Development Characteristics and Finite Element Simulation of Fractures in Tight Oil Sandstone Reservoirs of Yanchang Formation in Western Ordos Basin

Abstract

Structural fractures have a significant control effect on the large-scale accumulation of hydrocarbons in the Yanchang Formation. Previous studies have affirmed the important role of fractures in hydrocarbon accumulations in strongly deformed zones. However, for low-amplitude structural areas, the degree of fracture development is relatively low, and their control on sweet spots of hydrocarbons has not yet formed a unified understanding. In this paper, taking the Upper Triassic Yanchang Formation in the western Ordos Basin as an example, the development characteristics, prediction method, and the distribution of fractures in tight sandstone reservoirs in low-amplitude structural areas have been systematically studied using a large number of cores, thin sections, paleomagnetism, FMI logging, acoustic emission, productivity data, and finite element method. The research results showed that the Yanchang Formation in the study area mainly develop high-angle and vertical fractures, which were formed by regional tectonic shearing. Fractures are mainly developed in the fine-grained and ultra-fine-grained sandstones of the distributary channel and estuary bar microfacies, while the fractures in the medium-grained sandstones of the distributary channel and the mudstones of the distributary bay are relatively underdeveloped. The core fractures and micro-fractures of the Yanchang Formation all have the regional distribution characteristics, and the fracture strikes are mainly between NE50° and NE 70°. Moreover, the finite element method was used to predict the fractures in the target layer, and the prediction results are consistent with the actual distribution results of the fractures. The coupling analysis of fractures and tight oil sandstone distribution showed that the existence of fractures provided conditions for the accumulation of hydrocarbons in the Yanchang Formation. The confluence and turning areas of the river channels were repeatedly scoured by river water, and the rocks were brittle and easy to form fractures. The thickness of the fractured sandstone in these areas is usually greater than 0.4 m. Moderately developed fracture zones are prone to form hydrocarbon accumulation “sweet spots,” and the fracture indexes of these areas are usually distributed between 0.8 and 1.2. However, when the fracture index exceeds 1.2, over-developed fractures are unfavorable for the accumulation of hydrocarbons.

Characterization of fractures in low‐permeability thin tight sandstones: A case study of Chang 6 Member, Yanchang Formation, western Ordos Basin

“…Natural fractures in reservoirs have an important impact on oil and gas productivity (Liu et al, 2020; Tian et al, 2017). However, due to the limited observation range of downhole data, it is difficult to determine the distribution characteristics and heterogeneity of fracture systems by core observations (Chen et al, 2022; Wang et al, 2022).…”

Pore structure and fractal characteristics of the marine shale of the longmaxi formation in the changning area, Southern Sichuan Basin, China

“…The fracture shape coordinates with the crystal boundary, and the opening is approximately 0.01-0.10 μm (Figure 2G). Intragranular fractures, also known as mineral joint fractures, are caused by lattice dislocation or fracture in the crystal under tectonic action (Figure 2H) (Ameen, 2016;Cheng et al, 2021;Shan et al, 2021;Song et al, 2021;Liu et al, 2022a;Wang et al, 2022). 2) Diagenetic contraction fractures and bedding fractures are formed by diagenesis, in which diagenetic contraction fractures are formed by a series of physical and chemical actions such as dehydration, contraction, dry cracking, phase transformation, or recrystallization of sediments during diagenesis, which is characterized by a long extension distance and large opening variation range (Figure 2I).…”

Densification mechanism of deep low-permeability sandstone reservoir in deltaic depositional setting and its implications for resource development: A case study of the Paleogene reservoirs in Gaoshangpu area of Nanpu sag, China

“…Diagenesis is an important factor affecting reservoir quality. Reservoir types with different pore structures and physical properties result from distinct diagenetic sequences, diagenetic stages, and diagenetic evolution (Zhu et al, 2009;He et al, 2022b;Wang et al, 2022). Based on the assemblage of diagenetic minerals of deep low-permeability Gaoshangpu reservoirs, it Frontiers in Earth Science frontiersin.org can be preliminarily determined that the sequence of diagenetic minerals or diagenesis is as follows: Microcalcite-silica I stage overgrowth-weak dissolution of feldspar, debris-silica II stage overgrowth-kaolinite precipitationlarge-scale feldspar, and lithic dissolution-silica III stage overgrowth-intergranular pores filled with quartz-(ironbearing) calcite filling-dolosparite-weak dissolution.…”