The Research Of Seismic Response Characteristic Of The Tight Sandstone Layer

With the decrease of the conventional oil and gas resources, the exploration and development of tight sandstone reservoir has received considerable attention. And this kind of unconventional energy plays an important role in the present gas reservoir development. Gas-bearing tight sandstone is widely distributed in our country and the world range and characterized by low porosity and low permeability, especially the gas resources which contained is very appreciable.For the tight sandstone gas reservoir, the Ordos basin He-8section is a very typical example. Sulige gas field is an important oil and gas field of the Ordos basin, the field is located at the northeast of the Yishan slope of the basin, and the main gas-bearing formation is the bottom of upper Paleozoic Permian He-8. He-8section belongs to typical Fluvial sedimentary facies, and has the characters of large thickness, strong heterogeneity and the reservoir rock’s main content is quartz sandstone. The character of low porosity and low permeability of He-8section seriously restricts the large-scale commercial development for the gas in this region.By using seismic technology to conduct the pre-drilling prediction is the chief means to improve tight sandstone reservoir exploration and development benefits and based on rock physic research. Aiming at the practical requirements of the tight sandstone exploration in the Sulige, under the well logging geology, seismic exploration theory and AVO technology’s guidance and basing on the reservoir geological feature as well as petrophysical feature, this paper combine seismic with well log, take two-way comparison analysis between AVO forward and inversion, to realize the seismic response characteristic study for the tight sandstone reservoir.Basing on logging data and seismic exploration theory, using AVO forward method to study the AVO anomaly of gas-bearing reservoir and the results are used to guide AVO abnormal attributes interpretation of pre-stack data inversion. The purpose of building AVO model is to analyze the influence of tight sandstone physical property, reservoir-seal assemblage and fluid property for AVO response. On the other hand, the analysis for AVO forward model can guide for the inversion. Therefore, whether the forward model is correct or not is very important, it would affect the accuracy interpretation for AVO inversion results. But when doing inversion, correct forward analysis is equally important.Tight sandstone reservoir’s seismic response research is on the premise of petrophysical analysis. This paper through studying Ordos basin a series of development wells’forward model summarized He-8tight sandstone porosity, reservoir-seal assemblage abnormal response laws. By supposing the well’s target strata gas-bearing under the condition of0%、20%、40%、60%、80%, simulating these cases seismic response respectively and taking comparison with the actual curves, to determine the AVO response feature variation under the different gas saturation, and then determine the gas potential condition as well as AVO response type. On the basis of forward model research, the paper analyses the different porosity and reservoir-seal assemblage’s influence for tight sandstone reservoir seismic response.By studying Sulige tight sandstone, fully understand sandstone reservoir physical property, which are porosity, permeability and gas-bearing. Then from logging curve, verify them using the AVO forward and combine with angle trce gathering analysis, to get tight sandstone AVO response character. The purpose of AVO inversion based on pre-stack seismic data and the extraction of AVO abnormal attributes parameters and studying the anomalies of attribute section’s borehole-side gas-bearing section as well as AVO forward is to predict the gas bearing reservoir’s plane distribution of tight sandstone and gas bearing condition, to identify the abnormal response characteristics of tight sandstone gas-bearing section on the AVO abnormal attribute section and the attribute slices.Main conclusions as follows:1. In the case of low porosities, the majority of gas bearing section sandstone is consist of type II and type IV sandstone which is poor gas-bearing potential, with increasing porosity, type II and type IV sandstone gradually decreased, while type III gas filled sandstone becomes the main composition.2. There are four reservoir-seal assemblages including sand wrapped by mud, mud wrapped by sand, mud and sand wrapped by sand, sand and mud wrapped by sand, the former two are the best oil and gas reservoir, and60%is type III AVO abnormal response sandstone.3. He-8Section gas reservoir in Sulige gas field is a low pressure, low permeability and low abundance and widely distributed lithologic gas reservoir with the channel sand body as the main reservoir sand. It has obvious wave impedance difference and Poisson’s ratio difference with the surrounding rock. The effective thickness of the reservoir is identifiable by AVO technology.4. Summing up AVO forward results of the typical gas content wells in Sulige area, obviously seismic data AVO abnormal response is closely relative with tight sandstone gas bearing attributes, it mainly manifest two side, the first is gas saturation, the second is the porosities and reservoir-seal assemblages which limited the gas-bearing total volume.5. The tight sandstone gas bearing section has obvious rule of amplitude various with offset on the angle trace gathering, it has obviously AVO anomaly. It’s evidently that He-8section has corresponding AVO anomaly while gas content wells also locate in the favorable gas-bearing area.