SEISMIC TRACE SHAPE ATTRIBUTE ANALYSIS IN BELANAK ARANG-3 SAND RESERVOIR, WEST NATUNA BASIN INDONESIA

摘要

Well data from the Belanak Field, Block B, Natuna Sea, show that the Arang-3 reservoir was formed as a single sand package and that it is only present over a portion of the field. Seismic data can help delineate the extent of the sand presence but it must be combined with a geological depositional model to fully understand the hydrocarbon resource potential of the Arang-3 reservoir.Arang-3 is a gas bearing sand within the Arang Formation with average thickness of 20 ft and currently buried at about -6000 ft TVDSS. It is a thin sand unit deposited between shales and in close proximity to coal layers. Well-seismic correlation is an important procedure before starting seismic interpretation. Forward modeling showed that the seismic response is actually dominated by the coals and that seismic expression of the Arang-3 is a result of interference between sand-shale-coal rather than a simple shale over sand interface. Fortunately the coal layers are consistently developed across the field and do not introduce lateral variation in the seismic expression. Simple forward modeling validated by well-seismic correlation showed that the presence of Arang-3 sand generated additional dim reflection events in between two strong and continuous reflections of the coal layers. This specific character has a different seismic trace shape compared with surrounding areas that have no sand present.Seismic trace shape classification produced maps that showed good correlation between the seismic trace shape attribute and the presence of sand or shale from well data. This classification, combined with well log stratigraphic correlation and seismic sections flattened at the top Arang-3 zone lead to the interpretation that the Arang-3 sand was deposited in an incised valley system environment.An understanding of the trace shape developed from multiple forward models validated that these empirical observations were indeed consistent with variations in sand presence. This allowed the trace shape classification to be broken down into areas of better or worse sand deposition which was then used to influence development well planning.