f Research on petrophysical characteristics and pre-stack inversion of tight gas reservoirs in Ordos Basin, China

Daniudi gas field in Ordos basin is of typical characteristics of tight gas reservoirs. The average porosity of gas reservoirs is less than ten percent. Through the petrophysical researches, we found that it is difficult to distinguish sand from shale by P wave impedance only. A set of petrophysical parameters was studied to find those that are sensitive to gas bearing sands. We concluded that Poisson ratio is an effective parameter to distinguish sand form shale in research area. AVO behavior of gas reservoirs were also studied in the paper. As the P wave impedance of sand very close to that of shale, AVO behavior of gas reservoir belongs to the second AVO class. By careful examination of field data, we found that it was difficult to watch such AVO behavior because of the low resolution of the data. Another difficulty occurred in inversion is the presentation of strong reflections caused by coal bed, and the reflection of reservoirs is overwhelmed by the strong reflections caused by coal bed. To overcome this problem, a model based pre-stack inversion technique was employed to get P wave impedance and S wave impedance simultaneously. Wavelets that were gotten form tie well to seismic data have high similarities. Log curves must be handled carefully to build a suitable initial model for inversion. The result shows that pre-stack inversion can provide a partial solution to the seismic prediction of tight gas reservoir in the research area.