Development characteristics of natural fractures in tight sandstone reservoirs and their controlling factors: upper Triassic Xujiahe Formation, western Sichuan Basin

IF 2 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Yunzhao Zhang, Rongjun Zhang, Le Qu, Hao Wu, Quanqi Dai, Zhe Zhang, Tao Shen, Ruijun He
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Abstract

Natural fractures are widely developed and distributed in tight sandstone reservoirs in the western Sichuan Basin, China, influenced by complex tectonic movements and diagenetic processes. These natural fractures serve as the main flow channels and important storage spaces in such reservoirs. Understanding their development characteristics and controlling factors determines the positioning and production efficiency of tight gas wells. This paper first explores the types and characteristics of natural fractures in the second member of the Upper Triassic Xujiahe Formation (T3x2) in the western Sichuan Basin through outcrop investigations, core observations, image log interpretations, and thin section analyses. We study the main influencing factors controlling the development and distribution of tectonic shear fractures by combining the characterization of fracture attributes. The results show that the tight sandstone reservoirs in the T3x2 mainly contain tectonic fractures, diagenetic fractures, and overpressure fractures, among which shear fractures in tectonic fractures are the main types. There are four sets of tectonic shear fractures in the study area, oriented in the E-W, N-S, NE-SW, and NW-SE directions. The dip angles of fractures range from 20° to 80°, with an average dip angle of 53°, predominantly consisting of high-angle fractures. The lengths of fractures are mainly within 100 cm, with a mean value of 47.53 cm, and 76.2% of fractures have lengths less than 50 cm. Fracture apertures range from 2.51 to 163.19 μm, with an average of 30.54 μm. The proportion of effective fractures in tectonic shear fractures reaches 85.7%. The development of tectonic shear fractures is primarily influenced by lithology, rock mechanics stratigraphy, diagenetic processes and facies, and faults. Lithology stands as the fundamental factor influencing the degree of fracture development in tight reservoirs. In rocks with the same composition, as the grain size decreases, the degree of fracture development increases. Simultaneously, with the rise in muddy content, fracture development diminishes in siltstone, argillaceous siltstone, silty mudstone, and mudstone. The formation and distribution of fractures are governed by rock mechanics stratigraphy, primarily manifested as a decrease in fracture density with an increase in rock mechanics layer thickness within a certain range. Diagenetic processes affect the abundance of tectonic shear fractures by influencing the mechanical properties of rocks. The stronger the compaction and cementation, the poorer the physical properties, and the higher the brittleness of the rocks, resulting in a greater abundance of tectonic shear fractures. Rocks with strong compaction and robust cementation facies exhibit high brittleness, rendering them more susceptible to fracturing under identical stress conditions. Fractures are highly prevalent in both the hanging wall and footwall of faults. As the distance from the fault increases, the linear density of fractures significantly decreases, owing to the uneven distribution of stress perturbation caused by fault activity.
致密砂岩储层天然裂缝发育特征及其控制因素:四川盆地西部上三叠统徐家河地层
受复杂构造运动和成岩过程的影响,天然裂缝在中国四川盆地西部致密砂岩储层中广泛发育和分布。这些天然裂缝是此类储层的主要流动通道和重要储集空间。了解其发育特征和控制因素决定了致密气井的定位和生产效率。本文首先通过露头调查、岩心观察、图像测井解释和薄片分析,探讨了四川盆地西部上三叠统徐家河组(T3x2)第二段天然裂缝的类型和特征。结合断裂属性特征,研究了控制构造剪切断裂发育和分布的主要影响因素。结果表明,T3x2致密砂岩储层主要包含构造裂缝、成岩裂缝和超压裂缝,其中构造裂缝中的剪切裂缝是主要类型。研究区内有四组构造剪切断裂,走向分别为 E-W、N-S、NE-SW 和 NW-SE。断裂倾角从 20°到 80°不等,平均倾角为 53°,主要为高倾角断裂。断裂长度主要在 100 厘米以内,平均值为 47.53 厘米,76.2% 的断裂长度小于 50 厘米。断裂孔径从 2.51 微米到 163.19 微米不等,平均为 30.54 微米。构造剪切断裂中有效断裂的比例达到 85.7%。构造剪切断裂的发育主要受岩性、岩石力学地层、成岩过程和岩相以及断层的影响。岩性是影响致密储层裂缝发育程度的基本因素。在成分相同的岩石中,随着粒度的减小,裂缝的发育程度也会增加。同时,随着泥质含量的增加,粉砂岩、镁质粉砂岩、淤泥质泥岩和泥岩的裂缝发育程度减弱。裂缝的形成和分布受岩石力学地层的影响,主要表现为在一定范围内,随着岩石力学层厚度的增加,裂缝密度降低。二迭纪过程通过影响岩石的力学性质来影响构造剪切裂缝的数量。压实和胶结作用越强,岩石的物理性质越差,脆性越大,从而导致构造剪切裂缝越多。具有强压实和强胶结面的岩石具有高脆性,因此在相同的应力条件下更容易发生断裂。断裂在断层的悬壁和底壁都非常普遍。由于断层活动造成的应力扰动分布不均,随着与断层距离的增加,断裂的线性密度明显降低。
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来源期刊
Frontiers in Earth Science
Frontiers in Earth Science Earth and Planetary Sciences-General Earth and Planetary Sciences
CiteScore
3.50
自引率
10.30%
发文量
2076
审稿时长
12 weeks
期刊介绍: Frontiers in Earth Science is an open-access journal that aims to bring together and publish on a single platform the best research dedicated to our planet. This platform hosts the rapidly growing and continuously expanding domains in Earth Science, involving the lithosphere (including the geosciences spectrum), the hydrosphere (including marine geosciences and hydrology, complementing the existing Frontiers journal on Marine Science) and the atmosphere (including meteorology and climatology). As such, Frontiers in Earth Science focuses on the countless processes operating within and among the major spheres constituting our planet. In turn, the understanding of these processes provides the theoretical background to better use the available resources and to face the major environmental challenges (including earthquakes, tsunamis, eruptions, floods, landslides, climate changes, extreme meteorological events): this is where interdependent processes meet, requiring a holistic view to better live on and with our planet. The journal welcomes outstanding contributions in any domain of Earth Science. The open-access model developed by Frontiers offers a fast, efficient, timely and dynamic alternative to traditional publication formats. The journal has 20 specialty sections at the first tier, each acting as an independent journal with a full editorial board. The traditional peer-review process is adapted to guarantee fairness and efficiency using a thorough paperless process, with real-time author-reviewer-editor interactions, collaborative reviewer mandates to maximize quality, and reviewer disclosure after article acceptance. While maintaining a rigorous peer-review, this system allows for a process whereby accepted articles are published online on average 90 days after submission. General Commentary articles as well as Book Reviews in Frontiers in Earth Science are only accepted upon invitation.
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