The application of sedimentary microfacies on the fracability of tight sandstone reservoir in Chang 7 member of Longdong area in the Ordos Basin
[Objective]Sedimentary differences are the key factor in controlling reservoir heterogeneity.Analyzing reservoir heterogeneity through sedimentary microfacies is crucial for oil and gas field development and sweet spot prediction,and it also informs the evaluation of fracturing in tight sandstone reservoirs.There are many types and complex lithologies of unconventional oil and gas reservoirs in the Ordos Basin as well as many factors controlling reservoir fracability.At present,mechanical experiments are used to comprehensively characterize the fracturing property;however the research cost is high and the experimental process is complicated,making it unnsuitable for large-scale oilfield development and use.Therefore,this study attempted to analyze and compare the fracability of tight sandstones with different sedimentary microfacies from the perspective of sedimentary microfacies controlling the lithology and reservoir development to provide a reference for oilfield development plans.[Methods]Taking the compact sandstone of Chang 7 member of Yanchang Formation in the Longdong area of Ordos Basin as the research object,the different types of microfacies are identified through the data of core and cast slice,the mineral composition and structural parameters of rock samples were obtained by X-ray diffraction(XRD)analysis,and rock mechanics experiments were conducted to quantitatively described the fracturing property.[Results]The results are as follows:(1)Two sedimentary microfacies,namely underwater distributary channel and sheet sand,mainly developed in Chang 7 Member of Yanchang Formation in the study area.Among them,the single sand body thickness of the underwater distributary channel is greater than 2 m,the sheet sand is mostly a medium thin and thick sand mudstone interlayer,and the single sand body thickness is generally less than 2 m.(2)The composition and structure of the two sedimentary sandstone microfacies are obviously different:the content of carbonate minerals,clay minerals,and heterobases in the sheet sand microfacies are relatively high,the particle size is finer,and the sorting is worse,which are the main internal factors that cause the difference in tight sandstone fracability and are the basis for judging the fracability of tight sandstone by sedimentary microfacies.(3)The fracability index was related to the composition and structure of sandstone.In terms of composition,the fracability index was positively correlated with quartz mineral content as well as carbonate mineral content and negatively correlated with feldspar mineral content.In terms of structure,there is a negative correlation between the fracability index and the average particle diameter φ.The larger the particle size,the higher the fracability index.The fracability index was positively correlated with the standard deviation of the particle size,indicating that the worse the particle separation,the higher the fracability index.(4)Through grey correlation analysis,it was found that the degree of influence of sandstone parameters on fracability was in the order of carbonate mineral content,quartz content,standard deviation of particle size,and average particle size from high to low,while clay minerals and feldspar content were in a relatively weak position.[Conclusion]The results indicate that the higher the contents of carbonate and quartz and the higher the standard deviation of particle size(the worse the sorting),the better the fracability.The finer the particle size,the higher the feldspar content and the worse the fracability.Grey correlation analysis showed that the carbonate mineral content,separation and particle size play a major role in the fracability of tight sandstone.Compared with distributary channel microfacies,sheet sand has a higher carbonate minerals content,worse sorting,and little difference in quartz content.Although the microphase particle size of the sheet sand is slightly finer,the average particle size has a relatively minor effect on the fracability;thus,sheet sand as a whole shows better fracability.[Significance]Since the standard deviation(sorting)and particle size of sandstone particles are controlled by sedimentary microfacies,and the content of carbonate minerals is directly controlled by sandstone thickness and indirectly affected by sedimentary microfacies,the change in the tight sandstone fracability index can be judged according to the difference in sedimentary microfacies in practical engineering,and the working process can be simplified.
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