Abstract
The buried depth of the gas-producing reservoir in the Kuqa foreland thrust belt of the Tarim Basin exceeds 6000 m. The average matrix porosity of the reservoir is 5.5%, and the average matrix permeability is 0.128 × 10−3 μm2. In order to reveal the characteristics and effectiveness of ultra-deep fractures and their effects on reservoir properties and natural gas production, outcrops, cores, thin section, image logs and production testing data are used to investigate the effectiveness of tectonic fractures in ultra-deep reservoirs in the Kuqa foreland thrust zone, and the corresponding geological significance for oil and gas exploration and development are discussed. Tectonic fractures in the thrust belt include EW-trending high-angle tensile fractures and NS-trending vertical shear fractures. The former has a relatively high filling rate, while the latter is mostly unfilled. Micro-fractures are usually grain-piercing-through cracks with width of 10–100 microns. In the planar view, the effective fractures are concentrated in the high part and wing zones of the long axis of the anticline, and along the vertical direction, they are mainly found in the tensile fracture zone above the neutral plane. The adjustment fracture zone has the strongest vertical extension abilities and high effectiveness, followed by the nearly EW longitudinal tensile fracture zone, and the netted fracture zone with multiple dip angles. The effectiveness of fracture is mainly controlled by fracture aperture and filling degrees. Effective fractures can increase reservoir permeability by 1–2 orders of magnitude. The higher part of the anticline is associated with high tectonic fracture permeability, which control enrichment and high production of natural gas. The netted vertical open fractures effectively communicate with pores and throats of the reservoir matrix, which forms an apparent-homogenous to medium-heterogeneous body that is seen with high production of natural gas sustained for a long term.
Highlights
Tight sandstone gas is an important unconventional hydrocarbon resource (Jia et al 2012; Guo et al 2013; Dai et al 2012; Zhang et al 2014; Yang et al 2018; Zhang et al.Edited by Jie-Hao and Chun-Yan Tang2014; Li et al 2012; Zeng 2009; Zhan et al 2017; Zeng et al 2012)
The study area—the Keshen gas field is located in the Kelasu tectonic belt of the Kuqa depression, the Tarim Basin, which is rich in natural gas resources, with multiple faulted anticline gas reservoirs such as KeS5, KeS6, KeS2, KeS8, KeS9, and KeS13 (Fig. 1)
Through the observation of similar outcrops in the field (Fig. 3) and the analysis of downhole measurements, it is found that the fractures in the long axis of the anticline are mostly dominated by large vertical fractures penetrating through strata
Summary
Tight sandstone gas is an important unconventional hydrocarbon resource Regarding the foreland thrust belt of the Kuqa depression and the tectonic fractures in Keshen gas field, numerous studies have carried out in various aspects before, including quantitative description and prediction of fractures (Zeng and Wang 2014; Zhao et al 2014; Wang et al 2014; Lai et al 2019), indicative implications of regional stress field inversion provided by fractures (Zhang and Wang 2004; Zeng et al 2004), formation mechanisms of fractures and influential factors (Zeng et al 2010a, b; Liu et al 2017; Chang et al 2014; Gong et al 2015) These studies have revealed that fractures lead to obvious modification of ultra-deep tight sandstone reservoirs (Wang et al 2018). On the basis of analysis of tectonic fracture types and characteristics as well as information obtained from cores, thin sections and image logs, the effectiveness of tectonic fractures is studied, with the aim to provide geological guidance on the efficient development of the Keshen gas field
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