Fold-Related Fracture Distribution in Neogene, Triassic, and Jurassic Sandstone Outcrops, Northern Margin of the Tarim Basin, China: Guides to Deformation in Ultradeep Tight Sandstone Reservoirs

Ultradeep (6-8 km depth) low-porosity sandstone oil and gas reservoirs, in the Kuqa thrust belt of Tarim Basin, are an important natural gas supply source of China. Both opening and shear-mode fractures are extremely necessary reservoir elements for gas production in these rocks but are challenging to characterize with sparse core and well log observations. Here, we use outcrops of some of the same units from four exposed folds to describe fracture types and patterns. These four folds have a range of shapes that are representative of folds at depth. At the Dongq, Kuqa, Misib, and Tuger anticlines, we analyzed fracture type, cross-cutting and abutting relations, density, spatial arrangement, kinematic aperture, orientation, and mineral fill. One-dimensional inventories, field sketches, photographs, and LiDAR imagery documented fracture patterns. Most dips of all fractures shown everywhere on the fold are greater than 60 degrees (over 50%). Fracture kinematic apertures are 3 mm to similar to 6 mm, and fracture density is 0.5 traces/m to similar to 1.5 traces/m. All fractures are divided into shear mode (or small faults) and opening mode. Shear-mode fractures, with high dips that strike N-S, mutually crosswise arranged with intersection angles of 30-60 degrees, are found extensively on the flanks of these folds. In contrast, most opening-mode fractures strike E-W, are arranged parallel to each other, and are localized in fold hinges. Besides, exposed folds (reservoir analogues, figure 1) in a proximal (hinterland, Tuger and Misib) position have fracture abundance distributions and aperture size patterns compatible with fold-related fracture development whereas distal (basinward, Kuqa and Dongq) folds lack this correlation, but patterns in distal folds might be explained by overprinted effects of lithological heterogeneity on fracture abundance or the effects of nearby faults. The positive correlation between fold-related strain and fracture spatial distribution in the northern (proximal) folds permitted inference of fracture patterns in deep wells.

Automated summary

The ultra-deep low-porosity sandstone oil and gas reservoirs in the Kuqa thrust belt of Tarim Basin are important sources of natural gas supply in China. Fracture types and patterns, including shear-mode and opening-mode fractures, were described using outcrops from four exposed folds in the study area. Fracture distribution, density, aperture size, and kinematics varied between different locations on the folds, with shear-mode fractures dominating the flanks and opening-mode fractures localized in fold hinges.