| In this paper,the Wufeng-Longmaxi shales in Sichuan Basin and its adjacent areas,west of Xuefeng Mountain,have been studied.Lots of research work has been done to analyze the key-controlling factors of shale gas enrichment.These research works include literature review,field survey,drilling core observation,and experiment analysis(mineralogy,geochemistry,porosity,permeability,and specific surface area).After mapping the ichnography of each key-controlling parameters and then converting them into dataset,three mathematical methods(multiple linear regression analysis,multi-level fuzzy recognition,and neural networks)has been applied to calculate the weight of each parameter and establish the corresponding multi-parameter weighted equation to quantitatively screening the favorable areas for shale gas exploration.The study area has been undergone multi-phase tectonic movements,and the Caledonian and Yanshanian movements are especially intensive.As a result,the structural features are mainly north-east trending,followed by some north-south and few northwest trendings.Five types of sedimentary subfacies(foreshore,nearshore,offshore,shallow water shelf,and deep water shelf)have been identified in Wufeng-Longmaxi Formations in the study area.Particularly,the organic-rich mudstone is the target for shale gas exploration and well developed in the deep water shelf.They are characterized by thin-layered black carbonaceous and siliceous mudstones interbedded with several layer of millimeter-scale bentonite,and with abundant graptolite fossils(WF2-LM4 biozones)and pyrite nodules.The contents of brittle minerals in these mudstones are relatively high and the silica is partially derived from biogenic origins.Even though the porosity and permeability of the organic-rich mudstones are low,microfractures develop well,which is good for gas storage.The influence of shale reservoir on gas content has been analyzed,and shale gas geological parameters have been summarized including reservoir parameters(sedimentary facies,thickness,TOC,Ro,brittle mineral content,porosity,permeability and gas content),preservation parameters(depth,distance of outcrop and fracture distribution and structural feature,etc.)and the pressure coefficient distribution pattern.The sedimentary facies were influenced by the paleo-tectonics.The Yibin-Chongqing area was mainly deep-water shelf to shore facies environment.The buried depth is over 4000 m in the basin and less than 3500 m in the mountain area outside the basin.The thickness of the organic rich shale is over 30 m in the deep water shelf facies environment.The distribution of TOC is coupling well with the distribution of shelf facies and thickness with a maximum value of over 6%.With the influence of historical burial depth,Ro value is high inside the basin and relatively moderate outside the basin.The porosity and brittle mineral content showed a trend of gradual increase from east to west.Based on the comprehensive study of data derived from experiments and literature review,the characterization of geological parameters for Wufeng-Longmaxi shale gas has been summarized in Sichuan Basin and its adjacent areas,west of Xuefeng Mountain.After mapping,gridding and converting the geological parameters into dataset,vectorgraphes of geological parameters have been generated.Considering the geological features of shale,three algorithms have been applied to screening the favorable areas for shale gas exploration in Sichuan Basin and its adjacent areas.BP neural network is the algorithm simulating human-like thinking,and identifying the weights of influence factors.Multiple regression analysis is a statistical method that evaluating the weight value of each influence factor objectively without geological meanings.While multi-level fuzzy recognition is a method based on the experience of geologists and the weights are calculated subjectively.All three algorithms predict that the southern and eastern regions in Sichuan Basin are the favorable areas for shale gas exploration,and the high-steep structural belts in eastern Sichuan Basin are potential favorable areas.However,compared with industrial exploration,the BP neutral networks exhibit higher fittingness than other two algorithms and less restrictions on it.Compared to traditional method,the quantitative screening method in this study is clearer and more intuitive.Meanwhile,the plurality of favorable zones obtained by the method can be visually compared and sorted,and within the favorable zone,the potential resources could be calculated and manifested well. |
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