Analysis and Interpretation of the gravity data to delineate subsurface structural geometry of the Olkaria geothermal reservoir, Kenya

Abstract

In this study, about 255 gravity data stations collected between September to November 2021 were used to generate a Bouguer anomaly map, edge detection filters, and 3D inversion using an estimated Bouguer density of 2.47 g/cm3, an average of the density estimates methods of G-H, F-H, and CVUR. The edge detection filters show the field is structurally fractured, with the faults trending in the NNE-SSW, N-S, NE-SW, NW-SE, and some in the E-W direction. It is more likely that the subsurface structure in the area is affected by complex sets of faults within the field. Some new geological contact zones and their possible orientations were determined using HD and INH filter methods. 3-D gravity inversion constrained by geological data and previous geophysical data reveals a high-density anomalous body in the reservoir region (∼2.2–3 km) of an estimated average volume of about 130.6km3 with some low dense bodies extending to reservoir zone from near surface observed as the faults, dipping nearly 90° at the reservoir. Thus, the structural geometry and the subsurface fault orientation matrix, varying in dimension and depth, were delineated as characterized by reservoir dimension and depth. The first inversion of gravity modeling constrained by wells data, past geological and other geophysical findings to image the Olkaria geothermal reservoir system was presented. The gravity analysis methods applied entail a rich interpretation and understanding of the density dynamics of the Olkaria subsurface structural orientation subjected to reservoir characterization, revealing a unique conical-shaped low dense anomalous body extending increasingly with depth in the Olkaria Domes field around wells OW-903, 904, 905, and OW-921 trending northeastwards, which may be a highly fractured zone filled with fluid. The applied gravity filter and inversion methods portray detailed dynamical changes in the Olkaria subsurface density structures, revealing the characterization of the reservoir.