Lower Longmaxi Shale Research Articles

Multi-Scale Pore Structure Characterization of Silurian Marine Shale and Its Coupling Relationship With Material Composition: A Case Study in the Northern Guizhou Area

Investigation of pore structure is vital for shale reservoir evaluation and also “sweet spot” prediction. As the strong heterogeneity in pore types, morphology, and size distributions of organic matter-rich shales, it is essential to combine different approaches to comprehensively characterize them.Field emission-scanning electron microscopy (FE-SEM), low-pressure gas (CO2 and N2) adsorption, and high-pressure mercury intrusion (HPMI) were employed to systematically investigate the pore structure of the lower Longmaxi shale reservoirs in the northern Guizhou area. The results show that the shales can be divided into four lithofacies based on mineral composition, namely, siliceous shale (SS), clay shale (CS), carbonate shale (CAS), and mixed shale (MS), among which siliceous shale is the primary lithofacies of the Longmaxi shale. Numerous organic matter (OM)-hosted pores, clay interlayer pores, interparticle pores, and intraparticle pores were identified within shale reservoirs. The specific surface area ranges from 11.3 to 27.4 m2/g, with an average of 18.1 m2/g. It exhibits a strong positive correlation with TOC contents, suggesting that organic matter is the major contributor to the specific surface areas. A wide range of pore size distribution was measured by integration of gas adsorption and HPMI. It is shown that the pore size is primarily distributed within ∼100 nm, corresponding to micropores, mesopores, and part of macropores. The total pore volume, which is mostly derived from the contribution of micropores and mesopores, remains within a range of 0.11 to 0.025 ml/g, with an average of 0.018 ml/g. Furthermore, the volume of micropores and mesopores is mainly controlled by organic matter contents. The dissolution pore contributes most to the macropore space within shale reservoirs, based on the positive correlation with macropore volume and easily dissolved minerals, including carbonate and feldspar. Also, the total pores volume is mainly dominated by organic matter and carbonate contents. This is possibly attributed to the easily dissolved and rigid features of carbonate, which can protect the primary interparticle pores due to its high compression resistance and is conducive to forming abundant dissolution pores. OM-rich carbonate-bearing mixed shale may be the most favorable lithofacies for gas storage in the northern Guizhou area.

Pore development of the Lower Longmaxi shale in the southeastern Sichuan Basin and its adjacent areas: Insights from lithofacies identification and organic matter

X-ray diffraction (XRD), field emission/focusing ion beam scanning electron microscopy (FE-/FIB-SEM), nitrogen gas adsorption (N2GA), and geochemical analyses were performed on the Lower Longmaxi shale in the southeastern Sichuan Basin and its adjacent areas to reveal the controls of shale lithofacies and organic matter (OM) on pore development. The results showed that the Lower Longmaxi shale primarily comprises three types of lithofacies based on shale compositions and sedimentary environment: OM-rich siliceous shale (ORSS), OM-moderate mixed shale (OMMS) and OM-lean argillaceous shale (OLAS). ORSS comprises high contents of OM and siliceous minerals (quartz and feldspar), deposited in a deep-water shelf and early semi-deep-water shelf environment, while OLAS consists of high clay mineral and low OM contents, primarily formed in a shallow-water shelf environment. OMMS witness a transition from ORSS to OLAS in sedimentary environment and shale components. OM hosts a higher volume of pores per unit weight compared to inorganic minerals, corresponding to 0.1878 ml/g and 0.00396 ml/g, respectively. OM-hosted pores are dominant in ORSS, constituting 72%, whereas OLAS mainly consists of mineral-hosted pores, comprising 63.2%. OM-hosted and mineral-hosted pores in OMMS exist in similar proportions, namely, 56.4% and 43.6%, respectively. Differentiated diagenetic paths resulting from sedimentary environment are responsible for the differences in pore development under various shale lithofacies. The thermal maturity, macerals, and abundance of OM all play a significant role in the morphology and development of OM-hosted pores. OM-hosted pores are primarily founded in migrated OM (mainly sapropelic type), instead of structured OM (mainly humic type), in the form of secondary pores that are derived from dry gas-generation stage. Furthermore, OM-hosted pore development in shale gas reservoirs is slowed or even reduced because of the weakening of shale's resistance to compaction resulting from excessively high OM content.

Factors influencing shale gas accumulation in the lower Silurian Longmaxi formation between the north and South Jiaoshiba area, Southeast Sichuan Basin, China

Abstract Owing to successful shale gas exploration and development, wells were drilled into the Longmaxi shale formation in the Jiaoshiba area, Southeast Sichuan Basin, China. However, shale gas production in Longmaxi varies greatly between the North and South Jiaoshiba areas. To determine the factors controlling the shale gas accumulation mechanism, the total organic carbon (TOC) content, mineralogy, porosity, permeability, total gas content, gas preservation conditions, and tectonic activities of Longmaxi Shale in both the North and South Jiaoshiba areas were studied in detail. The results show that the concentration of organic matter and minerals in the Longmaxi shale differed slightly between the South and North Jiaoshiba areas, and the lower Longmaxi Shale (Long 1 Member), which is used in commercial shale gas production, is characterized by a high quartz content and low clay content. In addition, the Longmaxi shale in the North Jiaoshiba area is characterized by high porosity and low permeability, while that in the South Jiaoshiba area has low porosity and high permeability. Two different reservoir types were identified in the Jiaoshiba area. The major storage spaces are pores in the Longmaxi shale reservoirs located in the North Jiaoshiba area, while the storage capacity of shale reservoirs in the South Jiaoshiba area is predominantly provided by fractures. Shale gas mainly exists as free gas in the Northern Jiaoshiba area due to the good gas preservation conditions. However, owing to the development of abundant faults in the South Jiaoshiba area, a large amount of free gas was lost, causing absorbed gas to dominate the shale reservoir in this area.

Effect of sedimentary environment on the formation of organic-rich marine shale: Insights from major/trace elements and shale composition

Sedimentary environment and redox conditions play a significant role in the formation of organic-rich shale. The vertical variations of major/trace elements, TOC (total organic carbon) and mineral compositions were investigated for the Lower Silurian Longmaxi typical marine shale in the Upper Yangtze Platform, South China, to decipher sedimentary environment (redox conditions, detrital input, sedimentation rate (SR), paleoproductivity and watermass restriction). This gives insight to the effects of variations in sedimentary environment on the formation of organic-rich shale. The results show that the Lower Longmaxi Formation is composed of three systems tracts: TST (transgressive systems tract), EHST (early highstand systems tract) and LHST (late highstand systems tract). From TST to LHST, the contents of TOC, quartz and pyrite decline steadily, whereas clay minerals and carbonate show an increasing trend. Thus, organic-rich siliceous (ORSS), organic-moderate mixed (OMMS) and organic-lean argillaceous (OLAS) shales are dominant for TST, EHST and LHST, respectively. Due to a continues fall of the relative sea-level, a deep-water shelf environment with anoxic condition, high paleoproductivity and low detrital input in TST gradually evolves into a semi-deep-water environment with dysoxic-oxic conditions, moderate paleoproductivity, and moderate detrital influx in EHST, further to an environment of oxic condition, low paleoproductivity, and high detrital flux in LHST. As evidenced by Mo-TOC and MoU patterns, ORSS was deposited in a moderately restricted watermass, weaker than Black Sea, which progressively evolved into an enhanced restriction degree during the deposition of OMMS and OLAS due to a fall of relative sea-level. Major/trace elements and excess silica (EX-SiO2) concentrations illustrate that origins of quartz in the Lower Longmaxi shale are primarily terrigenous, biogenic and authigenic (smectite illitization). The average proportions of EX-SiO2 (biogenic and authigenic quartz) are 52.34%, 24.86% and 6.57% for ORSS, OMMS and OLAS, respectively. Biogenic quartz is the dominant contributor to EX-SiO2, while the contribution of authigenic quartz is limited, and the formation of the former is earlier than the latter according to the diagenetic transformation temperature. As both biogenic quartz and organic matter are firmly related to the abundance and preservation of paleoorganisms, anoxic condition, high paleoproductivity, low detrital flux and moderate restriction collectively control the high enrichment and preservation of organic matter and biogenic quartz, while oxic condition, low paleoproductivity, high detrital flux and stronger watermass restriction are unfavorable for the enrichment of biogenic quartz and organic matter.

Characterizations of pore, mineral and petrographic properties of marine shale using multiple techniques and their implications on gas storage capability for Sichuan Longmaxi gas shale field in China

Pore, mineral and petrographic properties of shale conjunctionally determine the gas storage and transport properties of gas shale reservoirs. To investigate how these characteristics and pore structure influence the methane adsorption capability of shale formation, a total of forty-nine over-matured shale outcrop samples, thirty samples from Upper Longmaxi Formation and nineteen samples from Lower Longmaxi Formation from southern Sichuan Basin in China, were collected. Multiple techniques, including geochemical and mineralogical measurements, field emission-scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), low-pressure CO2 and N2 adsorption and high-pressure methane adsorption, were employed to characterize the geo-properties, pore structure and their impacts on methane adsorption capacity under different temperatures. Geochemical and mineralogical results show that both Upper and Lower Longmaxi shales are over-matured which have oil-prone type I kerogen. Based on the FE-SEM observation, most of the organic matter (OM) pores in the tested Longmaxi shales are spongy OM pores due to the over-maturity stage with Ro between 2.36% and 3.15%. In general, Lower Longmaxi shales are rich in micropores with larger pore volume and specific surface area (SSA) compared to those of the Upper ones. A combination of CO2/N2 adsorption and MIP analyses on the pore structure show a whole-aperture pore size distribution (PSD) ranging from 0.4 nm to 10 μm. The methane adsorption capacity increases with increasing TOC for Lower and Upper Longmaxi shales, respectively, while increases with increasing SSA for combined Lower and Upper ones. It is noted that, the numerous OM pores in Lower Longmaxi shale are the primary contribution to a better pore network and greater methane adsorption capacities compared to that of the Upper Longmaxi shale, but it all depends on the surface area. Shale samples with higher surface area have higher methane adsorption capacity no matter which Longmaxi shale Formations. In addition, temperature has an apparently negative effect on methane adsorption capacity for both Upper and Lower Longmaxi shales, where temperature is more sensitive to the shale sample with higher TOC content.