The Lower Cretaceous Xiagou Formation is an important tight oil reservoir in the Qingxi Depression of the Jiuxi Basin. The micro-nanopore system within the reservoir requires a comprehensive analysis to improve the production of tight oil there. Nuclear magnetic resonance (NMR) experiments have been widely used for the petrophysical characterization of sandstones and carbonates. In the present study, the NMR experiment was applied to obtain the characteristics of the micro-nanopore system and permeability in the Lower Cretaceous Xiagou pelitic dolomite reservoir. According to the distribution shape of the transversal relaxation time (T₂) obtained under the 100% water-saturated condition (Sw), the samples are divided into four groups: (i) group I, two obvious peaks (P1 and P2); (ii) group II, an obvious high peak of P1 at 0.1˜1.0 ms and a relatively low peak of P2; (iii) group III, an obvious high peak of P2 and a relatively low peak of P1; and (iv) group IV, three peaks. In general, the distribution shape of T₂ under the initial condition (Sini) is unimodal, with all its peaks lower than those under the Sw condition. The NMR T₂ spectrum reflects the distribution of the rock pore radius. Most of the pore radius distributions are bimodal, and the main pore radius ranges from 10 nm to 70 nm. Three patterns can be identified and determined based on the distribution of the pore radius: I-unimodal distribution, II-bimodal distribution and III-trimodal distribution. The results indicate that the porosity in the Xiagou reservoir ranges from 1.17% to 6.89%, with an average of 3.33%. The permeability ranges from 0.03×10-3 μm² to 22.56×10-3 μm², with an average of 2.95×10-3 μm².

Stress sensitivity is the variation of rock petrophysical parameters resulting from changes in effective stress. In fractured reservoirs, experimental methods exhibit a certain amount of error in the quantitative analysis of reservoir rock stress sensitivity. In addition, fracture-bearing experimental rock samples are difficult to obtain and prepare. Therefore, in the present study, reservoir rock stress sensitivity in naturally fractured reservoirs was investigated based on geomechanical modeling using a case study of the Lower Cretaceous Xiagou Formation in the Qingxi Oilfield. The results indicate that the Xiagou fractured reservoir experiences strong stress sensitivity with a fracture permeability damage rate reaching 94.38%. Natural fractures influence reservoir rock stress sensitivity. The degree of filling and type of filled minerals within natural fractures have great effects on the permeability damage rate. A higher permeability damage rate suggests stronger rock stress sensitivity. Generally, I) for reservoir rocks with unfilled natural fractures, the permeability damage rate is extremely high; II) for reservoir rocks with partially filled natural fractures, the permeability damage rate is high, and if the minerals within the natural fractures are insoluble, the permeability damage rate is slightly higher than if the materials are soluble; III) for reservoir rocks with completely filled natural fractures, the permeability damage rate is extremely low if the minerals within the natural fractures are insoluble; however, if the materials are soluble, the permeability becomes slightly higher with the increase of effective stress. Most importantly, this study provides a practical method for analyzing stress sensitivity in naturally fractured reservoirs.

Generally, induced hydraulic fractures are generated by fluid overpressure and are used to increase reservoir permeability through forming interconnected fracture systems. However, in heterogeneous and anisotropic rocks, many hydraulic fractures may become arrested or offset at layer contacts under certain conditions and do not form vertically connected fracture networks. Mechanical layering is an important factor causing anisotropy in sedimentary layers. Hence, in this study, with a shale gas reservoir case study in the Longmaxi Formation in the southeastern Chongqing region, Sichuan Basin, we present results from several numerical models to gain quantitative insights into the effects of mechanical layering on hydraulic fracturing. Results showed that the fractured area caused by hydraulic fracturing indicated a linear relationship with the neighboring layer’s Young’s modulus. An increase of the neighboring layer’s Young’s modulus resulted in better hydraulic fracturing effects. In addition, the contact between two neighboring layers is regarded as a zone with thickness and mechanical properties, which also influences the effects of hydraulic fracturing in reservoirs. The initial hydraulic fracture was unable to propagate into neighboring layers under a relatively low contact’s Young’s modulus. When associated local tensile stresses exceeded the rock strength, hydraulic fractures propagated into neighboring layers. Moreover, with the contact’s Young’s modulus becoming higher, the fractured area increased rapidly first, then slowly and finally became stable.

Natural fractures serve as significant storage spaces for hydrocarbons and are favorable for fluid flow in shale gas reservoirs; therefore, predicting the intensity and distribution of natural fractures within reservoirs are of extreme significance for shale gas exploration and development. Natural fractures in the Nanchuan region are generally in high dip angles, and the dominant fracture strikes are in the ∼NE-SW and ∼NW-SE directions. Most natural fractures within the Longmaxi Formation are formed in the Late Yanshanian period. Therefore, in this study, the Late Yanshanian paleotectonic stress field was numerically investigated. The comprehensive rupture rate (CRR) was defined and calculated as an indicator to quantitatively predict the intensity and distribution of natural fractures within the Longmaxi Formation. The results indicated that regions with well-developed fractures were mainly located in/around fault zones and fold zones, among fault/fold orientations change, and at fault tips. The intensity and distribution of natural fractures in the Longmaxi Formation of Nanchuan region were influenced by many factors including tectonic activities, mineral compositions, etc. The present study provides preliminary insights into natural fractures within the Longmaxi Formation of Nanchuan region, South China, which can guide and support shale gas exploration and development.

The Lower Jurassic low porosity and low permeability Ahe Formation is the major reservoir of Dibei Gasfield in the Kuqa Depression, Tarim Basin. Natural fractures are important spaces for storage of hydrocarbons in low permeability reservoirs and can significantly improve the fluid flow capability; therefore, predicting the location and intensity of natural fractures in the Ahe Formation are of extreme importance. In the present study, the Late Himalayan paleotectonic stress field, the period of time when the majority of natural fractures generated in the Dibei Gasfield, was simulated and investigated with a three dimensional finite element (3D FE) model, which serves as a starting point for the prediction. Based on the principle of energy conservation and simulated paleotectonic stress field, the relationship between fracture density and stress parameter was established, and hence, natural fractures in the Ahe Formation of Dibei Gasfield were predicted. The results indicated that the development and distribution of natural fractures were primarily fault-controlled. Regions with well-developed natural fractures were mainly located in fault zones and around faults. Tectonic activities and ultra-high pressures were the dominant factors for natural fractures in the Ahe Formation. Regions with higher development degree of natural fractures in the Ahe Formation usually have a larger gas production; therefore, regions among Well Y1, B3, X1 and B2 should be focused in the Dibei Gasfield.

Earthquakes occurred on the surface of the Earth contain comprehensive and abundant geodynamic connotations, and can serve as important sources for describing the present-day stress field and regime. An important advantage of the earthquake focal mechanism solution is the ability to obtain the stress pattern information at depth in the lithosphere. During the past several decades, an increasing number of focal mechanisms were available for estimating the present-day stress field and regime. In the present study, altogether 553 focal mechanism data ranging from the year 1976 to 2017 with Mw \(\ge \)7.0 were compiled in the Global/Harvard centroid moment tensor (CMT) catalogue, the characteristics of global strong earthquakes and the present-day stress pattern were analyzed based on these data. The majority of global strong earthquakes are located around the plate boundaries, shallow-focus, and thrust faulting (TF) regime. We grouped 518 of them into 12 regions (Boxes) based on their geographical proximity and tectonic setting. For each box, the present-day stress field and regime were obtained by formal stress inversion. The results indicated that the maximum horizontal principal stress direction was \(\sim \)N–S-trending in western North America continent and southwestern Indonesia, \(\sim \)NNE–SSW-trending in western Middle America and central Asia, \(\sim \)NE–SW in southeastern South America continent and northeastern Australia, \(\sim \)NEE–SWW-trending in western South America continent and southeastern Asia, \(\sim \)E–W-trending in southeastern Australia, and \(\sim \)NW–SE-trending in eastern Asia. The results can provide additional constraints to the driving forces and geodynamic models, allowing them to explain the current plate interactions and crustal tectonic complexities better.

Tectonic fractures are important reservoir spaces for storage of hydrocarbons in low permeability sandstone reservoirs and can significantly improve the permeability; therefore, understanding and predicting their location and intensity in reservoirs are of extreme importance for both the exploration and exploitation planning activities. In the present study, the Early Himalayan Dongying period paleotectonic stress field, the period of time when the majority of tectonic fractures generated in the Dongying Depression, Bohai Bay Basin, China, was simulated and investigated with a three dimensional finite element (3D FE) model. Estimation of the Comprehensive Rupture Rate (CRR) and the relationship between the measured tectonic fracture densities and CRRs were undertaken to quantitatively predict the development and distribution of tectonic fractures in the Es-3(m) low permeability sandstone reservoir. The results indicated that well developed tectonic fractures were located in regions within and between fault zones, near fault tips, and the eastern parts of Block Shishen 100 and adjacent regions. Fault activities were critical to the development and distribution of tectonic fractures in the Es-3(m) reservoir of Block Shishen 100 and adjacent regions. In addition, abnormal high fluid pressures in the Es-3(m) reservoir can promote the development of tectonic fractures.

In the Xiagou low permeability reservoir, tectonic fractures are important reservoir spaces for storage of hydrocarbon and can significantly improve the permeability; therefore, understanding and predicting the development and distribution of tectonic fractures in the Xiagou reservoir are important for both petroleum exploration and exploitation activities in the Qingxi Oilfield. In the present study, the Himalayan paleotectonic stress field, the period of time when the majority of unfilled tectonic fractures generated in the Xiagou Formation of Qingxi Oilfield, was simulated with a three dimensional (3D) finite element (FE) model. Estimations of rock failure criteria (RFC) and comprehensive rupture rate (CRR) were undertaken to quantitatively determine the development and distribution of tectonic fractures in the Xiagou reservoir. Horizontally, areas with well-developed tectonic fractures were primarily located in fault zones, regions among faults, around Well G107 and with changes in the orientation of faults. Vertically, the K1g3 layer showed a relative higher CRR, indicating that it was highly fractured. Factors of faults and the lithology controlled the development and distribution of tectonic fractures in the Xiagou reservoir of Qingxi Oilfield. Abnormal high fluid pressures in the Xiagou Formation can promote the development of fractures in reservoirs and the transformation from shear fractures to tensile fractures under certain conditions.

Future earthquake potential in the Bohai–Zhangjiakou Seismotectonic Zone (BZSZ) in North China deserves close attention. Tectonic stress accumulation state is an important indicator for earthquakes; therefore, this study aims to analyse the stress accumulation state in the BZSZ via three-dimensional visco-elastic numerical modelling. The results reveal that the maximum shear stress in the BZSZ increases gradually as the depth increases, and the stress range is wider in the lower layer. In the upper layer, the maximum shear stress is high in the Zhangjiakou area, whereas in the lower layer, relatively high values occur in the Penglai–Yantai area, which may be affected by the depth of the Moho surface. Besides, weak fault zones will be easily fractured when the maximum shear stress is not sufficiently high due to their low strengths, resulting in earthquakes. Therefore, based on the modelling results, the upper layer of the Zhangjiakou area and the lower layer of the Penglai–Yantai area in the BZSZ in North China are more likely to experience earthquakes.

The Lower Cretaceous Xiagou Formation of Qingxi Oilfield is a typical reservoir with low porosity and permeability in China. Fractures within the Xiagou Formation play an important role in the petroleum exploration and development. In the present study, analysis data of conventional drill core, thin section and imaging logging were used to determine the characteristics and controlling factors of tectonic fractures in the Xiagou Formation. The majority of tectonic fractures in the Xiagou Formation were unfilled, and their apertures were less than 1.0×10−4m. The dominant strikes of tectonic fractures were in the NE-SW and NW-SE directions. Approximately 68.8% of tectonic fractures were bedding fractures, followed by oblique fractures (about 18.2%) and vertical fractures (about 13.0%). In different places, the densities of tectonic fractures varied greatly. The controlling factors, including tectonic and non-tectonic factors, for tectonic fractures in the Xiagou Formation were confirmed, which have been analyzed qualitatively or semi-quantitatively. Analysis of tectonic factors indicated that tectonic fractures were more probably formed (or the fracture density was high) in regions with larger stress gradients and/or closer to faults. Within the same tectonic setting and stress field, non-tectonic factors of the lithology and mineral composition became the dominant factors governing the development of tectonic fractures. Analysis of non-tectonic factors showed that tectonic fractures were most developed in dolomitic mudstone. The fracture density was positively related to the proportion of brittle minerals in rocks of the Xiagou Formation.

致密油资源是青西油田非常规油气中最现实、也是最容易获得突破的领域.从烃源岩、储层、盖层及储盖组合等方面,深入探讨青西油田下白垩统下沟组致密油的成藏条件.结果表明:青西油田下白垩统下沟组和赤金堡组烃源岩厚度较大,有机质丰度较高,干酪根类型较好,生烃潜力大;储层物性较差,为低孔低渗致密储层.但研究区内构造活动强烈,不仅有利于形成富集油气的圈闭,而且还能改善储层的品质,发育的构造裂缝以及构造溶蚀裂缝是下白垩统下沟组致密油藏的主要储集空间;下白垩统下沟组及中沟组泥岩、古近系泥岩和膏盐岩为主要盖层,生储盖组合包括自生自储型和下生上储型两类.

The distribution and intensity of tectonic fractures within geologic units are important to hydrocarbon exploration and development. Taken the Upper Triassic Yanchang Formation interbedded sandstone-mudstone in the Ordos Basin as an example, this study used the finite element method (FEM) based on geomechanical models to study the development of tectonic fractures. The results show that the sandstones tend to generate tectonic fractures more easily than mudstones with the same layer thickness, and the highest degree of tectonic fractures will be developed when the sandstone-mudstone thickness ratio is about 5.0. A possible explanation is proposed for the tectonic fracture development based on two important factors of rock brittleness and mechanical layer thickness. Generally, larger rock brittleness and thinner layer thickness will generate more tectonic fractures. In interbedded sandstone-mudstone formations, the rock brittleness increases with the increasing mechanical layer thickness, hence, these two factors will achieve a balance for the development of tectonic fractures when the sandstone-mudstone thickness ratio reaches a specific value, and the development degree of tectonic fractures is the highest at this value.

This paper, taking lacustrine carbonate rocks of Guanzhuang Group of Pingyi Basin for example, aims at studying the paleogeographic significance of lacustrine carbonate rocks. Taking the sedimentary theory as guiding theory and based on the outcrop section,we analyze the formation and sedi- mentary features of Guangzhuang Group. Combined with the formation and evolution of the basin, we also analyze the conditions of sedimentary environ- ment, such as the paleontology, ancient climate, content sources, physical and chemical conditions of water medium, and so on, and clarify the paleogeo- graphic patterns and characteristics when Gaanzhuang Group was deposited. The study suggests that the basin was a warm inland environment when lacus- trine carbonate rocks were deposited. And the tectonic movement was relatively static, paleotopography was relatively flat, which was favorable to the deposi- tion of the lacustrine carbonate rocks, and carbonate concentration in the environment of shallow water, low and middle energy,little terrigenous fragmental materials. Thus it is demonstrated that the favorable paleogeographic setting for lacustrine carbonate strata was the sedimentary environment with relatively static tectonic movement, flat terrain, shallow water and low energy.

The Shanxi Formation at the Zhengzhuang block is relatively thin and separated with upper and lower Formations by sandstone K7 and K8 and contains the No 3 main coal seam.It was formed in the transitional phase from an epicontinental sea to a land.A sequence of deltaic plain deposits,consisting of deltaic plain,distributary channel,interdistributary bay,natural levee,crevasse splay and swamp deposits,were developed in large area of the basin,even the delta front and prodelta deposits were not developed.According to the sedimentary environment,this paper divided the Formation into three sand groups(Ⅰ,Ⅱ,Ⅲ).Careful study has been made on the spatial distribution and evolution of micro-facies in this block.It is revealed that with the retreat of sea water,distributary channels become wider and more straight as bifurcates reduced and mouth bars gradually disappeared.Then the Formation changed into land facies at the upper part.

By analysing all the phasestate kinds and transformational rules of natural gas that comes from coal measure strata in Qinshui Basin, we summarize the accumulation rules. Natural gas of coal measure strata in this area comes from many kinds of source rocks, such as coal, dark mudstone and dark carbonate rock; Also there're many kinds of natural gas phasestates in this area like free gas and dissolved gas, as well as, the most important one, coal-bed gas. So we call that "multi-source and multi-phase state". During Indo-Chinese, Yanshanian and Himalayan periods, different phasestates of natural gas have different processes of transformation. And, of course, they're interconvertible. That is so-called "dynamic transformation", while the "unidirectionally accumulate and escape" emphasizes the irreversibility of the process of accumulation.

In this article, based on research results about the strata and welwitschiopsida of Eastern Shandong area, and in combination with the analysis on the sedimentary and tectonic paleogeographic of Western Shandong area, we point out that there was no uplifted area existing in Eastern Shandong area in the Palaeozoic Period. The area was a tectonic unit independent of the North China during Cambrian to early Late Carboniferous. Controlled mainly by Sulu ocean basin, the region was composed of a set of marine and paralic deposits, but part of the strata of Upper Jurassic-Middle Devonian might not exist. During late Late Carboniferous to Permian, resulting from collision and convergence between Eastern and western Shandong area, continental sedimentary formation was formed. Since Triassic, Eastern Shandong area had brought about the end of sedimentation, with strong uplifting, erosion and absence of Palaeozoic formation, which didn't have deposits until Cretaceous.

During Ordovician period, Huanghua Depression was a part of Huabei Epicontinental Sea, in which a set of carbonate formation was formed. The Lower Ordovician contains the Yeli Formation and Liangjiashan Formation; and the Middle Ordovician contains the Majiagou Formation and Fengfeng Formation; but the Upper Ordovician is missing. The paleogeography was characterized by two division zones of the south and north in early Ordovician, but in the Middle Ordovician, the paleogeography changed-there was a dolomitic lime flat along North-East strike in open Epicontinental Sea. The reservoirs of Ordovician in this area are changing from relatively good to relatively bad ones, which were characterized by higher porosity but low permeability due to post stage multi-deformation and multiple factors including lithology, sedimentary facies, diagenesis, fault and crack. Favorable reservoirs are distributed in Shenqingzhuang-Tanggu, Kongxi, Wangguantun, Nanpi-Wuqiao and Xuhei regions.