Fractures of multi-scales and multi-origins are primary storage space and effective seepage channels for metamorphic buried-hill reservoirs. They not only allow communication between various pores to enhance storage and seepage capacity, but also are essential for high yields. Fracture development and controlling factors at metamorphic buried-hill reservoirs of Bozhong 19-6 Gas Field were investigated based on imaging log, core data, experiments, e.g., thin sections and scanning electron microscope, and outcrop description. Results show that structural fractures, weathering fractures and dissolution fractures were developed in the metamorphic buried-hill reservoirs in the study area, among which structural fractures and weathering fractures are the most popular ones. However, fracture types varied obviously among different structure positions. Lithology, fault, weathering crust and ancient landform are primary factors affecting fracture distribution. Fractures were prone to be developed at lithology with high bright mineral contents. Faults were characterized by a dual structure including fault core and damage zone. Fracture density followed a decreasing trend with increasing distance from faults. Damage zone could be identified when fracture density was consistent with regional fracture density. The width of the damage zone was determined by factors such as fault scale and structure location. Well-connected weathering fractures were popular at the leached unit, with structural fractures of secondary importance. The unweathered unit was dominated by structural fractures with low density.

The Qingshankou Formation is the primary shale oil exploration and development target in the Songliao Basin, but controlling factors of shale oil enrichment, especially the role of faults and fractures, are still poorly understood. We identify fault geometry in the Qingshankou Formation based on seismic interpretation. We characterize natural fractures with outcrop and core observations, and thin-sections petrography. By integrating faults and fracture observations at various scales, tectonic history of the basin, and burial history of the studied reservoir formation, we explain the control of faults and fractures on shale oil enrichment. Results show that high-density fault systems, namely “T11″ and “T2″ fault systems, were developed at the top and bottom of Qingshankou Formation in the north of Songliao Basin. Both fault systems is characterized as a group of high-density, closely spaced small faults forming a fault belt, with evident multi-stage activities. Nature fracture categories identified including folding and faulting related fractures, bed-parallel fractures and overpressure fractures, while the former two are better developed. Tectonic movements have caused the structural uplift and subsidence to take place at different times in different parts of the basin. The structural uplift happened late in the study area, consequently the source rocks have gone through a long burial history and are well matured, which favors shale oil enrichment. Tectonic uplifts generated large numbers of fractures locally, effectively increase oil production by improving the migration pathways and storage space of shale oil. Fault systems of different scale also provided two-way channels for oil migration in Qingshankou Formation. Large-scale faults that cut through the full shale column allow the shale oil to migrate to the conventional reservoirs above and below the Qingshankou Formation. The smaller faults in the high-density fault belt within the Qingshankou Formation (Qing-1 Member) enhanced seepage capacity and connectivity of the shale reservoir. As a result, shale oil and conventional oil development in the study area can collaborate with one another. High-density, well connected natural fractures also played an important role by providing storage space and fluid flow conduit which is essential in shale oil reservoirs. However, these fractures along faults may migrate oil from shale reservoirs to neighboring conventional reservoirs.

Natural fractures are regarded as important reservoir spaces and effective seepage channels at metamorphic buried hills. Continuous networks associated with multi-scale fractures with good connectivity is critical for high-quality reservoirs as well as high and stable production in the tight metamorphic rocks. The multi-scale fractures in Bozhong 19–6 metamorphic buried hills were well characterized through integrating image logs, cores, thin-sections, and scanning electron microscope, etc. After that, power-law distribution of multi-scale fractures was established to understand contribution of fractures to reservoir quality and figure out structure models of fracture networks as well as their impact on production. Results show that parameters of fracture systems vary regularly with fracture scales. Fracture development degree, e.g., cumulative areal density, increases as a power law function with decreasing fracture size from macro to micro (e.g., aperture and/or length), where storage space associated with micro fractures is also increased. Reversely, fracture connectivity and permeability follow a significant decreasing trend. Five structure models of fracture network were established based on combination pattern of multi-scale fractures: multi-scale fracture network with high-density and multi-sets, large-scale fracture network with medium-density and multi-sets, small-scale fracture network with high-density and multi-sets, large-scale fracture network with low-density and multi-sets, and small-scale fracture network with low-density and single-set. The former two fracture networks can be widely developed into high-quality reservoirs, contributing greatly to high and stable yields. Fracturing is required for the third and the fourth fracture networks to obtain stable production, while it is difficult for the fifth fracture network to obtain industrial oil and gas flow.

Mechanical stratigraphy plays an important role in controlling the nucleation, propagation, and development of fractures. The fracture development pattern and development mode were defined in this study via a detailed description. Fracture growth and evolution in bedded rocks were numerically simulated. The results show that fracture growth and propagation are controlled by the mechanical layer. Fractures are divided into bed-confined fractures and throughgoing fractures based on their spatial configuration with the mechanical layer. Fractures preferentially nucleate and expand in the mechanical layer and terminate at mechanical contact. After that, new fractures are generated between two adjacent fractures. No new fractures are further developed when stress reaches a certain value, indicating a saturation state. Finally, throughgoing fractures are developed. Under the same stress field, the bed-confined fracture density is positively correlated with Young’s modulus of the mechanical layer and is negatively correlated with its thickness. Bed-confined fractures can extend to another mechanical layer to develop throughgoing fractures only under significant stress fields or at mechanical contact with small thickness or minor difference in mechanical properties. Exploring the impact of mechanical stratigraphy on fracture development and propagation in bedded rocks is of great significance to investigate the fracture distribution in hydrocarbon reservoirs.

Natural fracture growth plays an important role in shale-oil enrichment. Systematically investigating fracture features and their controlling factors in shale-oil reservoirs is essential for accurately predicting fracture distribution. The controlling factors of fracture distribution in the continental shale of the Qingshankou Formation in the Songliao Basin, China, were systematically analyzed based on the quantitative fracture characterization of outcrops and cores. Strata-confined fractures, throughgoing fractures, bedding-parallel fractures, and stylolites can be observed in the Qingshankou shale reservoir in the study area. Fracture distribution is not only controlled by internal factors, e.g., mineral composition, mechanical stratigraphy, and lithofacies, but also by external factors, e.g., faults and abnormally high pressure readings. Mineral composition is the primary factor governing fracture development, and it not only controls fracture abundance, but it also affects fracture filling and effectiveness. Mechanical stratigraphy determines the spatial morphology and developmental pattern of a fracture. Fractures are well-developed in brittle strata, with fracture spacing being proportional to bed thickness. Lithofacies can determine fracture development by controlling the variation of mineral composition, rock structure, bed thickness, etc. Stress concentration is commonly high at fault tips, intersections, and overlaps, where fracture density is high and has good connectivity. The existence of abnormally high pressure reduces effective stress, promoting shear fracture development. Tensile overpressure fractures can also be generated under small levels of differential stress.

鄂尔多斯盆地陇东地区上三叠统延长组长7段储集层致密,裂缝发育程度是影响其油气分布和单井产能的主要因素.通过露头剖面、岩心、铸体薄片及成像测井等资料,对长7段储集层裂缝的分布特征进行定量表征,明确裂缝发育的主控因素,并结合岩石力学测试和数值模拟,对裂缝分布进行定量预测.研究区长7段储集层主要发育高角度构造裂缝,矿物充填性较差,有效裂缝发育,宏观裂缝平均线密度为0.31条/m.微观裂缝平均面密度为0.25～0.50μm/μm2,平均孔隙度为0.32％,增加了致密储集层的储集空间,并沟通了粒间和粒内孔隙,增强了孔隙连通性.研究区主要发育4组裂缝,分别为北东—南西向、北西—南东向、近东西向和近南北向,其中北东—南西向裂缝最为发育.裂缝主要发育在能干性强的岩石力学层内,并终止于岩性界面或层理面,裂缝高度主要分布在5～20 cm,最大可达110 cm.岩石中脆性矿物含量越高、颗粒越细、岩石越致密、岩层厚度越小,裂缝发育程度越高.通过有限元数值模拟,对陇东地区长7段裂缝的分布规律进行定量预测,预测结果与实际测量结果一致.

Natural fractures are the key factors controlling the enrichment of shale oil. It is of great significance to clarify the distribution of natural fractures to guide the selection of sweet spots for shale oil. Taking the Qing-1 Member shale oil reservoir in the northern Songliao Basin, China as an example, a new method considering the factors affecting fracture distribution was proposed to quantitatively predict the structural fractures. And the effect of natural fractures on shale oil enrichment was discussed. Firstly, the types and characteristics of fractures in shale oil reservoirs are characterized by using core and outcrop data. Combined with the experimental analysis, the influences of fault, mechanical stratigraphy, mineral composition and content, TOC, and overpressure on fracture intensity were clarified. Then, the number and density of fractures are quantitatively predicted according to the power-law distribution of fault length. Next, geomechanical simulation and fracture prediction were carried out on the model which was established with comprehensive consideration of the influencing factors of fracture distribution. Finally, the fracture distribution is evaluated comprehensively based on above prediction. The prediction results in this work are consistent with the core measurements.

Fractures are well developed in tight sandstone reservoirs, which are the main reservoir space and important seepage channel. Fractures control the migration, accumulation, preservation and single well productivity of tight oil and gas, and affect the drilling and completion methods, fracturing reconstruction measures and production effect of tight oil exploration and development. This paper mainly studies the genetic type, development characteristics and control factors of fractures in tight reservoirs. According to the geological origin of fractures, there are two types of structural fractures and diagenetic fractures in the study area, among which high angle structural fractures are the main one, and it is layer controlled fracture. The degree of fracture development is controlled by lithology, formation thickness, sedimentary micro and other rock heterogeneity factors.

Natural fractures control the migration and accumulation of oil and gas in the tight sandstones. Understanding the characteristics and formation mechanism of natural fractures has important guiding significance to the comprehensive prediction and evaluation of the fracture distribution. Focusing on the tight sandstones of the Xujiahe Formation (T3x) of Jiulongshan gas field, northwest Sichuan Basin, China, the characteristics of natural fractures were characterized quantificationally, and a comprehensive evaluation method of fracture formation mechanism is proposed. Two types of natural fractures were identified in the T3x, namely tectonic fractures and diagenetic fractures. Most of the natural fractures are tectonic shear fractures, which can be subdivided into steep fractures and nearly horizontal fractures according to their dip angles. Three sets of tectonic fractures were identified in the study area, namely: NW-SE trending, NNE-SSW trending and NEE-SWW trending fractures. According to the characteristics of fracture sets, crosscutting relationships, acoustic emission tests and fluid inclusion analysis, and combined with the tectonic evolution history of the study area, the natural fractures were formed in four periods. Among them, the diagenetic fractures formed at the process of diagenesis are the first period fractures. The second to fourth periods are tectonic fractures formed at the end of the Triassic to the Early Jurassic, the Late Cretaceous and the end of Pliocene to the early of Pleistocene, respectively. Horizontal tectonic compression, uplift denudation and overpressure were the main force sources for the formation of the steep tectonic fractures. The formation of nearly horizontal fractures is related with the thrusting or inter-formational sliding caused by tectonic compression.