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11. Structural Trap and Fault-seal Analysis, Offshore Myanmar: A Case Study
R. K. Davies,1 L. An,2 D. A. Medwedeff,3 D. Yarwood4
1Rock Deformation Research USA, Inc., McKinney, Texas, U.S.A.
2Consultant, Flower Mound, Texas, U.S.A.
3Chevron Petroleum Technology Company, San Ramon, California, U.S.A.
4BP, Houston, Texas, U.S.A.
We thank Willam F. Dula Jr. and two anonymous reviewers for edits that improved the manuscript and ARCO for permission to publish the results.
The objectives of this study are to (1) describe the prospect trap style in Block M-9 near the continental shelf break offshore Myanmar in the Gulf of Martaban, (2) analyze the geometry and sealing mechanisms and risks for fault S1, and (3) use these results as a calibration for trap style and fault-seal risks on additional prospects in the block. Fault S1 is a key trapping component to seven prospective hydrocarbon accumulations of which six are delineated by anomalously bright seismic amplitudes interpreted from 3-D seismic data in the footwall of the fault. All six anomalies correlated with mud log gas peaks during the drilling of the exploration well Shwe Pyi Htay-1 (SPH-1) in 1997. Only one anomaly (DHI 6), however, tested recoverable gas reserves.
We analyzed the seal potential of fault S1 using an ARCO-proprietary, PC-based fault-seal analysis program integrated with 3-D visualization techniques. Fault-seal potential (FSP) is a parameter determined from the fault geometry, the throw distribution, and the reservoir quality adjacent to the fault surface that indicates the likelihood of a fault to seal hydrocarbons. The results indicate that the fault has high seal potential (65%) in the upthrown DHI 6 sand interval and lower seal potentials (30–45%) in the other prospective sand intervals. These results agree with the drilling results from SPH-1 well and might be applied to other prospects in the same area to assess exploration risk. Our analysis suggests that the gas-water contact (GWC) at the depth of 1497 m (4910 ft) in the upthrown DHI 6 interval may result from gas leakage across the fault through a window of low seal potential along a region of low throw associated with a connecting fault splay. If this is the case, then the threshold FSP for the DHI 6 interval is 60%. Another possibility is that the small faults below seismic resolution that intersect the S1 fault control the GWC. If this is true, the threshold FSP would be lower than 60% but higher than 45%, which is the lowest seal potential calculated in the remaining reservoir intervals with residual gas.
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