Recently, significant achievements have been made in the gas exploration of marine Longmaxi shale in China. As exploration efforts have advanced, the exploration targets have gradually expanded to other sedimentary systems (marine-continental transitional and lacustrine). Compared with marine shale, shale in other sedimentary systems shows stronger heterogeneity, rendering previous exploration experiences of marine shale ineffective in guiding exploration efforts. Therefore, there is a pressing need for comparative studies to support future exploration practices. In this paper, the marine Longmaxi Formation and the marine-continental transitional Longtan Formation shales in the Lintanchang area of the southeastern of the Sichuan Basin are selected as the research objects. The study aims to compare the mineralogical characteristics, pore systems, and methane adsorption capacities of these two sets of shales, thereby revealing the differences in controlling factors that affect their physical properties and methane adsorption capacities. Our results show that the Longtan shale exhibits a higher clay mineral content, while the Longmaxi shale demonstrates significantly higher siliceous mineral content. Compare with Longmaxi shale, the Longtan shale exhibits a wider distribution range and higher average value of TOC content. The pore system in the Longmaxi shale is primarily dominated by organic matter-related pores, whereas the Longtan shale is characterized by clay mineral-related pores as the primary pore type. Given the variance in sedimentary environments, the controlling factors of physical properties differ significantly between the two sets of shales. In the case of the Longmaxi shale, TOC content is the most influential factor governing physical properties, while clay mineral content exerts the most significant influence on physical properties in the Longtan shale. Furthermore, TOC content emerges as the primary factor affecting methane adsorption capacity in both the Longmaxi and Longtan shales, despite the presence of significant variations in their pore systems. Nevertheless, the specific mechanisms through which TOC content impacts methane adsorption capacity exhibit variations between the two distinct shale types under investigation. The difference in sedimentary environment leads to various effects of mineral composition on methane adsorption capacity. Therefore, in the future research, the influences of different factors on methane adsorption capacity should be studied in combination with the sedimentary background.
Quartz is the main component of marine shale. However, the influence of quartz on shale gas accumulation is not yet fully understood, especially for deep shale reservoirs. In this paper, the deep Longmaxi shale recently drilled samples were taken from the Luzhou area of southern Sichuan, and their geochemical properties and pore characteristics were investigated, their quartz origin types were identified and quantified, and the effects of different kinds of quartz on the organic matter accumulation and physical properties of the shale were discussed. There are three types of quartz, namely, terrigenous quartz, clay-mineral-transformed quartz, and biogenic quartz in the shale. The biogenic quartz is concentrated in the siliceous shale, and the terrigenous quartz is dominant in the mixed shale. Different types of quartz have various effects on organic matter accumulation and physical properties. Biogenic quartz is positively related to total organic carbon (TOC), porosity, and brittleness. Clay-mineral-transformed quartz is not particularly conducive to the high TOC content and development of porosity. Although terrigenous quartz has a dilution effect on the TOC content, its effect on porosity is complex, with seeming dependence on lithofacies. Terrestrial quartz has a positive effect on the development and preservation of organic pores in the mixed shale, but it has no obvious impact on the siliceous shale. The high content of biogenic quartz in the studied shale indicates the enrichment of organic matter and excellent physical properties, which will be significant to conduct the exploration and development of deep and ultra-deep shale gas.
Significant achievements have been reported for deep oil and gas exploration in the Bohai Bay Basin. Among them, the Nanpu Sag has achieved good deep (greater than 3.5 km) hydrocarbon exploration results. However, the hydrocarbon formation mechanisms and the diagenetic evolution of these high‐quality sandstone reservoirs remain unclear. Mineralogical, petrographic, and fluid inclusion data were selected to investigate the deeply buried sandstone reservoir physical properties, pore systems, and diagenetic features and the main formation mechanisms of the deep high‐quality reservoirs of the Es1 (the top member of the Paleogene Shahejie Formation) sandstones in the Nanpu Sag, eastern China. The results show that the Es1 sandstone reservoirs are dominated by lithic arkoses and have low porosities (average porosity 13.4%) but medium to high permeability (average permeability 188 mD). The pore systems consist of primary intergranular pores, secondary dissolution pores, and microfractures. Loss of porosity is greater due to compaction (average 20.89%) than due to cementation (average 4.82%), and dissolution contributes an improvement in porosity of approximately 5.85%. Two peaks in the inclusion homogenization temperatures were observed: The first fluid inclusion homogenization temperature term ranges from 130°C to 145°C and exhibits yellow fluorescence; the other peak occurs from 155°C to 165°C, showing green fluorescence. Eodiagenesis is due to mechanical compaction, precipitation of early calcite cements and quartz overgrowths, and dissolution of feldspar and carbonate cements. Mesodiagenesis occurs due to mechanical compaction, framework grain (mainly feldspar) dissolution, and precipitation of quartz and late carbonate cements. The main factors controlling the formation of the deep high‐quality reservoirs are the presence of a high hydrodynamic depositional environment (subaqueous distributary channel), the type of provenance (Archean granites), and the creation of secondary porosity by feldspar dissolution.
Petrological analysis, thin-section observation and laboratory analysis data were selected to systematically study the physical and diagenetic features of the first member of the Paleogene Shahejie Formation (Es 1 ) in the No. 3 structural belt of the Nanpu Sag, Bohai Bay Basin. The intensities of different diagenetic processes were determined, the diagenetic evolution sequence was reconstructed, the typical diagenetic facies were identified and the effects of different diageneses on the reservoir were quantitatively analyzed. The results show that the main intergranular fillings include authigenic-quartz, quartz secondary enlargement, clay minerals, carbonate cement and matrix. The pore types include intergranular porosity, dissolution porosity and microfractures. The reservoir has experienced compaction, early cementation, dissolution and late cementation, among which compaction is the most important porosity reducer. Compaction was the main diagenetic process involved in porosity reduction, accounting for about 24.4% of the loss of thin-section porosity. The dissolution process clearly improved the porosity, increasing thin-section porosity by 2.7%. Five diagenetic facies were identified on the basis of petrographic analyses, namely, (a) strongly compacted-weakly cemented-weakly dissolved facies; (b) weakly compacted-strongly cemented-weakly dissolved facies; (c) moderately compacted-moderately cemented-weakly dissolved facies; (d) strongly compacted-weakly cemented-moderately dissolved facies; and (e) strongly compacted-weakly cemented-strongly dissolved facies. According to the analysis of diagenesis intensity, the porosity evolution model of various diagenetic facies was reconstructed, and the reservoir quality of various diagenetic facies was quantitatively predicted. The reservoir quality of different diagenetic facies clearly changed with depth. The best reservoir quality was in strongly compacted–weakly cemented–strongly dissolution facies, which have good sorting, contain a large amount of feldspar and soluble debris, and are mainly developed in the main part of the river channel. Our study can provide a reference for the subsequent exploration and development of deep petroleum systems.
Natural gas hydrate (NGH) has attracted much attention as a new alternative energy globally. However, evaluations of global NGH resources in the past few decades have casted a decreasing trend, where the estimate as of today is less than one ten-thousandth of the estimate forty years ago. The NGH researches in China started relatively late, but achievements have been made in the South China Sea (SCS) in the past two decades. Thirty-five studies had been carried out to evaluate NGH resource, and results showed a flat trend, ranging from 60 to 90 billion tons of oil equivalent, which was 2–3 times of the evaluation results of technical recoverable oil and gas resources in the SCS. The big difference is that the previous 35 group of NGH resource evaluations for the SCS only refers to the prospective gas resource with low grade level and high uncertainty, which cannot be used to guide exploration or researches on development strategies. Based on the analogy with the genetic mechanism of conventional oil and gas resources, this study adopts the newly proposed genetic method and geological analogy method to evaluate the NGH resource. Results show that the conventional oil and gas resources are 346.29 × 108 t, the volume of NGH and free dynamic field are 25.19 × 104 km3 and (2.05–2.48) × 106 km3, and the total amount of in-situ NGH resources in the SCS is about (4.47–6.02) × 1012 m3. It is considered that the resource of hydrate should not exceed that of conventional oil and gas, so it is 30 times lower than the previous estimate. This study provides a more reliable geological basis for further NGH exploration and development.
Abstract The Lower Jurassic Ziliujing Formation in China’s Sichuan Basin is a significant shale target for exploration; however, the strong heterogeneity of the properties of organic matter (OM) in shale makes it challenging to identify the target area for exploration, and the mechanism of OM enrichment is still unclear. Furthermore, the mechanisms of the response of the Da’anzhai member to the Toarcian Oceanic Anoxic Event (T-OAE) are controversial. Previous studies have focused on sedimentary facies analysis based on mineralogy and elemental abundances and have provided minimal information about organic geochemistry, which adds to the challenge of deeply understanding the influence of the T-OAE on the molecular geochemical characteristics of the Da’anzhai member. In this study, the Da’anzhai member of the Lower Jurassic Ziliujing Formation in the Langzhong area, Sichuan Basin, is studied via X-ray diffraction, total organic carbon, gas chromatography–mass spectrometry, organic carbon isotope, organic petrographical and pyrolysis analyses. To accurately identify the trend of the paleosedimentary environmental proxies, the Mann‒Kendall test is utilized to identify the trend of the data. Our results show that the Da’anzhai shale was deposited in a dysoxic transitional environment to an intermittent reducing environment with freshwater to brackish conditions. The response to the T-OAE can be identified in the middle and upper parts of the middle submember and the bottom of the upper submember of the Da’anzhai member. The T-OAE influenced the redox conditions, salinity, and OM origins during deposition in the middle of the Da’anzhai member, which resulted in the enrichment of OM. The abnormally high C 30 diahopane/C 30 hopane (C 30 D/C 30 H) ratio can be considered a potential proxy for locating the section of strata that responded to the T-OAE in the Da’anzhai member. In the study area, the mechanism of the response of the Da’anzhai shale to the T-OAE manifested as an improvement in hydrological cycling rather than a marine incursion. Our study provides new information that deepens the understanding of the mechanisms of the response of lacustrine shales to oceanic anoxic events from the perspective of molecular organic geochemistry.
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