Sensitivity analysis aids in determining important factors affecting pipeline safety. Sensitivity analysis of stress inside gas pipelines with corrosion defects in a landslide region can provide a theoretical basis for the safe operation of pipelines. This study considered an X80 high-grade steel gas pipeline model with corrosion defects using finite element analysis (ABAQUS software) under lateral landslide conditions. Particularly, we studied the six major engineering elements of soil cohesion to understand the stress variations in buried gas pipelines and performed a sensitivity analysis of each influencing parameter. The calculation results revealed that all the factors influencing the stress in corroded gas pipelines during landslide conditions were positively correlated to the internal pipe stress, except for the axial position of corrosion defects. The factors in the descending order of influence on the sensitivity coefficient are stated as follows: landslide displacement, axial position of corrosion defect, soil cohesion, depth of corrosion defect, pressure, and length of corrosion defect. The results of this study will aid in the design and implementation of such pipelines in mountainous or other landslide-prone terrains. Full article

Continental faulted basins are widely distributed in eastern China. Many of these basins, in which the faults block oil and gas reservoirs, have been explored. The heterogeneity of the reservoirs in fault block is very strong, shich restricts the further efficient development of these kinds of oil and gas fields. In this study, porosity and permeability tests, the use of thin sections of rock, mercury injection experiment and CT scan were used to investigate reservoir quality characteristics and control factors. The results showed that the content of quartz, feldspar, and debris in rock had a significant control function on the quality of the reservoir. Reservoir performance improved with increase of quartz and feldspar content, and worsened with increase of debris content. Taking the Ek1 reservoir in the Wang Guantun area as the specific research object, we developed the following understanding. On the one hand, the main compaction in the study area was mechanical compaction. When the compaction rate was greater than 60%, the porosity and permeability were inversely proportional to the compaction rate. On the other hand, dissolution pores were relatively developed in the study area, and the main types of dissolution were intragranular and intergranular dissolution pores. When the surface porosity of the dissolution pore was over 9.2%, porosity increased significantly the increase of dissolution surface porosity. This showed that dissolution surface porosity had greatly improved the reservoir porosity in this range. Full article

Establishing a suitable classification and evaluation scheme is crucial for sweet spot prediction and efficient development of shale oil in the Chang 7_1-2 sub-member of the Longdong area. In this paper, a series of experiments, such as casting thin sections (CTS), scanning electron microscopy (SEM), low-temperature nitrogen adsorption (LTNA), high-pressure mercury intrusion porosimetry (HMIP), and nuclear magnetic resonance (NMR), were integrated to classify the pore throats and shale oil reservoirs in the study area. Moreover, the pore structure characteristics of different types of reservoirs and their contributions to productivity were revealed. The results show that the pore-throat system can be divided into four parts: large pore throats (>0.2 μm), medium pore throats (0.08~0.2 μm), small pore throats (0.03~0.08 μm), and micropore throats (<0.03 μm). Based on the development degree of various pore throats, the reservoir is divided into four types: type I (Φ ≥ 10%, K > 0.1 mD), type II (Φ ≥ 8%, 0.05 mD < K < 0.1 mD), type III (Φ ≥ 5%, 0.02 mD < K < 0.05 mD) and type IV (Φ < 5% or K < 0.02 mD). From type I to IV reservoirs, the proportion of dissolved pores and intergranular pores gradually decreases, and the proportion of intercrystalline pores increases. The proportion of large pore throats gradually decreases, and the proportions of medium pore throats and small pore throats increase initially and then decrease, while the proportion of micropore throats increases successively. The NMR pore size distribution changes from the right peak to the left peak. The developed section of the type I reservoir corresponds to the oil layer, and the developed section of the type I and II reservoirs corresponds to the poor oil layer. In contrast, the developed section of the type III and IV reservoirs corresponds to the dry layer. The daily production from single wells is primarily attributable to type I and II reservoirs. Full article

Coal and organic-rich shale in the Yan’an Formation in the southwestern margin of the Ordos Basin are widely developed, which is an important fact for oil and gas exploration in China that has been widely explored for a long time. In this paper, detailed sequence division and sedimentary environment analyses of the Yan’an Formation in the Pengyang area on the southwestern margin of the Ordos Basin were conducted using field outcrops, drilling cores, logging, wavelet transform and organic geochemistry. The results showed that the succession consists of some units with distinctly different characteristics. Based on the petrographic assemblage and transform wavelet characteristics, the Yan’an Formation in this area can be divided into a long-term cycle, five medium-term cycles, and eleven short-term cycles, among which coal and carbonaceous shale were mainly developed in the short-term cycles I₂, III₁, III₂, V₁ and V₂. Coal and organic-rich mud shale have been developed in the Yan’an Formation and plant debris in mudstone and coal is common, indicating the development of swamps and shallow water-covered depressions in this area. The sandstones showed parallel bedding, cross-bedding and scours, thus indicating fluvial deposits. The saturated hydrocarbon gas chromatographic parameters of mud shale showed that the pristane/phytane (Pr/Ph) ratio is 2.24–6.22, the Ph/nC18 ratio is 0.15–0.93, and the Pr/nC17 ratio is 0.97–2.78, supporting the finding that the organic matter has mainly originated from terrestrial sources. Full article

Three-dimensional (3D) seismic data and well log data were used to investigate the sandstone architecture of the Middle Jurassic deltaic reservoirs of the Zhetybay Oilfield, Mangeshrak Basin, Kazakhstan. The base-level cycles of different scales were identified and divided using well log and 3D seismic data. Five types of sedimentary boundaries were identified in the mouth bar sandstones. The boundaries divide single mouth bars. Vertically, the spatial distribution of sand bodies can be divided into superposed, spliced, and isolation modes. Laterally, contact modes can be divided into superposition, lateral, and isolation modes. We found that the base-level cycle controls the evolution of the delta front sand body architecture. In the early decline or late rise of the base-level cycle, the superimposed or spliced modes dominate the sand body. By contrast, the lateral or isolation modes dominate the sand body in the late decline or early rise of the base-level cycle. This paper proposes an architecture model of the delta front sand bodies controlled by the base-level cycle. The spatial distribution and morphological variation of deltaic sand bodies could be linked to the base-level cycles. Full article

During the production process, water phase incursion into the reservoir causes water blocking damage and seriously affects the production of tight gas reservoirs. Recently, nanoemulsions have been used as highly effective water blocking removing agents in the field, but their mechanism is still unclear. In this research, a series of nanoemulsions with different droplet sizes were synthesized, and their water blocking removing performance was intensively investigated. To begin, the relationship between the droplet size and the chemical composition of the nanoemulsion was determined by dynamic light scattering. Second, the influence of the nanoemulsion droplet size on the surface tension and the contact angle experiments was studied. Finally, NMR and permeability recovery experiments were used to study the relationship between the droplet size and the water locking removing effect of the nanoemulsions. Simultaneously, the surfactant release process was investigated using the static adsorption curves of the nanoemulsions. The experimental results show that the droplet size of nanoemulsion has an exponential relationship with the oil phase content. The surface tension decreases with the increase in droplet size, but the wetting reversal effect decreases with the increase in droplet size. The nanoemulsion with an oil phase content of 5 wt.% has the best water locking removing effect, and the permeability recovery value of the core reaches 59.54%. The adsorption control of the nanoemulsion on the surfactant is the key to its water blocking removing ability. This comprehensive study shows that the nanoemulsion with an oil phase content of 5 wt.% has optimum adsorption control capability. Thus, it can be used as a promising candidate for removing water blocking in tight gas reservoirs. Full article

The wave field characteristics of thin reservoirs are extremely complex due to the tuning and interference between the top and bottom interfaces of the reservoirs, which leads to large uncertainty in thin layer AVO (Amplitude Versus Offset) analysis. In order to reduce the uncertainty of thin layer AVO analysis, we study the uncertainty dominant factors of the effect of thin layer on the AVO response characteristics from the aspects of theoretical derivation and forward simulation. Based on the research results, we use the AVO fitting forward method with offset and tuning utility as the joint inversion operator to establish an AVO detuning effect method, based on the sparse fitting inversion strategy, and study the objective function of the fitting inversion method. We optimize the sparsity constraints and the sparsity method to reduce the non-independence of multiparameter variables and seismic data, and the noise of inversion. Through the verification analysis of the model using actual data, the AVO detuning effect method studied in this paper has a correct and reasonable technical theory and obvious application effect. Full article

The Yinchuan Graben is an important potential exploration area that is located on the western margin of the Ordos Basin. Over 8000 m of Cenozoic strata have been formed since the Cretaceous. With an integrated approach of cores observation, logging analysis, and geochemical analysis, we analyzed the characteristics of the Cenozoic source rocks in the Yinchuan Graben and determined the formation and destruction of the fossil oil reservoirs. With type III kerogen, the TOC of the dark mudstone in the Qingshuiying Formation is up to 7.5%, and the Ro is 0.95–1.04%, indicating the source rocks have entered the mature stage but the hydrocarbon generation potential is insufficient. A quantity of reservoir bitumen and oil-bearing fluid inclusions (GOI = 1.67–4%) were found in the Qingshuiying Formation sandstone in Well YQ-1, which indicates a fossil oil reservoir had existed. The fossil oil reservoir and reservoir bitumen were generated by the unexplored pre-Cenozoic strata in the Yinchuan Graben. The reservoir bitumen has high maturity and is associated with many fluid inclusions with a high homogenization temperature or CO₂. This indicates that the bitumen was formed by the pyrolysis of the oil which was caused by the hot fluid migrating along with the deep fault belts. Full article

With the successful development of unconventional hydrocarbons, the production of unconventional hydrocarbons has increased rapidly. However, a single conventional or unconventional model is not suitable for the mechanism of hydrocarbon accumulation in a given basin or sag. Based on data from drilling, logging, and geophysical analysis, the hydrocarbon accumulation mechanism in the Hari sag in the Yingen-Ejinaqi basin, China, was analyzed. There are three sets of source rocks in the Hari sag: the K₁y source rocks were evaluated as having excellent source rock potential with low thermal maturity and kerogen Type I-II₁; the K₁b² source rocks were evaluated as having good source rock potential with mature to highly mature stages and kerogen Type II₁-II₂; and the K₁b¹ source rocks were evaluated as having moderate source rock potential with mature to highly mature stages and kerogen Type II₁-II₂. Reservoir types were found to be conventional sand reservoirs, unconventional carbonate-shale reservoirs, and volcanic rock reservoirs. There were two sets of fault-lithologic traps in the Hari sag, which conform to the intra-source continuous hydrocarbon accumulation model and the approaching-source discontinuous hydrocarbon accumulation model. The conclusions of this research provide guidance for exploring multi-type reservoirs and multi-type hydrocarbon accumulation models. Full article

The thermal evolution process of organic matter is associated with the complete hydrocarbon generation and expulsion process in shale, however, the thermal evolution of organic matter is a long process and cannot be realized without experimental simulations. Although several scholars have substantially studied the thermal evolution of organic matter, it remains a challenging and much debated issue in the studies of organic geochemistry. Volcanic events are crucial in the enrichment of organic matter, and appropriate heating accelerates the thermal evolution of organic matter. However, how strong-rock baking restricts the evolution of organic matter in shale has not been specifically studied. The South Qilian Basin in China is a typical superimposed basin where complex tectonic movements have induced multiple volcanic events, which makes it a favorable location to perform the aforementioned research. This study used the Galedesi Formation shale in the Hala Lake Depression of the South Qilian Basin as an example for investigating the constraints of the volcanic events related to the thermal evolution of organic matter by integrating the results obtained using the geochemical and petrological methods. Our results demonstrate that the lacustrine Galedesi Formation shale of the Hala Lake Depression in the Late Triassic is a typical deep-lake facies deposit with good hydrocarbon generation potential. However, because of the influence of regional tectonic evolution, the burial depth of shale is not deep and the thermal evolution of organic matter is insufficient. Due to the influence of multiple volcanic thermal events in the later stages, the thermal maturity of organic matter in the Galedesi Formation shale generally exceeds 3.0%, which is abnormally high. The apparent carbonization of organic matter can be observed via scanning electron microscopy. Rapid magma baking typically cannot effectively promote the hydrocarbon generation of shale organic matter. Finally, the burial depth of lacustrine shale of the Galedesi Formation in the Hala Lake Depression of South Qilian Basin is too shallow. Organic matter hydrocarbon generation and later shale preservation conditions are not conducive to the enrichment, accumulation, exploration, and development of shale gas. Full article

The intrusive rock buried-hill reservoir is one of the main targets for oil and gas exploration in the offshore sedimentary basins of China. In order to discover the reservoir as early as possible for decision-making to save costs, reservoir interpretation with mud-logging data needs to be studied. Previous studies have showed that the homogeneity rocks reservoir can be well interpreted with mud-logging data u the K_b (mechanical specific energy ratio) method and the W_L (tangential power)-W_H (vertical power) intersection method. However, reservoir interpretation with mud-logging data for intrusive rock buried-hill has not been reported. The key steps or parameters of these two kinds of methods used for reservoir interpretation need to be modified for the vertical variation of intrusive rock buried-hill. Furthermore, confirming the interpretation of these two kinds of methods has not been reported. Taking the Y area in the Qiongdongnan Basin of the South China Sea as an example, the intrusive rock buried-hill can be divided into four zones based on its characteristics and genesis in descending order: the sand–gravel weathering zone, the weathering fracture zone, the inner fracture zone, and the base rock zone. The reservoir can be well interpreted when taking the M_SE (mechanical specific energy) geometrical mean of the base rock zone as a basic value to calculate K_b. The reservoir will also be well interpreted when W_H ranges from 0 to 4.5 MPa and W_L ranges from 0 to 99 MPa in the column while intersecting. The layers of the reservoir can be interpreted as K_b < 1 in the sand–gravel weathering zone and the weathering fracture zone. The K_b < 1 and effective intersection of W_L-W_H layers at the same time could be interpreted as a reservoir in the inner fracture zone and the base rock zone. After combining the K_b method with the W_L-W_H intersection method, the reservoir of intrusive rock buried-hill can be well interpreted. The total thickness of the uninterpretable reservoir ratio is less than 20% compared to reservoir interpretation with well-logging for each well. Full article

Low permeability tight gas resources account for 90% of the Xihu Sag. Under the background of extensive development of low permeability and tight reservoirs, the key to economic and effective development is to find sweet reservoir formation. To clarify the origin and distribution of a sweet reservoir in the study area, it is important to study the formation and evolution mechanisms of chlorite. In this study, based on the analysis of thin section, X-ray diffraction and SEM, through the analysis of the key factors in the formation of authigenic chlorite of the Huagang Formation in the middle and north of the central inversion structural belt, we reasoned the formation and evolution process of chlorite in the whole life cycle. According to the sedimentary diagenetic response characteristics of chlorite, two types of favorable sedimentary facies belts of chlorite are identified. The results showed that the development of pore-lined chlorite is a natural advantage of reservoirs in the East China Sea. Chlorite is formed under the joint action of three factors: the source of iron and magnesium ions, the alkaline environment in the early diagenetic stage and the open fluid field. After the formation of pore-lined chlorite, the sweet spots developed under the protection of four mechanisms: inhibiting quartz cementation, enhancing compression resistance, protecting macropore throat and primary pores, and promoting secondary intergranular dissolved pores. When the content of chlorite in the pore lining is high (relative content > 35%), the lining thickness is moderate (4–10 μm). A high degree of wrapping and good crystallization are conducive to the formation and preservation of sweet spots. Full article

The Lower Paleozoic carbonate reservoir in the Tarim Basin is a hotspot area for deep oil and gas exploration in China. Although the Lower Cambrian of the Bachu Uplift has not encountered industrial oil flow, rich bitumen has been found there. As the most direct trace of petroleum migration, the effective identification of bitumen is the key to studying the hydrocarbon transportation path. In this study, the Adobe Photoshop quantification (PSQ) method is used to identify the bitumen content in the Xiaoerblak Formation in Well Shutan 1, and, combined with the bitumen characteristics of the Shihuiyao section, a high-quality petroleum transverse transport layer is determined. The results indicate the following: (1) In Well Shutan 1, bitumen is mainly concentrated in the middle and upper parts of the Xiaoerblak Formation with high porosity and high permeability. (2) The shale of the Yuertus Formation in the Shihuiyao section has low hydrocarbon generation potential. However, the overlying Xiaoerblak Formation has developed multistage bituminous veins and bitumen-encapsulated gravels, which is the result of multiple instances of horizontal hydrocarbon migration. (3) After combining the bitumen characteristics of Well Shutan 1 and the Shihuiyao section, it is confirmed that there are high-quality lateral transport conductors in the middle and upper parts of the Xiaoerblak Formation, and the Subsalt Cambrian dolomite reservoir has great exploration potential. Full article

There are rare comparative studies on the geological characteristics of shale in different members of Permian Fengcheng Formation in Mahu Sag, Junggar basin, China. In order to compare the mineral composition, physical properties, and geochemical characteristics of shale in three members of Fengcheng Formation in Mahu Sag, a large number of test data such as X-ray diffraction, high-pressure mercury injection, organic carbon, rock pyrolysis, and vitrinite reflectance were collected and analyzed. Results showed that the content of clay minerals in the shale of the third member of Fengcheng Formation (P₁f₃) is the highest. The content of carbonate minerals is the highest and the content of clay minerals is the lowest in the shale of the second member of Fengcheng Formation (P₁f₂). The content of felsic minerals is the highest and the content of carbonate minerals is the lowest in the shale of the first member of Fengcheng Formation (P₁f₁). The physical properties of the shale of P₁f₃ are the best, and the porosity of the shale of P₁f₂ is the smallest, but its permeability is relatively large, and the permeability of shale of P₁f₁ is the lowest. The organic matter abundance of shale of P₁f₂ is the highest, while that of P₁f₁ is relatively the lowest. Most of the organic matter types of shale of P₁f₃ are type I–II, those of P₁f₂ are mainly type II, and those of P₁f₁ section are distributed from type I–III. On the whole, the shale of Fengcheng Formation in the peripheral fault zone and slope area of Mahu Sag has reached the low mature to mature stage, and the shale in the central area of the sag has reached the mature stage. More than half of the shale samples of Fengcheng Formation belong to fair to good source rocks, especially the samples of P₁f₂. A few samples from P₁f₃ and P₁f₁ belong to non-source rocks. This study indicates that the shale of Fengcheng Formation in Mahu Sag has good hydrocarbon generation potential, especially the shale of P₁f₂, and can become the target of shale oil exploration. Full article

Shale oil is an unconventional oil resource that needs to be developed and utilized urgently. However, the Chang 7 shale in the Ordos Basin, as the most typical continental source rock in China, is limited by the study of organic matter (OM) enrichment factors in continental lacustrine facies, and there are still controversies about the controlling factors, which limit the progress of oil and gas exploration. This paper aims to reconstruct the paleoenvironment of Chang 7 shale in the southern margin of Ordos Basin and reveal the controlling factors of organic rich shale by organic and elemental analysis, X-ray diffraction (XRD) analysis, thin section observation, and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) analysis. The results show that during the deposition period of Chang 7 shale, the climate was warm and humid, the lake water has strong reducing, low salinity and rapid depth changes. Total organic carbon (TOC) is positively correlated with salinity and hydrothermal action and inversely proportional to terrigenous input. The high productivity, low consumption and low dilution result in high enrichment of shale OM in the southern margin of Ordos Basin. Full article

Investigations into the connectivity and complexity of pore systems in shales are essential for understanding the flow of shale gas and the capacities of the associated reservoirs. In the present study, eight shale samples from the Wufeng-Longmaxi (WF-LMX) Formation that were collected from Well Yucan-6 in the southeast of the Sichuan Basin were analyzed for microstructural, pore network, and pore connectivity characteristics. The measurement results of low-pressure nitrogen adsorption illustrated that all shale samples contain micropores, mesopores, and macropores. Micropores and mesopores account for a high proportion of the total pores, and the dominant pore throat size is in the range of 2–6 nm. High-pressure mercury injection tests reveal that the porosity, total pore volume, and total specific surface area of pores for samples from the WF Formation are higher than those for samples from the LMX Formation. In spontaneous absorption experiments, the slopes of the absorption curves of n-decane (oil-wetting) and deionized water (water-wetting) in the WF and LMX Formations varied from 0.254 to 0.428 and from 0.258 to 0.317, respectively. These results indicate that shales in both formations exhibit mixed wettability characteristics, but lipophilic pores are better connected relative to hydrophilic pores. The total organic carbon and silica contents are the main factors controlling the pore connectivity in these shales, while the effects of other minerals are not significant. The findings of this work can improve our understanding of the pore structure characteristics of black shale. Full article

A prediction method of hydrocarbon distribution associated with fault-shale caprock configuration leakages is established through superposing hydrocarbon distribution at deep basin fault-shale caprock configuration leakages, and conducting faults to understand distribution of shallow reservoirs of “lower generation, upper accumulation” in petroliferous basins based on fault-shale caprock configuration leakage mechanism. Prediction of hydrocarbon distribution at the Ed-3 Member in the south of the northern Dagang area of Bohai Bay Basin was used to demonstrate the application of the proposed method. Results show that predicted oil and gas at the Ed-3 Member are mainly distributed in the middle of the north and in the middle of the south edge, where fault-shale caprock configuration leakages in the middle of the Sha-1 Member contributed positively to oil and gas migration from the hydrocarbon reservoir at deep basin to the shallow reservoirs. The prediction matches well with discovered oil and gas in this area, proving the validity of this method. Full article

The water blocking damage to the reservoir caused by the invasion of external fluid is one of the main factors that affect the efficient development of tight sandstone gas reservoirs. In this paper, microfluidic chip technology is used to explore the causes of water blocking damage in porous media and find suitable recovery solutions. The research results show that reducing the gas-liquid capillary pressure can effectively reduce the rate and quantity of spontaneous speed of cores. After chemical treatment, the liquid phase fluidity of the non-fractured matrix core is increased by 1.72 times, and that of the fractured core is increased by 2.13 times. In water wetting porous media, there are mainly four types of liquid hold-up: (1) Liquid hold-up in the dead volume of a non-connected pore; (2) The water phase in the pore throat with a small inner diameter cannot be driven away due to its larger capillary pressure; (3) Adsorption viscous force, the wetting phase is adsorbed on the surface of the solid phase; (4) Reservoir heterogeneity. The water blocking damage can be removed to a certain extent by changing the gas injection pressure, the gas injection method, or adding a wetting modifier. Full article

Member 1 of the Cretaceous Qingshankou Formation (K₂qn¹ Member) in the Southern Songliao Basin, composed of mainly semi-deep and deep lacustrine shale layers, is rich in shale oil. Previous studies on shale reservoir characteristics mainly focused on marine shale strata, but few studies have considered lacustrine shale strata, so the pore-throat features and differences between the lacustrine shale reservoir and marine shale reservoir need to be studied. Taking the Class-I and II sweet spot sections and Class-III non-sweet spot section of Da’an shale oil demonstration area as examples, SEM (scanning electron microscopy) was used to qualitatively and semi-quantitatively describe the morphology and occurrence characteristics of the shale. Full-scale pore size distributions of lacustrine shale samples were quantitatively measured by N₂GA (nitrogen absorption) combined with dominant pore size segments tested by experiments. Finally, the lacustrine shale reservoir was compared with classical marine shale reservoirs, and factors influencing semi-deep lacustrine and deep lacustrine shale oil in a large depression basin were analyzed by XRD (X-ray diffraction). The results show that Class-I and II sweet spots are rich in organic matter, quartz, and carbonate minerals, have mainly type H2 nitrogen adsorption hysteresis loops, and contain mainly inorganic pores, such as intergranular and intragranular pores in nano-scale, forming nano-scale reservoirs. Lacustrine shale is obviously different from marine shale in terms of pore structure, and the development characteristics of the lacustrine shale pore structure are more influenced by mineral components. Factors affecting the development of shale oil reservoirs in K₂qn¹ member include mineral components, TOC (total organic carbon), and diagenetic processes. Quartz and carbonate minerals are good for enhancing reservoir quality, while clay minerals are destructive to the development of reservoirs. TOC is the material foundation and main factor for forming organic pores, but the higher the TOC, the smaller the diameter of the organic pores will be. Compaction, cementation, and dissolution are the main diagenetic processes controlling the development of reservoir space. Full article

Multistage hydraulic fracturing is one of the most prevalent approaches for shale reservoir development. Due to the complexity of constructing reservoir environments for experiments, numerical simulation is a vital method to study flow behavior under reservoir conditions. In this paper, we propose a numerical model that considers a multistage fractured horizontal well with a hilly-terrain trajectory in a shale reservoir with the presence of natural fractures. The model was constructed based on the MATLAB Reservoir Simulation Toolbox and used the Embedded Discrete Fractured Model (EDFM) to describe the interrelationship between the matrix, fractures, and wellbore. The model was then applied to an actual condensate gas well producing from a shale reservoir, and the effects of reservoir parameters on the simulation data were studied based on this well case. The simulation results were highly consistent with the actual production data, which validates the accuracy of this model and proves its potential for predicting future production trends. We extended the discussion to two examples with extreme well trajectories by reviewing the inflow contribution of each hydraulic fracture with respect to fracture pressure, and the changes in static pressure with time. In conclusion, the proposed model is capable of providing simulation results close to reality and thus guiding field design and operation in the fracturing and drilling process. Full article

Nitrogen and oxygen (NO) compounds are important compositions in shale source rocks, and they carry an abundance of geochemical information for hydrocarbon generation. Due to technical limitations, the significance of NO compounds has not been paid enough attention. In this paper, the NO compounds from shale rocks of the Dongpu Depression are analyzed to explore the compositional characteristics and geochemical significance of using geological and organic geochemical ways of rock-eval, gas chromatography-mass spectrometry (GC/MS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results show that shale rocks are rich in NO compounds, with twelve types of compounds that were detected: N₁, N₁O₁, N₁O₂, N₁O₃, N₁S₁, N₁S₂, N₂O₁, O₁, O₂, O₃, O₃S₁, and O₄. Of these compounds, O₂ and O₃ predominated, followed by N₁ and N₁O₁. Of the N₁ species, the most abundant classes are DBE of 9, 12, 15, and 18, which changed with maturity. Of the O₂ species, compounds of DBE of 1 (fatty acids) are the predominant class. Classes of DBE 5 and 6 in the O₂ species are naphthenic acids with special biological skeleton structures, which are usually appear in immature and low-mature oils. N₁, O₂, and N₁O₁ compounds are affected by their maturity and they often run to polarization with enhanced DBE species and a shorter carbon chain as their maturity increases. The parameters of DBE_18–25/DBE_9–18-N₁ and DBE_12–20/DBE_5–12-O₂ increase with the increase of buried depth and maturity. The NO compounds that were revealed by FT-ICR MS may have a promising application in distinguishing between the different depositional environments. Source rocks of saline lacustrine are rich in O₂ and N₁O₁, but less N₁ and O₁ compounds. The research results are of vital importance for expanding the application of the NO molecular compounds in petroleum exploration. Full article

The combination of lidar and digital photography provides a new technology for creating a high-resolution 3D digital outcrop model. The digital outcrop model can accurately and conveniently depict the surface 3D properties of an outcrop profile, making up for the shortcomings of traditional outcrop research techniques. However, the advent of digital outcrop poses additional challenges to the 3D spatial analysis of virtual outcrop models, particularly in the interpretation of geological characteristics. In this study, the detailed workflow of automated interpretation of geological characteristics of fractures and cavities on a 3D digital outcrop texture model is described. Firstly, advanced automatic image analysis technology is used to detect the 2D contour of the fractures and cavities in the picture. Then, to obtain an accurate representation of the 3D structure of the fractures and cavities on the digital outcrop model, a projection method for converting 2D coordinates to 3D space based on geometric transformations such as affine transformation and linear interpolation is proposed. Quantitative data on the size, shape, and distribution of geological features are calculated using this information. Finally, a novel and comprehensive automated 3D quantitative characterization technique for fractures and cavities on the 3D digital outcrop texture model is developed. The proposed technology has been applied to the 3D mapping and quantitative characterization of fractures and cavities on the outcrop profile for the Dengying Formation (second member), providing a foundation for profile reservoir appraisal in the research region. Furthermore, this approach may be extended to the 3D characterization and analysis of any point, line, and surface objects derived from outcrop photos, hence increasing the application value of the 3D digital outcrop model. Full article

This paper presents an effective numerical simulation method for production prediction of in situ recovery of oil shale reservoirs with steam injection. In this method, finite volume-based discretization schemes of heat and mass transfer equations of the thermal compositional model are derived and used. The embedded discrete fracture model is used to accurately handle the fractured vertical well. A smooth non-linear solver is proposed to solve the global equations, then cell pressure, temperature, saturation, component mole fractions, and well production rates can be obtained. Compared with the existing commercial software, this new method can have a smoother non-linear solution and handle the complex fracture geometry theoretically. A numerical example is used to test this presented method and can realize accurate calculation results compared with CMG. Another numerical case with a hydraulic fracture and an open thermal boundary condition is implemented to validate the presented method and can effectively handle the actual situation of steam injection-assisted in situ recovery of oil shale, which was difficult to handle using previous methods. Full article

In the Benxi Formation of the Carboniferous system of the Upper Paleozoic in the Ordos Basin, there are many sets of coal measures dark organic-rich shale, being marine continental transitional facies, with significant unconventional natural gas potential. Previous studies are only limited to the evaluation of tight sandstone reservoir in this set of strata, with no sufficient study on gas bearing and geological characteristics of organic-rich shale, restricting the exploration and evaluation of shale gas resources. In this study, analysis has been conducted on the organic carbon content, the major elements, the trace elements, and the mineral composition of core samples from the Benxi Formation in key drilling sections. In addition, qualitative and quantitative pore observation and characterization of core samples have been conducted. The sedimentary environments and reservoir characteristics of the shale of the Benxi Formation have been analyzed. Combined with the gas content analyzing the results of the field coring samples, the shale gas resource potentials of the Benxi Formation have been studied, and the geological characteristics of the Benxi Formation shale gas in the eastern Ordos Basin have been made clear, to provide a theoretical basis for shale gas resource evaluation of the Benxi Formation in the Ordos Basin. The results show that (1) in the Hutian Member, Pangou Member, and Jinci Member of the Benxi Formation, organic-rich shale is well developed, with the characteristics of seawater input as a whole. There is a slight difference in sedimentary redox index, which shows that the reducibility increases gradually from bottom to top. (2) There is an evident difference in the mineral characteristics of shale in these three members. The Hutian Member is rich in clay minerals, while the Jinci Member is high in quartz minerals. (3) The pores are mainly inorganic mineral intergranular pores, clay interlayer fractures, and micro fractures, and organic matter pores are developed on the surface of local organic matter. (4) The mud shale in the Jinci Member has a large cumulative thickness, has relatively high gas-bearing property, and is rich in brittle minerals. The Jinci Member is a favorable section for shale gas exploration of the Benxi Formation. Full article

The shale oil reservoir of the Lucaogou Formation in the Jimsar Sag has undergone tectonic movement, regional deposition and complex diagenesis processes. Therefore, various reservoir space types and complex combination patterns of pores have developed, resulting in an intricate pore throat structure. The complex pore throat structure brings great challenges to the classification and evaluation of reservoirs and the efficient development of shale oil. The methods of scanning electron microscopy, high-pressure mercury injection, low-temperature adsorption experiments and thin-slice analysis were used in this study. Mineral, petrology, pore throat structure and evolution process characteristics of the shale oil reservoir were analyzed and discussed qualitatively and quantitatively. Based on these studies, the evolution characteristics and formation mechanisms of different pore throat structures were revealed, and four progressions were made. The reservoir space of the Lucaogou Formation is mainly composed of residual intergranular pores, dissolved pores, intercrystalline pores and fractures. Four types of pore throat structures in the shale oil reservoir of the Lucaogou Formation were quantitatively characterized. Furthermore, the primary pore throat structure was controlled by a sedimentary environment. The pores and throats were reduced and blocked by compaction and cementation, which deteriorates the physical properties of the reservoirs. However, the dissolution of early carbonate, feldspar and tuffaceous minerals and a small amount of carbonate cements by organic acids are the key factors to improve the pore throat structure of the reservoirs. The genetic evolution model of pore throat structures in the shale oil reservoir of the Lucaogou Formation are divided into two types. The large-pore medium-fine throat and medium-pore medium-throat reservoirs are mainly located in the delta front-shallow lake facies and are characterized by the diagenetic assemblage types of weak compaction–weak carbonate cementation–strong dissolution, early medium compaction–medium calcite and dolomite cementation–weak dissolution. The medium-pore fine throats and fine-pore fine throats are mainly developed in shallow lakes and semi-deep lakes. They are characterized by the diagenetic assemblage type of strong compaction–strong calcite cementation–weak dissolution diagenesis. This study provides a comprehensive understanding of the pore throat structure and the genetic mechanism of a complex shale oil reservoir and benefits the exploration and development of shale oil. Full article

In the Carboniferous–Permian period, several organic-rich black shales were deposited in a marine–continental transitional environment in the Linfen area on the eastern margin of the Ordos Basin. Integrated sedimentological and organic geochemical analyses are performed on an outcrop in order to clarify the relationship between paleoenvironment and organic matter accumulation. The results of this study show that the marine–continental transitional strata of the Upper Carboniferous Benxi Formation to Lower Permian Taiyuan and Shanxi Formation exposed in the Linfen area are composed of sandstone, shale, coal, and limestone. Total organic carbon (TOC) contents of the studied samples were mainly distributed in the range of 0.59%–35.4%, with an average of 7.32%. From Benxi Formation to Shanxi formation, the humidity gradually increased, and the climate gradually changed from hot and humid to warm and humid during Carboniferous to Permian. The deposition of the Shanxi Formation ended with the climate returning to hot and humid, having an oxic-suboxic conditions and a high paleoproductivity. Paleoredox conditions and paleoproductivity are the two vital factors controlling the formation of organic matter in black shales. The transitional environment characterized by oxic-suboxic, relatively high deposition rate, and various source of organic matter, although different from the marine environment, provides a good material basis for the deposition of organic-rich shales. Full article

Lipid biomarkers play an important role in defining oil-source rock correlations. A fundamental assumption is that composition (or ratios) of biomarkers in oil is not significantly different from that in bitumen in the source rock. In order to compare the geochemical characteristics of expelled oil and residual oil, a Permian Tasmanite oil shale was used for an artificial maturation experiment to simulate the oil generation period. The results show that the Tasmanite oil shale generated high amounts of hydrocarbons (731 mg HC/g TOC) at low maturation temperatures (340 °C). The hydrocarbon (HC) group compositions are different between the expelled oil (with more aromatic HC and saturated HC) and the residual oil (with more resin fraction and asphaltene). The Pr/Ph ratio (up to 4.01) of the expelled hydrocarbons was much higher than that in residual oil (<1.0). Maturity-related biomarkers Ts/(Ts + Tm), and αααC29-20S/(20S + 20R) and C29-αββ/(ααα + αββ), also showed complicated variations with pyrolysis temperature, especially at post peak oil generation. C27-, C28-, and C29- sterane distributions showed variations with pyrolysis temperature. Therefore, without considering the influence of maturity on the abundance of compounds, either source, maturity and/or organic matter type from the chemical characteristics may not be correct. Full article

Shale gas accumulates in reservoirs that have favorable characteristics and associated organic geochemistry. The Wufeng-Longmaxi formation of Well Yucan-6 in Southeast Chongqing, SW China was used as a representative example to analyze the organic geochemical and reservoir characteristics of various shale intervals. Total organic carbon (TOC), vitrinite reflectance (Ro), rock pyrolysis, scanning electron microscopy (SEM), and nitrogen adsorption analyses were conducted, and a vertical coupling variation law was established. Results showed the following: the Wufeng-Longmaxi formation shale contains kerogen types I and II₂; the average TOC value at the bottom of the formation is 3.04% (and the average value overall is 0.78%); the average Ro value is 1.94%; the organic matter is in a post mature thermal evolutionary stage; the shale minerals are mainly quartz and clay; and the pores are mainly intergranular, intragranular dissolved pores, organic matter pores and micro fractures. In addition, the average specific surface area (BET) of the shale is 5.171 m²/g; micropores account for 4.46% of the total volume; the specific surface area reaches 14.6%; and mesopores and macropores are the main pore spaces. There is a positive correlation between TOC and the quartz content of Wufeng-Longmaxi shale, and porosity is positively correlated with the clay mineral content. It is known that organic pores and the specific area develop more favorably when the clay mineral content is higher because the adsorption capacity is enhanced. In addition, as shale with a high clay mineral content and high TOC content promotes the formation of a large number of nanopores, it has a strong adsorption capacity. Therefore, the most favorable interval for shale gas exploration and development in this well is the shale that has a high TOC content, high clay mineral content, and a suitable quartz content. The findings of this study can help to better identify shale reservoirs and predict the sweet point in shale gas exploration and development. Full article

At present, there are two main methods for solving oil and gas seepage equations: analytical and numerical methods. In most cases, it is difficult to find the analytical solution, and the numerical solution process is complex with limited accuracy. Based on the mass conservation equation and the steady-state sequential substitution method, the moving boundary nonlinear equations of radial flow under different outer boundary conditions are derived. The quasi-Newton method is used to solve the nonlinear equations. The solutions of the nonlinear equations with an infinite outer boundary, constant pressure outer boundary and closed outer boundary are compared with the analytical solutions. The calculation results show that it is reliable to solve the oil-gas seepage equation with the moving boundary nonlinear equation. To deal with the difficulty in solving analytical solutions for low-permeability reservoirs and numerical solutions of moving boundaries, a quasi-linear model and a nonlinear moving boundary model were proposed based on the characteristics of low-permeability reservoirs. The production decline curve chart of the quasi-linear model and the recovery factor calculation chart were drawn, and the sweep radius calculation formula was also established. The research results can provide a theoretical reference for the policy-making of development technology in low-permeability reservoirs. Full article