Geophysical constraints on the Luhoi (Tanzania) geothermal conceptual model

Abstract

We present the results of the gravimetric, magnetic and magnetotelluric geophysical campaign aimed to constrain the geothermal conceptual model of the Luhoi geothermal field, a poorly known system located in the Tanzanian southeastern coastal sedimentary basin. Regional assessment by existing potential field data reveals that Luhoi lies at the margin of a wide regional NE trending uplifted block that involves the Precambrian metamorphic basement complex. In the geothermal field area, 3D geophysical modelling has imaged a local asymmetric horst of Cretaceous age, 1 km wide and 5 km long, with the same regional NE trend. The horst is formed by the denser Kipatimu Lower Cretaceous sandstones surrounded by the Upper Cretaceous Ruaruke claystones that are down thrown up to 1 km by normal faulting. Far from the horst, lithology mainly controls the modelled electrical resistivity distribution: the Ruaruke claystones show low resistivity (1–6 Ωm) primarily due to its clay content, while the Kipatimu sandstones have generally higher resistivity (10–30 Ωm) because of their prevalent electrolytic conduction. However, anomalous low resistivity values (1–5 Ωm) in the shallower portion of the Kipatimu sandstones forming the horst indicate enhanced electrical conduction due to the cation exchange capacity (CEC) of clay. The CEC estimated values from electrical resistivity are in the range 2−30 meq/100 g, suggesting the presence of kaolinite and illite and excluding the occurrence of smectite, which is the typical clay mineral in the cap-rock of high-temperature geothermal systems. This shallow clay-rich layer together with the Ruaruke claystones act as a regionally continuous clay cap confining the reservoir formed by moderately hot water (close to 100 °C from water geothermometers) in the permeable Kipatimu sandstones layer that reach a maximum depth of ca. 2 km, likely heated by a slightly enhanced geothermal flux attributable to crustal extension. At Luhoi, the local horst structure imaged by the geophysical models pushes up the sandstone reservoir at very shallow depth (about 500 m from the surface), strongly enhancing the local geothermal gradient and originating the surface thermal manifestations that are controlled by the system of normal faults.