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首页> 外文期刊>Marine and Petroleum Geology >Analysis of the developmental characteristics and major regulating factors of fractures in marine-continental transitional shale-gas reservoirs: A case study of the Carboniferous-Permian strata in the southeastern Ordos Basin, central China
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Analysis of the developmental characteristics and major regulating factors of fractures in marine-continental transitional shale-gas reservoirs: A case study of the Carboniferous-Permian strata in the southeastern Ordos Basin, central China

机译：海洋-陆相过渡页岩气藏裂缝发育特征及主要调控因素分析-以鄂尔多斯盆地东南部石炭系-二叠系为例

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摘要

Fractures play an important role in the formation of shale-gas reservoirs because they can enlarge the transport channels and aggregation spaces and increase the specific surface area of the gas shale. For artificial hydraulic fracturing of these reservoirs, the natural fracture system must be fully integrated with the artificial fracture system to form an intact fracture system. In this study, we first comprehensively examined the fractures in 42 shale-gas wells using several approaches, including a systematic examination and description of the cores and the casting of thin sections, a compilation of the statistics of fracture feature parameters, and observation of various analytical and test data, such as the mineral composition, the organic carbon contents, and the rock mechanics properties for specimens from the corresponding fractured intervals. The data enabled us to thoroughly explore the developmental features and major factors affecting organic-rich shale fractures in the upper Paleozoic Carboniferous-Permian marine-continental transitional coal-bearing formation, which is located in the southeastern Ordos Basin. Our results reveal that, in comparison with the Paleozoic marine shale in the United States and southern China, as well as the Paleozoic basin transitional shale in northern China, the upper Paleozoic black shale in the Ordos Basin is primarily characterized by a relatively low content of brittle minerals and a high content of clay ingredients. The total content of brittle minerals, e.g., quartz, feldspar, and siderite, was approximately 33%, which included 27% quartz and 0.3% K-feldspar but did not include carbonate. The total content of clay minerals reached 64% and was dominated by mixed-layer illite -smectite (I/S), which accounted for more than 41% of the total clay ingredients. The shale accommodated the widespread development of various types of macro- and microfractures. In the core specimens, medium-angle slip fractures and horizontal bedding cracks were the most common types of fractures, whereas vertical and high-angle fractures and horizontal bedding cracks were underdeveloped. In the thin sections, microfractures arising in organic matter laminations or at their edges as well as those of tectonic origin were the predominant type of fractures, and they were mainly short, narrow, and open. Overall, the surface/fracture ratios of the thin sections were concentrated in a range of approximately 0.1 -0.3%. The developmental level of the fracture was influenced by various factors, including tectonism, lithology, rock mechanics, and organic matter and mineral content. Thus, increased developmental level of fractures was correlated with higher paleostructural elevation and increased sand content whereas the developmental level of microfractures was correlated with high Iamellation development high level of organic carbon (leading to more pronounced laminations), and high contents of quartz, mixed-layer I/S, and illite (leading to low levels of kaolinite). These findings were corroborated by other data generated in this study, including the rock mechanics results for the upper Paleozoic black shale and silty shale, the observation results from the cores and from the thin sections sampled from more than 40 shale-gas wells, and the anomalies of gas logging.
机译：裂缝在页岩气储层的形成中起着重要作用，因为它们可以扩大运输通道和聚集空间，并增加页岩气的比表面积。为了对这些油藏进行人工水力压裂，必须将自然压裂系统与人工压裂系统完全整合以形成完整的压裂系统。在这项研究中，我们首先使用多种方法全面检查了42个页岩气井的裂缝，包括系统地检查和描述岩心和薄层铸件，汇编裂缝特征参数的统计数据以及观察各种方法。分析和测试数据，例如来自相应裂缝间隔的标本的矿物成分，有机碳含量和岩石力学特性。这些数据使我们能够深入探索影响鄂尔多斯盆地东南部的上古生代石炭纪-二叠纪海陆陆过渡含煤地层的富含有机质页岩裂缝的发育特征和主要因素。我们的研究结果表明，与美国和中国南部的古生代海相页岩以及中国北方的古生代盆地过渡页岩相比，鄂尔多斯盆地的上古生代黑色页岩的主要特征是相对含量较低。易碎的矿物质和高含量的粘土成分。脆性矿物例如石英，长石和菱铁矿的总含量约为33％，其中包括27％的石英和0.3％的钾长石，但不包括碳酸盐。粘土矿物的总含量达到64％，并以混合层伊利石-蒙脱石（I / S）为主，占混合粘土成分总数的41％以上。页岩适应了各种类型的宏观和微观裂缝的广泛发展。在岩心标本中，中角度滑移裂缝和水平层理裂缝是最常见的裂缝类型，而垂直和高角度裂缝和水平层理裂缝则不发达。在薄层中，有机质叠层或其边缘以及构造起源的微裂缝是裂缝的主要类型，主要为短，窄和开放型。总体而言，薄部分的表面/断裂比集中在约0.1 -0.3％的范围内。裂缝的发育水平受多种因素影响，包括构造，岩性，岩石力学，有机质和矿物质含量。因此，裂缝的发展水平与较高的古构造高度和含砂量有关，而微裂缝的发展水平与高的搪瓷发展，有机碳的含量高（导致更明显的叠层）以及高含量的石英，混合- I / S层和伊利石（导致高岭土含量低）。这些发现得到了本研究产生的其他数据的证实，包括上古生界黑色页岩和粉质页岩的岩石力学结果，岩心和从40多个页岩气井取样的薄层的观测结果，以及天然气测井异常。

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