Methods of Reservoir Identification for Iso-Velocity Sand-Mud Strata
When velocity or wave impedance of sandstone is equal or close to that of mudstone, it is difficult to use 3-D seismic amplitude related attributes or wave impedance inversion to identify sandstone reservoir thickness Cretaceous Qingshankou Formation in Songliao Basin, Upper Paleozoic- Mesozoic in Ordos Basin and Silurian Kepingtage Formation in the Tarim basin are all belong to iso-velocity sand-mud strata. After years of research, we consider three methods are effective to identify this kind of sandstone reservoir based on 3-D post stack seismic data.
The first method is to use amplitude acceleration attribute. Its principle is that velocities of mudstone and sandstone are not absolutely equal, and generally there are three kind of lithology: sandstone, calcareous (or silty) mudstone and shale. The former two lithologic velocity is very close to, but not exactly the same. Despite rare in content, shale normally with quietly small velocity, contribute greatly to wave impedance contrast or reflection amplitude. Amplitude acceleration method suppress the amplitude contribution from shale, but widen the contribution of amplitude from the difference between sandstones and silty mudstone.
Second method is to use coherent integral ratio attribute. Straight lineation anomalies of low coherent value on a horizon map are well known to be generally caused by faults and curved or irregular low coherent anomaly are caused by channel or deltaic sandstone. But for iso-velocity sand-mud strata coherence value of channel or deltaic sandstone is not low enough to form a microfacies morphologic feature to be recognized. Coherent integral ratio attribute along a horizon window allow to sum up all samples of the window coherence value and compute the low coherence ration, so as to highlight the occurrence of different planar geometry of sandstone reservoirs on a coherent integral ration map.
Third kinds of method are log reconstruction inversion. This method need to reconstruct acoustic log curves based on gamma ray to increase the sonic log different between sandstone and mudstone, and then carry out well constraint stochastic simulation inversion. For some iso-velocity sand-mud strata, vivid plane morphologic distribution and thickness variation of sandstone reservoirs of a research interval can be obtained through this method. It usually requires over 7-10 wells data at a 3-D seismic survey hundreds of square kilometers.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013