Knowledge of the spatial characteristics of stress and fractures in reservoirs is important for op- timising! production and injection processes. Serni-permanent passive microseismic monitoring is being conducted in the Yibal field, Oman, to better understand reservoir geomechanics. The network comprises 12 4C stations in 5 monitoring wells which can be used for focal mechanism and anisotropy studies. In this Study, I analyse 22 days of data, containing over 600 located events. In the first analysis, 43 reliable fault plane solutions (FPSs) are determined using polari- ties and amplitudes of direct P-, SV- and SH-waves based on a pure double-couple source. The principal stress directions are estimated using the method ot'Gephart and Forsyth (1984) from FPSs. Stress Mag"llitUdesare then estimated based on a friction model, and stresses are finally modelled based on a passive basin model. In the second analysis, nearly 400 reliable S-wave splitting measurements of time lag and fast shear-wave strike are determined. Shear-wave split- fingt7modellingCýis used to interpret the results in terms of fracture orientation and fracture density. In the final analysis. 19 examples of frequency-dependent S-wave splitting are determined and the results are interpreted Using the Chapman (2003) theory to estimate the fracture size. I observe a transition in faulting repme from strike-slip (with a thrustino component) in the shale Fiqa cap rock to pure thrusting in the gas-charged Natih A chalk reservoir. Deeper in the held I observe another transition from strike-slip in the Nalir Urnr shale cap rock to normal faulting in the oil-bearin- Shuaiba chalk reservoir. The transition at each shale/chalk interface may be attributed to variations in the Friction angles: from low in the shales (12' and 18', respectively) to high in the chalks (39'). The Natih A results suggest a positive anomaly in Poisson's ratio (0.37), which is consistent with the ongoing compaction in this unit. The maximurn compressive stress direction varies with depth: horizontal E in Fiqa, horizontal NNE in Natih-A, sub-horizontal E' in Nahr Umr, and sub-vertical in Shuaiba. The splitting rnaonitudes are high (5-10%) in the SE footwall of" the large eastern-most graben fault that runs through the field and low (11/c) in the opposite hanging wall. The highest fracturing (517caverage anisotropy) and largest fracture sizes (2 rn) are predicted in the Natih A reservoir. In contrast, the Fiqa exhibits moderate Fracture density (31Y() with fine-scale fractures (<O. I jim in size). Weaker anisotropy is found in the Nalih B-G, which is attributed to moderate fracture density in the Lipper layers and preferred crystal orientation in the lower layers. The splitting orientation results are interpreted in terms of a sin,Z_vle set ofnear- vertical fractures trending: 19'NNE in the Natih A, 90'E in the Fiqa and the lower part of Natih B-G, and 45'NE in the Lipper part of' Natih B-G. The fractures are ali-nedCý parallel to the direction of the maximum compressive stress, as determined by the HIS-based

Knowledge of the spatial characteristics of stress and fractures in reservoirs is important for op- timising! production and injection processes. Serni-permanent passive microseismic monitoring is being conducted in the Yibal field, Oman, to better understand reservoir geomechanics. The network comp...