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Quantitative Evaluation of Diagenesis and Porosity Evolution of Tight Sandstone Reservoirs: A Case Study of Yanchang Formation in the Ordos Basin, China


Tight oil is an important component in the unconventional resources, which has huge reserves and wide prospects for exploitation and application. Currently, previous studies on tight sandstone mostly focused on characterization of tight reservoir and factors controlling its formation. However, studies on the effect of various stages of diagenesis on reservoir quality and quantitative evaluation of porosity evolution are still very weak. Ordos Basin is one of important tight oil plays in China, of which the producing layer is lacustrine sandstones from the 6th and 7th members of the upper Triassic Yanchang Formation. In order to study the diagenesis and porosity evolution of tight sandstone reservoirs, a comprehensive study of Yanchang 6th and 7th members based on core and optical observations, logging data, as well as porosity and permeability data has been carried out in this work, and the following conclusions can be drawn. (a) The Yangchang 6th and 7th members received a succession of delta-lacustrine sediments, which have various pores and throats belonging to low-porosity and low-permeability reservoirs. The pore structures of tight sandstones mainly consist of primary pores, followed by secondary pores. (b) The diagenesis is the primary cause for the formation of tight reservoirs. Diagenetic stages of Yangchang 6th and 7th members sandstone reservoirs can be divided into the early compaction, the early chlorite and carbonate cementation, quartz overgrowth, the late carbonate cementation, and weak dissolution of feldspar and debris. The compaction made the reservoir quality worse, directly leading to the reduction of porosity and permeability. The early cementation had partly counterbalanced the effect of compaction, favoring for preservation of primary pores, while the late carbonate cementation significantly deteriorated the reservoir quality. The formation of secondary pores induced by dissolution can improve the reservoir quality to some extent. (c) Taking Yangchang Fm. as an example, a model for quantitative evaluation of porosity evolution has been established. That is, Φpred=Φo*e-0.00056*h+ΔΦRPIC+ΔΦIPID. Among them, Φpred refers to the ultimate predicted porosity of reservoir, Φo refers to the original porosity of reservoir, h refers to burial depth of reservoir, ΔΦRPIC refers to the reduced porosity induced by cementation, and ΔΦIPID refers to the increased porosity induced by dissolution.