Fracture detection via correlating P-Wave avoa analysis and well data in eastern central Saudi Arabia

The late Carboniferous clastic Unayzah-C in eastern central Saudi Arabia is a low porosity, possibly fractured reservoir. Mapping the Unayzah-C is a challenge due to low signal-to-noise ratio (SNR) and limited bandwidth in the conventional 3D seismic data. A related challenge is to delineate and characterize fracture zones within the Unayzah-C. Full-azimuth 3D broadband seismic data were acquired using point receivers, low frequency sweeps down to 2 Hz, and 6-km patch geometry. The data show significant enhancement in continuity and resolution of the reflection data, leading to improved mapping of the Unayzah-C. Since the dataset has rectangular patch geometry with full inline offsets to 6000 m, amplitude variation with offset and azimuth (AVOA) may be effective to delineate and characterize fracture zones within Unayzah-A and Unayzah-C. The study was undertaken to determine the improvement of wide azimuth seismic data in fracture detection in clastic reservoirs. The results were validated with available well data including borehole images, well tests and production data in the Unayzah-A. There are no production data or borehole images within the Unayzah-C. For validation, we had to refer to a comparison of alternative seismic fracture detection methods: mainly curvature and coherence. Anisotropy was found to be weak, which may be due to noise, clastic lithology and heterogeneity of the reservoirs, in both reservoirs except for along the western steep flank of the study area. These may correspond to some N-S trending faults suggested by circulation loss and borehole image data in few wells. The orientation of the ellipses is NW-SE and not in agreement with the N-S structural trend. No correlation was found between curvature, coherence and AVOA in Unayzah-A or Unayzah-C. Some possible explanation for the low correlation between AVOA ellipticity and natural fractures are noisy dataset, overburden anisotropy, heterogeneity, granulation seams and deformation.