FLUID SUBSTITUTION AS AN AVO CALIBRATION FOR QUANTIFYING UNCERTAINTY OF GAS PRESENCE IN LATE MIOCENE BINIO SANDSTONE, KAMPAR AREA, CENTRAL SUMATRA BASIN

摘要

The Binio Formation in Kampar Block, Central Sumatra Basin is relatively thick, approximately 1500 feet, but has less than five thin shaly sandstone reservoir layers. The uppermost sands have thickness ranging from 10 to 15 feet and have proved to flow a significant rate of biogenic gas. The sandstones are believed to have been deposited in the lowstand fan sequence during Late Miocene in the South Bengkalis Trough.An attempt has been made to assess the amplitude response of these sandstones on seismic line 745-82. The bright spot is located on the crest of the structure. The evaluation includes Amplitude Versus Offset (AVO) gather analyses in this Pre-Stack Time Migration (PSTM) seismic line. The result indicates that the amplitude is class III low impedance sandstone with negative intercept (Rp) and negative gradient (G). The fluid substitution technique of the Gassmann equation has been applied to calibrate the AVO analytical results through the rock physics parameters by inputting the new reservoir condition, including effective porosity (Φ_(eff)), mineral composition, pressure, and by changing the value of pore water saturation (Sw) of 100 % for the wet case and 50-70 % for gas case. Other variations in the rock matrix compositions are also utilized, including bulk modulus (K), shear modulus (μ), and density (p) to provide predicted wireline logs for gas and wet reservoir conditions. The computed outputs of Vp, Vs and p in gas and wet conditions are used to generate synthetic seismogram for comparison purposes.The AVO analysis and fluid substitution methods are also applied to confirm the high amplitude response in the untested structure located at the NWPanduk area. The same work process has been performed in seismic line 745-82. The results are very similar to that in line 783-82, with a higher degree of confidence as the seismic quality improved.Most of the wells in Kampar Block is old well which lack of a complete data either log data or PVT (Pressure, Volume, Temperature) data. The old data mostly does not concern in shallow formation. To optimize the limited data is a big challenge for AVO and fluid substitution analysis. The aim of this study is to convince AVO analysis about presence of hydrocarbon (gas) in the undrilled structures. Therefore, the exploration drilling success in the region can be improved.