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Jiu K.,China University of Geosciences | Jiu K.,Key Laboratory of Strategic Evaluation of Shale gas Resources | Jiu K.,Key Laboratory of Geological Evaluation and Development Engineering of Unconventional Natural Gas Energy | Ding W.,China University of Geosciences | And 8 more authors.
Marine and Petroleum Geology | Year: 2013

Fractures play a vital role in the exploration and development of shale oil and gas by providing effective space for shale reservoirs and significantly improving the fluid flow capability. Core observations, microscopic analyses of thin sections, scanning electron microscopy, and Formation MicroScanner Imaging (FMI) were used to determine the types, causes of formation, and development characteristics of the fractures in lacustrine shale reservoirs in the lower part of the Paleogene Shahejie Formation (Es3) in the Zhanhua Depression, Bohai Bay Basin, eastern China. X-ray diffraction (XRD) analysis, total organic carbon (TOC) measurements, and porosity and permeability measurements were used to study the controlling factors of the fractures in the shale reservoirs, and to analyze the impact of the fractures on the shale reservoirs properties and subsequent exploration and development. The studied shale reservoir mainly displays tectonic fractures as well as various types of non-tectonic fractures. The non-tectonic fractures mainly include over-pressure fractures, diagenetic fractures, inter-layer bedding fractures, and fractures of mixed origins. In the study area, the tectonic fractures which were formed under the combined action of tensile and shear stress display the following characteristics. The dip angle of the tectonic fractures varies significantly. Unfilled or half-filled effective fractures have a high proportion. These fractures are mainly oriented in the NE-SW, NNE-SSW, and WNW-ESE directions, with fractures in the NE-SW direction accounting for the highest proportion. The tectonic and non-tectonic fracture development is affected by multiple types of factors such as the presence of faults, mineral composition, lithology, abnormal pressure and organic matter content. Abnormally high pore pressure is a very important factor in the development of non-tectonic fracture. It is inferred that the over-pressure is mainly related to hydrocarbon generation during thermal evolution. Fractures effectively improve the porosity and permeability of the shale reservoirs, and the enhancement of permeability is particularly significant. The current stress field affects the fluid flow capability of the fracture reservoirs, and the present maximum principal stress in Zhanhua Depression is oriented in the NEE-SWW direction, which has a small angle with fractures in NE-SW direction. We propose that the fractures in this direction have the greatest connectivity and thus are a high-priority target for petroleum exploration and development. © 2013 Elsevier Ltd. Source

DING W.,China University of Geosciences | DING W.,Key Laboratory of Strategic Evaluation of Shale gas Resources | DING W.,Key Laboratory of Geological Evaluation and Development Engineering of Unconventional Natural Gas Energy | DAI P.,China University of Geosciences | And 13 more authors.
Geological Magazine | Year: 2015

Fractures are important for shale-gas reservoirs with low matrix porosity because they increase the effective reservoir space and migration pathways for shale gas, thus favouring an increased volume of free gas and the adsorption of gases in shale reservoirs, and they increase the specific surface area of gas-bearing shales which improves the adsorption capacity. We discuss the characteristics and dominant factors of fracture development in a continental organic matter-rich shale reservoir bed in the Yanchang Formation based on observations and descriptions of fracture systems in outcrops, drilling cores, cast-thin sections and polished sections of black shale from the Upper Triassic Yanchang Formation in the SE Ordos Basin; detailed characteristics and parameters of fractures; analyses and tests of corresponding fracture segment samples; and the identification of fracture segments with normal logging. The results indicate that the mineral composition of the continental organic-matter-rich shale in the Yanchang Formation is clearly characterized by a low brittle mineral content and high clay mineral content relative to marine shale in the United States and China and Mesozoic continental shale in other basins. The total content of brittle minerals, such as quartz and feldspar, is c. 41%, with quartz and feldspar accounting for 22% and 19% respectively, and mainly occurring as plagioclase with small amounts of carbonate rocks. The total content of clay minerals is high at up to 52%, and mainly occurs as a mixed layer of illite-smectite (I/S) which accounts for more than 58% of the total clay mineral content. The Upper Triassic Yanchang Formation developed two groups of fracture (joint) systems: a NW–SE-trending system and near-E–W-trending system. Multiple types of fractures are observed, and they are mainly horizontal bedding seams and low-dip-angle structural fractures. Micro-fractures are primarily observed in or along organic matter bands. Shale fractures were mainly formed during Late Jurassic – late Early Cretaceous time under superimposed stress caused by regional WNW–ESE-trending horizontal compressive stress and deep burial effects. The extent of fracture development was mainly influenced by multiple factors (tectonic factors and non-tectonic factors) such as the lithology, rock mechanical properties, organic matter abundance and brittle mineral composition and content. Specifically, higher sand content has been observed to correspond to more rapid lithological changes and more extensive fracture development. In addition, higher organic matter content has been observed to correspond to greater fracture development, and higher quartz, feldspar and mixed-layer I/S contents have been observed to correspond to more extensive micro-fracture development. These results are consistent with the measured mechanical properties of the shale and silty shale, the observations of fractures in cores and thin-sections from more than 20 shale-gas drilling wells, and the registered anomalies from gas logging. Copyright © Cambridge University Press 2015 Source

Ding W.,China University of Geosciences | Ding W.,Key Laboratory of Strategic Evaluation of Shale gas Resources | Ding W.,Key Laboratory of Geological Evaluation and Development Engineering of Unconventional Natural Gas Energy | Zhu D.,China University of Geosciences | And 11 more authors.
Marine and Petroleum Geology | Year: 2013

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 lamellation development, high levelof 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. © 2013 Elsevier Ltd. Source

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