Deep shale gas reservoir (3,500 ∼ 4,500 m) of Wufeng–Longmaxi Formation in southern Sichuan Basin in China has great potential for exploration and development, with resource of 16.31 × 1012 m3, ...counting 84% of the total resources in this area. And the deep shale gas wells in Luzhou and west Chongqing have achieved good development results in the past two years. We systematically summarized the geological characteristics of deep shale gas reservoir of Wufeng–Longmaxi Formation, and discussed the facing challenges and proposed some corresponding strategies. The deep shale gas reservoir in southern Sichuan Basin has six major characteristics. The reservoir is concentrated in depositional center and deep-water shelf environment. The reservoir is rich in silica and organic carbon while low in calcium and clay. Organic pores, inorganic pores, and microfractures are developed and well interconnected to form networks. Surface porosity is generally > 5% and increasing with depth. The gas content is 4.7 ∼ 7.5 m3/t, higher than that in Changning, Weiyuan, and Zhaotong area. The deep shale gas pressure coefficient is greater than 2.0 and is the highest in the southern Sichuan Basin, indicating its best preservation condition. Up to now, deep shale gas exploration is still encountered with many problems and challenges, including great difficulty in accurately obtaining reservoir parameters, unclear of high production mechanism, urgent need for optimal and quick drilling technology, great difficulty in reservoir fracturing, and unclear of hydro-fracture expansion rules. We proposed two strategies corresponding to the challenges, including deepen the delicate reservoir evaluation and clarify the rules of high production and enrichment of shale gas wells, and strengthen field experiments and explore effective and applicable technologies for drilling and fracturing of deep shale gas in China.
•High pressure coefficient is the most important geological characteristic of deep shale gas in China.•Four main challenges about geology and engineering are encountered in deep shale gas exploration.•Two main strategies were proposed for deep shale gas development in China.
A continuously cored well covering the Carboniferous-Permian Benxi, Taiyuan, and Shanxi formations in the eastern Ordos Basin provides a rare window into environmental conditions of the ...marine-continental transitional strata. Integration of petrographic, mineralogical, and high-resolution geochemical data allows marine-continental investigating the depositional environment, organic abundance, detrital influx, paleoclimate, paleo-water salinity, and paleoredox conditions of transitional facies. The marine-continental transitional strata in the Upper Carboniferous to Lower Permian consist of complex interbedding of sandstone, shale, coal and limestone. TOC contents display a considerable change in the vertical direction, with high TOC content in coal, moderate TOC in shale, low TOC content in limestone and no noticeable TOC enrichment in sandstone. The Upper Carboniferous Benxi Formation is dominated by marine facies then changing into transitional facies. The Lower Permian Taiyuan Formation is characterized by multiple alternation of marine and transitional facies. Marine limestones are characterized by low detrital influx proxies, whereas transitional shales show high detrital influx input and oxic environment. The Lower Permian Shanxi Formation is a typical marine-continental transitional depositional system, which is characterized by complex interbedding of shales, sandstones and coals. Sandy intervals were deposited in deltaic and tidal flat environment and characterized by high Ti and Al content, low TOC (0.09–0.97%, avg. 0.31%), relatively low U/Th (0.11–0.32, avg. 0.17), showing limited influence of seawater. Shale intervals were deposited in a lagoonal environment with high TOC (0.94–40.64%, avg. 5.43%), Sr/Ba and trace elemental proxies values (U/Th, 0.18–3.27%, avg. 0.44%; MoEF, 0.95–32.3, avg. 8.56; UEF, 0.67–15.17, avg. 2.53). This study shows that high-resolution geochemical characteristics of elements could be reliable indicators for sedimentary facies analysis, and hence could provide broader implications for utilizing trace element geochemistry from marine-continental transitional periods in which sedimentary environment changes frequently and organic-rich sediment accumulated under varied conditions.
•A systematic sedimentological study was conducted on C-P transitional shales.•We establish a high-resolution paleoenvironmental profile of transitional strata.•Depositional models were established for of the C-P transitional facies.•Geochemical characteristics of elements useful for sedimentary facies analysis
The shales in the 2nd Member of Shanxi Formation in the east margin of the Ordos Basin were deposited in a marine-nonmarine transitional environment during the Permian. Based on the recent ...breakthroughs in the shale gas exploration and theoretical understandings on the shale gas of the study area, with a comparison to marine shale gas in the Sichuan Basin and marine-nonmarine transitional shale gas in the U.S., this study presents the geological characteristics and development potential of marine-nonmarine transitional gas in the study area. Four geological features are identified in the 2nd Member of the Shanxi Formation in the study area has: (1) stable sedimentary environment is conductive to deposition of widely distributed organic shale; (2) well-developed micro- and nano- scale pore and fracture systems, providing good storage capacity; (3) high content of brittle minerals such as quartz, leading to effectively reservoir fracturing; and (4) moderate reservoir pressure and relatively high gas content, allowing efficient development of shale gas. The 2nd Member of Shanxi Formation in the east margin of Ordos Basin is rich in shale gas resource. Three favorable zones, Yulin-Linxian, Shiloubei-Daning-Jixian, and Hancheng-Huangling are developed, with a total area of 1.28×104 km2 and resources between 1.8×1012 and 2.9×1012 m3, indicating a huge exploration potential. Tests of the 2nd Member of Shanxi Formation in vertical wells show that the favorable intervals have stable gas production and high reserves controlled by single well, good recoverability and fracability. This shale interval has sufficient energy, stable production capacity, and good development prospects, as evidenced by systematic well testing. The east margin of the Ordos Basin has several shale intervals in the Shanxi and Taiyuan formations, and several coal seams interbedded, so collaborative production of different types of natural gas in different intervals can be considered. The study results can provide reference for shale gas exploration and development and promote the rapid exploitation of shale gas in China.
To study organic matter and pore characteristics of the Lower Cambrian Niutitang organic-rich shales, 166 shale samples from the Niutitang Formation were collected from the studied well, drilled in ...Kaiyang county in the Northern Guizhou area, Upper Yangtze region. All of the 166 shale cores were analysed for total organic carbon (TOC), total sulfur (TS), and the stable carbon isotope of kerogen (δ13Corg). Shale samples with varying TOC values were chosen for mineral composition analysis, helium porosity measurement and low pressure nitrogen adsorption measurement. The Lower Cambrian Niutitang Formation features high organic matter content, type I oil-prone organic matter, ultra-thick organic-rich shales, and an abundance of brittle minerals exhibiting favourable characteristics for the Lower Cambrian shale gas exploration in the Upper Yangtze region. Helium porosity measurement and low pressure nitrogen adsorption measurements were performed on selected shale samples with varying TOC content in order to analyse and characterize pore characteristics. Significant differences in mineral composition and pore characteristics were found between clay-rich and clay-poor Niutitang shales. Quartz weakly correlated with TOC content in clay-rich shales, which contained minor biogenically contributed quartz and performed better in terms of reservoir properties than clay-poor shales, and exhibited higher porosity, maximum nitrogen quantity adsorbed, pore volume, and specific surface area. In clay-rich shales, the maximum nitrogen quantity adsorbed, total pore volume, and specific surface area positively correlated with TOC content (R2=0.77, 0.79, and 0.75, respectively). The pore volume of clay-rich shales was dominated by mesopores, and both micropore and mesopore volume of clay-rich shales correlated significantly with TOC content. In clay-poor shales, quartz positively correlated with TOC content, indicating biogenic contribution to quartz. Micropore volume and mesopore volume of clay-poor shales were weakly positively correlated with TOC content. Pore volume, especially mesopore volume, may be affected by the recrystallization of biogenic silica, exhibiting identical micropore volume with clay-rich shales but lower mesopore volume.
•The Lower Cambrian shales displayed high-quality organic matter concentrations.•The quartz content positively correlated with TOC content in the clay-poor shales.•Organic matter plays a positive role in the pore structure of Niutitang shales.•Significant differences exist in the pore characteristics of clay-rich and clay-poor shales.
Through lithofacies analysis and architecture anatomy of the Carboniferous Ross Sandstone turbidites outcropped at western Ireland, the depositional model of deepwater turbidite lobes is established. ...Seven types of lithofacies are recognized including goniatites-rich shale, laminated shale, laminated siltstone, massive sandstone, fine-medium sandstone with mud-clast, basal gravel, and chaotic mudstone, which can be subdivided into units of three origins, turbidite lobe, turbidite channel, and slide-slump; and four hierarchical levels, lobe complex, lobe, lobe element and single sandstone layer. The lobes show apparent compensational stacking pattern, lobe elements display typical thickening-upward cycles on vertical profile, and the higher the hierarchical level, the better the preservation of the hierarchical boundary is. In general, turbidite lobe deposits appear as tabular, parallel/sub-parallel sandstone and mudstone interbeds, and change from thick, massive sandstone in the proximal end to thinner sandstone and mudstone interbeds from axis to fringe, with the sand-shale ratio and degree of sandstone amalgamation decreasing.
The dominant factors controlling development of microfractures in the black shale and the origin of microfractures in the sweet spot intervals were discussed of the Ordovician Wufeng ...Formation−Silurian Longmaxi Formation in Shuanghe outcrop profile, Changning, Sichuan Basin. For the target interval, holographic photograph statistics of microscopic composition of 203 big thin sections and 203 small thin sections, TOC content of 110 samples, 110 whole rock X-ray composition, and main trace elements of 103 samples were tested and analyzed. The results show that the microfractures include bedding microfractures and non-bedding microfractures. The bedding microfractures are mostly plane slip microfractures, lamellation microfractures and echelon microfractures. The non-bedding microfractures are largely shear microfractures and tension microfractures. Vertically, the density of microfractures is the highest in SLM1 Member of Longmaxi Formation, decreases from SLM2 Member to SLM5 Member gradually, and drops to the lowest in Wufeng Formation. The microfracture density is positively correlated with siliceous content and negatively correlated with the carbonate content. The finer the grain size of the black shale, the higher the density of the microfractures is. The microfracture density is controlled by biogenic silicon: the higher the content of biogenic silicon, the higher the microfracture density is. Under the effect of ground stress, microfractures appear first in the lamellar interfaces. Regional tectonic movements are the key factor causing the formation of microfractures in the sweet spot interval, diagenetic contraction is the main driving force for lamellation fractures, and the pressurization due to hydrocarbon generation is the major reason for the large-scale development of microcracks.
Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper ...Ordovician Wufeng Formation—Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation—Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation—Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces (i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.
The paleoceanic redox conditions and the vertical and spatial distribution of organic-matter-rich lower Cambrian Niutitang shales are varied in the Upper Yangtze region, China. An integrated ...approach, including organic geochemistry, major and trace elemental geochemistry, and X-ray diffraction (XRD) mineralogy, was used to characterize changes in redox conditions and to determine key controls on organic-matter accumulation during deposition of the lower Cambrian Niutitang shales. A 202-m-long cored well located in Kaiyang County in northern Guizhou, China, was thoroughly characterized by the integrated approach in this study. Vertically, total organic carbon (TOC-) rich siliceous and argillaceous black shales were developed in the Lower and Middle Niutitang Formation, and TOC-lean silty shales were found in the Upper Niutitang Formation. The TOC content in the Lower and Middle Niutitang shales ranges from 0.27% to 6.7%, with an average value of 2.1%, and it decreases from 0.30% to 0.11% in the Upper Niutitang Formation shales. The TOC-rich siliceous shales in the Lower Niutitang Formation were characterized by high total sulfur content, enrichment in redox-sensitive trace elements, high Corg:P ratios, and high authigenic MoU covariation, indicating they were deposited under persistent anoxic conditions. Most organic-rich shales from the Middle Niutitang Formation were deposited under anoxic conditions, whereas a few intervals having low TOC contents were deposited in a predominantly oxic depositional environment. In contrast, the Upper Niutitang Formation were deposited under fully oxic conditions. A combination of high primary productivity and enhanced organic preservation under euxinic water column conditions is the key control on organic-matter accumulation in the Lower Niutitang Formation shales. However, a coupling of redox depositional environment and terrestrial mineral dilution plays an important role in organic-matter accumulation and preservation in the Middle Niutitang Formation. Similar organic-matter abundance distribution and carbon isotope excursion of organic matter in the sediments deposited at different depositional facies on the Yangtze Platform suggest regional isotopically homogenous organic carbon input due to the widespread transgressive event with deep basinal waters being transported into the shallow shelf environments at the base of the lower Cambrian. As a result, δ13Corg distribution patterns can be used as a stratigraphic correlation marker in the Upper Yangtze region.
•Similar δ13Corg patterns as stratigraphic correlation marker in the Yangtze region.•Lower and Middle Niutitang TOC-rich shales were deposited under anoxic conditions.•MoU covariation indicates an unrestricted marine environment in the early Cambrian.•The controls on organic-matter richness in the Niutitang shales are different.
The effect of clay minerals on the methane adsorption capacity of shales is a basic issue that needs to be clarified and is of great significance for understanding the adsorption characteristics and ...mechanisms of shale gas. In this study, a variety of experimental methods, including XRD, LTNA, HPMA experiments, were conducted on 82 marine shale samples from the Wufeng–Longmaxi Formation of 10 evaluation wells in the southern Sichuan Basin of China. The controlling factors of adsorption capacities were determined through a correlation analysis with pore characteristics and mineral composition. In terms of mineral composition, organic matter (OM) is the most key methane adsorbent in marine shale, and clay minerals have little effect on methane adsorption. The ultra-low adsorption capacity of illite and chlorite and the hydrophilicity and water absorption ability of clay minerals are the main reasons for their limited effect on gas adsorption in marine shales. From the perspective of the pore structure, the micropore and mesopore specific surface areas (SSAs) control the methane adsorption capacity of marine shales, which are mainly provided by OM. Clay minerals have no relationship with SSAs, regardless of mesopores or micropores. In the competitive adsorption process of OM and clay minerals, OM has an absolute advantage. Clay minerals become carriers for water absorption, due to their interlayer polarity and water wettability. Based on the analysis of a large number of experimental datasets, this study clarified the key problem of whether clay minerals in marine shales control methane adsorption.
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