very low-grade metamorphism in lower Palaeozoic sediments: A case study from deep borehole Tobolka 1, the Barrandian Basin, Czech Republic

The diagenetic grade of a 2.7km thick sequence of lower Palaeozoic marine sediments in the Tobolka 1 deep exploration borehole was investigated by analysing organic and mineral components and fluid inclusions embedded in vein minerals.The reflectance of organic matter (Rr %) increases down-section from values of about 0.77% at the top of the Devonian Srbsko Formation to values around 3% at the base of the Ordovician Letná Formation. In the Devonian and Silurian sediments the reflectance values of vitrinite-like and graptolite fragments (0.77–1.13% Rr and 1.09–1.46% Rr respectively) and the composition of fluid inclusions entrapped in vein minerals (CH4+CO2+complex saline solutions±light gaseous hydrocarbons±liquid petroleum; Th=170–230°C) collectively indicate organic maturation within the high-temperature part of the oil window to the wet gas stage. In the Ordovician sediments below 1000m, chitinozoan reflectance values (1.38–3.03% Rr) and the CH4-dominated composition of the fluid inclusions (Th=250–310°C) indicate a wet gas/condensate stage to an advanced dry gas stage.The clay mineral assemblage remains uniform throughout the whole succession and consists of illite, mixed-layered illite–smectite (IS), chlorite and kaolinite. Below 1400m the illite content gradually increases down-hole, forming up to 45–55% of the clay fraction in the deeply-buried Ordovician sediments. Throughout the borehole, the IS displays uniformly low expandability (7–12±3% of smectite layers) and an ISII ordering. The illite crystallinity broadly correlates with organic matter reflectance values and improves with depth (from 1.1 to 0.54° Δ2θ according to the Crystallinity Index Standard (CIS) scale) indicating high-grade diagenesis to incipient anchimetamorphism. The chlorite crystallinity (ChC index), however, shows no systematic correlations with depth.The combined evidence from organic matter, clay minerals and fluid inclusions points to maximum paleotemperatures of ~100–230°C in the Silurian–Devonian strata, and up to ~250°C or even higher in the Ordovician strata. Apatite fission track analysis (AFTA) data suggests that maximum heating occurred between 340 and 380Ma, i.e. during Variscan orogenic deformations. These paleotemperatures and the degree of diagenesis may have been produced by a post-Middle Devonian burial or tectonic load that was subsequently eroded, or by higher paleogeothermal gradients, or by a combination of both.

Estimating depth to the bottom of magnetic sources at Wikki Warm Spring region, northeastern Nigeria, using fractal distribution of sources approach

Abstract: The Wikki Warm Spring (WWS) is located on the Kerri-Kerri formation of tertiary age. This forms part of the Benue Basin, which links up with the Chad Basin in the north and extends southwest to the Anambra Basin in Nigeria. The stratigraphy of the WWS region consists of alluvium, the Kerri-Kerri formation, the Gombe formation, the Pindiga formation, the Yolde formation, the Bima formation, and basement rocks as the oldest. We estimate the depth to the bottom of magnetic sources (DBMS) in the WWS region using the fractal sources distribution approach on aeromagnetic data to identify the geothermal system of the WWS region. The adopted computational method is based on statistical methods of depth determination from the radial power spectrum assuming a fractal distribution of magnetic sources. The average estimated DBMS at the WWS source location is 10.72 ± 0.54 km. The obtained results imply an average thermal gradient of 54.11 °C km-1 and heat flow value of 135.28 mW/m2. Generally, shallow DBMS values are obtained in the northeastern region of the WWS area and these increase towards the southwestern region when regional variation patterns of estimated depths are considered. The generally shallow DBMS obtained in the study area is attributed to magmatic intrusion or diapirism in the subsurface and emphasizes the effects of large-scale tectonic events, particularly the basin-initiating event, as major influences on the thermal history. Results will contribute to decisions on where to drill boreholes for further geothermal energy exploration in the region.