Geology, Exploration Wells Siting and Resource Estimation Using Power Density Method in Paka Geothermal Field

Abstract

This paper presents the results of a recent geological field survey highlighting Paka eruptive history and structural geology. The findings are integrated to asses current exploration wells sites and give insights to Paka geothermal resource capacity. The new stratigraphic framework identifies seven (7) volcanic units comprising trachyte eruptions separated by basalt lava flows. The eruptive events in Paka were initiated in middle Pleistocene at 582 ka with climatic phase at 8 ka. The caldera formed between 70 ka and 35 ka, a much earlier period than proposed by earlier researchers (11 ka). The area has experienced intense tectonism between 278-205 ka during a period of hiatus in volcanism, creating much of the Paka localized faults as seen today. The faults are geothermally active and forms interesting localities to target for exploration drilling. Structural considerations were employed during the siting and ranking of the 3 exploration wells sites PS-01 (PW-02), PS-02 (PW-01) and PS-03 (PW-03). Well site PS-02 was ranked higher compared to well site PS-01 and PS-03 due to fault intersection, soil gas anomaly and is at the intersection between the caldera and the eastern crater. Well PW-01 was drilled of a depth of 2,553 m and discharged on test. The high potential anomalous area in Paka is about 20 km2 based on structures, surface manifestations and soil gas anomalies. Resource assessment using Power Density method indicates a most likely (P50) resource capacity of approximately 387 MWe. The optimistic (P10) high end of the range of power capacities is over 500 MWe. It is worth noting that these estimations are based on limited surface and well data and as more geoscientific and wells data is acquired, and a 3-D numerical model is conducted, a more accurate estimate will be determined.