Development characteristics and main controlling factors of bedding-parallel lamellated fractures in shale in 7th member of Triassic Yanchang Formation, southwestern Ordos Basin

Abstract

Bedding-parallel lamellated fractures are widely developed in shale in the 7th member of Triassic Yanchang Formation (hereinafter referred to as Chang 7) in the southwestern Ordos Basin, which holds significant importance for sweet spot selection, fracturing operations, and development planning. In this paper, based on the surface outcrop and core observations in the Qingcheng to Huachi region of the southwestern basin, combined with analysis and testing of organic matter content, mineral composition and fabric characteristics, the developmental characteristics of bedding-parallel lamellated fractures in different lithologies in the Chang 7 shale were identified, and the main controlling factors of fracture development were analyzed. Results show that the morphology and distribution of the bedding-parallel lamellated fractures are mainly controlled by the laminates, exhibiting characteristics such as continuous flatness, wavy bending and branching due to the different characteristics of the laminae. Sandstone bedding-parallel lamellated fractures are mostly distributed along the biotite laminae, with good continuity and large aperture, and are generally unfilled. Shale bedding-parallel lamellated fractures are most developed in black shale, mostly distributed along the bedding laminates composed of organic matter layers, with a few partially or completely filled by calcite and organic matter. The aperture is smaller than that of sandstone, but the density is higher. Bedding-parallel lamellated fractures are also controlled by organic matter content, lithology, mineral composition, and laminate structure. The sandstone bedding-parallel lamellated fractures are mainly controlled by the content of biotite and the laminates formed by it. When sandstone sorting is good and biotite content is high with a layered distribution, the degree of fracture development is high. As the density of the laminates increases, the degree of development of bedding-parallel lamellated fractures also increases. Shale bedding-parallel lamellated fractures mainly develop in organic matter laminates and tuffaceous laminates, and are controlled by organic matter content and mineral components. Fracture density increases first and then decreases with the density of the layers. Fracture density in thin laminates is higher than that in thick laminates.

Keywords

• shale oil
• bedding-parallel lamellated fracture
• development characteristics
• main controlling factors
• 7th member of yanchang formation
• ordos basin