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Three Dimensional Seismic Inversion and Neural Net Impedance Facies Map in Reservoir Characterization
Three-dimensional seismic inversion was performed on a 3D survey in central Lavaca County around Hallettsville, Texas in an area with several oil and gas fields. The study showed that in Upper Edwards limestone a 4 to 6 km fairway of low impedance extending 2 to 3 km on each side of the shelf-edge in the land ward direction and in the direction of the slope. The shelf-edge is approximately at 2800 ms and the fairway extends from ~2780 ms to ~2880 ms. A neural-net-generated seismic facies map based on trace shape classification of a seismic interval in the Upper Edwards shows this fairway. Other attribute analyses also show this feature. The fairway has been interpreted as a karsted area, probably caused by circulating meteoric water during a low sea level stand. A seismic inversion procedure using a global optimization technique was utilized to derive a framework of acoustic impedance layers from the 3D seismic data. The layered impedance data show increased vertical resolution, increased coherency and less noise as compared to the original seismic data and provide a better understanding of porosity distribution in the Edwards limestone. Well logs were used to validate the inversion results but were not used directly in the inversion process. An impedance facies map for the Upper Edwards was generated by combining several layers with their impedance and thickness values using a neural net classification method and this map was compared with previously reported measured porosity data from logs in eight wells. Impedance attribute facies map shows interpretable patterns distinguishing areas of high porosity from low porosity within the fairway. While the seismic facies map based on trace shape of regular data recognizes the karsted area from the non-karsted area, further distinction between low and high porosity within the karsted fairway was not possible. Porosity also increases when the location of the well is on or near a fault, as a result of fracture porosity development. It is concluded that wells within the recognized impedance attribute facies and drilled on or near a fault should have higher porosity and better production. The distribution of low impedance in time and space should be considered in developing the Upper Edwards reservoir.
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