Seismic anisotropy in a hydrocarbon field estimated from microseismic data

摘要

The study of seismic anisotropy in exploration seismology is gaining interest as it provides valuable information about reservoir properties and stress directions. In this study we estimate anisotropy in a petroleum field in Oman using observations of shear-wave splitting from microseismic data. The data set was recorded by arrays of borehole geophones deployed in five wells. We analyse nearly 3400 mi-croearthquakes, yielding around 8500 shear-wave splitting measurements. Stringent quality control reduces the number of reliable measurements to 325. Shear-wave splitting modelling in a range of rock models is then used to guide the interpretation. The difference between the fast and slow shear-wave velocities along the raypath in the field ranges between 0-10% and it is controlled both by lithology and proximity to the NE-SW trending graben fault system that cuts the field formations. The anisotropy is interpreted in terms of aligned fractures or cracks superimposed on an intrinsic vertical transversely isotropic (VTI) rock fabric. The highest magnitudes of anisotropy are within the highly fractured uppermost unit of the Natih carbonate reservoir. Anisotropy decreases with depth, with the lowest magnitudes found in the deep part of the Natih carbonate formation. Moderate amounts of anisotropy are found in the shale cap rock. Anisotropy also varies laterally with the highest anisotropy occurring either side of the south-eastern graben fault. The predominant fracture strikes, inferred from the fast shear-wave polarizations, are consistent with the trends of the main faults (NE-SW and NW-SE). The majority of observations indicate subvertical fracture dip (>70°). Cumulatively, these observations show how studies of shear-wave splitting using microseismic data can be used to characterize fractures, important information for the exploitation of many reservoirs.