Tight Gas Reservoir in the Upper Triassic Sichuan Basin, Western China

Abstract

The western Sichuan Basin is a foreland basin formed in the Late Triassic at the front of the Longmen Mountain in the western Sichuan Province of China. The Upper Triassic Xujiahe Formation in the basin is an ultralow-permeability and low-porosity tight sandstone and shale gas reservoir.

Tight gas reservoirs are often defined as gas-bearing sandstones or carbonates having in situ permeabilities to gas less than 0.1 mD. This paper offers an integrated approach to describe microstructure characteristics of tight sandstone and shale gas reservoir. In particular, the primary and secondary porosity of a tight gas sandstone are identified and quantified in three dimensions using X-ray Nano-CT imaging and visualization of core material at the pore scale. Once the tomographic images are combined with SEM images from a single plane within the cubic data set, the nature of the secondary porosity can be determined and quantified. In-situ mineral maps measured on the same polished plane are used to identify different microporous phases contributing to the secondary porosity. Once these data sets are combined, the contribution of individual clay minerals to the microporosity, pore connectivity, and petrophysical response can be determined. Insight into the producibility may also be gained. The existence of overpressure reduced effective stress, promoting opening-mode fracture growth. The existence of tension fractures can also be used as an indicator of ancient overpressure in a sedimentary basin. Diagenetic fractures formed from the Late Triassic, when the foreland basin of the western Sichuan Basin formed, to the Early Cretaceous. Under rapid sedimentation and intense compaction, intragranular microfractures formed because of the crushing of quartz grains and cleavage in feldspar. At the same time, under the influence of compaction and pressure solution, diagenetic transgranular microfractures formed along the microbedding planes and are parallel to the directional fabric of platy minerals. The intragranular and grain-boundary fractures are developed discretely at different structures, have small size and permeability, and are a major factor in storage of gas.

Tectonic microfractures controlled by fold thrust and lithology are developed at all of the gas fields and have the largest contribution to gas production. Diagenetic microfractures distributed along the microbedding plane and microfractures related to overpressure have little contribution to gas production.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018