Southern Sichuan Basin Research Articles

Detailed Imaging of a Seismogenic Fault That Potentially Induced the Two 2019 Weiyuan Moderate Earthquakes in the Sichuan Basin, China

Abstract Induced earthquakes are becoming a significant issue. Several destructive induced earthquakes (Ms ≥5.0) have occurred in shale gas fields located at the southwestern Sichuan basin, China, in the recent years. Specifically, two moderate earthquakes with magnitudes of Ms 5.4 (occurred on 8 September 2019) and 5.2 (occurred on 18 December 2019) exceeded the largest earthquake ever recorded in the Weiyuan. It remains unclear which seismogenic faults were responsible for the two moderate events due to insufficient knowledge of the subsurface structure and contradictory hypocenter depths obtained from different seismological methods. In this study, mainshock locations, focal mechanisms, and precise relocations of microseismicity were used to map the 3D geometry of the seismogenic fault. The result shows that the two moderate earthquakes occurred on the same fault. High-resolution seismic reflection profiles indicate several preexisting faults at various depths and a potential seismogenic fault. No large-scale faults are identified in the sedimentary cover above the Cambrian at the shallow centroid depth of these earthquakes. However, the seismic reflectors show phase discontinuous feature, indicating the existence of preexisting faults in the basement. A preexisting fault in the basement is consistent with a seismogenic fault that may have induced the two moderate earthquakes. Our study suggests that poroelastic stress propagation is likely responsible for the basement fault reactivation. The rupture on a stressed basement fault was triggered and produced two moderate earthquakes in a short period during hydraulic fracturing. The previous strategy of avoiding direct injection of fluids into preexisting faults to reduce seismic risk in operation region is not enough to avoid the possibility of the coupled poroelastic stress triggering deep fault rupture. We need to reevaluate the potential seismic risk of shale gas development in Weiyuan, the southern Sichuan basin shale gas field considering the preexisting faults in sedimentary and in basement and the effect of the fully coupled poroelastic stress field.

Enhanced 3D velocity structure, seismicity relocation and basement characterization of Changning shale gas and salt mining regions in southern Sichuan basin

Current extensive seismicity in southern Sichuan Basin is ascribed to the reactivation of pre-existing faults, as a result of prolonged fluid injection for salt mining and shale gas development, respectively. However, the structural framework of the region remains poorly understood. Here, we apply Vp/Vsconsistency-constrained double-difference seismic tomography to high quality phase data from 36,314 earthquakes jointly recorded by our local array and a regional seismic network to determine high-resolution velocity models. Earthquake relocations reveal shallow hypocenters for the Ms>5.0 earthquakes and two distinct seismogenic zones corresponding to the salt mine and shale gas regions, with most induced seismic events forming widespread lineaments some of which extend to the basement and are remarkably similar to the fault and fracture trends interpreted on reflection seismic and outcrops, respectively. Our 3-D crustal velocity analyses show that seismicity beneath the Changing salt mining area is associated with a combination of relatively low Vp/Vs(1.6–1.74) and high Vp/Vs(1.75–1.86) expressions, while most of small earthquakes within the Xingwen shale gas block are associated with relatively high Vp/Vsvalues (1.77–1.87), indicating the earthquakes in these two areas are caused by unique inducing mechanisms. The two moderately strong 2018 Xingwen Ms5.7 and 2019 Gongxian Ms5.3 earthquakes in the Xingwen shale gas block are located around low Vp/Vs. zones, suggesting they could be structurally controlled. In comparison, the 2019 Changning Ms6.0 earthquake in the Changning salt mining area is associated with high Vp/Vs. expression, suggesting its occurrence is related to fluid injections. In addition, top of the crystalline early Neoproterozoic (pre-Sinian) Sichuan basement is characterized by the 6.5 km/s Vpcontour, which is new for earthquake tomographic studies in the region. Combined with outcrop analysis, we are able to construct a structural framework for induced seismicity in southern Sichuan basin, which unravels the structural architecture of induced seismicity.

Research on the differential tectonic-thermal evolution of Longmaxi shale in the southern Sichuan Basin

The southern Sichuan Basin in China holds abundant shale gas resources; however, the shale gas bearing property shows great differences due to the multiple stages of tectonic transformation. The key to revealing the shale gas differential enrichment mechanism is to explore the thermal evolution characteristics during tectonic evolution. Therefore, taking the Luzhou and Changning blocks as an example, which have obvious differences in tectonic evolution, the organic geochemical conditions of Longmaxi shale were firstly compared with the test data. Then, the thermal evolution characteristics under the background differential tectonic uplift-erosion were recovered using basin modeling techniques. The results showed that the two blocks contain similar organic geochemical conditions of the Longmaxi shale. Moreover, the hydrocarbon generation condition in Luzhou Block is greater than that in the Changning Block. Influenced by the differential tectonic evolution, the study area experienced a complex burial history and the formation of multiple unconformities. As a result, the present burial depth of Longmaxi Formation in the Luzhou Block is significantly greater than that in the Changning Block. The thermal evolution history of Longmaxi shale in the study area could be divided into three stages, including a low-temperature stage from Caledonian to Hercynian, a middle-temperature stage from Hercynian to Indosinian, and a high-temperature stage from Yanshanian to Himalayan. In addition, it was found that the Himalayan period is the main stage resulting in the differential gas bearing property of Longmaxi shale in the southern Sichuan area. Under the differential structural modification, the peak time of hydrocarbon generation in the Luzhou Block occurred earlier and the conversion rate was slightly higher than that in the Changning Block. Cited as: Zhao, L., Mao, W., Liu, Z., Cheng, S. Research on the differential tectonic-thermal evolution of Longmaxi shale in the southern Sichuan Basin. Advances in Geo-Energy Research, 2023, 7(3): 152-163. https://doi.org/10.46690/ager.2023.03.02

Key factors affecting hydrocarbon accumulation in ancient dolomite gas reservoirs of Xixiangchi formation (Southern Sichuan Basin, China)

The Upper Cambrian Xixiangchi Formation in the Southern Sichuan Basin, China, has favorable hydrocarbon accumulation conditions. The accumulation factors and enrichment conditions of this formation were key considerations in this study. By analyzing core, thin section, seismic, and geochemical data, the research shows that there are many sets of granular and crystalline dolomite reservoirs in the Xixiangchi Formation vertically, with thin thickness of single-reservoir. During the transformation of karst and tectonism, dissolution pores and fractures developed to form an ideal reservoir space. The reservoir of the Xixiangchi Formation is connected to the Lower Cambrian source rock through a fault system. The high-energy shoal facies of the Xixiangchi Formation are located on the oil and gas migration path, providing an appropriate reservoir space for forming the source reservoir configuration relationship between the lower generation and upper reservoir. The key factors affecting hydrocarbon accumulation in the Xixiangchi Formation are sufficient oil and gas supply, development of inherited paleo-uplift, effective transportation system, and favorable reservoir-forming combination. The inherited paleo-uplift controls the distribution of gas reservoirs. Owing to the short migration distance of oil and gas, hydrocarbon is found near source hydrocarbon accumulation, and the paleo-uplift slope area should be targeted for exploration in future studies.

Well-logging evaluation of the shale gas potential in the lower Silurian Longmaxi Formation, Weiyuan, southern Sichuan Basin, China

Recent observations of shale gas breakthroughs in the Weiyuan marine shale gas play in the Sichuan Basin have attracted great interest. To better understand these breakthroughs, we have used core description, cyro-focused ion beam scanning electron microscopy data, X-ray diffraction data, organic geochemistry, and well-logging data to better understand the reservoir characteristics carbonaceous shale, calcareous shale, and siliceous shale lithology, with a focus on organic-rich shale units. We find that conventional well-log methods are effective in mapping the spatial distribution of organic-rich shale in the Weiyuan area where the total organic carbon (TOC) content in the Longmaxi Formation ranges from 1.35% to 6.95%, averaging 4.42%. The kerogen is type I-II, and the vitrinite reflectance (Ro) is greater than 2.57%, which indicates that the formation is susceptible to shale gas accumulation. The clay mineral content ranges from 48 to 63 wt% (avg. 51 wt%) with illite and chlorite averaging 73.8% and 25.7%, respectively. The brittle mineral quartz and plagioclase content ranges from 32 to 61 wt% (avg. 47 wt%). Compared to the surrounding lithologic units, the marine shale exhibits relatively high gamma ray, neutron, sonic, resistivity, potassium, and uranium (U) values and relatively low density, photoelectric effect, and thorium/U values, allowing us to construct crossplots to define the units of interest. Using the same process, we quantify the TOC content providing a spatial distribution of organic-rich shale using conventional well logging.

Magnetic Anomaly Characteristics and Magnetic Basement Structure in Earthquake-Affected Changning Area of Southern Sichuan Basin, China: A New Perspective from Land-Based Stations

The Changning area is located in the southern Sichuan basin and the western Yangtze Plate and is the most abundant shale gas exploration area in China. In recent years, Changning has experienced frequent earthquakes with moderate magnitudes, attracting extensive interest. To investigate the magnetic characteristics in Changning, 952 land-based stations were employed to establish a magnetic anomaly model with a resolution of 2 km, and the subsurface magnetic basement structure was obtained by an iterative algorithm in the Fourier domain. The magnetic anomaly model shows significant distinctions between the northern salt mine area and the southern shale gas area. The magnetic basement includes the crystalline basement and the Sinian sedimentary rock metamorphic basement, which has strong magnetism. The large intracratonic rift that developed in the Sinian–Early Cambrian plays an important role in the evolution of Changning, which also impacts magnetic anomalies and the magnetic basement structure. Finally, by comparing the seismic wave velocity ratio structure, the deeper magnetic basement that corresponds to the higher seismic wave velocity ratio can be explained. This article implies that magnetic anomalies and magnetic basement depth have a certain correlation with earthquakes in Changning, and it provides a geodynamic reference for Changning and the southern Sichuan basin.

Adsorbed and free gas occurrence characteristics and controlling factors of deep shales in the southern Sichuan Basin, China

Deep shale gas (3500–4500 m) will be the important succeeding field for the growth of shale gas production in China. Under the condition of high temperature and high pressure in deep shale gas reservoirs, its gas occurrence characteristics are markedly different from those of medium and shallow layers. To elucidate the gas occurrence characteristics and controlling factors of deep shales in the Wufeng-Longmaxi Formation, methane adsorption, low-temperature N2, and CO2 adsorption experiments were conducted. The results show that in deep shales, the mesopores provide approximately 75% of the total specific surface area (SSA) and 90% of the total pore volume (PV). Based on two hypotheses and comparing the theoretical and actual adsorption capacity, it is speculated that methane is adsorbed in deep shale in the form of micropore filling, and free gas is mainly stored in the mesopores. Correlation analysis demonstrated that TOC is the key material constraint for the adsorption capacity of deep shale, and micropore SSA is the key spatial constraint. Other minerals and mesopore parameters have limited effect on the amount of adsorbed gas. Moreover, the free gas content ranges from 2.72 m3/t to 6.20 m3/t, with an average value of 4.60 m3/t, and the free gas content ratio is approximately 58%, suggesting that the deep shale gas reservoirs are dominated by free gas. This ratio may also increase to approximately 70% when considering the formation temperature effect on adsorbed gas. Gas density, porosity, and gas saturation are the main controlling factors of free gas content, resulting in significantly larger free gas content in deep shale than in shallower formations.

Differential sedimentary mechanisms of Upper Ordovician-Lower Silurian shale in southern Sichuan Basin, China

The Wufeng-Longmaxi Formations are the focus of shale gas exploration in China. Their sedimentation is affected by global and regional geological events, and the resulting heterogeneity hinders the expansion of exploration results. This study combines logging, mineralogy, and geochemistry to reconstruct the sedimentary mechanisms of shale in the southern Sichuan Basin and discusses its implications for exploration. The shale can be divided into third-order sequences, in which the Wufeng Formation and lower Long-1 Member are transgressive systems tracts (TST1 and TST2), and the Guanyinqiao Member and the middle-upper Long-1 Member are highstand systems tracts (HST1 and HST2). Volcanic eruption and glaciation evolution are the driving factors of the sedimentary environment during the TST. The area is in a cold-warm climate, characterized by anoxic conditions, slow sedimentation, booming productivity, and low terrigenous influx, thereby depositing four types of siliceous shale. The siliceous shale is rich in biogenic microcrystalline quartz and high TOC, which are the keys to becoming the “sweet spot” of shale gas. Microcrystalline quartz fills the intergranular pores to support the entire pore network framework. The organic matter undergoes thermal evolution to produce abundant organic matter pores, which constitute the main place for adsorbed gas and free gas. For the highstand systems tract, HST1 is the product of the peak of glaciation, mainly composed of calcareous shale and carbonate rocks. The sedimentation of HST2 is affected by turbidity currents and violent orogeny. The area is in a warm climate, characterized by high terrigenous influx, dysoxic-oxic conditions, medium productivity and rapid sedimentation rates. The lithofacies gradually changed from siliceous shale to mixed shale and argillaceous shale. Excessive clay minerals and low TOC make it difficult for HST2 to realize the commercial development of shale gas.

“Extreme utilization” development of deep shale gas in southern Sichuan Basin, SW China

To efficiently develop deep shale gas in southern Sichuan Basin, under the guidance of “extreme utilization” theory, a basic idea and solutions for deep shale gas development are put forward and applied in practice. In view of multiple influencing factors of shale gas development, low single-well production and marginal profit of wells in this region, the basic idea is to establish “transparent geological body” of the block in concern, evaluate the factors affecting shale gas development through integrated geological-engineering research and optimize the shale gas development of wells in their whole life cycle to balance the relationship between production objectives and development costs. The solutions are as follows: (1) calculate the gold target index and pinpoint the location of horizontal well drilling target, and shale reservoirs are depicted accurately by geophysical and other means to build underground transparent geological body; (2) optimize the drilling and completion process, improve the adaptability of key tools by cooling, reducing density and optimizing the performance of drilling fluid, the “man-made gas reservoir” is built by comprehensively considering the characteristics of in-situ stress and fractures after the development well is drilled; (3) through efficient management, establishment of learning curve and optimization of drainage and production regime, the development quality and efficiency of the well are improved across its whole life cycle, to fulfil “extreme utilization” development of shale gas. The practice shows that the estimated ultimate recovery of single wells in southern Sichuan Basin increase by 10%-20% than last year.

The putative earliest Cambrian trilobite–brachiopod association in southern Sichuan Basin of South China

ABSTRACT Parabadiella is considered the earliest trilobite representative in South China and was originally recovered from the mid-upper part of the Guojiaba Formation, in Liangshan section. Previous studies suggested that the earliest brachiopod co-occurred with the oldest trilobite, and the Parabadiella–Eoobolus association from the Xihaoping Member (Cambrian Series 2, Stage 3), in southern Shaanxi was considered the first trilobite–brachiopod assemblage zone in South China. Therein, brachiopods occur together with trilobites recovered in Well JS103 seemingly extending the First Appearance Datum deeply in the lower Cambrian at the central Yangtze Platform. Well JS103 is a shale gas exploration well that targets the lower Cambrian Jiulaodong Formation in southern Sichuan Basin. The uppermost fossiliferous layers are siliciclastic rocks nearly 10 metres thick. Here, two distinct trilobite zones are found, including the lower Parabadiella Zone and the upper Wutingaspis–Eoredlichia Zone. Additionally, the lowermost part of the Guojiaba Formation is poorly fossiliferous, and ranges from 200 to 250 metres in thickness. Thus, the discovery of the brachiopod–trilobite assemblage provides new insights into the biostratigraphy in Sichuan and is of great potentials for biostratigraphic correlation of the siliciclastic rocks in the western margin of the Yangtze Platform.

Estimating high-resolution PM2.5 concentration in the Sichuan Basin using a random forest model with data-driven spatial autocorrelation terms

The Sichuan Basin (SCB) is severely polluted by fine particulate matter (PM2.5). Accurate PM2.5 concentration is important for pollution control and epidemiological studies. Evidence indicates that the distribution of PM2.5 is spatially clustered. Additionally, the high local variation in PM2.5 in densely populated areas indicates the necessity of high-resolution PM2.5 estimation. However, spatial clustering and local variation are not considered in current studies in the SCB, which may limit the prediction accuracy of PM2.5 estimation. In this study, we estimated the PM2.5 concentration at 0.01° (approximately 1 km) resolution using a random forest model with data-driven spatial autocorrelation terms (DDW-RF) considering both the first-law-of-geography-based similarity and spatial clustering of PM2.5. The repeated 10-fold cross-validations revealed that compared to the traditional RF model, the optimal model had an 18.31% decrease in the root mean square error (RMSE) and a 4.68% increase in the coefficient of determination (R2). The distribution of PM2.5 revealed another heavily polluted area in the northeastern SCB, including Nanchong and Dazhou besides the two commonly known heavily PM2.5 polluted areas in the western and southern SCB. Then, we built a downscaled model in the megacity Chengdu, which estimated PM2.5 at 0.001° resolution with a 0.156 μg/m3 (1.72%) decrease in RMSE compared to those of 0.01° estimations. The accurate and high-resolution PM2.5 estimates generated by DDW-RF and downscaled models in this study could be beneficial for accurate health effect estimation not only in the whole SCB but also in the city areas with high variable concentrations.

A Numerically Sensitive Study of Two Continuous Heavy-Pollution Episodes in the Southern Sichuan Basin of China

To explore the causes of pollution formation and changes in the complex topography of the Sichuan Basin, China, and improve the comprehensive simulation capability of pollution models, we use two online coupling models, WRF/Chem and WRF/CUACE, to simulate two heavy pollution episode that successively occurred in the southern part of Sichuan Province from 15 December 2016 to 11 January 2017 in this paper. Additionally, two sets of meteorological physics parameterization schemes MET1 and MET2 are designed, and four groups of experiments are carried out. The results suggest that the two models are good at simulating the static weather parameters such as temperature, low speed of winds and boundary layer height. The four groups of tests can accurately simulate the beginning, maintenance and turning point of the two pollution episodes’ life cycles. CUACE shows better performance in terms of higher correlation coefficients and lower errors in most of the particles and particulate components evaluations. It also performs better in the competitive mechanism of sulfate and nitrate against ammonium in the thermodynamic equilibrium mechanism. In addition, the evaluation of PM2.5 and the component simulation show that CUACE is more capable of simulating the mechanisms of heavy pollutions in southern Sichuan. Meanwhile, MET2 scheme is more appropriate for the simulation than MET1 dose. Based on the simulated concentrations of components and their precursors, the models overestimate the conversion of NO2 to nitrate but underestimate the conversion of SO2 to sulfate, which is the essential cause of the general overestimation of nitrate. Therefore, reducing the overestimation of nitrate is one major target for future model improvement.

Geochemical characteristics and genesis of natural gas in the Upper Triassic Xujiahe Formation in the Sichuan Basin

Abstract In recent years, the geochemical characteristics, genesis and sources of natural gas in the Upper Triassic Xujiahe Formation in the Sichuan Basin have received extensive attention, but their genesis and sources are still controversial. In this study, taking the natural gas from the Xujiahe Formation in the Sichuan Basin as an example, the source and genesis of the natural gas have been systematically analyzed. The results show that the natural gas of the Xujiahe Formation in the Sichuan Basin is dominated by methane, followed by a small amount of CO2 and N2; only the southern Sichuan area contains a small amount of H2S, which comes from the supply of the underlying carbonate source rocks. Except for the western Sichuan Basin, the drying coefficient of the natural gas is generally less than 0.95 (wet gas). Furthermore, the composition of the natural gas is mainly controlled by the maturity of source rocks. The carbon isotope of ethane in natural gas ranges from −33.9 to −21.5‰, and the hydrogen isotope of methane ranges from −188‰ to −151‰. The carbon and hydrogen isotope values are higher in the western Sichuan Basin than in the central, northeastern and southern Sichuan Basin. The identification of the origin of natural gas and the comparison of gas sources show that the natural gas in the Xujiahe Formation is mainly coal-derived gas from its own coal-measure source rocks; the natural gas in the northern part of the southern Sichuan Basin is oil-derived gas originating from the Changxing Formation and the Silurian marine source rocks; however, the natural gas in the northeastern Sichuan Basin is a mixture of coal-derived and oil-derived gases. In addition, the carbon and hydrogen isotopes in some natural gas samples from the Xujiahe Formation have inversions of δ13C1 > δ13C2, δ13C2 > δ13C3, δ13C3 > δ13C4, and δD2 > δD3, and the magnitude of the inversions is small. It is considered to be caused by the mixing of gases from the same source, as well as the mixing of coal-derived and oil-derived gases.

Characterization and Modelling of Multiscale Natural Fractures in Shale Reservoirs: A Case Study from a Block in the Southern Sichuan Basin

Natural fractures are vital to the efficiencies of drilling and completion operation. The purpose of this paper is to characterize and model multiscale natural fractures in shale reservoirs. Based on the seismic and log responses, as well as outcrop and core observations, we divide natural fractures into Macro, Meso, and Micro three scales. Macroscale fractures are the faults picked directly in seismic profiles. Also, Mesoscale fractures are the natural fracture corridors analyzed by ant tracking technique in 3D seismic data. Furthermore, Microscale fractures are the fractures observed in imaging logs and cores. The fracture intensity is obtained by the correlation between ant tracking attributes and fracture density in borehole. The fracture aperture, dip, and azimuth are three main parameters, which are recognized by the loggings and cores. Stochastic modelling is applied to factures. We find that faults identified by the ant tracking result are excellent in line with Macroscale faults interpreted directly from seismic data. In addition, Mesoscale fractures are indicated from the ant tracking result, which are in accord with breakpoint in the well and in keeping with tectonic history of the area. Such high consistency indicates the ant tracking result is reliable. Moreover, image logs and cores reveal that it mainly develops high angle natural fractures and the fracture aperture is about 1 mm. The fracture strike includes three sets (NNW-SSE, NE-SW, and NNE-SSW). The distribution of the natural fractures in discrete fracture network (DFN) system is distributed controlled by the ant tracking result. Comparing the histograms of DFN results and fracture characterized by seismic and logging responses, as well as outcrop and core observation, it suggests that the major part of the observed natural fractures is retained into our DFN model.

Quantitative evaluation and influencing factors analysis of the brittleness of deep shale reservoir based on multiply rock mechanics experiments

Due to a great increase in the plasticity of deep marine shale reservoirs in southern Sichuan under high-temperature and high-pressure conditions, the single brittleness evaluation method is difficult to effectively characterize its fracability, which significantly limits the selection of sweet spots and fracturing reconstruction in the area. In the case of the deep marine shale reservoir of the Wufeng-Longmaxi formations in the southern Sichuan Basin, through triaxial high-temperature and high-pressure experiments, fracture toughness and X-ray diffraction experiments, the mechanical properties and its influencing factors in the shale reservoir are studied, and the rock fracture morphology under various loading conditions is quantified. According to the morphological characteristics of shale, the analysis of influencing factors and comprehensive quantitative evaluation of the brittleness has been carried out. The deep marine shale resource in the southern Sichuan Basin is likely to be characterized by its high elastic modulus and low I fracture toughness. The mineral composition, temperature, pressure, and degree of bedding development are the primary factors for determining the brittleness; with high quartz mineral content (>50%), low confining pressure (<20 MPa), medium and low temperature (<60 °C) and high density of the shale bedding, the fractal dimension of the sample after the experiment is higher; whereas the geometry of cracks are mainly complex shear cracks, and brittleness is higher. The analytic hierarchy approach establishes a comprehensive evaluation index by analyzing the relationship between the normalized rock mechanical parameters, the stress–strain curve's brittleness index, and the fractal dimension. The brittleness of deep marine shale can be more accurately described by this evaluation index. The primary target layer for future shale gas exploitation, the 31 sublayer of the first member of the Longmaxi Formation, is shown to have a high brittleness index.

The new multistage water adsorption model of Longmaxi Formation shale considering the spatial configuration relationship between organic matter and clay minerals

The water adsorption by shale significantly affects shale gas content and its seepage capacity. However, the mechanism of water adsorption by shale is still unclear due to its strong heterogeneity and complicated pore structure. The relationship between the adsorbed water content at different relative humidities (RHs) and shale compositions, as well as shale pore structure and the spatial configuration relationship between organic matter (OM) and clay minerals, was investigated to clarify the controlling factors and mechanisms of water adsorption by Longmaxi Formation shale from the Southern Sichuan Basin in China. Consequently, the water adsorption process could be generally divided into three different stages from 0%RH to 99%RH. Furthermore, the Johnston's clay mineral interlayer pore structure model (JCM), the Freundlich model (FM) and the Dubinin–Astakhov model (DAM) were tested to fit the three water adsorption stages from low RH to high RH, respectively. The fitting results of the JCM and FM at lower RHs were far from good, while the fitting results of DAM at higher RHs were acceptable. Accordingly, two revised models (LRHM and MRHM) considering the spatial configuration relationship between OM and clay minerals were proposed for the two stages with lower RHs, and performed better fitting results indicating the pronounced effect of the spatial configuration relationship between OM and clay minerals on the water adsorption process of Longmaxi Formation shale. The outcomes of this study will contribute to clarifying the water distribution characteristics in the pore network of shale samples with variable water contents.

Influence of Multi-Period Tectonic Movement and Faults on Shale Gas Enrichment in Luzhou Area of Sichuan Basin, China

The Luzhou area in the southern Sichuan Basin has experienced multiple tectonic movements, forming a complex fault system; the activity has an important impact on the enrichment of shale gas in this area. In order to reveal the influence of the fracture system on the differential enrichment of shale gas, this paper takes the southern Sichuan Basin as the research object. The structural evolution process and fracture development characteristics of the different tectonic units in Luzhou area of southern Sichuan were characterized by conducting a seismic profile analysis, structural recovery using a back-stripping method, and core hand specimen description. We clarified the control effect of the structural deformation and fracture on the differential enrichment of shale gas, and we established a differential enrichment model of shale gas in the Luzhou area. The results show that: (1) The Luzhou area has undergone the transformation of a multi-stage tectonic movement. There are many sets of detachment structures in the longitudinal direction, and the plane structural form is a thin-skin fold-thrust belt composed of wide and narrow anticlines in the north–south direction. (2) The faults in the study area are affected by the Himalayan tectonic movement. The high-angle reverse faults are developed, and the number of large faults is small. The second and third faults are mainly developed. The second faults are only developed at the high position of the structure, which has a significantly destructive effect on shale gas reservoirs, while the third and fourth faults have no significant destructive effect on shale gas reservoirs. (3) In the study area, the types of cracks are categorized into transformational shear cracks, bed-parallel shear cracks, intraformational open cracks, lamellation cracks, shrinkage cracks, and abnormal high-pressure cracks. The thickness of the shale rock mechanical layer, brittle mineral content, and organic matter content jointly control the crack development degree in the shale of the Wufeng–Longmaxi Formation. (4) The uplift erosion, structural deformation, and fracture development caused by the structural evolution have affected the preservation of shale gas, resulting in the differential enrichment of shale gas reservoirs in the region. Based on the enrichment factors of shale gas, we established a differential enrichment model of shale gas in typical structural units and optimized the favorable enrichment areas, which are important contributions for guiding shale gas exploration and development in the Sichuan Basin.

Pore structure and fractal characteristics of the marine shale of the longmaxi formation in the changning area, Southern Sichuan Basin, China

The pore structure is an important factor affecting reservoir capacity and shale gas production. The shale reservoir of the Longmaxi Formation in the Changning area, Southern Sichuan Basin, is highly heterogeneous and has a complex pore structure. To quantitatively characterize the shale’s pore structure and influencing factors, based on whole rock X-ray diffraction, argon ion polishing electron microscopy observations, and low-temperature nitrogen adsorption-desorption experiments, the characteristics of the shale pore structure are studied by using the Frenkel-Halsey-Hill (FHH) model. The research reveals the following: 1) The pores of the Longmaxi Formation shale mainly include organic pores, intergranular pores, dissolution pores and microfractures. The pore size is mainly micro-mesoporous. Both ink bottle-type pores and semiclosed slit-type pores with good openness exist, but mainly ink bottle-type pores are observed. 2) The pore structure of the Longmaxi Formation shale has self-similarity, conforms to the fractal law, and shows double fractal characteristics. Taking the relative pressure of 0.45 (P/P0 = 0.45) as the boundary, the surface fractal dimension Dsf and the structural fractal dimension Dst are defined. Dsf is between 2.3215 and 2.6117, and the structural fractal dimension Dst is between 2.8424 and 2.9016. The pore structure of micropores and mesopores is more complex. 3) The mineral components and organic matter have obvious control over the fractal dimension of shale, and samples from different wells show certain differences. The fractal dimension has a good positive correlation with the quartz content but an obvious negative correlation with clay minerals. The higher the total organic carbon content is, the higher the degree of thermal evolution, the more complex the pore structure of shale, and the larger the fractal dimension. The results have guiding significance for the characterization of pore structure of tight rocks.

Natural Fracture Development Characteristics and Their Relationship with Gas Contents-A Case Study of Wufeng-Longmaxi Formation in Luzhou Area, Southern Sichuan Basin, China.

Natural fractures are critical factors that should be considered in shale reservoir evaluation, storage condition analysis, horizontal well design, and fracturing stimulation, which also play a non-negligible role in the occurrence state of shale gas in the reservoir. This paper discussed the influence of fracture development on gas-bearing properties based on the analysis results of core observation, scanning electron microscopy, mineral composition, and gas-bearing properties after the development characteristics of fractures and their longitudinal variation law were clarified. In this way, the development characteristics of organic-rich marine shale fractures in the Longmaxi Formation in the 203 well area of the Luzhou member of the Sichuan Basin and their effects on the gas-bearing properties can be analyzed. The results show that the Longmaxi Formation shale develops shear fractures, extensional fractures of tectonic origin, bedding fractures, dissolution fractures, and abnormally high-pressure fractures of nonstructural origin. Specifically, interlayer fractures, intercrystalline fractures, organic matter contraction fractures, and fractures between clay layers are microfractures. Fracture development is characterized by short longitudinal extension, small opening, high degrees of composite filling, and large density changes, with calcite and pyrite as the filling materials. The fracture density has a “three-stage” variation pattern longitudinally, and the bottom is dominated by thin siliceous shale development, together with a small amount of shale mixed with calcareous and calcareous materials. Moreover, the fracture is dominated by “splitting” and “shearing” failure, crossing stratification with the fracture density. The highest fracture density was found in the 2 sub-layer, featuring the joint development of horizontal and vertical fractures, which form the mesh fracture system through mutual cutting and restriction. The lithofacies in the 4 sub-layer are dominated by clay siliceous shale with a small amount of mixed shale of calcareous and siliceous materials. The formation of fractures always expands along the lamellation direction, which has concentrated development members of top and bottom fractures, with the development of horizontal fractures dominated and vertical fractures less developed. Furthermore, a synergistic effect can be found among the total organic carbon (TOC) content, fracture density, and gas-bearing property of the shale in Longmaxi Formation. It is worth noting that a high TOC content and siliceous content are conducive to the formation of microfractures, while the development of fracture contributes to the total gas-bearing property, especially to the increase in free gas content. To be concrete, the free gas content in the fracture development member accounts for more than 55% of the total gas content, thanks to a channel provided by fractures for the desorption of shale gas.

Qualitative–quantitative multiscale pore characteristics of black shale from the Upper Ordovician and Lower Silurian black shale in the Southern Sichuan Basin, China

Accurate investigation of shale pores is critical to the assessment of shale-gas reservoir properties. Qualitative and quantitative methods have been commonly conducted in shale pore studies. In this study, we take the black shale from the Upper Ordovician and Lower Silurian black shale in the Southern Sichuan Basin as an example. Both qualitative and quantitative methods were used to study the pores at multiple scales, and we discuss the effectiveness of shale pore detection by multiple methods. The following results were obtained: 1) The black shale is composed of quartz, feldsaper, calcite, dolomite and clay minerals in the study area, and its average TOC and porosity are 2.19 and 2.12%, respectively. 2) The CT results show that the pore size of macropores (pore size &amp;gt;0.5 μm) mainly ranges from 2 to 16 μm. Nitrogen adsorption analysis shows that organic-rich shale has abundant organic matter pores with narrow necks and wide bodies. The pores of organic-poor shale are mainly composed of mineral-associated pores with narrow slit shapes. The NMR results show that with increasing TOC, the left peak T2 relaxation times move leftwards, meaning that organic matter pores have a larger pore size than matrix pores at the mesopore scale. 3) CT data identify lamellation fractures and bed-cutting fractures, and their development is controlled by mineral type and content, TOC and laminae density. 4) The determination of segmentation thresholds in CT and transverse surface relaxivity in NMR seriously affects the accuracy of the results.