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In this paper, the potential seven sets of geothermal resources in the Sichuan Basin were evaluated by using a method for rapidly calculating geothermal resources in the sedimentary basins by multi data fusion, and suggestions on geothermal development and utilization are made based on the basic characteristics of abandoned wells in the oil and gas fields. The results show that the Sichuan Basin is rich in geothermal resources. The total resources of 7 sets of thermal reservoirs reach 4.26 × 1013 GJ, of which the Qixia-Maokou Formation has the largest potential geothermal resources, reaching 1.36 × 1013 GJ. The Permian Qixia-Maokou Formation in southern Sichuan Basin has the conditions to prioritize the development of geothermal resources and has the characteristics of the highest temperature of formation water and the largest current and cumulative water production. At the same time, according to the formation temperature of abandoned oil and gas wells, they are divided into two types of wells, and different geothermal resource utilization methods are proposed. The research results can provide a model for the evaluation of geothermal resources in sedimentary basins and a resource basis for the development of geothermal resources in the Sichuan Basin. Method diagram for fast calculation of geothermal resources in the sedimentary basins by multi data fusion. [Display omitted] [ABSTRACT FROM AUTHOR]

• A new research framework for identifying the key Ecological nodes was proposed. • Achieved dynamic analysis on ecological networks and their components. • Weighted complex networks have been used to obtain ecological network topology information with higher credibility. • Rethinking landscape function from the perspective of animal movement. Regional ecological security is a pressing issue in the context of escalating human-environment conflicts. Ecological networks(ENs), the fundamental tool for characterizing ecosystems, have enabled further quantitative analysis at the micro level by integrating with complex networks in recent years. However, most studies neglect the unreliability of unweighted complex networks and the dynamic characteristics of ENs. This paper takes the Sichuan Basin as the research area and adopts the following methods. Firstly, it integrates landscape ecology and ecosystem services to construct the ENs using Linkage Mapper. Secondly, it introduces the cost-weighted distance as the weight to build complex networks and identifies potential pivot ecological sources and key ecological corridors based on the topological features of the weighted complex networks in 2000, 2010, and 2020. Thirdly, it applies circuit theory to detect ecological pinchpoints and ecological barrier points within the corridors as priority areas for ecological conservation and restoration. The results show that the ENs in the study area are denser on the northern and southern sides, and some ecological corridors change direction due to variations of resistance surfaces and landscape morphology. Through dynamic analysis of the weighted complex networks, 27 potential pivot ecological sources and 25 key ecological corridors are identified; then, 28 priority conservation areas and 10 priority restoration areas within these ecological corridors are extracted based on circuit theory. The study reveals a certain correlation between the distribution of ecological nodes and water bodies. Furthermore, comparing the weighted and unweighted complex network, we find that the weighted complex network is more reasonable, with 64.2% of ecological sources showing lower betweenness centrality than that in the unweighted network, reflecting the obstacles that urbanization poses to ecological networks. This study explores the impact of constantly changing resistance surfaces on the overall ENs and their components through dynamic analysis. The evolving topological features reflect the feedback of the ENs to external environmental changes, as well as the dynamic characteristics of the real ENs. Therefore, the findings of this study provide valuable references for ecological conservation and governance efforts in Sichuan Basin, promoting regional ecological security and the advancement of ecological civilization. [ABSTRACT FROM AUTHOR]

Yibin area is located in the southwestern margin of Sichuan Basin of upper Yangtze River basin, close to the southeastern boundary of the Qinghai-Tibetan Plateau. The paleoclimatic characteristics recorded by Quaternary river sediments in this area can display the impact of uplift of the Qinghai-Tibetan Plateau on paleoclimate change of Sichuan Basin. In this paper, the macro-characteristics, clay minerals and the diagnosis geochemical elements characteristics of the sediments of terraces and floodplain were used as climatic proxies to explore the evolutionary process of the Quaternary paleoclimatic change in the study area. Key findings can be shown as follows: 1) warm and humid paleoclimate occurred during the Middle Pleistocene and the early Late Pleistocene (730–90 ka), shown by the high values of Al 2 O 3 , Fe 2 O 3 , Rb/Sr, chemical index of alteration (CIA, 82–97), illite chemical index (ICI, >0.5), kaolinite and the low values of Na 2 O, K 2 O, MgO, K 2 O/Al 2 O 3 , (CaO + Na 2 O)/Al 2 O 3 , illite, smectite, chlorite from the brown or brownish-red sediments of the terraces T5 to T3. 2) The reversal values of these geochemical and clay mineral proxies (e.g., values of CIA between 50 and 64) derived from grey and greyish-yellow sediments in terrace T2 and floodplain denote the dry and cold paleoclimate, emerging in the middle-late Late Pleistocene (50 ka), corresponding to the Baiyu glacial period of the Qinghai-Tibetan Plateau. Comparing the paleoclimatic evolutionary characteristics in Yibin area with those of the Three Gorges area of northeastern margin of Sichuan Basin from previous researches, this paper considers that during the Gonghe tectonic movement (0.15 Ma), the uplift of the Qinghai-Tibetan Plateau and its southeastern boundary intensified the Plateau winter monsoon and caused the southwesterly withdrawal of the Indian monsoon from Sichuan Basin. Consequently, the warm and humid paleoclimate at the northeastern and southwestern margins of the Basin had successively become dry and cold in the early and middle-late periods of the Late Pleistocene. • Paleoclimate information can be well preserved in terrace sediments in Yibin area. • The paleoclimate in Yibin area had changed from warm and wet to cold and dry since 50 ka. • Quaternary paleoclimate changed to dry and cold from northeast to southwest in the Sichuan Basin. • Tibetan Plateau's Gonghe tectonic movement enhanced the Plateau monsoon and blocked the Indian summer monsoon. • Influence of Indian summer monsoon weakened in the Sichuan Basin along the SW direction after 110 ka. [ABSTRACT FROM AUTHOR]

Clumped isotope thermometry is an effective tool which allows to understand the post-depositional alteration, including recrystallization and solid-state reordering, of carbonate rocks. However, as both processes are controlled by elevated temperatures, it is frequently a challenge to distinguish between the relative importance of these two processes in deeply buried carbonate successions (depth > 5000 m or formation temperature > 150°C). In this paper, we have established a clumped isotopic framework of paired dolomite and calcite in order to decipher the relative influence of solid-state reordering versus recrystallization and consequently offer an improved understanding of the mechanisms involved. The material studied was derived from two deeply buried successions (Jurassic-Ediacaran) drilled in the central Sichuan Basin of South China. The calcite and dolomite minerals from these cores were separated using buffered acetic acid, and based on a two-component mixing model, the carbon, oxygen and clumped isotopic (Δ 47) compositions of the two endmembers were determined. Based on an evaluation of solid-state reordering models, we conclude that micritic carbonates (dolomite and calcite) from the Jurassic to the Cambrian have been altered during burial by the geothermal gradient as determined by vitrinite reflectance. The Ediacaran micritic dolomite records a hydrothermal event in the transition between the Ediacaran and Cambrian. In contrast, the clumped isotopic temperature of Ediacaran saddle dolomites reflects subsequent hydrothermal activity during the Silurian and Devonian and shows compositional variations controlled by cooling rates. This study highlights the utility of the theoretical framework based on paired dolomite and calcite records to untangle overprinted signals of clumped isotopes in ancient carbonate rocks and gives insight to better understand the complexity of alteration processes in deeply burial carbonate successions. [ABSTRACT FROM AUTHOR]

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO 2 –Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO 2 –Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region. • The tectonic background of the Late Ordovician-Early Silurian was studied. • The northern margin of the Upper Yangtze Platform is an extensional background. • The extensional tectonics caused the cold seepage events. • The extensional fault controlled the distribution of volcanic clastic sediments. • Organic matter accumulation was influenced by the extensional fault depression. [ABSTRACT FROM AUTHOR]

In this paper, the genesis of dolomite in Longwangmiao Formation is analysed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ13C value, and higher δ18O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca2+ was consumed, resulting in an increase in the Mg2+/Ca2+ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata. • The highlights of this article are as follows: • This paper provides a case study of terrigenous clastic dolomite. • This paper analyses the genesis of the dolomites of Longwangmiao Formation and establishes a dolomitization model. • We analysed a large number of samples and provided basic geochemical data for Longwangmiao Formation. [ABSTRACT FROM AUTHOR]

The Nb-Ta-Zr-Hf-REY-Ga mineralization in the Late Permian coals in southwestern China has been well investigated by several studies and has generally been attributed to alkaline pyroclastics coupled with input from hydrothermal fluids. The Late Triassic is one of the important coal-forming periods but the critical element mineralization in coals of this geological age has not been well documented. In this paper, the mechanism of Nb-Ta-Zr-Hf-REE-Ga-Th-U mineralization in the K5 coal seam of the Upper Triassic Xujiahe Formation from the Gaosheng Mine, Sichuan Basin, southwestern China, was investigated. The K5 coal is a high-ash, low-sulfur, and low-volatile bituminous coal. The minerals in this coal include illite, kaolinite, quartz, and, to a lesser extent, anatase, calcite, chamosite, bassanite, and pyrite, along with traces of ankerite, rutile, and zircon. Compared with the averages of common world hard coals, the studied coals are enriched in critical elements including Zr (247 ppm), Hf (5.56 ppm), Nb (23 ppm), Ta (1.55 ppm), rare earth elements and yttrium (REY, 168 ppm), Ga (18.3 ppm), Th (14.1 ppm), and U (6.18 ppm) (all on a whole coal basis). The Al 2 O 3 /TiO 2 ratios, Eu anomalies, and the relationships of Al 2 O 3 /TiO 2 vs. Zr/TiO 2 and Nb/Yb suggest that the sediment-source rocks of the Gaosheng coals were Neoproterozoic and Early-Middle Triassic felsic-intermediate igneous rocks from the southeastern Qinling and Longmenshan Orogens, along with the Hannan and Micangshan-Dabashan Uplifts. Other probable sources are contemporaneous felsic-intermediate pyroclastics from the Late Triassic volcanic areas around the Sichuan Basin (i.e., East Kunlun Orogen, West Qinling Orogen, and East Tibetan Plateau). The relationships of Al 2 O 3 /TiO 2 vs. Zr/Al 2 O 3 , Al 2 O 3 /TiO 2 vs. Nb/Al 2 O 3 , Ga/Al 2 O 3 , and Al 2 O 3 /TiO 2 vs. LREY/Al 2 O 3 further indicate that the elevated concentrations of Nb(Ta)-Zr(Hf)-REY-Ga in the Gaosheng coal are probably sourced from both the erosion of sediment-source regions and contemporaneous pyroclastics. The illite-kaolinite aggregates with a vermicular texture and authigenic illite, quartz, anatase, pyrite, calcite, ankerite, barite, and REE-rich minerals, REY distribution patterns, as well as elemental variation trends of the coal seam, collectively indicate that the studied coals were likely influenced by multiple-stage infiltration of various solutions. These solutions leached the fresh pyroclastics and products of erosion derived from granites in the sediment-source regions, leading to redistribution of the critical elements (e.g., Zr, Nb, and HREY) between the roof, floor, and parting mudstones and the coal benches. The concentrations of (Nb,Ta) 2 O 5 , (Zr,Hf) 2 O 5 , and Ga in the coals are 107, 1093, and 57.6 ppm, respectively, close to or higher than the marginal grade of conventional ore deposits for these elements, and thus the Gaosheng coals have potential as raw materials for the recovery of the critical metals Nb, Ta, Zr, Hf, and Ga. [Display omitted] • The coal contains elevated concentrations of Nb, Ta, Zr, Hf, REE, Ga, Th, and U. • The sediment-source region of the coals was felsic-intermediate igneous rocks. • Elevated concentrations of critical elements in coal due to products of erosion and pyroclastics. • The coals are likely influenced by the multiple-stage infiltration of acid solutions. • The acid solutions leached the fresh pyroclastic and terrigenous materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pure microbial carbonates and marine fibrous cements can record the original Precambrian seawater chemistry. • Carbonate rocks of the Ediacaran Dengying Formation were deposited from seawater under suboxic conditions with high salinity. • Seawater oxygen concentration increased and salinity decreased gradually during the deposition of the Dengying Formation. • The suboxic and high-salinity seawater contributed to the development of the Precambrian microbial carbonates. Abundant microbial carbonates with well-preserved primary textures occur in the upper Ediacaran Dengying Formation in the Sichuan Basin, southwest China, providing an opportunity to study the paleoceanographic conditions and the origin of Precambrian microbial carbonates. However, paleoenvironmental studies of these microbial carbonates are relatively rare, especially with respect to the comparison between the upper and lower parts of the Dengying Formation. In this paper, we present new analyses of carbon and oxygen isotopes and major, trace, and rare earth elements (REEs) for carbonate rocks from the Dengying Formation, in order to reveal the paleoceanographic redox and salinity conditions of the seawater from which these rocks were deposited. These rocks have shale-normalized REE and Y patterns that display weak negative Ce anomalies, weak positive Eu anomalies, and moderate middle REE enrichment, indicating the suboxic seawater for their deposition. From the lower to upper parts of the Dengying Formation, negative Ce anomalies become more prominent, while positive Eu anomalies and middle REE enrichment weaken, suggesting a gradual increase in the seawater oxygen concentration. The heavier oxygen isotopes of the carbonate matrixes and fibrous cements in the second member of the lower part of the Dengying Formation reflect a higher seawater salinity, conforming to the thick evaporates in the second member. The elevated oxygen content and decreased salinity in the seawater promoted the emergence of metazoans that inhibited the growth of microbes, causing the decreasing development of microbial carbonates in the upper part of the Dengying Formation compared to the lower part. [ABSTRACT FROM AUTHOR]

Sichuan basin is a superimposed basin of marine sediment and continental sediment in Southwest China. The great potential of the deep reservoir in Sichuan basin motivated geophysical studies on the deep structure in this area. Limited by the penetration depth of seismic exploration, the gravity inversion is widely applied to study the deep structure and the density properties. This paper inverted the 3D density distribution of Sichuan basin based on gravity inversion with reweighted regularized conjugate gradient algorithm. The proposed algorithm was tested and verified with two synthetic models. For the application in Sichuan basin, an initial model based on former studies was built to reach a reasonable model in the shallow part structure. The inversion result gave the 3D density structure of Sichuan basin with 60 km in depth. It illustrated the basic layered structure and the horizontal inhomogeneous features from the sedimentary layer to the deep crust. The density features of 7–12 km in depth revealed the basement structure with three parts, which was consistent with the velocity structure on the whole. The high density feature of Dazu area might indicate the local mantle uplift. • 3D density of Sichuan basin is inverted with the reweighted regularized algorithm. • 3D density shows the layered structure and lateral heterogeneity of Sichuan basin. • Basement and deep crust of Sichuan basin are mainly divided into three parts. • High density in the deep part of Dazu area may indicate local mantle uplift. [ABSTRACT FROM AUTHOR]

[Display omitted] • The shale gas resource potential of the Longmaxi Formation was evaluated by using mud gas isotope logging technology. • The reservoir properties of shale were evaluated based on gas isotope fractionation effects. • The shale gas in the Changning area belongs to late kerogen cracking gas. • The lower part of the Longmaxi Formation has a better shale gas exploration potential. The Longmaxi Formation in the Sichuan Basin is one of the most important shale gas producing formations in China. Previous studies of shale gas were mostly carried out on based on laboratory analyses and did not provide effective guidelines for shale gas exploration. In this paper, we present a large amount of carbon isotopic data from mud gas isotope logging and rock-cutting headspace gas in wellsites. The δ13C 1 values showed a significant decrease at the bottom of the Longmaxi Formation. The δ13C 1 and δ13C 2 values in the Changning area were observed to be heavier than those in the Weiyuan and Jiaoshiba areas. The second half of the N216H27-3 horizontal well had obvious outgassing and carbon isotope fractionation effects. This indicates that the bottom of the Longmaxi Formation has better shale gas exploration potential. The shale gas of the Longmaxi Formation in the Changning area belongs to late kerogen cracking gas, where exploration prospects are poorer than those in the Weiyuan and Jiaoshiba areas. There were more oil cracking gas resources and better reservoir conditions at the end of the N216H27-3 horizontal well; therefore, drilling should be continued at this site. [ABSTRACT FROM AUTHOR]

Pores and throats are the key parameters for determining the pore structure and fluid flow behavior in tight sandstone reservoirs. In this paper, pores and throats of the tight sandstone reservoir of the Upper Triassic Xujiahe Formation in the northern Sichuan Basin were investigated using classical petrography, scanning electron microscopy, and confocal laser scanning microscopy. In addition, a method of pore as well as throat identification and quantitatification using high-pressure mercury injection (HPMI) measurements was worked out. The results show that clay mineral-related pores are well developed in lithic sandstone and feldspathic lithic sandstone in addition to pores produced by dissolution of calcite cement and clastic grains. Intergranular pores are connected by lamellar, curved lamellar, and narrow pore throats. The pores and throats associated with intragranular dissolution and clay minerals are mostly tube shaped. The rapid-increasing and slow-increasing profiles of the mercury injection curves represent the dominance of pores and throats, respectively, allowing to differentiate pores and throats. Calculations show, in addition to fractured lithic sandstone, that has micron-sized pore–throat systems, the other lithic sandstones possess dominatly nano-sized pore–throat systems. In case of feldspathic lithic sandstones, micron-sized and nano-sized pore throat systems are equally important. In micron-sized pore throat dominated reservoirs, the pore volume is slightly larger than the throat volume, whereas for the nano-sized pore throat system, the pore volume is considerably larger than the throat volume. The key factors that are favorable for the formation of lithic sandstone pore throat systems are early calcite dissolution, lithic fragments dissolution, and early chlorite cementation. In contrast the key factors that are favorable for the formation of feldspathic lithic sandstone pore throat systems are dissolution of feldspars and lithic fragments. • A new method to distinguish pores and throats within tight sandstone reservoirs is proposed. • Lithic and feldspathic lithic sandstone reservoirs host different pore and throat types. • Pore volume is slightly larger than throat volume in the micron-size pore network. • Pore volume is considerably larger than throat volume in the nano-size pore network. • Compaction, cementation, and dissolution directly affected the development of pore throat systems. [ABSTRACT FROM AUTHOR]

A new sauropod-dominanted dinosaur fauna, including several new species of sauropods and stegosaurs, was recently excavated from the Lower Shaximiao Formation in Yunyang, northeastern Chongqing, southwestern China. Although this dinosaur fauna could possibly provide significant clues for a better understanding of the basal sauropod's evolution and diversification of basal sauropods in the Jurassic, many aspects of this fauna are not well defined constrained in terms of detailed stratigraphy, geochronology, and comparison with similar faunas in other regions of the Sichuan Basin. This paper reports on the petrological features of this locality, using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS), with laser ablation-multi-collector-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb geochronology for a tuffaceous siltstone layer beneath the dinosaur fossil-bearing layers from the fossil site. The SEM and XRD results show a typical tuffaceous mineral composition of this layer. Zircon U-Pb data yielded a weighted average age of 166.0 ± 1.5 Ma, indicating a maximum burial age for this dinosaur fauna. This fauna is chronologically comparable with the Shunosaurus-Omeisaurus Fauna, which is primarily found in the central Sichuan Basin. • 166.0 ± 1.5 Ma as the maximum burial age for dinosaur fauna in Chongqing, SW China. • Precise age constraints on newly discovered dinosaur in Lower Shaximiao Formation. • Basal sauropods in Sichuan Basin are younger than biostratigraphic ages. • Providing age constraints on the radiation of basal sauropods in Jurassic. [ABSTRACT FROM AUTHOR]

An appropriate shut-in after fracturing can dramatically enhance performance of wells in shale gas reservoirs. In this study, a novel wettability index and mineral content curve based shut-in time optimization approach was proposed. First, a new experimental device for spontaneous imbibition of fracturing fluid under the conditions of formation temperature and confining pressure was designed. Then, we conduct a set of experiments on shale samples from the Longmaxi Formation (LF) in the Sichuan Basin to measure the water-wet index and determine the critical value of clay content. Finally, the low-field nuclear magnetic resonance (NMR) equipment was used to monitor the water flow in shale micro-pores, based on this, the shut-in time for different shale reservoirs was optimized. The experimental results show that LF shale hydration can induce new micro-fractures, and the fractures shape is mainly shale bedding fractures, with obvious directionality. The clay content is the key factor affecting water wettability of shale and the critical clay content value of type I and type II shale reservoirs was about 39.5%. The optimal shut-in time of type I and type II shale reservoirs are about 20 days and 15 days respectively. Field application of three wells showed that the average daily gas production was increased by 2–3 times. The research results can guide the development of shale gas reservoirs effectively. • In this paper, the water imbibition experiment of shale under the condition of formation temperature and pressure of shale reservoir was carried out, and the critical clay content affecting the imbibition capacity of shale was determined. • On this basis, the shut-in-time of different reservoirs types was further optimized. • The research results play an important guiding role in understanding the flow law of fracturing fluid in shale reservoirs. [ABSTRACT FROM AUTHOR]

This paper focuses on celestite-carbonate-cemented veins that developed in the Jialingjiang Formation of the Lower Triassic carbonates from the Libixia anticline in eastern Sichuan Basin, China. Comparative petrography and geochemistry of the host rock and vein minerals provide clues to the nature, origin, and evolution of the vein-cementing fluids. The host rocks are composed mainly of micrite dolomite which yields δ18O, δ13C and 87Sr/86Sr ratios close to the values that characterize contemporaneous seawater. Two generations of vein-mineralization were recognized based on textural relationships, fluid inclusion data and isotope geochemistry. The early generation is dominated by celestite and saddle dolomite, formed from hypersaline, Sr-enriched, hydrothermal fluids. The later generation is represented by calcites, precipitated from less saline fluids at lower temperatures. The 87Sr/86Sr ratios of vein dolomite and celestite vary from 0.7076 to 0.7081, significantly lower than those of host carbonate counterparts (0.7078 to 0.7081; average 0.7080). This isotopic discrepancy suggests that the majority of Sr was sourced from deeper sequences of the lower Triassic carbonates, probably associated with thermochemical sulfate reduction for Sr accumulation. It is argued that the fracture systems associated with the Huayingshan fault provided the heat source and served as conduits for migration of hydrothermal fluids through several hundred meters, and the final mineralization occurred as a result of mixture of Sr-enriched brines with sulfate-bearing solutions. The large celestite ore bodies in the studied area are proposed to form related partially to these hydrothermal fluids. [ABSTRACT FROM AUTHOR]

The Longmaxi Formation has abundant shale gas resources. High-yield wells have been obtained in medium-shallow and deep areas in the Sichuan Basin. Quantitative research on the brittle-ductile change in rocks is a basic geological component for caprock evaluation, and it can also be applied to shale gas exploration. This paper fully considers the difference between the lower shale (N1) and upper shale (N2) of the Longmaxi gas-bearing shale. Through total organic carbon (TOC), mineral components, field emission scanning electron microscopy (FE-SEM), and triaxial mechanics experiments, the two groups of shale samples, N1 and N2, were compared and analyzed, and the brittle-ductile transition was quantitatively studied. Based on previous work, a method for the fast calculation of the key over consolidation ratio (OCR) was proposed. The results show that N1 is mainly organic-rich siliceous shale, N2 is primarily organic-poor clay shale and mixed shale, and N1 is generally more brittle than N2. The key OCRs of N1 and N2 are 2.16 and 2.60, respectively. The key confining pressures at which N1 and N2 become fully ductile are 87.87 MPa and 70.28 MPa, respectively. Therefore, the key depths of the brittle zones of N1 and N2 (H b1 and H b2) and the key depths of the ductile zones of N1 and N2 (H d1 and H d2) are obtained. On this basis, a comprehensive model for the Longmaxi shale gas' brittle-ductile transition exploration zone in the Sichuan Basin and H b1 and H b2 maps of the Longmaxi gas-bearing shale have been established. Through comprehensive vertical and horizontal analysis, Longmaxi shale gas exploration risks in different blocks of the Sichuan Basin have been clarified. Therefore, this study of the variation in the brittle-ductile transition has an important guiding role in refining the exploration of Longmaxi shale gas in the Sichuan Basin. • There are obvious differences between the lower shale and the upper shale. • Difference of brittle-ductile transition makes shale gas exploration more complicated. • The new method can quickly calculate the threshold value of over consolidation ratio. • The new model provides more comprehensive guidance for shale gas exploration. [ABSTRACT FROM AUTHOR]

The estimated ultimate recovery (EUR) of a single shale gas well is one of the important evaluation indicators for the scale and benefit development of shale gas. Nevertheless, due to the lack of engineering and geologic data, as well as the limitations of the existing empirical methods, the current production forecasting methods cannot make reasonable predictions for EUR. Therefore, Duong method is used as the prototype in this paper, a rapid and accurate empirical model for improved production prediction is proposed based on the historical production profile analysis of mass producing wells in Sichuan Basin. The accuracy and universality of the new model are verified by production examples in the Weiyuan, Changning, Luzhou and Yuxi blocks of the Sichuan Basin. The examples illustrate that the new model can be applied well in typical wells, and has no limitation of flow regimes for the production prediction of gas wells. In order to solve the problem of production data fluctuation and decline curve deviation caused by stimulation measures in the production process, a multi-segment production prediction model analysis method is proposed based on the improved model in this study and has been well applied in the field. Most importantly, the new model has clear physical parameters and is easy to operate. Performance of the new model based on production data is as reasonable as the modern production decline methods (including analytical model), which is better than existing empirical methods. • Proposing an improved empirical model, which is rapid and accurate. • Less parameters were selected, the new model still has clear physical meaning. • Application condition of the new model is not limited by the flow regime. • Performance of the new model based on production data is as reasonable as the analytical model. • Multi-segment production prediction model is well applied to gas wells with production data fluctuation. [ABSTRACT FROM AUTHOR]

The upper Permian Longtan Formation (LTF) shale is a new shale gas exploration and development focus in China, which was formed in a transitional depositional environment. Because of the difference in the sedimentary environment between marine-continental transitional shale and marine shale, the enrichment characteristics and mechanism of organic matter (OM) are very different. Because of a weak research foundation, the enrichment mechanism of OM in the LTF shale in the Sichuan Basin is still unclear. In this paper, shale samples of the upper Permian LTF from the Well F5 were collected, and the abundance, source and sedimentary environment of organic matter were studied by using paleoclimate, detrital input, redox, paleoproductivity and sedimentation rate parameters. The results show that the TOC content of the LTF shale is high (average = 3.04%) and reaches the high to overmature stage (average = 2.46%), indicating a favorable source rock. The LTF shale was deposited in a humid and hot climate, which was conducive to the growth and prosperity of a large number of plants. The source of organic matter was mainly terrestrial plant fragments. The sedimentary environment of the LTF shale was oxic, it had a high sedimentation rate, which reduced the oxidation rate of OM, promoted the rapid deposition of OM, and ensured the preservation of organic matter. In summary, the enrichment of OM of LTF shale in the Sichuan Basin was affected by the sedimentary environment, which was complicated. The main controlling factors were detrital input, redox conditions and sedimentation rate. • The geochemical characteristics of Permian Longtan Formation (PLTF) shale in eastern Sichuan are analyzed. • The sedimentary paleoenvironment of PLTF shale in eastern Sichuan is reconstructed. • The main controlling factors of organic matter enrichment of PLTF in eastern Sichuan are studied for the first time. [ABSTRACT FROM AUTHOR]

Thermal regime and thermal history are of significant important in basin dynamics and hydrocarbon generation of source rocks. The sedimentary basins in three typical cratons of China, North China Craton (NCC), Yangtze Craton (YC) and Tarim Craton (TC), underwent completely different tectonic background and dynamic mechanism, which resulted in the differentials of their thermal regimes. In this paper, the thermal regimes of the Bohai Bay Basin in the eastern NCC, Tarim Basin in TC, and Sichuan basin in the Upper YC were studied. There has some lower thermal background in the area of western China than eastern side of the NCC. The mean heat flow value of Tarim basin is 42.5 mW/m2, 53.8 mW/m2 in the Sichuan basin and 66.7 mW/m2 in the eastern NCC. And the temperature at 10000 m ultra-depth in the Tarim Basin also shows lower values than that of Sichuan and Bohai Bay basins. The thermal history, modeled from thermal indicators of vitrinite reflectance (R o), equivalent vitrinite reflectance data (R equ), fission track and (U Th)/He data of apatite and zircon, evolved differently because of the different tectonic history in these basins. The Tarim basin underwent a cooling history with the heat flow of 55–65 mW/m2 in the Cambrian decreasing to the present-day of 40–50 mW/m2. The Meso-Cenozoic thermal history of the Bohai Bay basin experienced four evolutionary stages with two heat flow peaks in the late Early Cretaceous and in the Middle to Late Paleogene, which correspond to the destruction peaks of the eastern NCC and had heat flow values of 82–86 mW/m2 and 81–88 mW/m2, respectively. The Sichuan basin was stable with heat flow value of less than 65 mW/m2 from the Late Sinian to Late Paleozoic, and the heat flow increased rapidly to the peak value (75–100 mW/m2) at the end of Early Permian by thermal effect of the Emeishan mantle plume. From Mesozoic to present, the heat flow decreased to the present value (60–70 mW/m2). In addition, the comparation of thermal characteristics of typical cratons around the world, including heat flow, thermal history, lithospheric thermal structure and thermal lithospheric thickness, also have been caried out. The differential thermal histories of Tarim and Sichuan basins resulted in the different thermal evolution of Cambrian source rocks. This study may provide some new geothermal evidence for hydrocarbon generation and phase distribution of oil and gas in the deep and/or ultra-deep basin. Thermal regime and thermal history of typical sedimentary basins in three typical cratons of China underwent completely different evolution. The Tarim basin underwent a cooling history, the Bohai Bay basin experienced four evolutionary stages with two heat flow peaks, and the Sichuan basin was stable with heat flow but to a peak at the end of Early Permian. Nansheng Qiu, Chang Jian, Zhu Chuanqing, Liu Wen, Zuo Yinhui, Xu Wei, Li Dan. [Display omitted] • Three typical cratons of China show great differences in heat flow and deep-temperature. • Three typical cratons of China show great differences in lithospheric thickness from 76–170 km. • Thermal histories of Bohai Bay, Sichuan and Tarim basins are summarized. • Thermal regime between stable and destabilized cratons are compared. • Three thermal evolution patterns of source rocks are obtained. [ABSTRACT FROM AUTHOR]

The lamellation fractures in marine shales are important to the accumulation and preservation of shale gas, whose characteristics and controlling factors are still unclear. This paper focus on the characteristics descriptions and controlling factor analyses based on core observations, thin sections, and scanning electron microscope (SEM) experiments. The marine shales of Paleozoic Wufeng Formation and Longmaxi Formation in the Fuling Area of Eastern Sichuan Basin are taken as an instance. The lamellation fractures are occurring along the direction of the lamellations, partially of them curved, bifurcated, and converged. The apertures of fractures typically range from 1 to 500 μm. Only a few (<1%) lamellation fractures are filled with quartz, calcite, pyrite, and bitumen. After a series of analyses, developments of lamellation fractures are controlled by total organic carbon (TOC), brittle minerals, laminae, and pyrite. (1) The organic matters control the formation and evolution of lamellation fractures by generating gas and resulting in overpressure. An increase in the TOC content of 1% can increase the lamellation fracture density by 0.26 cm-1. (2) Brittle minerals, especially authigenic quartz, is also an important controlling factor to the development of lamellation fractures. When BI increases by 10%, the lamellation fracture density increases by 19%. (3) Besides, siliceous and carbonate laminae in the shale are more suitable for lamellation fractures to develop. When the laminae density is smaller than 4 cm−1, it is easier to develop lamellation fractures with higher laminae density. However, too densely-distributed laminae will suppress the formation of lamellation fractures. (4) Higher pyrite content is good for the development of lamellation fractures. Lamellation fracture density will increase by 9%, with pyrite content increased by 1%. For the shale at the bottom of the Wufeng Formation - Longmaxi Formation, lamellation fractures develop extensively due to the high TOC and pyrite content, great brittleness, and moderate laminae density. The density of lamellation fractures decreases from the bottom to the top. • Quantitative characterize about development characteristics of the lamellation fractures. • The lamellation fractures is controlled by brittle mineral, TOC, laminae and pyrite. • TOC is the main one that affects the development of lamellation fractures. • Clarify vertical distribution characteristic of lamellation fractures in the Wufeng-longmaxi Fm. [ABSTRACT FROM AUTHOR]

The Emeishan mantle plume is supposed to play an important role in the Permian thermal regime and source rock hydrocarbon generation evolution in the Sichuan Basin, Southwest China. In this paper, we provide new valuable insights into the spatial thermal effect of the Emeishan mantle plume to the Sichuan basin and maturity evolution of the Paleozoic source rocks. Cambrian core samples from four wells were selected for zircon fission track and zircon (U–Th)/He analysis in order to determine their thermal histories. Then the thermal histories of typical wells throughout the basin were obtained based on the above result and previous studies. According to the peak heat flow value (q), four Permian geothermal units were classified: a stratospheric heat flow unit (q ≥ 90 mW/m2), a high heat flow unit (80 ≤ q < 90 mW/m2), a medium heat flow unit (70 ≤ q < 80 mW/m2), and a normal heat flow unit (q < 70 mW/m2). The interpretation of the abnormal Permian heat flow indicates that the enormous heat flow anomalies with values of >20 mW/m2 occurred in the stratospheric and high heat flow units was mainly caused by the mantle plume, while the weak Permian thermal perturbation with heat flow anomalies of <15 mW/m2 in other two units was more likely associated with crustal thinning. The thermal effect of the Emeishan mantle plume may be contained within the stratospheric and high heat flow units. By comparing the maturation histories of the Paleozoic source rock, we identified the thermal effect on the maturity evolution. The thermal perturbation had no obvious impact on the maturity evolution of the Lower Permian and Lower Silurian Longmaxi Formation. Only the Lower Cambrian Qiongzhusi Formation within high geothermal unit suffered significant thermal effect of the mantle plume and matured rapidly during the end of the Early Permian. • Four Permian geothermal units in the Sichuan basin were first classified. • New insight for the spatial thermal effect of Emeishan mantle plume. • Formation mechanism of Permian heat flow anomalies was identified. • The effect of mantle plume on the maturity evolution of source rocks was characterized. [ABSTRACT FROM AUTHOR]

Details of the Lower Cambrian succession of the newly-discovered intraplatfrom basin of the Yangtze Platform (South China), source rocks of the giant Ediacaran-Cambrian gasfields of the Sichuan Basin, including its stratigraphic framework, redox and organic matter accumulation histories, remain unresolved. This paper describes results of a petrological and geochemical investigation of Lower Cambrian strata near the center of the Sichuan Basin with the goal of elucidating those processes that controlled the sedimentary history of the succession. We recognize two intervals of organic-rich source rocks deposited in the intraplatfrom basin, including the Maidiping Formation and overlying Qiongzhusi Formation. The Maidiping accumulated in a carbonate platform setting during Fortunian-Early Stage 2 time (~541–526.5 Ma) and comprises a variety of rock types, including carbonate, chert, phosphorite, shale, and Mn-ore layers. Total organic carbon (TOC) value ranges from 0.47 to 5.06 wt.%; the highest quality source rocks are found in its lower member. The Qiongzhusi Formation was deposited as part of a clastic shelf during Middle Stage 2-Middle Stage 3 (~526.5–515 Ma) time. This unit is dominated by shale and argillaceous siltstone and displays TOC values varying from 0.30 to 4.28 wt.%. The most organic-rich deposits are found in its lower and upper members. Redox and hydrographic proxies suggest that the intraplatfrom basin was dominated by a strongly restricted environment and anoxic bottom water conditions that experienced intermittent episodes of euxinic or dysoxic-oxic conditions during the Fortunian-Middle Stage 3 time. However, bottom water conditions appear to have been oxic during Late Stage 2 time. Accumulation of Lower Cambrian source rocks across the Yangtze Platform appears to have been closely linked to depositional setting. Our findings suggest that organic matter accumulation in the intraplatfrom basin was fundamentally redox-controlled whereas productivity exerted the greatest control on deposition of organic matter near the slope-deep basin of the Yangtze Platform. In summary, Early Cambrian high-quality source rocks preferentially accumulated in topographic lows, including the intraplatfrom basin and slope-deep basin, an association useful to targeting conventional and unconventional hydrocarbons in the Sichuan Basin. • Stratigraphic framework of the Lower Cambrian deposits in an intraplatfrom basin on the Yangtze Platform were established. • The intraplatfrom basin was dominated by anoxic bottom water with intermittent episodes of euxinic/dysoxic-oxic conditions. • A model depicting controls on OM accumulation across the Yangtze Block during the Early Cambrian was proposed. [ABSTRACT FROM AUTHOR]