Zambia: Conceptual Model and Drilling Update of the Deep Circulation Bweengwa River Geothermal Prospect
Nicholas HINZ, Peter VIVIAN-NEAL, Jill HAIZLIP, Gabe MATSON, William CUMMING, Peter HARRISON, Stuart CAMPBELL
[Geologica Geothermal Group, Inc., USA]
Kalahari GeoEnergy Ltd (KGE) has continued its exploration of the Bweengwa River geothermal prospect, following up its 2012 to 2017 exploration program of geoscience surveys and six slim hole wells with a 2018-2019 program of LiDAR, structural mapping and eight shallow temperature gradient holes (TGH) wells that have been used to update conceptual models that can support targeting of slim holes to discover a 130 to 150°C reservoir upflow. As of early 2017 the exploration program included geologic mapping, analysis of fluid geochemistry, geophysical surveys (AMT, gravity, magnetics), developing conceptual models, drilling six slim hole wells to 250-550m depth, and analyzing core and cuttings to update conceptual models. The six wells were located 0.6 to 1.5 km northwest of the Southern Bounding fault (SBF) and reached a maximum temperature of 102 °C but, because of their locations and challenging drilling, did not reach the SBF. Quartz geothermometry ranged from 128 to 149°C for the sulfate-dominated waters from the hot springs and slim holes. The initial conceptual models were of a deep circulation system with the reservoir hosted within the steeply dipping SBF with possible lateral stratigraphic continuity into parts of the Precambrian marble within the basement and local updip outflow into the Karoo sediments in the basin. A follow-up slim hole drilling program was planned to discover and test the reservoir upflow. However, because no exploration wells had yet intersected the fault, significant uncertainty remained in the structural model for the SBF and so LiDAR data was collected in 2018 to help map Quaternary fault scarps along the SBF. The results of the LiDAR analysis were integrated with existing geologic mapping of exposed fault zones, the stratigraphic data from the first five core holes and the geophysics to update the structural model. The updated structural model included a previously unrecognized breached fault-propagation fold in the hanging wall of the SBF, along with key step-overs along the SBF within the breached fold. This data was used to update the conceptual model, which included several targets with structure and formation properties potentially favorable for encountering a permeable reservoir along the SBF. To reduce ambiguity in the upflow location along the SBF prior to targeting deep slim hole wells, eight TGH were drilled to 150 m depth. Based on the integration of the TGH and earlier slim hole drilling data with the updated geology and structure, the SBF likely has a steep dip, in the order of about 65 degrees. The updated conceptual models are consistent with a slim hole encountering a permeable reservoir with temperatures of 130 to 150oC at ~1,000 m depth. The new data for the Gwisho hot springs (HS) area is consistent with a narrowly confined upflow within part of the main SBF. New data for the Bwanda HS area supports a range of possible upflow models along one or more strands of the SBF and likely lateral flow in the deeper sediments, the Gwembe Coal Formation (GCF), and/or along fault strands in the upper 500 m to connect the upflow with the hot springs and with the hot aquifers encountered by wells in the GCF. The new upside model for the Bwanda HS area includes the possibility that the lower part of the GCF may reach ≥125°C at ~300 to 500 m depth in part of the basin, making this a possible exploration target for developing a shallower resource separately from the deeper upflow resource. The most significant risks for exploration include challenges with defining a potentially complex 3D upflow pathway that depends on both structure and formation properties and on the possibility of intersecting cool downflow along parts of the SBF better connected to the surface than the subsurface.
|        Topic: Exploration||Paper Number: 11135|