Paleoenvironments, stratigraphic evolution and reservoir characteristics of the Upper Cretaceous Yingjisha Group, southwest Tarim Basin

摘要

The Upper Cretaceous Yingjisha Group in the southwestern Tarim Basin is an important host of hydrocarbons with the highest-quality reservoirs are a product of depositional environment, diagenesis and tectonics. The Yingjisha Group is composed, in ascending order, of the Kukebai, Wuyitak, Yigziya and Tuyiluok formations. Strata are thickest in the Kashgar Sag in the northwest and the Yecheng Sag (up to 635 m) in the southeast and thin over the central Qimugen Uplift (less than 70 m). Six facies associations (1-6) are interpreted, respectively, as braided fluvial, lagoon, nearshore-shelf, playa-lake, restricted carbonate platform and open carbonate platform to platform margin. Stratigraphic evolution of the Yingjisha Group, recorded in vertical changes in inferred depositional environments, is related to changes in water depth and climate. At least two transgressive (Kukebai and Yigziya formations - regressive (Wuyitak and Tuyiluok formations) cycles are recognized that can be correlated with the global eustatic curve of Haq et al. (1988). Detailed paleoenvironmental analysis forms the basis for understanding and predicting the occurrence of reservoir rocks in this mixed siliciclastic-carbonate succession. High-energy, braided-fluvial sandstones and carbonate platform and platform-margin grainstones (locally dolomitized) comprise the best reservoirs. The most important diagenetic processes in sandstones that resulted in porosity and permeability changes are: 1) mechanical compaction, 2) cementation, and 3) replacement and dissolution of unstable clastic grains and cements, whereas in carbonate rocks the dominant diagenetic processes are cementation and dissolution. Fracturing played an important role in enhancing the quality of sandstone and especially carbonate reservoirs notably in terms of their permeability by connecting previously isolated pores. Stratigraphic evolution of the Yingjisha Group resulted in stacking of seal rocks above reservoirs during the late transgressive and regressive phases of sedimentation. (C) 2015 Elsevier Ltd. All rights reserved.