The Cambrian System in the Tabei area underwent multiple phases of tectonic movement. The paleostructures have been significantly reconstructed, which makes geomorphological restoration very difficult. Previous studies in this area have mostly focused on traditional, single technical methods, resulting in limitations in the characterization of complex paleogeomorphology. For this reason, this paper proposes a new paleogeomorphology restoration method based on combining the shoreline trajectory method with the stratigraphic thickness method. First, the denudation volume at the end of the Late Cambrian sedimentation is calculated using the layer flattening method and the impression method, constrained by the double-sided restoration method. Then, on this basis, the tectonic uplift of the Xiaoerbulake sedimentary period is restored using the stratum thickness method. Finally, the relationship between accommodation space and sediment supply rate is used to derive the law of shoreline migration. Shoreline migration indicates the direction of tectonic movement and identifies the period of tectonic uplift during the Xiaoerbulake sedimentary period. The results indicate that the Xiaoerbulake Formation's sedimentary period in the early Cambrian was the main uplift period. The amount of tectonic uplift increased in a northeasterly direction, with the maximum located in the northern part of the Yuqi-6 Well. This controls the direction in which the Xiaoerbulake sediments prograde and the formation of the platform-margin. The Xiaoerbulake sedimentary period experienced two phases of uplift and three sedimentary stages, causing the northern sedimentary line to move eastward.

Assessing the formation sequence and genetic mechanism of natural fractures in dual-porosity carbonate reservoirs is crucial for evaluating the percolation potential of fractures within geological evolution. Multiple types and orientations of natural fractures are widespread in the Middle Permian Maokou carbonates of the Zigong area, southern Sichuan Basin, China. Hitherto, the geological genesis, forming times and formation mechanisms of these fractures remain insufficiently constrained, although they are closely associated to hydrocarbon migration and enrichment in the Maokou Formation. In this study, the geological, geophysical, geochemical and geomechanical data were integrated to investigate the genetic types, characteristic parameters, forming stages, and formation mechanisms of natural fractures, and their impacts on hydrocarbon accumulation was discussed. Four types of natural fractures, including the tectonic shear fractures (TSF), tectonic tensile fractures (TTF), bedding-parallel fractures and stylolites, were identified from the Maokou Formation in Zigong area. TSF and TTF are dominant types, and show high heterogeneity in occurrence, scale, filling, and density. Three formation periods of tectonic fractures were detected by fracture crosscutting relationship, acoustic emission, stable isotope, fluid inclusion and U-Pb dating. The first-period fractures, consisting of NNE- and ENE-striking TSF, were formed under the SW-NE tectonic compression during the Early-Middle Indosinian movement (similar to 243.0 Ma to similar to 228.0 Ma). The second-period fractures were produced by SE-NW tectonic compression and overpressure during the Late Yanshanian movement (similar to 117.0 Ma to similar to 95.0 Ma), forming WNW- and NNW-striking TSF and NE-striking TTF. The third-period fractures, including nearly E-W and NE-striking TSF and NNW-striking TTF, were generated in the Late Himalayan movement (similar to 21.0 Ma to similar to 16.0 Ma) related to WSW compressional thrust and substantial uplift. The second- and third-period fractures are more favorable for hydrocarbon accumulation benefiting from their high average density and good effectiveness during hydrocarbon migration, although some third-period TTF with large scales may cause gas escape.

Identification of reservoir types in deep carbonates has always been a great challenge due to complex logging responses caused by the heterogeneous scale and distribution of storage spaces. Traditional cross-plot analysis and empirical formula methods for identifying reservoir types using geophysical logging data have high uncertainty and low efficiency, which cannot accurately reflect the nonlinear relationship between reservoir types and logging data. Recently, the kernel Fisher discriminant analysis (KFD), a kernel-based machine learning technique, attracts attention in many fields because of its strong nonlinear processing ability. However, the overall performance of KFD model may be limited as a single kernel function cannot simultaneously extrapolate and interpolate well, especially for highly complex data cases. To address this issue, in this study, a mixed kernel Fisher discriminant analysis (MKFD) model was established and applied to identify reservoir types of the deep Sinian carbonates in central Sichuan Basin, China. The MKFD model was trained and tested with 453 datasets from 7 coring wells, utilizing GR, CAL, DEN, AC, CNL and RT logs as input variables. The particle swarm optimization (PSO) was adopted for hyper-parameter optimization of MKFD model. To evaluate the model performance, prediction results of MKFD were compared with those of basic-kernel based KFD, RF and SVM models. Subsequently, the built MKFD model was applied in a blind well test, and a variable importance analysis was conducted. The comparison and blind test results demonstrated that MKFD outperformed traditional KFD, RF and SVM in the identification of reservoir types, which provided higher accuracy and stronger generalization. The MKFD can therefore be a reliable method for identifying reservoir types of deep carbonates.

Siliceous minerals of the Dengying Formation in the Gaoshiti–Moxi area in the central Sichuan Basin exhibit four types of quartz crystals (cryptocrystalline quartz, chalcedony, microcrystalline quartz, and megacrystalline quartz) and three structural types: cryptocrystalline, microcrystalline, and mosaic (laminated mosaic, window-hole interrupted mosaic, and arc-laminated mosaic). Siliceous minerals have a great influence on the storage performance of the reservoirs in the Dengying Formation. According to the petrophysical parameters of the Dengying Formation and porosity intersection diagrams, the siliceous dolomite and the reservoirs have low impedance characteristics, which makes it difficult to distinguish between them and leads to difficulties in the characterization and prediction of the reservoirs. The transverse wave velocity is favorable for reservoir characterization. Currently, the main method used to estimate the transverse wave velocity is petrophysical modeling, which establishes a relationship between the elastic and physical parameters of the reservoir. In this paper, the siliceous minerals in the dolomite in the study area are regarded as solid inclusions, and the calculation method of the rock matrix modulus is improved by using solid replacement. Then, an improved petrophysical model is constructed by combining the KT (Kuster–Toksöz) model, the DEM (Discrete Element Method) model, the Gassmann equation, and the Wood equation. The transverse wave velocity is estimated using the improved model under the constraint of the longitudinal wave velocity. The shapes of the transverse wave velocity curves obtained by the improved model and the deviations from the measured velocities are significantly better than those of the Xu–Payne model and other models. The results show that the improved model can effectively estimate the transverse wave velocity of the reservoir in this area, which provides a basis for future reservoir predictions in this area.

Understanding the types, characteristics, and controlling factors of natural fractures in deep and ultradeep carbonates is crucial for evaluating reservoir quality, optimizing well deployment, and comprehending their impact on hydrocarbon exploitation. Multiple types of natural fractures are widespread in the deep carbonates of the Feixianguan Formation in the Puguang area, northeastern Sichuan Basin, China, and the main controlling factors are complex. Based on geological, geophysical, and experimental data, this study defined fracture types and analyzed the fracture development characteristics in the deep Feixianguan carbonates. On this basis, the main geological factors that control the development and distribution of tectonic fractures were discussed by combining statistical and experimental analyses. Results indicate that natural fractures in the Feixianguan Formation can be genetically classified into tectonic and diagenetic fractures. Specifically, tectonic fractures include shear fractures and tensile fractures, in which the former are predominant. In the Feixianguan carbonates, tectonic shear fractures are mainly developed in the NE-SW and near E-W strikes, with dip angles mostly ranging from 30 to 70°. The majority of shear fractures appear on a small scale and have good effectiveness. The fracture heights and apertures are commonly less than 50 cm and 25 μm, respectively, and unfilled fractures account for more than half. The distribution and development of tectonic fractures are mainly controlled by lithology, mechanical stratigraphy, reservoir physical properties, diagenetic cementation, and faults. In the Feixianguan carbonates, tectonic fractures are more developed in crystalline dolomite. With the increasing thickness of mechanical stratigraphy, the fracture density decreases and the scale increases. The presence of early dissolution pores can prevent the formation of later tectonic fractures. Tectonic fractures in the NNW-SSE and near N-S strikes generally possess poor effectiveness due to multiple cementations after their formation. In the vicinity of faults, tectonic fractures generally stretch subparallel to the extension direction of the fault, and the development degree of fractures close to major faults is higher.

Objective Fractures commonly occur in the marine carbonate reservoirs of Permian Maokou Formation in Zigong area of Sichuan Basin and have important impacts on reservoir properties, seepage patterns and hydrocarbon enrichment. Methods Cores, thin sections, image logs and experimental test data were used to clarify the type of fracture genesis and development characteristics, analyse the time of fracture formation, and determine the period of fracture formation. Results The results show that the marine carbonate reservoirs of Maokou Formation in Zigong area are divided into two types, namely, tectonic fractures and diagenetic fractures, among which the tectonic fractures include tectonic shear fractures and tectonic tensile fractures, and the diagenetic fractures include horizontal bedding fractures and diagenetic sutures. Tectonic shear fractures dominate Maokou Formation reservoir and occur mainly in the NEE and NNE directions. The fracture inclination angle ranges from 20° to 80°, and the extension length is less than 60 cm. The fracture degree of filling is low, and the validity is good. Combined with the analysis and test data, the reservoir fractures of Maokou Formation in Zigong area were determined to have formed by 3 stages of structural movement. The first stage included the late Hercynian and early Indosinian periods, approximately 240-220 Ma. Under the SW stress derived from the clockwise movement of the South China Plate, a small number of shear fractures developed, and the fractures were mostly filled with minerals, representing the secondary development period of fractures. The second stage occurred in the late Yanshan-early Himalayan period, approximately 78-69 Ma, and a large number of tectonic fractures developed under the NW-trending stress generated by the Xuefeng uplift in Jiangnan, which was the main period of fracture formation in the study area. The third period was the late Himalayan period, approximately 13-0 Ma. Fractures were formed under the NEE compressive stress generated by the collision of Indian Ocean Plate, and most of the fractures were unfilled, indicating good effectiveness. Conclusion The above fracture-related research provides the basis for establishing favourable exploration zones in the study area.

Structural fracture distribution is essential in oil and gas transportation and development in passive continental margin basins. In this paper, taking as an example the clastic reservoirs in the A-Basin, a passive continental margin in northeastern South America, the paleotectonic stress field of the Late Cretaceous Maastrichtian formation in Basin A was numerically simulated by finite element technique through the integrated interpretation of seismic total data, logging data and core data, and the distribution of tectonic fractures was later predicted based on rock fracture criterion. The results of the study show that: (1) The distribution of tectonic stress and fractures during the Late Cretaceous Maastrichtian formation of Basin A is affected by the fracture zone, mechanical properties of rocks and tectonic stress, regions with extensive fracture development are susceptible to stress concentrations, resulting in significant stress gradients. (2) The development of structural fractures in the study area was predicted using the Griffiths criterion, and the tensile rupture coefficient T was introduced to quantitatively characterise the intensity of fracture development, with larger values reflecting a higher degree of fracture development. The well-developed and relatively well-developed fractures are mainly located in the fracture zones and the interior of submarine fans. (3) Fracture zones and sedimentary phases mainly control structural fractures in Basin A; within 5 km outside the fracture zones, the development of fractures is controlled by the fracture zones, beyond which the regional tectonic stress field controls them; inside the sedimentary fan, the development of fractures is controlled by the sedimentary subphase, which decreases in the order of the upper fan, the middle fan, and the lower fan; inside the subphase, they are controlled by the regional tectonic stress field, and the fractures show the increasing trend in the direction of NW-NE.

Karst reservoirs in the Ordovician carbonates of Tarim Basin are currently major targets of deep and ultra-deep hydrocarbon exploration and development in China. New drilling and seismic data show that these deep-buried carbonate karst reservoirs are spatially dependent on strike-slip faulting. Based on the geological and geophysical data, this study developed an attempt to determine the impact and spatial distribution of strike-slip faulting on carbonate karst reservoirs in the Tahe oil field. Strike-slip faults in the Taha area can be classified as first order, second order, and third order. Fault damage zones are distributed along the strike-slip faults, and their widths have a positive correlation with fault order. Influenced by strike-slip faults, fault-related fractures, and rock mechanical properties, the development degree and spatial distribution of carbonate karst reservoirs display strong heterogeneity. The horizontal heterogeneity of karst reservoirs is controlled by strike-slip faults with different scales and strikes, and the vertical heterogeneity is combined results of the strike-slip faulting and mechanical properties of carbonates. In the Tahe area, ultra-deep carbonate karst reservoirs are generally concentrated within 550 m (?1800 ft) of the damage zone of north-northeast- and north-northwest-striking first-order and second-order strike-slip faults. Fewer and smaller-scale karst reservoirs are developed further away from the fault core. Additionally, fracture intensity is greater in more brittle facies enhancing the karstification. The results from this research provide an analogue for understanding the distribution of fault-controlled karsting in deep carbonate reservoirs around the world.

An integration of outcrop observations,as well as data from drilling,logging,and seismic surveys in an oilfield is applied to analyze the filling types and filling cycle assemblages of karst caves associated with paleokarst buried rivers;accordingly,the filling sequences and patterns of the paleokarst buried rivers,as well as the discussion on their petroleum geological implications.The results show that the Ordovician buried-river karst caves with a filling rate of 89.9%in the Tahe oilfield,are predominantly filled with sandy mudstones and collapse breccias.These karst caves host multiple combination cycles featuring coarse-grained lower parts and fine-grained upper parts,which can be classified into polycyclic sedimentary assemblages and polycyclic collapse-sedimentary assemblages for filling.The former is distributed in the karst slope's lower reaches of flat landform,where wells with lost circulation and stringers account for small and high proportions,respectively.In contrast,the latter is situated in the karst slope's upper reaches featuring landform of great drops,where wells with lost circulation are of high proportion together with multiple high-yielding wells.The following conclusions can be reached through analysis:(1)The tortuous spatial structure of buried rivers,combined with their strong runoff transport capacity,facilitate the filling of large amounts of karst detrital materials,resulting in an extremely high filling rate;(2)The seasonal fluctuations in the phreatic surface lead to the formation of cyclic and comparable fillings.This,coupled with water erosion and tectonic activities,gives rise to multi-phase collapses of karst caves.Consequently,polycyclic collapse-sedimentary filling assemblages are formed in the upper reaches,with unfilled spaces developed;(3)The relatively closed underground environment supersaturated with calcium carbonate,results in severe calcareous cementation of fillings,decreasing the intergranular porosity;(4)The unfilled spaces serve as the major targets with potential for oil and gas exploitation.

Carbonate reservoirs in the Sinian Dengying Formation, central Sichuan Basin, are a hot spot for hydrocarbon exploration and development in deep and ultra-deep sequences in China. Multi-type and multi-scale natural fractures are widely seen in the Dengying reservoirs, and of significant impacts on the seepage flow pattern and well productivity. In this study, the genetic types and developmental characteristics of natural fractures in the fourth member of Dengying Formation (Deng 4 Member), Gaoshiti-Moxi area, are clarified utilizing the cores, thin sections, image logs, experimental tests and production testing data. Major geological factors controlling the development of natural fractures are used to analyze the impact of natural fractures on well productivity. Results show that natural fractures in the Deng 4 Member reservoirs in Gaoshiti-Moxi area are mainly of tectonic and diagenetic types. The tectonic fractures can be divided into shear fractures and tension fractures, and the diagenetic fractures are mainly bedding fractures and stylolites, among others. The shear fractures dominate the Deng 4 Member reservoirs, and are mainly of high-angle ones and striking in the NNW-SSE, nearly EW, NE-SW and nearly SN directions. Factors controlling the shear fracture effectiveness include the timing of fracture formation, cementation, dissolution, fracture occurrence and current in-situ stress. The shear fractures in NNW-SSE and nearly EW orientation are more effective than others. The development degree of shear fractures is closely related to the lithology, mechanical layer thickness and faults. The shear fractures are more highly developed in micritic dolomites, and their density decreases and scale enlarges along with the increasing thickness of mechanical layers. The shear fractures near the main strike-slip faults, in particular within the tip and superimposed part of faults, are better developed. The development degree and effectiveness of fractures jointly determine gas well productivity. The effective fractures in different occurrences and scales can form fracture network, and get relatively isolated pores interconnected, which greatly improves the seepage capacity of reservoirs and elevate well productivity. In the Deng 4 Member reservoirs of Gaoshiti-Moxi area, the shear fractures in NNW-SSE and nearly EW orientation are of stronger ability to enhance reservoir permeability, and make greater contribution to the improvement of well productivity.

Coal structure is a critical parameter in coalbed methane (CBM) development due to its significant impacts on methane enrichment, fluid flow and hydraulic fracturing. Traditional statistical analysis and data-driven machine learning methods for coal structure identification are highly dependent on the labeled logging data and have potential limitations when labeled logging data is limited. To address this issue, this paper proposed a semi-supervised learning method based on Laplacian support vector machine (LapSVM) to identify coal structure by using few labeled logging data. By mining the structure information from abundant unlabeled data, LapSVM can improve the model performance and alleviate the over-reliance on labeled data. To evaluate and verify the effectiveness and reliability of the proposed LapSVM method in coal structure identification, datasets collected from 32 CBM wells in the southern Qinshui Basin, China, are utilized in this study. The particle swarm optimization (PSO) is adopted for parameter optimization of LapSVM models. For the LapSVM model, the addition of unlabeled data is conducive to enhance model accuracy, and unavoidably increases the computational cost at the same time. The comparison of training, testing and blind-well test results between the LapSVM and standard support vector machine (SVM) models indicates that the LapSVM outperforms traditional SVM and possesses higher accuracy and generalization in coal structure identification. It has been demonstrated that the LapSVM can be a reliable tool for coal structure identification when limited labeled logging data is available.

The deeply buried carbonates in the Sinian Dengying Formation are one of the crucial hydrocarbon exploration targets in the central Sichuan Basin of China, and are characterized by strong heterogeneity in reservoir quality, which is closely related to natural fractures. To reveal the impacts of natural fractures on the reservoir quality of Deng-4 Member carbonates in the Gaoshiti-Moxi area, the types, characteristics, and factors controlling the development and effectiveness of fractures, were systematically analyzed based on core samples, thin sections, image logs, petrophysical measurements, and drill stem tests. Tectonic shear fractures are the dominant fracture type affecting the petrophysical properties of the Deng-4 Member reservoirs. The distribution and development of shear fractures are mainly controlled by lithology, mechanical layers and faults. Factors influencing the effectiveness of shear fractures include the timing of fracture formation, cementation, dissolution and current in-situ stress. Early-formed shear fractures are more susceptible to filling during subsequent cementation, whereas fractures formed at later stages are rarely filled and, also, have the potential to be solution-enlarged by dissolution. The relatively large apertures of unfilled shear fractures parallel to the orientation of SHmax are associated with the small effective normal stress exerted on fracture surfaces. Shear fractures effectively improve the overall permeability of Deng-4 Member carbonate reservoirs by connecting relatively isolated pores but have only minor impacts on reservoir porosity. The development degree and effectiveness of fractures together determine the contribution of fractures to reservoir permeability. In the Gaoshiti-Moxi area, shear fractures in the NWW-SEE and nearly E-W strikes provide the major flow pathways and contribute the most of permeability of the tight carbonate reservoirs in Deng-4 Member. These results provide insights for deeper understanding of the impact on reservoir quality of natural fractures in tight carbonates, and are significant for the evaluation of high-quality carbonate reservoirs in the Sichuan Basin.

The Upper Triassic Xujiahe Formation is a typical tight gas reservoir in which natural fractures determine the migration, accumulation and production capacity of tight gas. In this study, we focused on the influences of natural fractures on the tight gas migration and production. We clarified characteristics and attributes (i.e. dips, apertures, filling degree and cross-cutting relationships) of the fractures based on image logging interpretations and core descriptions. Previous studies of electron spin resonance, carbon and oxygen isotopes, homogenization temperature of fluid inclusions analysis and basin simulation were considered. This study also analysed the fracture sequences, source of fracture fillings, diagenetic sequences and tight gas enrichment stages. We obtained insight into the relationship between fracture evolution and hydrocarbon charging, particularly the effect of the apertures and intensity of natural fractures on tight gas production. We reveal that the bedding fractures are short horizontal migration channels of tight gas. The tectonic fractures with middle, high and nearly vertical angles are beneficial to tight gas vertical migration. The apertures of fractures are controlled by the direction of maximum principal stress and fracture angle. The initial gas production of the vertical wells presents a positive correlation with the fracture abundance, and the intensity and aperture of fractures are the fundamental factors that determine the tight gas production. With these findings, this study is expected to guide the future exploration and development of tight gas with similar geological backgrounds.

Compared with conventional reservoirs, tight reservoirs experience more intense diagenesis; thus, their properties are extremely poor. Nevertheless, natural fractures with high‐strength can appear in these reservoirs and could play an essential role in the accumulation and production of tight oil. In this study, we focused on the effects of different diagenetic processes in the formation and transformation of natural fractures to determine the effects of natural fractures on tight reservoirs. Various observation techniques and analysis methods were applied (i.e., observations of cores, cast thin sections, scanning electron microscopy; field emission scanning electron microscopy; and X‐ray diffraction analysis). We clarified characteristics of the fractures and distribution patterns based on core descriptions and microscopic observations and explained the diagenetic stage and evolution sequence of reservoirs. Past studies regarding carbon and oxygen isotope analysis and basin simulation were considered. This study also reviewed the fracture formation time, source of fracture fillings, and oil charging time. We obtained insight into the coupling relationship of fracture states, diagenetic sequences, hydrocarbon charging, and, in particular, the controlling effect of diagenesis on natural fractures. The results revealed that formation, preservation, and destruction of natural fractures in tight reservoirs were closely related to diagenesis. Compaction and cementation in the reservoirs decreased the porosity and altered the petrophysical properties of the reservoir. They also provided favorable conditions for the development of tectonic fractures, while dissolution did not. The influences of dissolution and cementation on natural fractures depended on the duration for which these processes were active. Compaction and cementation formed related types of diagenetic fractures, while dissolution increased the effectiveness of natural fractures. The purpose of this study was to evaluate the influence of diagenesis on the formation, controlling effects, and effectiveness of natural fractures, as well as the effects of natural fractures on tight reservoirs through geological history. This study is expected to provide guidance for future exploration and development of tight oil and gas with similar geological origins.

沁水盆地南部煤岩储层天然裂缝普遍发育,天然裂缝有效性是决定煤岩储层渗流能力、影响煤层气井能否高产的重要因素.综合利用野外露头、岩心和扫描电镜等资料,在分析沁水盆地南部上古生界煤岩储层天然裂缝类型和发育特征的基础上,对裂缝有效性及其主控因素开展研究,并结合单井生产动态资料,探讨了裂缝有效性对煤层气开发的影响.结果表明:沁水盆地南部煤岩储层主要发育割理和构造裂缝,其中割理包括面割理和端割理,构造裂缝包括剪切裂缝和张性裂缝.受多种地质因素影响,不同类型裂缝的有效性存在明显差异.割理形成于煤化作用阶段,多被方解石和粘土矿物等全充填或半充填,整体上其裂缝有效性较差.构造裂缝多未被矿物充填,其有效性主要受控于裂缝规模、倾角、储层埋深及与现今地应力最大主应力方向夹角.构造裂缝开度大、延伸较远,裂缝有效性好;高角度构造裂缝有效性好于斜交缝;受地应力状态变化影响,随储层埋深的增加,裂缝有效性依次表现为较差、较好、较差;NE-SW向裂缝有效性最好,其次为近EW向和近NS向裂缝,NW-SE向裂缝有效性最差.相比于割理,构造裂缝在规模和裂缝有效性方面均好于前者,可作为煤层主要的渗流和产出通道,对煤层气的开发具有明显积极作用.然而,过大规模尺度的构造裂缝,尤其当构造裂缝穿过煤层在顶底板发育时,不仅会造成煤岩储层内煤层气散失和储层压力降低,同时还将导致外来地层水对煤层的补充,不利于煤层气的保存和有效开采.

Natural fractures, as the effective storage space and fluid flow pathway of hydrocarbons, play a critical role in the exploration and development of unconventional gas resources in the coal bearing strata. In the present study, to reveal the effects of natural fractures on coalbed methane (CBM) preservation and development, the types, origins and development characteristics of natural fractures in the upper Paleozoic Permian Shanxi Formation in the southern Qinshui Basin were systematically described and analyzed based on outcrops, cores, borehole image logs, thin sections, and SEM. Four types of natural fractures have been identified in the Shanxi Formation according to their geologic origins, including the endogenic fractures (cleats) and exogenous fractures in the No.3 coal seam, and tectonic fractures and diagenetic fractures in its roof and floor. Tectonic fractures can be further divided into small faults, intraformational shear fractures, intraformational open fractures and slip fractures, while diagenetic fractures are mainly bed-parallel fractures. The statistical results show that the development and distribution characteristics of those types of fractures are significantly different, which are mainly influenced by lithology, layer thickness and geological structure. Further, natural fractures with different types and distributions show different influences on CBM preservation and well productivity. Exogenous fractures can effectively improve the seepage capability of coal reservoirs and are conducive to CBM wells productivity, whereas endogenic fractures have less contributions to coal permeability due to their poor effectiveness. The intraformational open fractures in the fold axis zones, and intraformational shear fractures and small faults in the fault zones are generally excessive-developed. Those fractures may result in gas dissipation and external water supplement, which have negative effects on CBM preservation and production. However, moderate-developed intraformational shear fractures in the sandstone roof can favorably affect the pressure reduction of coal reservoir and improve the recovery potential of unconventional gas resources. These investigations may serve as a geological basis for unconventional gas exploration and development of the upper Paleozoic coal bearing strata in the southern Qinshui Basin.

利用野外露头、岩心、测井和铸体薄片资料,对塔里木盆地塔河地区中-下奥陶统碳酸盐岩储层天然裂缝发育特征及其影响因素进行研究.塔河地区中-下奥陶统碳酸盐岩储层天然裂缝主要分为构造裂缝、成岩裂缝和复合成因裂缝3种类型.其中,构造裂缝以剪切裂缝为主,其次为张裂缝;成岩裂缝主要为水平层理缝,其次是成岩缝合线;复合成因裂缝主要包括构造-成岩裂缝、构造-表生裂缝和岩溶裂缝.构造剪切裂缝是该区的主要裂缝类型,发育有北东-南西向、北西-南东向、近东-西向和近南-北向4组,它们在不同层位的发育程度存在明显的差异性;其中,北东-南西向裂缝为层位的优势裂缝发育方向.裂缝的倾角主要在70°以上,以高角度裂缝为主.裂缝的纵向延伸长度受岩石力学层控制,裂缝在岩石力学层内发育,并终止于力学层界面上.裂缝密度在纵向上和平面上存在明显的非均质性,主要受沉积作用、构造作用和岩溶作用等地质因素控制.随着塑性矿物含量与岩石层厚的增加,裂缝密度呈逐渐降低的变化规律.断层与褶皱构造对裂缝的发育程度有重要影响,使得不同构造部位的裂缝密度存在明显的差异;距断层面与褶皱轴面的距离越远,裂缝密度也逐渐降低.岩溶作用影响溶洞上部岩层中的裂缝发育程度;在溶洞上部岩层中,主要发育近直立裂缝、斜交裂缝和近水平裂缝3种产状类型的裂缝系统.

Coal structure is closely related to the porosity and permeability of coal reservoirs, which not only affects the enrichment of coalbed methane (CBM), but also influences the hydraulic fracturing and efficient development of CBM. The accurate identification of the coal structure would be a critical issue in CBM exploration and development, and is always a challenge. Compared with traditional methods for identifying coal structure base on borehole cores or mining seam observation, geophysical logging has become the most economic and efficient technique. Several linear correlations have been established to describe the relationships between the coal structures and well logs. However, those correlations cannot accurately reflect nonlinear relations between them. Therefore, a reliable and efficient method to identify coal structure is needed. As a powerful nonlinear classifier, the kernel Fisher discriminant analysis (KFD) method has been widely used due to its strong generalization ability. In this paper, a new quantitative coal structure identification model was developed based on the KFD method by using geophysical logging data. The model was trained, tested and optimized using 178 logging data sets from 15 CBM wells in Qinshui Basin, China. The approach accounted for all the available well logging attributes, and the training data sizes and kernel parameters were analyzed to get the most appropriate model in practice. In addition, the built model was validated individually by employing logging data of a new CBM well. The results indicate that the KFD based identification model has high prediction accuracy, which can be used as a reliable method for coal structure identification. The KFD method exhibits strong capability during the modeling and generalization in the determination of nonlinear relationships, which provides an efficient way for the coal structure prediction in basic research of coal reservoirs.

This article takes low-rank coal reservoirs in the southern margin of Junggar Basin as a research object.Based on the mercury intrusion method,a comparative and quantitative study of heterogeneity of pore structure in low-rank coal reservoirs is carried out by using three fractal models.Fractal dimension D has no obvious correlation between D m and D s,and less correlation between pore connectivity of coal and rock reservoirs.But with the increase of fractal dimension Dm,Dshas a tendency to increase,and with the increase of D m and Ds,the connectivity of micro-pores in coal reservoirs is also obviously better than that of medium and large pores.The fractal dimensions of different fractal models have different geological significance for low-rank coal reservoirs,the fractal dimension (D) of the fractal porous model mainly indicates the effect of large-scale pore on pore volume change,but it does not reveal the change rule between fractal dimension and physical properties of coal reservoir.The fractal dimension (Dm) of classical geometry model is mainly characterized by pore space variation,and pore volume decreases with the increase of D m.Fractal dimensions of fractal model of the thermal (Ds) indicates pore surface roughness,surface tension of pore increases along with Ds,and pore surface has the greater roughness.Low porosity and permeability of low-rank coal reservoir,and micro-pores with relatively good connectivity increase the volume of the connected pores.According to the geological significance of the fractal dimension of different fractal models,the variation law of low-rank coal reservoir permeability can be characterized by different fractal models according to different permeability ranges,which provides a geological basis for the extraction of coalbed methane and the prediction of effective permeability for low-rank coal.