Abstract: Sedimentary difference is a 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 lithology of unconventional oil and gas reservoirs in the Ordos Basin, and there are also many controlling factors for reservoir fracability. At present, mechanical experiments are used to comprehensively characterize fracturing property, but the research cost is high and the experimental process is complicated, which is not suitable for large-scale oilfield development and use. Therefore, this paper attempts to analyze and compare the fracability of tight sandstones with different sedimentary microfacies from the perspective of sedimentary microfacies controlling lithology and reservoir development, so as to provide reference for oilfield development plans.
Taking the compact sandstone of Chang 7 member of Yanchang Formation in Longdong area of Ordos Basin as the research object, the different microfacies types are identified through the data of core and cast slice, the mineral composition and structural parameters of rock samples were obtained by X-yay diffraction analysis (XRD) and rock mechanics experiments quantitatively described the fracturing property.The results are as follows：(1) Two sedimentary microfacies, namely underwater distributary channel and sheet sand, are 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, and the sheet sand is mostly 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 microfacies sandstones are obviously different: the content of carbonate minerals, clay minerals and heterobases in the sheet sand microfacies is relatively high, the particle size is finer, and the sorting is worse.These differences are the main internal factors that cause the difference of tight sandstone fracability, and are the basis for judging the fracability of tight sandstone by sedimentary microfacies. (3) The fracability index is related to the composition and structure of sandstone. In terms of composition, the fracability index is positively correlated with quartz mineral content and 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 frac index was positively correlated with the standard deviation of particle size, indicating that the worse the particle separation, the higher the frac index. (4) Through grey correlation analysis, it is found that the degree of influence of sandstone parameters on fracability is 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 are in a relatively weak position.
The results indicate that the higher the content 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 is, the higher the feldspar content is, and the fracability tends to be worse. The grey correlation analysis shows that 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 higher content of carbonate minerals, worse sorting, and little difference in quartz content. Although the microphase particle size of sheet sand is slightly finer, the average particle size has a relatively minor effect on the fracability, so sheet sand as a whole shows better fracability.
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 of tight sandstone frac  index can be judged according to the difference of sedimentary microfacies in practical engineering, and the working process can be simplified.