Identification of sweet spot is of great significance in confirming shale gas prospects to realize large-scale economic shale gas development. In this paper, geological characteristics of shale gas reservoirs were compared and analyzed based on abundant data of domestic and foreign shale gas reservoirs. Key elements of sweet spots were illustrated, including net thickness of gas shale, total organic carbon (TOC) content, types and maturity (Ro) of organic matters, rock matrix and its physical properties (porosity and permeability), and development characteristics of natural fractures. After the data in Changning and Weiyuan blocks, the Sichuan Basin, were analyzed, the geologic laws of shale gas enrichment were summarized based on the economic exploitation characteristics of shale gas and the correlation between the elements. The elements of favorable "sweet spots" of marine shale gas reservoirs in the Changning block and their distribution characteristics were confirmed. Firstly, the quality of gas source rocks is ensured with the continuous thickness of effective gas shale larger than 30 m, TOC > 2.0% and Ro = 2.4–3.5%. Secondly, the quality of reservoir is ensured with the brittle minerals content being 30–69%, the clay mineral content lower than 30% and a single lamination thickness being 0.1–1.0 m. And thirdly, the porosity is higher than 2.0%, the permeability is larger than 50 nD, gas content is higher than 1.45 m3/t, and formation is under normal pressure–overpressure system, which ensures the production modes and capacities. Finally, the primary and secondary elements that control the "sweet spots" of shale gas reservoirs were further analyzed and their restrictive relationships with each other were also discussed.
Accurate prediction of gas planar distribution is crucial to selection and development of new CBM exploration areas.Based on seismic attributes, well logging and testing data we found that seismic absorption attenuation, after eliminating the effects of burial depth, shows an evident correlation with CBM gas content; (positive) structure curvature has a negative correlation with gas content; and density has a negative correlation with gas content.It is feasible to use the hydrocarbon index (P*G) and pseudo-Poisson ratio attributes for detection of gas enrichment zones.Based on seismic multiple-attribute analyses, a multiple linear regression equation was established between the seismic attributes and gas content at the drilling wells.Application of this equation to the seismic attributes at locations other than the drilling wells yielded a quantitative prediction of planar gas distribution.Prediction calculations were performed for two different models, one using pre-stack inversion and the other one disregarding pre-stack inversion.A comparison of the results indicates that both models predicted a similar trend for gas content distribution, except that the model using pre-stack inversion yielded a prediction result with considerably higher precision than the other model.
Porous-type sandstone reservoir is shorter and shorter and gas reservoir in tight sand is more and more important.The study of the 8th Member of the Shihezi Sandstone indicates that AVO anomaly for the tight sandstone is characteristic of gas or water content in the sandstone.The gas-bearing sandstone and water-bearing sandstone are characterized by negative P value of zero offset.AVO anomaly for gas-bearing sandstone is obvious with negative G value due to decrease in Poisson’s ratio,while AVO anomaly for water-bearing sandstone is characterized by positive G value due to increase in Poisson’s ratio.
Using core,logging,seismic data and rock flakes,and based on the results of laboratory analysis and other information,the microfacies of the Member 2 of the Nantun Formation from the Tarbell depression were studied.Results show that in the Tarbell depression,there developed various depositional systems,such as alluvial fan,fan delta,nearshore submarine fan,lake floor fan and lacustrine systems dominated by the fan-delta and lacustrine systems.Through seismic facies analysis,seismic attribute analysis,study of sandstone percentage,diplog,and heavy minerals,the spatial distribution of the above mentioned depositional systems were presented in this paper.
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