--> Quantitative Evaluation of Diagenesis and Porosity Evolution of a Tight Sandstone Reservoir — A Case Study of the Southwestern Part of the Ordos Basin, China

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Quantitative Evaluation of Diagenesis and Porosity Evolution of a Tight Sandstone Reservoir — A Case Study of the Southwestern Part of the Ordos Basin, China

Abstract

Abstract

The Ordos basin is an important region of dense oil in central China, and the formation contains the Chang-6 and Chang-7 members of the Yanchang Formation, formed during the Triassic period. However, most existing knowledge of the basin is from dense reservoir characteristics and control factors of its formation. Currently most research focuses on the characteristics of dense reservoir and the control factors of its formation. The understanding of pore evolution within the reservoir is relatively limited, which is influenced by different diagenesis stages. This study is aimed at discussing the influences of different diagenetic stages and how they affect the reservoir storage performance. The study found that Chang 63–Chang 72 reservoir rock types were primarily lithic arkose and feldspathic lithic sandstone, with lesser amounts of quartz and higher amounts of filler content. The reservoir porosity was mainly distributed in the range of 4%–16%. The permeability was generally distributed in the range of 0.01–1 × 10−3 µm2. Therefore, the reservoir was categorized as a low- porosity, low-permeability reservoir. By diagenesis, structural evolution, and paleogeothermal information, the Chang 63–Chang 72 sandstone reservoir from the oil diagenetic stage in the study district was divided into early quickly strong compaction to early chlorite and carbonate cementation, quartz overgrowth to silica, clay minerals and late carbonate cementation to carbonate, and feldspar and weak dissolution of debris. It is believed that diagenesis was the main cause of the low-porosity and low-permeability observed in the reservoir. The compaction effect also directly reduced the porosity and permeability. In addition, the multiperiod and multitype cementation was attributed to the protection and destruction of the pores. The secondary porosity formed by corrosion that was more common but not very strong to improve the reservoir porosities of sandstone in different degrees. Based on this observation, we further established a diagenetic-porosity evolution model of the Chang 63–Chang 72 sandstone oil reservoir study district. According to a variety of diagenesis intensities, we established a quantitative model for evaluating the generation and loss of porosity Finally, the model was applied to forecast the variation of dense reservoir porosities in the study area, and the favorable distribution area of the dense reservoir was noted.