Spatial Variations in Microseismic Focal Mechanisms, Yibal Field, Oman

Fault plane solutions (FPSs) are estimated for microseismic events in the Yibal field, west-central Oman. We analyse 22 days of data, containing over 600 located events recorded by a maximum of 5 monitoring wells (10 downhole 3-component stations in total). 43 reliable FPSs are determined using polarities and amplitudes of direct P-, SV- and SH-waves and based on assumption of a pure double-couple source. The analysis was guided by tests with synthetic data, which were generated by the reflectivity method. We observe a transition in faulting regime from strike-slip (with a thrusting component) in the shale Fiqa cap rock to pure thrusting in the gas-charged Natih A chalk reservoir. Deeper in the field we observe another transition from strike-slip in the Nahr Umr shale cap rock to normal faulting in the oil-bearing Shuaiba chalk reservoir. Collectively the results suggest that the fault regime is controlled by style of deformation and lithology.