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A Case Study from Barrier Volcanic Complex, Kenya

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A Case Study from Barrier Volcanic Complex, Kenya GRC Transactions, Vol. 42, 2018 Early Stage 3D Model Construction for Well Planning: A Case Study from Barrier Volcanic Complex, Kenya. Abraham., S1, Kimani., F1, Njau, K1, Baxter., C2, O’Brien., J3 1Olsuswa Energy, Mayfox House, P.O Box 14991 – 00800, Garden Road off Riverside Drive, Nairobi, Kenya 2Seequent UK Limited, Building 1, Chalfont Park, Gerrard’s Cross, Buckinghamshire SL9 0BG, United Kingdom 3Seequent Limited, 20 Moorhouse Ave, Addington, Christchurch 8011, New Zealand Keywords Leapfrog, Olsuswa Energy, Barrier Volcanic Complex, Conceptual Model, Well. ABSTRACT Integration of surface data and inferred subsurface structure is a crucial part of geothermal resource delineation prior to beginning drilling campaigns. Inputs at this stage often include geological maps and cross sections, geochemical data, lidar or land survey data, and geophysical data. The integration of this early information informs the conceptual model and therefor drilling targets. Often this process is completed in a 2D environment where sometimes spatial context can be left behind. This paper highlights how incorporating surface exploration data into a 3D environment at the earliest stage can shed light on resource uncertainty and key features at the Barrier Volcanic Complex in Kenya. The Barrier Volcanic Complex (BVC) is a shield volcano located in Turkana, Kenya. The volcano Last erupted in 1921 and is the northernmost geothermal prospect in Kenya, lying in the Gregory Rift Valley at 2° 20’N, 36° 37’E. The volcanic complex forms a natural dam between Lake Turkana and the Suguta Valley. North to South topographic profiles indicate that this 20km long and 15 km wide, E-W trending ridge is a broad symmetrical feature with gently sloping flanks. BVC is a composite structure composed of four distinct volcanic centers namely; Likaiu East, Likaiu West, Kakorinya, and Kang’olenyang’. In the build up to the 3D and spatial integration processes, Olsuswa utilised data from earlier geoscientific works by the British Geological Survey (BGS) and Kenya’s Geothermal Development Company (GDC). This was in addition to data from Olsuswa’s detailed surface exploration program. A three dimensional representation of the system has helped with Abraham et al. communicating with stakeholders and delineating key subsurface elements key to planning the initial exploration drilling strategy. 1. Introduction The Barrier Volcanic Complex is located in Turkana, Kenya and lies in the Gregory Rift Valley at 2° 20’N, 36° 37’E (Figure 1). Barrier volcanic complex is the northernmost geothermal prospect in Kenya at the southern shores of Lake Turkana. It is a complex composite of four volcanoes, namely; Kakorinya, Kang’olenyang’, Likaiu West and Likaiu East. Figure 1: Map showing the location of the Barrier Geothermal Field. Except in Kakorinya’s main caldera formation where pyroclastic deposits constitute the youngest rock types, a suite of basalt, hawaiite, mugearite, and trachyte lava flows constitute the recent lithologic formations in the other three volcanic centers. The project site covers the central area of the BVC surrounding the Kakorinya volcano. This area is transacted by a series of curvilinear N-NE trending normal faults which extend in an en echelon right-stepping fashion across the volcano. Geothermal surface manifestations in the complex are fumaroles, hydrothermally altered grounds, hot springs, silica sinter, and geothermal grass. Estimated gas geothermometry temperatures have given mean subsurface temperatures of > 281°C. Abraham et al. This paper aims to investigate how incorporating surface exploration data into a 3D environment at an early stage. Historically collected data by BGS and GDC as well as local understanding of the area were used to infer the sub surface lithological 3D model in Leapfrog Geothermal. The model has helped delineating key subsurface elements key to planning the initial exploration drilling strategy. 2. Geothermal System in BVC Field 2.1 Geological Setting The geology of the BVC complex is comprised of a wide spectrum of lava types including basanite, basalt, hawaiite, mugearite, benmorite, trachyte and phonolite. Trachytic pyroclastic deposits cover much of the western slopes of Kakorinya and the summit area of Likaiu West. The oldest exposed rocks of the BVC are massively faulted porphyritic olivine basalts. These Pliocene foundation rocks were dated at about 4.53 Ma (Dunkley et al, 1993) and are well exposed in the adjacent rift margins. They cover most of the east and are called the Parkati Basalts. They occur in the faulted ground far east of Lake Logipi and around Latar, Southeast of Lake Turkana. The oldest lavas on the west are Lotikipi Basalts dated to be between 4.0-1.86Ma, (Dunkley et al, 1993). As observed by Dunkley et al. (1993), trachytic volcanism constructed the centres of Kang’olenyang’ and Likaiu East and major trachytes lavas are exposed within the inner trough. The youngest trachytes form the domes to the west of Kakorinya caldera around the caldera rim and these trachytes are dated to be about 0.05 Ma (Dunkley et al, 1993). Upper trachytes of 0.09 Ma (Dunkley et al, 1993) are exposed in the east of the caldera running in a N-S direction, with a minor outcrop of this formation being noted in the north west with a strip in the southwest. Older trachytes are exposed closer to the flanks in the east and are dated to be about 1.37 Ma (Dunkley et al, 1993). Pyroclastics cover most parts of the western side of Kakorinya while spots of alluvial sediments are scattered within this area. Some alluvials are found on top of cones inferring that the lake levels were much higher than they are at present. The youngest pyroclastic deposits on Kakorinya are airfall pumice lapilli and are best exposed in a thick wedge, which infills the western dipping slope between the caldera rim and the outer ring fractures. These deposits bank against and mantle the ring fracture escarpments and the pre-caldera domes. They are cut by the caldera wall in the west and bury the northern wall. This relationship indicates that the eruption of these trachytic tuff was broadly contemporaneous with the caldera collapse. Lacustrine sediments provide evidence of the existence of former Lake Suguta, which infilled the inner trough northwards from Emuruangogolak. The latest eruption in the area is historic and occurred in 1921. The erupted material was scoria basalts of Teleki Cone, which is still fresh and unvegetated (Figure 2). Abraham et al. Figure 2: Geological map of the Barrier Volcanic Complex and adjacent areas. 2.2 Structural Setting Studies of the structural setting of the Barrier volcanic field and associated Kakorinya volcano were presented by Dunkley et al. (1993). These structures are part of the rift floor structural system. Structurally, the BVC is characterized by faults, steep ridges and four eruption centres. The general trend of the faults in the area are N-S and NNE-SSW, which is consistent with the regional stress of the area. The mostly faulted areas are to the east, from the south eastern tip of Lake Turkana where dense faults trending NNE-SSW intersect with the eastern rim of Kakorinya Caldera. Southeast of the rim, the faulting takes a sudden turn to the south. The western half of the complex is less intensely faulted, with the faults trending NNE-SSW just southwest of Kakorinya and NNW- SSE around Kang’olenyang’ volcano. Abraham et al. The heat source is postulated to be bound by the caldera system at Kakorinya. This is further supported by the higher frequency of surface manifestations around this caldera than around the other three volcanic centres within the area. 2.3 Surface Manifestations Dunkley et al. (1993) delineated the thermal manifestations in the area. They include hotsprings, fumaroles, altered grounds, silica sinters and Fimbristylis exilis (“Geothermal grass”). Geothermal manifestations generally occur along a series of NNE-trending faults and fissures on the caldera floor and also around the caldera walls in the east and southeast (Figure 3). The hottest and most vigorous activity is associated with the caldera ring fractures and trachytic lava domes in the west of the caldera. The maximum recorded fumarole temperature is 96.4°C and occurs within the caldera floor (GDC, 2011). Outside the caldera there are a few fumaroles which exhibit low temperatures of up to 78.2°C (GDC, 2011). Hot springs frequently occur along the northern shores of Lake Logipi and around the eroded tuff cone of Naperito, although some of the hot springs occur under the lake (Figure 3). A maximum temperature of 70.0°C has been recorded for the hot springs (Dunkley et al, 1993). Silica sinters are common at many of the geothermal areas on Kakorinya and indicates former hot spring activity. The most spectacular development of sinter occurs on the trachyte lava domes cut by southwest wall of the caldera. They have sub-vertical dips, strike 008-028° and are parallel to a series of open fissures and faults. The veins extend up to the top surface of the lava domes where mounds of botryoidally sinter occur. Outside the caldera chalcedonic silica veins occur at several localities where the ring fracture meets a northeast-trending fault at weak fumaroles and steaming ground. Thermal indicators extracted from remote sensing data inform the PC4 image indicate the presence of hydrothermal alteration minerals southeast of Kakorinya caldera and in the Kang’olenyang’ area (Mutua et al. 2011). Thermal infrared imagery also shows thermal areas within Kakorinya area. 3. Historical Work Two historical studies have been used by Olsuswa to define the conceptual model and delineate the investigatory area. The BGS in 1993 conducted reconnaissance surveys which revealed the occurrence of strong surface manifestations. Subsequently during 2011, GDC carried out a preliminary reconnaissance surface exploration studies to establish the geothermal potential of the prospect (GDC, 2011).
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