--> Diagenetic Characteristics and Quantitative Evaluation of the Porosity Evolution of Tight Sandstone Reservoirs: A Case Study of the Yanchang Formation in the Southern Ordos Basin, China

AAPG ACE 2018

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

Abstract

Abstract: In recent years, tight oil has made a breakthrough in numerous fields worldwide and is prospective for exploitation. Widespread diagenesis has occurred in the Yanchang Formation in the southern Ordos Basin during its long geological evolution, creating a tight reservoir; however, identifying the genetic mechanism of the tight sandstone is challenging. Evaluation of the pore evolution is key to gaining a better understanding of oil migration and accumulation in tight oil exploration; To study the diagenesis and porosity evolution of tight sandstone reservoirs, a comprehensive case study of the 8th member of the Yanchang Formation (Chang 8) was carried out through core observation, fluorescent thin section observation, scanning electron microscopy (SEM), cathodoluminescence analysis, electron probing, and logging data analysis, which includedporosity and permeability data. The following conclusions can be drawn. (1) The Chang 8 of the upper Triassic Yanchang Formation in the southern Ordos Basin is a typical tight sandstone reservoir. The pore type includes mainly secondary dissolved pores and residual intergranular pores. Most of the Yanchang Formation in the southern part of the basin underwent an early period of the middle diagenetic stage A. (2) Compaction was the most influential factor of reservoir density and decreased the average pore size by 24.8%. Carbonate cementation decreased the porosity by 5%-10%. The most important diagenetic process for increasing the reservoir porosity was dissolution, which increased the pore size by 5.2%. (3) The Yanchang Formation is used to establish a model for quantitatively evaluatingthe porosity evolution; Φpre=Φo*e-0.001449*H-Φcem+Φdis, where Φpre refers to the ultimate predicted porosity, Φo refers to the original porosity, H refers to the burial depth, Φcem refers to the reduced porosity due to cementation, and Φdis refers to the increased porosity due to dissolution. The deviation of the calculated results from the measured porosity is 1.5%. The established model can be used to predict high-quality reservoirs in tight sandstone.

Key words: Reservoir Characteristics, Diagenesis, Pore Evolution, Ordos Basin