Stratification model of an ultra-deep tight sandstone fracture reservoir under tectonic stress: A case study of a Cretaceous reservoir in the Kuqa foreland thrust belt of the Tarim Basin

Abstract

The Kuqa foreland thrust belt is a recent hot topic of ultra-deep oil–gas exploration in China, and many natural gas fields with high and stable yields, have been found in subsalt clastic reservoirs with a buried depth of 6500–8000 m. In this paper, the well logging, core, and thin slice data are used to analyse the characteristics of the ultra-deep tight sandstone reservoir of the Bashijiqike Formation in the Kuqa foreland thrust belt and to discuss the relationship between tectonic stress and reservoir development. The results show that the sandstone reservoir of the Bashijiqike Formation can be vertically divided into 3 zones; from the top down, the surface porosity and the average pore aperture decrease, the rock density increase, and the permeability change. Such vertical differences are mainly affected by tectonic compression. Significant differences exist between the stress above and below the neutral plane in the same anticline. Above the neutral plane, the presence of local tensile stress and the dominant development of a large number of high-angle tension fractures with large apertures are conducive to dissolution; below the neutral plane, the presence of local compressional and torsional stress and the main development of reticulated shear cracks with small openings indicate the enhanced compaction of the reservoir. The variation of subsalt tectonic stress from the northern oblique to the southern transverse compression processes, development of pre-Mesozoic paleo-uplift, induction of the overlying salt body with low stress and arched extension of the underlying mudstone induced ultra-deep Cretaceous reservoir stress. A stratification model of the faulted anticline reservoir affected by the stress neutral plane is constructed and is significant for predicting a favourable reservoir in a compression basin.