Delineation of Shallow Fault Damage Zones by Analyses of Drilling Loss Zones- A Case Study of Olkaria East and North-East Geothermal Fields

Abstract

Fault zones are primary controls on permeability architecture, fluid circulation, and heat extraction in high-enthalpy geothermal systems hosted in volcanic terrains. In the Olkaria East and North-East geothermal fields of Kenya, these zones are characterized by intervals of intense fracturing that are routinely intersected during drilling operations. This study presents a systematic analysis of drilling fluid and circulation returns loss zones recorded in 32 geothermal wells to delineate shallow fault damage zones and assess their spatial distribution, geometry, and structural controls. The loss zones are interpreted as zones of enhanced permeability associated with fault-related fracture networks. Correlation of loss intervals with lithological logs, well trajectories, and regional structural trends shows a strong spatial association between the loss zones and the N-S (Oloolbutot fault & Olkaria fracture), ENE–WSW (Olkaria fault), and NW–SE striking faults and fractures, which constitute the dominant structural fabric of the Olkaria Volcanic Complex. Statistical analysis indicates that 70–75% of all loss zones in the sampled wells occur at depths shallower than 300 m. Loss zone thicknesses range from less than 20 m to greater than 500 m, with the thickest zone at OW-51A having an 800m loss zone. Lateral continuity of loss zones across adjacent wells (e.g., the 742, 731, and 710 well clusters) supports interpretation as coherent fault damage zones rather than isolated fractures. The results demonstrate that drilling loss data provide a cost-effective, high-resolution dataset for characterizing shallow structural permeability and refining fault models in active geothermal field development.