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The dolomite reservoir of the Middle Permian Qixia Formation in the Shuangyushi structure in northwestern Sichuan Basin is deeply buried, thin, and highly heterogeneous. With low-resolution seismic data, it is not easy to identify the reservoir signal and it is difficult to accurately describe the plane distribution of thin reservoirs. Based on high-resolution processing of seismic data, well-seismic combination technology, forward simulation technology, and cross-well profiles, in this study, we defined the waveform characteristics that correspond to different modes of reservoir development, summarized the seismic response modes of reservoirs in the area, optimized sensitive attributes, and finely depicted their plane distribution. The ability of seismic data to identify thin reservoirs was significantly improved after high-resolution processing. According to a comprehensive analysis, the reservoirs of the Qixia Formation in the study area had two response modes: "weak peak" and "complex wave." The waveform clustering attribute was optimized to describe the plane distribution characteristics of the Qixia Formation reservoirs in the study area. Type-I and type-II waveforms were favorable waveforms for reservoir development, and they mainly were distributed in the west of the Devonian denudation line, in a strip along the north–south direction, and mostly along faults. The predictions made with this method agreed well with the actual reservoir development, which provided a reference for the prediction of deep and thin reservoirs in this area. [ABSTRACT FROM AUTHOR]

Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites. [ABSTRACT FROM AUTHOR]

This study employs the Muller geological accumulation index method and the Hakanson risk assessment method in combination with ArcGIS technology to assess the potential heavy metal pollution in soil. The study aims to investigate the geochemical characteristics, pollution degree, and potential ecological risks associated with soil heavy metal element content and distribution across various geological formations. The concentrations of eight heavy metals, namely Cu, Cd, Ni, Pb, Zn, Hg, As, and Cr, were analysed by collecting samples of the soil layer, parent material layer, and parent rock layer. The proportions of samples with different risk levels are accurately calculated to locate areas with high pollution risks. The findings are as follows: Firstly, the soil pH value in the study area is low, and the Cd and Cr concentrations are 1.38 and 1.20 times the background concentrations in soil in Sichuan Province, respectively. Moreover, Cu, Cd, Ni, Zn, and Cr concentrations are 1.23, 1.10, 1.20, 1.10, and 1.55 times the background concentrations in soil in China. Secondly, the distribution of heavy metal elements varies across different geological formation units, and the heavy metal contents of different samples from the same formation also varies significantly. The concentrations of heavy metal elements are influenced by different geological formations, with the pedogenic parent rock in the soil determining the initial concentrations of heavy metal elements. Thirdly, the spatial distribution of heavy metal elements indicates a good spatial distribution coupling relationship among the soil layer, parent material layer, and rock layer. The study suggests that the contents of the eight heavy metal elements in the soil inherited the characteristics of the parent material to a large extent and are influenced and controlled by the pedogenic parent rock (geological formation). Finally, the evaluation results reveal that all eight heavy metal elements exhibit moderate pollution, with a relatively large distribution area exhibiting moderate Cd, Hg, and As pollution and above. The overall study area exhibits a mild potential ecological risk level, with some areas at a moderate potential ecological risk level, distributed in patches across the study area. Cd and Hg exhibit relatively high potential ecological risks. The potential ecological risks of heavy metals are relatively high in the JKms and Tss units. While the overall quality of the soil in the study area is good, heavy metal pollution and potential ecological risks of some elements are still above ideal levels. [ABSTRACT FROM AUTHOR]

The influence of karstification on gas reservoir formation is an area of interest in petroleum geology, and karst paleogeomorphology restoration is a topic of reservoir research. However, most previous studies of karst paleogeomorphology have focused on karst reservoirs, ignoring the influence of the karst paleogeomorphology on the overlying strata and gas accumulations. Therefore, the value of the petroleum geology information associated with karst paleogeomorphology is not maximized. The carbonate strata of the Leikoupo Formation were uplifted and exposed by the Indosinian movement, leading to the development of a weathered karst stratum (Lei 4 3 sub-member). Many commercial gas wells have been developed in the Lei 4 3 sub-member. Based on the latest drilling, logging, and two- and three-dimensional seismic data for the northwestern Sichuan Basin, we selected the residual thickness and the impression methods to restore the karst paleogeomorphology of the Lei 4 3 sub-member. The karst paleogeomorphology was generally high in the southeast and low in the northwest. The area can be successively divided into three secondary karst geomorphological units, namely, karst highlands, karst slopes, and karst depressions, and several tertiary geomorphological units. The karst geomorphology had the function of controlling reservoir, hydrocarbon source, and gas accumulation distribution. The karst processes were different in the various geomorphological units, further controlling the distribution and development of the karst reservoirs. The inherited geomorphology also controlled the sedimentation of the clastic rocks in the overlying formation. Due to the paleogeomorphological setting of higher in the east and lower in the west, the hydrocarbon generation intensity of the early Xujiahe Formation increases from the karst highland to the karst depression. Due to the cutting and lateral sealing of the formation by faults and karst valleys, there are no unified gas-water boundaries in the Lei 4 3 sub-member. As a result of the overall tectonic setting of high in the east and low in the west, the natural gas is enriched in the karst monadnocks. • Karst palaeogeomorphology at the top of the Leikoupo Formation was restored. • The karst geomorphology controlled the reservoir, source rocks and gas accumulation. • The gas accumulations have characteristics of each karst monadnock is a gas accumulation. [ABSTRACT FROM AUTHOR]

Seismic data are an integrated reflection of the sedimentary interfaces and processes of paleo formations. In this study, seismic geomorphology was applied to analyze and reconstruct the paleogeomorphology of the sedimentation of the Cambrian Longwangmiao Formation in the central Sichuan area by combining three-dimensional (3D) seismic data, a large amount of drilling data, logging data, cores and thin sections. It is believed that the thickness of the Cang-2 member reflects the paleogeomorphology during the sedimentation of the Longwangmiao Formation. Forward seismic modeling was used to select the seismic physical attributes sensitive to the Cang-2 member, which were then used to reconstruct the paleogeomorphology during the sedimentation of the Longwangmiao Formation. The results of this study show that the larger the amplitude of the bottom trough of the Longwangmiao Formation, the higher the landform during the sedimentation period. Therefore, the Caledonian paleo-uplift had already taken shape before the sedimentation of the Longwangmiao Formation, creating a paleo-geomorphologic pattern containing two uplifts and one depression. The geomorphologic distribution controlled the development of the shoals and reservoirs. The geomorphic highlands not only contained shoals with large areas and thicknesses but were also susceptible to superimposed syngenetic and quasi-syngenetic exposures, which provided preferential flow channels for atmospheric fresh water in the later period and caused the reservoirs to be subjected to more intense karstification. Due to the differences in the potential energy of the topography, the quality of the reservoirs in the NW slope zone is the best, followed by that in the NW high zone and the SE sub-high zone, while the shoals and reservoirs of the sedimentary geomorphic lowlands are poorly developed. • The RMS attribute of Longwangmiao Formation bottom trough was used to reconstruct the sedimentary paleogeomorphology. • The distribution of sedimentary facies has been predicted precisely under the constraints of paleogeomorphology. • Development model of the shoals and reservoirs in the Longwangmiao Formation in the study area was established. [ABSTRACT FROM AUTHOR]

Assessment of production potential and prediction of sweet spots in unexploited shale gas wells are crucial technologies for achieving a high success rate in drilling. Most existing methods are only applicable for yield forecasting of shale gas wells with known geological and drilling/completion factors after production. In addition, the shale gas reservoir in the Longmaxi Formation in the southern Sichuan Basin of China is characterized by low porosity, low permeability, and diverse gas reservoir states. Thus, forecasting production from unexploited wells and predicting the exact location of the sweet spot under complex geological conditions have become challenging topic for both academia and industry. According to the first law of geography, shale gas wells that are closer in space have more similar geologic features. Based on this foundation, a machine-learning prediction model based on spatial correlation (SC-MLPM) is proposed for evaluating potential production of shale gas wells in the Changning play located in Longmaxi Formation in the southern Sichuan basin, China. Specifically, an improved K -nearest neighbor algorithm (SC-KNN) based on the spatial correlation characteristics of the exploited wells is designed to estimate the relevant geological factors of the unexploited wells in the same area. In addition, an extreme gradient lifting model based on Huber loss function (Hu-XGB) is developed to predict the potential production of unexploited shale gas wells, and then the shale gas sweet spot area in the research area is predicted quickly and accurately. The experimental results on 88 shale gas wells from Changning play show that the mean relative error of the prediction results of the proposed method is 9.8%. Furthermore, the results also determine that the sweet spots of shale gas wells in the research area are mainly developed in the northwest and northeast directions, and gradually become worse in the southeast direction. Eventually, experimental results for the Fox Creek play in Canada show that the proposed model obtains better predictive performance as the number of gas wells increases, and also verifies the generalizability. This work provides a basis for the efficient development of the research area. • A machine-learning prediction model based on spatial correlation is proposed. • An improved KNN algorithm is designed on the first law of geography. • An extreme gradient lifting model is developed to predict potential production. [ABSTRACT FROM AUTHOR]

Parakmeria omeiensis is a Critically Endangered tree species in the family Magnoliaceae, endemic to south-west China. The tree is functionally dioecious, but little is known about the species' status in the wild. We investigated the range, population size, age structure, habitat characteristics and threats to P. omeiensis. We located a total of 74 individuals in two populations on the steep slopes of Mount Emei, Sichuan province, growing under the canopy of evergreen broadleaved forest in well-drained gravel soil. A male-biased sex ratio, lack of effective pollinating insects, and habitat destruction result in low seed set and poor seedling survival in the wild. We have adopted an integrated conservation approach, including strengthening in situ conservation, cultivation of saplings, ex situ conservation and reintroduction, to protect this species. The successful conservation of P. omeiensis has important implications for the conservation of the genus Parakmeria and the family Magnoliaceae. [ABSTRACT FROM AUTHOR]

The Upper Triassic Xujiahe (须家河) Formation in the Sichuan (四川) Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by oldlands in multiple directions including the north, east and south. Thus, the basin was generally characterized by coastal and widely open and shallow lacustrine deposition during the Late Triassic Xujiahe period. This is similar to the modern well-known Poyang (鄱阳) Lake. Therefore, we investigated the framework and conceptual model of the Sichuan Basin during the Xujiahe period with an analogue to the Poyang Lake. Results show that the conceptual model of the deposition can be divided into transgressive and regressive stages. The first, third and fifth members of the formation are in transgressive stage and the deposits are dominated by shore and shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have implications for understanding the large-scale sand deposition elsewhere. [ABSTRACT FROM AUTHOR]

Presented a study on shale gas reservoir of Longmaxi Formation of Lower Silurian in southern Sichuan Basin. The reservoir porosity was measured using nitrogen adsorption method. The pore volume, pore specific surface area and pore size distribution were studied through the isotherms and DFF analysis. Based on all of these, discussed the main factors to control the nanopore structure and the nanopore's influence on shale gas accumulation. The results show that, the pore structure of the shale gas reservoir of Longmaxi Formation is relatively complex, which is composed of nanopores and possesses a certain ruleless structure (the amorphous pore), pores are open in style and mainly consist of the cylinder pores with two ends open and parallel-plate pores with four sides open. It is concluded that openness degree of the pores improved with the increase of the vertical depth, the sizes of the main nanopores are in a range of 2 ~40 nm, which take up 88.39% of the total volume of pores and 98.85% of specific surface area. Most of the pore volume is occupied by the mesopores which are 2 ~ 50 nm in size. The micropores and mesopores of which the diameters are less than 50 nm dominate the main pore specific surface. The main internal factor to control the volume and the pore specific surface of nanopores of the shale gas reservoir in Longmaxi Formation is TOC content. It is also the important substance to provide the main storage space. The nanopores have a strong ability to absorb shale gas, the interior of which is structured filled with the shale gas, thus increase shale gas storage capacity and finally make the model characteristic complicated. The open nanopores can improve the shale gas desorbing efficiency and the permeability of the reservoirs, and then improve the shale gas production. [ABSTRACT FROM AUTHOR]

To evaluate the risk of induced seismicity due to hydraulic fracturing, a detailed stress analysis in a seismically active region, i.e., the Changning shale-gas field in the southern Sichuan Basin, was conducted utilizing sonic wave velocity logs and high-resolution electrical resistivity image logs of shale gas wells. Wells with relatively higher magnitudes of horizontal stresses are spatially located in the vicinity of thrust faults, in agreement with the stress concentration at the upper tip of the basement-involved thrust fault. The prevailing orientation of maximum horizontal principal stress (S Hmax) is N110°–120°E on the southwestern Changning anticline, which aligns with the motion of the Sichuan Basin, as driven by the continuous eastward extrusion of the Tibet plateau, which initiated at approximately 50 Ma. However, the S Hmax orientation is N30°–70°E along the northern Changning anticline, implying an inheritance of the N–S geodynamic compression during the Jurassic to Late Cretaceous. Drilling-induced tensile fractures primarily appear in compliant layers that are rich in clay or organic matter in wells located adjacent to thrust faults, suggesting that stiff layers can efficiently inhibit the hydrofracture growth height owing to more intense tectonic stress near thrust faults. A natural fault with a strike of approximately 30° to the S Hmax orientation can be more easily activated by extreme pumping pressure in wells that are weakly affected by the stress concentration along the thrust fault. Nevertheless, a cyclic injection of fracturing fluid with a gradually increasing target pressure may effectively reduce the magnitude of induced seismicity. • Detailed stress analysis of Changning shale-gas field using geophysical data. • Wells with higher horizontal stresses spatially located near thrust faults. • Drilling-induced tensile fractures appear in layers rich in clay or organic matter. • Stiff layers inhibit hydrofracture growth due to more intense tectonic stress. • Cyclic fluid injection with increasing target pressure may reduce seismicity. [ABSTRACT FROM AUTHOR]