Triassic Xujiahe Formation Research Articles

Gas source and exploration direction of the Middle Triassic Leikoupo Formation in the Sichuan Basin, China

Natural gas exploration in the Middle Triassic Leikoupo Formation (T2l) in the Sichuan Basin of China has achieved a remarkable breakthrough in recent years and thus attracts wide attention. However, there is no consensus on the gas source. The genetic identification and gas–source correlation based on the geochemical characteristics of natural gas indicate that the natural gas in the Leikoupo Formation is mainly from the underlying Upper Permian Longtan/Wujiaping Formation (P3l/P3w) source rocks (Moxi). Nevertheless, it has assistance (Zhongba) or hydrocarbon supply condition (Chuanxi) brought by the source rocks in the 1st Member of the Upper Triassic Xujiahe Formation (T3x), or mainly from the T3x source rocks with a specific contribution by the T2l source rocks (Yuanba and Longgang). The T2l gas pools display three types of source-reservoir assemblages, and they suggest different exploration directions. The P3l source rocks serve as main source rocks that can constitute the lower-generation and upper-accumulation pattern. The positive structures around the deep faults connecting the P3l and T2l are the most favorable areas to explore large-scale T2l gas pools. The source rocks at the bottom of T3x and the reservoirs at the top of the T2l can constitute the side-generation and side-accumulation pattern, which is favorably developed in the residual karst hills or the local tectonic high position of the karst slope. The T2l can constitute self-generation and self-accumulation pattern, which assists in the formation of gas pools.

Palynofacies analysis for interpreting paleoenvironment and hydrocarbon potential of Triassic–Jurassic strata in the Sichuan Basin, China

The Sichuan Basin is one of the largest petroliferous basins in China. The continental fluvial‒lacustrine sediments of the Upper Triassic Xujiahe Formation and the Lower Jurassic Zhenzhuchong Formation yield diverse fossil organisms and host one of the most important gas reservoirs in the basin. However, paleontological implication for the paleoenvironment and hydrocarbon source is less documented. Here, we report, for the first time for this basin, results from palynofacies analyses combined with thermal alteration data in the Early Mesozoic deposit of the Qilixia section, northeastern Sichuan Basin. The results show that, organic matters in sediments are dominated by phytoclasts, with less abundant palynomorphs and a few amorphous organic matters (AOMs). Four palynofacies assemblages were identified, reflecting depositional settings in a general proximal and oxic fluvial‒deltaic environment, with two distal–proximal sedimentary cycles. The prominent dominance of opaque phytoclasts within the lower Zhenzhuchong Formation may be related to frequent wildfires across the Triassic–Jurassic transition. Palynofacies data (especially the relative abundance of opaque phytoclasts) may reflect 405 kyr eccentricity cyclicity pattern. The palynofacies and thermal alteration index (TAI), combined with geochemical data indicate the presence of type III kerogen in mature to post-mature phases, suggesting gas potential of the Xujiahe and Zhenzhuchong formations in the northeastern Sichuan Basin. This study provides significant implications for better understanding the paleoenvironment variations during the Triassic–Jurassic transition and the future gas exploration in this area.

“Exploring petroleum inside source kitchen”: Shale oil and gas in Sichuan Basin

The Sichuan Basin is rich in shale oil and gas resources, with favorable geological conditions that the other shale reservoirs in China cannot match. Thus, the basin is an ideal option for fully “exploring petroleum inside source kitchen” with respect to onshore shale oil and gas in China. This paper analyzes the characteristics of shale oil and gas resources in the United States and China, and points out that maturity plays an important role in controlling shale oil and gas composition. US shale oil and gas exhibit high proportions of light hydrocarbon and wet gas, whereas Chinese marine and transitional shale gas is mainly dry gas and continental shale oil is generally heavy. A comprehensive geological study of shale oil and gas in the Sichuan Basin reveals findings with respect to the following three aspects. First, there are multiple sets of organic-rich shale reservoirs of three types in the basin, such as the Cambrian Qiongzhusi Formation and Ordovician Wufeng Formation-Silurian Longmaxi Formation marine shale, Permian Longtan Formation transitional shale, Triassic Xujiahe Formation lake-swamp shale, and Jurassic lacustrine shale. Marine shale gas enrichment is mainly controlled by four elements: Deep-water shelf facies, moderate thermal evolution, calcium-rich and silicon-rich rock association, and closed roof/floor. Second, the “sweet section” is generally characterized by high total organic carbon, high gas content, large porosity, high brittle minerals content, high formation pressure, and the presence of lamellation/bedding and natural microfractures. Moreover, the “sweet area” is generally characterized by very thick organic-rich shale, moderate thermal evolution, good preservation conditions, and shallow burial depth, which are exemplified by the shale oil and gas in the Wufeng-Longmaxi Formation, Longtan Formation, and Daanzhai Member of the Ziliujing Formation. Third, the marine, transitional, and continental shale oil and gas resources in the Sichuan Basin account for 50%, 25%, and 30% of the respective types of shale oil and gas geological resources in China, with great potential to become the cradle of the shale oil and gas industrial revolution in China. Following the “Conventional Daqing-Oil” (i.e., the Daqing oilfield in the Songliao Basin) and the “Western Daqing-Oil & Gas” (i.e., the Changqing oilfield in the Ordos Basin), the Southwest oil and gas field in the Sichuan Basin is expected to be built into a “Sichuan-Chongqing Daqing-Gas” in China.

Hydrocarbon generation and expulsion features of the Upper Triassic Xujiahe Formation source rocks and their controlling effects on hydrocarbon accumulation in the Sichuan Basin, Central China

Understanding the features of the hydrocarbon generation and expulsion of the Upper Triassic Xujiahe Formation (T3x) source rocks is important to properly guide petroleum exploration in the Xujiahe Formation in the Sichuan Basin. Based on mass balance, the hydrocarbon generation and expulsion from the T3x source rocks are characterized in this paper through a method (i.e., hydrocarbon generation potential index using pyrolysis data) different from that used in previous studies. The threshold and peak of hydrocarbon expulsion are determined to be Ro = 0.9% and 1.15%, respectively. The amounts of hydrocarbon generated and expelled in the T3x source rocks are 4,525.5 × 108 and 2,420.5 × 108 t, respectively. Moreover, three hydrocarbon expulsion regions T3x have been identified in the Sichuan Basin. On the basis of the hydrocarbon generation and expulsion features of the source rocks and the distribution of the oil‐and‐gas reservoirs, the quantitative model of the hydrocarbon accumulation probability under the control of the T3x source rock is established via single‐factor regression analysis. The predictions obtained through this analysis show that the favourable oil‐and‐gas exploration areas in the Xujiahe Formation are distributed in the Qionglai, Chengdu, and Deyang regions, as well as in the west of Yilong region. Further, the results show that 78% of the discovered oil‐and‐gas reservoirs are located in the predicted favourable and relatively favourable oil‐and‐gas exploration areas.

Composition and origin of natural gases accumulated in the Upper Triassic tight sandstone reservoirs in the Western Sichuan depression, China: Gaseous hydrocarbons, noble gases and carbon dioxide

In this paper, the gas composition and isotopic characteristics of the tight sandstone of the Upper Triassic Xujiahe Formation in the western Sichuan depression were analyzed, and the origin of hydrocarbon gas, rare gas and CO2 in natural gas were determined. The hydrocarbon gas content in the tight sandstone gas of the Xujiahe Formation is the highest, ranging from 96.43% to 99.44 vol.%, while the non-hydrocarbon gas content is only 0.34–2.73 vol.%. The hydrocarbon gas has the highest content of CH4, which is distributed from 88.52% to 97.86 vol.%, and the content of other hydrocarbon gases is relatively low. The studied natural gas is mainly dry gas, and a small part is wet gas. According to the hydrocarbon gas component and carbon isotope analysis results, it showed that the natural gas of the Xujiahe Formation was a thermogenic gas, and there was no mixing of biogenetic gas. The natural gas was derived from the thermal degradation of the humus kerogen. Moreover, the rare gas in the natural gas of the Xujiahe Formation is a crustal origin organic gas, and there is no mantle origin gas, which indicates the lack of inorganic components in the natural gas. We also found that the CO2 in the natural gas is mainly inorganic gas, which is derived from the dissolution of carbonate minerals; and only a small part is organic thermogenic gas, which is derived from the thermal maturation process of organic matter.

Diagenesis of tight sandstone reservoirs of Xujiahe Formation (Upper Triassic), the Xinchang Gas Field, western Sichuan Basin, China

The tight sandstones of the Upper Triassic Xujiahe Formation are important gas reservoirs in the western part of the Sichuan Basin, China. However, their diagenesis and the factors controlling diagenetic processes are poorly constrained. The petrological and geochemical nature of diagenetic minerals of the Xujiahe sandstone reservoirs was analysed from 105 core samples of three wells in the Xinchang Gas Field. The lower member, Xu‐2, sandstones have an average framework composition of Q65F16R19, and 47% of the total lithic fragments are volcanic rock fragments (VRFs). In contrast, the upper member, Xu‐4, sandstones have an average framework composition of Q53F0R47 with a near absence of feldspar and VRFs. The main phase of carbonate cementation in the Xu‐2 sandstones is mesogenetic ferroan dolomite, whereas in the Xu‐4 sandstones, it is intergranular poikilotopic calcite. A fair amount (0.97% in bulk rock and 29% in clay fraction) of authigenic kaolinites appear in the Xu‐4 sandstones, but they are completely absent in the Xu‐2 sandstones. The observed textural occurrence of kaolinites, in combination with the positive relationship between δ13C and δ18O in the calcite cements, suggests that meteoric water flushed the Xu‐4 sandstones, resulting in the dissolution of feldspar to a negligible amount. Quartz cementation is more extensive (1.4 vol%) in the Xu‐2 sandstones than in the Xu‐4 sandstones (0.4 vol%). The fluid inclusions within the quartz cements in the Xu‐2 sandstones have a higher homogenization temperature interval (84–217°C) than those in the Xu‐4 sandstones (81–147°C), indicating that quartz cementation continued to affect the Xu‐2 sandstones during deep burial but ceased relatively early in the Xu‐4 sandstones. High‐intensity chlorite authigenesis, feldspar overgrowth, and albitization occur only in the Xu‐2 sandstones, suggesting that the source of these sandstones was primarily from the alteration of VRFs during late‐stage diagenesis. These clear variations in diagenetic features between the Xu‐2 and Xu‐4 members were most likely controlled by the significant variations in their depositional environments (marine vs. non‐marine) and in the original framework compositions.

Microscopic pore structures of tight sandstone reservoirs and their diagenetic controls: A case study of the Upper Triassic Xujiahe Formation of the Western Sichuan Depression, China

Abstract The study of microscopic pore structures is of central significance to the evaluation of the reservoir quality and well productivity of tight gas sandstone. The second and fourth members (abbreviated as T3x2 and T3x4, respectively) of the Upper Triassic Xujiahe Formation of the Western Sichuan Depression were used as cases to analyze microscopic pore structures and diagenetic controls in consideration of the disparities in reservoir quality and well productivity. Optical microscopy of thin sections, Micro-CT Scanning, scanning electron microscopy (SEM), High Pressure Mercury Injection (HPMI) and apex methods based on the HPMI results were used to determine the pore and throat geometries and the types and pore size distributions in terms of the quantity and levels of pore connectivity. Tight sandstone reservoirs of the study area are characterized by poor physical properties, with the T3x2 reservoirs presenting relatively higher levels of permeability and lower porosity levels than those of the T3x4 reservoirs. The pore spacing configurations, sizes and distributions and throat geometries reveal significant disparities. Primary pores and micro-fractures were found to be more developed in the T3x2 reservoirs with bending and narrow pore throats measured as smaller than 0.5 μm. Compared to those of the T3x2 reservoirs, secondary pores including moldic and intracrystalline pores were more developed in the T3x4 reservoirs with relatively larger, straighter and wider throats of 0.5–1.0 μm. An evaluation of the pore connectivity levels shows that only 26.86% and 28.90% pores are closely connected to the whole for T3x2 and T3x4 reservoirs, respectively. Four major diagenetic processes were found to control reservoir quality and the microscopic pore structure. Mechanical compaction is the primary cause of the densification of reservoirs in the study area. Compared to that observed in the T3x4 reservoirs, more intensive mechanical compaction was observed in the T3x2 reservoirs, changing the configuration of the throats present, reducing the pore throat radii, and ultimately lowering the porosity of T3x2 reservoirs as a whole. Quartz overgrowth and late period carbonate cements were observed in partly filled pore spaces, narrowing throats and reducing pore connectivity, but eogenetic cementation and chlorite cements can increase the pressure-resistance of sandstone and inhibit compaction quartz overgrowth. Thus, T3x4 reservoirs are relatively more porous as a result of eogenetic calcite cementation, and primary pores are more developed in T3x2 reservoirs with chlorite cements. Dissolution is a major factor that improves the tight sandstone reservoir quality because it generates a large volume of secondary pores. The more intensive levels of dissolution in T3x4 reservoirs not only creates more intergranular, intragranular, moldic and intracrystalline pores but also further increases the porosity of T3x4 reservoirs. Fracturing effects have developed in the study area to improve pore connectivity levels and thus the permeability of tight sandstone reservoirs, potentially explaining the pore connectivity and permeability of certain T3x2 reservoirs. This work may be able to be used as a useful reference for the reservoir quality evaluation and exploration of tight gas sandstone reservoirs for similar oil fields worldwide.

Significance and evolution characteristics of the isobutane/n-butane ratio of natural gas

In previous studies, two conflicting conclusions existed, which were: (a) the isobutane/n-butane ratio of natural gas increases with the increasing maturity (Ro) of source rocks and (b) decreases with the increasing Ro. In this paper, the correlations between the isobutane/n-butane ratios, dryness of natural gases, and the Ro values of source rocks of 77 gas samples from Cretaceous and Tertiary in Kuqa Depression, Tarim Basin, Triassic Xujiahe Formation in central Sichuan Basin, Carboniferous–Permian in Sulige and Yulin gas field, Ordos Basin, China, and 80 shale gas samples from Mississippian Barnett Shale in the Fort Worth Basin, the United States are analyzed to reveal the evolution of the isobutane/n-butane ratios, then mathematical models of the isobutane/n-butane ratios and Ro are attempted to be established. Results show that the isobutane/n-butane ratio initially increases and then decreases with increasing Ro, both coal-derived gas and oil-type gas. Diverse types of kerogens may be responsible for the different corresponding Ro values when the isobutane/n-butane ratios of gases reach their maximum values. The initial increase in the isobutane/n-butane ratios with increasing Ro is the reason that isobutane is mainly generated at a higher rate by carbonium ion reaction of α-olefins with protons during kerogen primary cracking at lower maturity, whereas free radical reactions to form n-butane relatively quickly during oil cracking at higher maturity and isobutane cracked at a higher rate during the wet gas cracking stage may result in the terminal decreases in the isobutane/n-butane ratios. Besides, mathematical models of the isobutane/n-butane ratios of different types of natural gas and maturity are established. Therefore, the maturity of gas source rock can be obtained quickly based on the models using the isobutane/n-butane ratio combined with other component information (such as dryness, wetness, etc.), which is of great significance to the characterization of natural gas maturity and gas source rock correlation.

Hydrogen isotope of natural gas from the Xujiahe Formation and its implications for water salinization in central Sichuan Basin, China

Based on the analysis of the molecular composition as well as carbon and hydrogen isotopes of gases from the Upper Triassic Xujiahe and other formations in the Sichuan Basin in China, by combining previous research results and regional geological background, the gas origin, gas source, and significance of hydrogen isotopes are studied. The results show that gas from the Triassic Xujiahe Formation in the Sichuan Basin is mainly hydrocarbon gas. The methane content ranges 67.89%–98.05%, whereas the content of heavy hydrocarbon gas (C2+) ranges 0.42%–16.62%. Meanwhile, the non-hydrocarbon gas content (CO2, N2) is low. Except for the relatively high content of N2 and CO2 in Well Yue121, the average content of N2 and CO2 are 0.82% and 0.26%, respectively. The gas from the Xujiahe Formation in the Yuanba area has a high average dryness coefficient of 0.991, which indicates that it is dry gas. It is also worth noting that gases from other areas of central Sichuan have a dryness coefficient of less than 0.95, which is indicative of wet gas. The δ13C1 value varies from −43.8‰ to −29.2‰, the δ13C2 value ranges from −33.5‰ to −20.7‰, δ13C3 value ranges from −33.6‰ to −19.3‰, and the δ13C4 value ranges from −27.2‰ to −22.2‰. The δD1 value ranges from −191‰ to −148‰, δD2 value ranges from −165‰ to −115‰, and δD3 value ranges from −153‰ to −107‰. Methane and its homologues (C2-4) become more enriched in 13C and D as the carbon number increases (δ13C1 < δ13C2 < δ13C3 < δ13C4, δD1 < δD2 < δD3); the aforementioned is consistent with the carbon and hydrogen isotopic characteristics of thermogenic gases. The thermal maturity RO of the natural gas from the Xujiahe Formation in the Yuanba area ranges 1.09%–1.78%, whereas the RO value of the natural gas from the Xujiahe Formation in other areas of central Sichuan ranges 0.64%–0.92%. The natural gas from the Xujiahe Formation in central Sichuan is mainly from the coal measure source rocks of the Xujiahe Formation. The gas from the 2nd Member of the Xujiahe Formation (Xu-2 Member) in the Yuanba area is mixed gas; it is a mixture of the high-mature coal-derived gas from the coal measure source rock of the Xujiahe Formation and the oil-cracked gas from the Lower Cambrian (and Lower Silurian). The δD value of methane from the Xujiahe Formation in the Sichuan Basin is relatively high as it is higher than −200‰. Compared to the coal-derived gas from the source rocks formed in limnetic facies with freshwater in the Taibei Sag, Turpan-Hami Basin, though at similar thermal evolution stage, the difference of the δD values of methane can be as high as 90‰, indicating that the source rock of the Xujiahe Formation is formed in an environment with water salinization.

Formation of the Late Triassic western Sichuan foreland basin of the Qinling Orogenic Belt, SW China: Sedimentary and geochronological constraints from the Xujiahe Formation

The Upper Triassic Xujiahe Formation is a succession of continental clastic rocks in the western Sichuan Basin, China. This formation has previously been regarded as deposited in a foreland basin linked to the NE-SW-striking Longmen Shan Thrust Belt. However, this hypothesis is questionable due to the presence of faults within the Longmen Shan Thrust Belt. Focusing on the Late Triassic proto-basin, we obtained new data from both outcrops and boreholes, which provide information on the sedimentary facies and provenance in the basin. The results show that the basin was originally striking E-W and that the depocentre migrated southward during the Late Triassic. Palaeocurrent indicators and detrital zircon age data reveal that the Upper Triassic strata were derived mainly from the Qinling Orogenic Belt, with a minor component of detritus derived from the South China Block. Detrital zircon U-Pb ages of ~228 Ma, from the bottom of the Xujiahe Formation, constrain the timing of onset of clastic deposition. Our results suggest that the Late Triassic western Sichuan Basin was a foreland basin at the southern front of the Qinling Orogenic Belt.

Geochemical characteristics and organic matter enrichment mechanism of black shale in the Upper Triassic Xujiahe Formation in the Sichuan basin: Implications for paleoweathering, provenance and tectonic setting

Abstract The first, third and fifth members (henceforth referred to as T 3 x 1 , T 3 x 3 and T 3 x 5 , respectively) of the Upper Triassic coal-bearing Xujiahe Formation black shale are among the most significant hydrocarbon source rocks in the Sichuan basin. Here, we present geochemical data for the Upper Triassic black shales from core from Well LD-1 to determine their paleoenvironmental conditions, paleoweathering, provenance transitions and tectonic setting. The V/(V + Ni) vs. U/Th, V/Cr vs. U/Th and Ni/Co vs. U/Th bivariate plots and the TFe-TOC-TS (total Fe-total organic carbon-total Sulphur) ternary diagram indicate that the synsedimentary redox regime of almost all the shale samples was oxidizing. Ba/Al, Sr/Al and P/Ti data combined with quantitative biogenic Ba data indicate that moderate-high primary paleoproductivity levels prevailed during deposition. The Sr/Cu ratio and C-value combined with the sedimentary features are indicative of warm-humid climate conditions. The Ti/Al deposition rate proxy and decompacted sedimentation rate are positively correlated with the total organic carbon (TOC) content. According to our multiproxy approach, black shale development in the Xujiahe Formation was mainly controlled by the primary productivity level, foreland basin setting, high tectonic subsidence and sedimentation rate and exhibited a limited correlation with water column redox conditions. Through the compilation and calculation of various weathering indices for the Xujiahe black shale, we suggest that caution must be taken when inferring paleoclimate characteristics based on the chemical index of alteration (CIA) because of the influence of multiple nonweathering factors. Provenance-sensitive elemental ratios (Th/Sc vs. Zr/Sc and Co/Th vs. La/Sc) indicate that the clastic contribution to Xujiahe black shales was of a predominantly felsic character. The black shales exhibit a transition from T 3 x 1 to T 3 x 3 to T 3 x 5 , whereby T 3 x 1 was primarily sourced from the Proterozoic to early Paleozoic strata of the Qinling orogeny, T 3 x 3 was primarily sourced from the Neoproterozoic complex in the Longmen Shan, and T 3 x 5 was primarily sourced from the Songpan-Ganzi flysch strata, which contributed recycled material because of folding and strong southeastward thrusting.

Hydrocarbon generation and expulsion characteristics of the source rocks in the third member of the Upper Triassic Xujiahe Formation and its effect on conventional and unconventional hydrocarbon resource potential in the Sichuan Basin

Abstract The third member of the Upper Triassic Xujiahe Formation (T3x3) is a typical source-reservoir-cap unit for natural gas exploration in the Sichuan Basin. However, most previous studies mainly focused on the T3x3 as a source rock, ignoring its role as a reservoir. As a result, the evaluation of hydrocarbon resource potential was not accurate. This study established a hydrocarbon generation and expulsion model through analyzing the geological and geochemical characteristics of the T3x3 source rocks, and calculated the resource potential of conventional gas, tight gas and shale gas systematically with combining the lower boundary of buoyancy-dominated accumulation (LBBA). The results indicated that the thickness of the source rocks dominated by mudstone was distributed widely, with an average value of more than 45 m. The sedimentary environment was salty lacustrine deposits. They have a high content of total organic carbon (TOC) with an average value of 4.80 wt%, the kerogen is mainly type III with minor type II, and the thermal evolution has entered the high to over mature stage. The hydrocarbon generation threshold was determined to be 0.5%Ro, while the hydrocarbon expulsion threshold and the LBBA were determined to be 0.83%Ro and 1.12%Ro, respectively. The efficiency of hydrocarbon expulsion was relatively high, with an average of about 56%. Also, the maximum hydrocarbon generation and expulsion intensity reached to 110 × 108 m3/km2 and 53 × 108 m3/km2. The amounts of hydrocarbon generation, expulsion and residual associated with the T3x3 source rocks were 125.68 × 1012m3, 70.38 × 1012m3, and 55.30 × 1012m3, respectively. Under the criterion condition of the LBBA, the conventional gas resource was 7.21 × 1010m3, the tight gas resource was 294.34 × 1010m3, and the shale gas resource was 2488.50 × 1010m3. It indicated good prospects for exploration.

Climate and tectonic-driven deposition of sandwiched continental shale units: New insights from petrology, geochemistry, and integrated provenance analyses (the western Sichuan subsiding Basin, Southwest China)

How the depositional setting efficiently governs the characteristic sandwiched continental shale units remains uncertain, which restricts an integrated assessment of organic-rich fluvio-lacustrine shale reservoirs, and accurate estimation of potential natural gas resources. Here, we present new results from petrological observations, element geochemical fingerprinting, and integrated analyses of heavy mineral, basinal subsidence history, and sandstone / stratum ratio on typical and terrigenous sandwiched-like depositional systems of the targeted Upper Triassic Xujiahe Formation in the western Sichuan subsiding Bain, Southwest China. In view of the representative and environment-sensitive indices, we suggest that passive continental margin and continental island arc dominated by granite to granodiorite source are the major tectonic settings of the provenance, and both a more warm-humid climate characterized by intensified chemical weathering conditions and a calm tectonically quiescent setting are identified as two major drivers forcing the accumulation and preservation of organic matter in organic-rich continental shale units. Finally, a comprehensive depositional model is established for providing new insights into the linkage between palaeoclimatic conditions, tectonic pulses and terrigenous clastic sedimentation. Both the cyclic palaeoclimate fluctuations and episodic tectonic activities are believed to have exerted a very considerable force on development of the unique sandwiched-like stratigraphic framework, and the coupling interactions between the tectono-climatic evolution and fine-grained sedimentation are thus also stressed.

Affecting factors and application of the stable hydrogen isotopes of alkane gases

Abstract To study the composition, affecting factors of the stable hydrogen isotopes of alkane gases and their application to identification of the natural gas origin and maturities, the chemical and isotopic compositions of 118 gas samples of Carboniferous- Permian in the Ordos Basin, and of Triassic in the Sichuan Basin, combined with 68 gas samples from the Sinian and Cambrian reservoirs in the Sichuan Basin, and Ordovician and Siliurian reservoirs of Tarim Basin, are analyzed comprehensively. The following conclusions are obtained: (1) Natural gases in the study area and strata of the Ordos and Sichuan basins are dominated by alkane gases, and the dryness coefficients and maturities of the Carboniferous-Permian gases in the Ordos Basin are higher than the gases in the Triassic Xujiahe Formation of the Sichuan Basin, while the hydrogen isotopes of the latter ones are much enriched in 2 H than the former. (2) The δ 2 H CH4 -C 1 /C 2+3 genetic identification diagram of natural gas was drawn, and the diagrams of hydrogen isotopic differences between the heavy alkane gases and methane vs. hydrogen isotopes of alkane gases can also be used in natural gas genetic identification. (3) The δ 2 H CH4 -R o formulas of coal-formed gas in different areas of the two basins are given, and the δ 2 H C2H6 - δ 2 H CH4 is a new index for maturity, and the ( δ 2 H C2H6 — δ 2 H CH4 )-R o formula of the coal-formed gas can be used to calculate the maturity of the natural gas. (4) The stable hydrogen isotopes of alkane gases are affected by parent materials in source rocks, maturity, mixing and the aqueous medium conditions, among which the aqueous paleo-salinity is the key factor. To sum up, the hydrogen isotopes of alkane gases are affected by multiple factors, and they are significant to the identification of the origin, and maturity of natural gas, and the water environment during the deposition of source rocks.

Relationships of the iC4/nC4 and iC5/nC5 ratios with maturity of coal-derived gases of Triassic Xujiahe Formation in central Sichuan Basin, SW China

Abstract The original gas reservoirs in different areas and different layers of the Triassic Xujiahe Formation in the central Sichuan Basin are studied to reveal the relationships of iC4/nC4 and iC5/nC5 ratios in coal-derived gas components with maturity using conventional natural gas geochemical research methods. The testing results of 73 gas samples from 8 gas fields show that the iC4/nC4 and iC5/nC5 ratios in coal-derived gas have a good positive correlation, and the correlation coefficient is above 0.8. Both the iC4/nC4 and iC5/nC5 ratios become higher with the increase of natural gas dryness coefficient (C1/C1+) and the methane carbon isotope becoming less negative. These parameters are highly correlated. This study not only reveals characteristics of heavy hydrocarbon isomers generated by coal formation, but also puts forward new identification indicators reflecting the maturity of coal-derived gas, the regression between iC4/nC4, iC5/nC5 and Ro, which can provide an important reference for maturity, migration and accumulation of coal-derived gas, and late stage reformation of coal-derived gas reservoirs.

Impacts of lithologic characteristics and diagenesis on reservoir quality of the 4th member of the Upper Triassic Xujiahe Formation tight gas sandstones in the western Sichuan Basin, southwest China

Abstract The 4th member of the Upper Triassic Xujiahe Formation has become a primary target for tight gas exploitation in the western Sichuan Basin in past decades. Finding relatively high porosity reservoirs in tight sandstones is significant for successful tight gas exploration. In this study, the Xu4 sandstone was studied by a variety of experimental methods, including core observation, thin-section and cathodoluminescence observation, porosity and permeability measurement, mercury intrusion, scanning electron microscopy, X-ray diffraction, electron probe micro-analyzer and fluid inclusion, to determine the reservoir characteristics and also diagenesis evolution and to discuss their impacts on the reservoirs. The Xu4 sandstones are mainly litharenites, feldspathic litharenites and sublitharenites. Among these, calcarenaceous sandstone (CS sandstone) and sandstone with a high kaolinite content (KS sandstone) are classified by their special compositions. The reservoir quality of the Xu4 sandstone is poor, and among the five types of sandstone, the reservoir quality of the KS sandstone is generally better than that of the other types. The diagenetic variations of the Xu4 sandstone are complicated by compaction, kaolinite and quartz cementation, two stages of carbonate cementation, and two stages of dissolution of feldspar and some unstable rock fragments. The KS sandstone is the product of feldspathic litharenite that underwent early-stage dissolution of feldspar during eodiagenesis because of the acidic fluids in the sandstone. This process resulted in a low K+/H+ ratio of the pore fluids, i.e., acidic fluids derived from atmospheric fresh water and fluids of adjacent strata bearing coal seams. The determination of the diagenetic evolution indicated that the poor reservoir quality can be attributed to strong compaction or extensive cementation (mainly calcite cementation). However, the occurrence of relatively high porosity reservoirs varies among the sandstones. For CS sandstone, weak early carbonate cementation is essential. For sublitharenite, feldspathic litharenite, and litharenite, late dissolution of feldspar and less cement are necessary for the formation of relatively high-quality reservoirs. Due to their high porosity, almost all of the KS sandstones are relatively high-quality reservoirs.

Characteristics, formation periods and genetic mechanisms of tectonic fractures in the tight gas sandstones reservoir: A case study of Xujiahe Formation in YB area, Sichuan Basin, China

The main target for hydrocarbon exploration of terrestrial strata in Sichuan Basin is the tight gas sandstone reservoir of the Upper Triassic Xujiahe Formation. Xujiahe Formation in YB area is a major discovery of hydrocarbon exploration in recent years, which is characterized as tight with ultralow porosity, ultralow permeability, and intensive heterogeneity. The tectonic fractures have important influence on accumulation and productivity of gas in the area. In the paper, analysis data of outcrop, core, imaging logging and other experiments were used to determine the characteristics, formation periods and genetic mechanisms of tectonic fractures. There are mainly high-angle and low-angle shear fractures in the area, which caused by tectonic stress. The length of the fractures is mainly 0–4 m from the outcrop and 10–20 cm from the cores and fracture frequencies (number per meter) is mostly 2-7 m−1, 2-4 m−1 respectively. The fracture fillings are mainly calcite, argillaceous and carbonaceous, about 60% of the fractures is unfilled or incompletely filled. Without considering the diagenetic fractures, there are three periods of the fractures: the middle-late Yanshanian tectonic movement, the early-middle Himalaya tectonic movement and the late Himalaya tectonic movement. The first period of the fractures are mainly filled with fine calcite, homogenization temperature of inclusion is 75–85 °C and the maximum effective principal stress of paleostress is 71.64 Mpa. The second period of the fractures are half-filled (or unfilled) with coarse calcite, homogenization temperature of inclusion is 150–175 °C and the maximum effective principal stress of paleostress is 74.91 Mpa. The third period of the fractures are low filling degree and cuts formed fractures clearly, the maximum effective principal stress of paleostress is 86.52 Mpa. For these tectonic movement, there are four sets of the plane “X” shear fractures in the area: NE strike fractures, NW strike fractures, NEE strike fractures, SN strike fractures. There are one set of the profile shear fractures of NWW strike. Finally, the complex fault system of nearly NS, NW and NE strike are formed in YB area.

Diagenetic characteristics, evolution, controlling factors of diagenetic system and their impacts on reservoir quality in tight deltaic sandstones: Typical example from the Xujiahe Formation in Western Sichuan Foreland Basin, SW China

Abstract Deeply buried (3000–5000 m), deltaic sandstones of the Upper Triassic Xujiahe Formation are important tight gas reservoirs in the Sichuan Foreland Basin, China. The diagenesis of these tight sandstones was examined using a variety of petrographic and geochemical techniques, including thin section description, X-ray diffraction (XRD), whole-rock chemical analysis, scanning electron microscopy (SEM), Cathodoluminescence (CL) imaging, electron probe analysis, fluid inclusions and isotopic analysis. These integrated petrographic and geochemical techniques were used to determine the diagenetic history of the sandstones and its impact on the reservoir quality. The tight deltaic sandstones of the T3x2 and T3x4 (second and fourth members of Xujiahe Formation) have undergone a significant and complicated series of diagenetic alterations and changes in geochemical composition. Strong mechanical and chemical compaction together with carbonate cementation destroyed almost all the primary pores and the secondary dissolution pores now dominate the pore space. The T3x4 sandstones experienced a more open diagenetic system at near-surface and eodiagensis resulted in higher porosity than seen in the T3x2 sandstones. Both the T3x2 and T3x4 sandstones experienced closed-system diagenesis during middle-late mesodiagenesis. The early diagenetic dissolution, which mainly occurred in the open geochemical system, produced secondary pores and provided kaolinite and some K+ needed for the subsequent illitization of kaolinite and K-feldspar. The late dissolution of K-feldspar and illitization of K-feldspar in T3x4 sandstones and T3x2 sandstones during the mesodiagenesis, produced some effective secondary pores in the closed geochemical system or in the focused fluid flow zone along fractures. The diagenetic characteristics, size and evolution of (open vs closed) diagenetic system, which were constrained by the depositional environment, deep burial depth and tectonic activity, can be used to predict the reservoir quality ahead of drilling.

INVESTIGATION OF FRACTAL CHARACTERISTICS AND METHANE ADSORPTION CAPACITY OF THE UPPER TRIASSIC LACUSTRINE SHALE IN THE SICHUAN BASIN, SOUTHWEST CHINA

To better understand the nanopore characteristics and their effects on methane adsorption capacity of shales, we performed fractal analysis of nine shale samples collected from the fifth member of Upper Triassic Xujiahe Formation in the Sichuan Basin, southwest China. [Formula: see text] adsorption results show that shales have different adsorption characteristics at relative pressure of 0–0.5 and 0.5–1. Two fractal dimensions [Formula: see text] and [Formula: see text] were calculated using the Frenkel–Halsey–Hill (FHH) equation. Results show that the methane adsorption capacity increases with the increase of [Formula: see text] and [Formula: see text], of which [Formula: see text] has a more significant influence on adsorption capacity than [Formula: see text]. Further studies indicate that [Formula: see text] represents the pore surface fractal characteristics caused by the irregularity of shale surface, whereas [Formula: see text] represents the pore structure fractal characteristics, which is mainly affected by shale components (e.g. TOC, clay minerals) and pore parameters (e.g. average pore diameter, micropores content). A higher [Formula: see text] corresponds to a more irregular pore surface, which provides more space for methane adsorption. While a higher [Formula: see text] indicates a more complex pore structure and a stronger capillary condensation action on the pore surface, which in turn enhances the methane adsorption capacity.

Paleoseismic Slip Records and Uplift of the Longmen Shan, Eastern Tibetan Plateau

AbstractWe document the geological fault zone geometry and internal architecture of the Yingxiu‐Beichuan fault (YBF) in the Longmen Shan, eastern Tibetan Plateau, which hosted the catastrophic 2008 Wenchuan earthquake. The YBF has been repeatedly reactivated during its long‐term activity and contains widespread brittle fault rocks. Pseudotachylytes are recognized in the Pengguan Complex both from the Wenchuan earthquake Fault Scientific Drilling (WFSD) cores and at a surface outcrop. Microstructural evidence of embayed and rounded fragments, flow structures, vesicles, and microlites of various morphologies indicate that the pseudotachylytes were generated by frictional melting during earthquakes. Cataclasite and fault gouge with fragments of angular shapes and varied sizes observed within the fault zones are also considered as evidence of paleoearthquakes. These imply that the main fault zones in the WFSD cores record ancient seismic slip events. Geological field mapping at the surface and in WFSD cores shows that the YBF is a listric reverse fault, with a 155–290‐m thick fault zone that dips 73°–68° at depths &lt;700 m and 30° at depths &gt;700 m. Three faulted sets of alternating Neoproterozoic Pengguan Complex overthrusting upon the Late Triassic Xujiahe Formation in the WFSD‐2 core imply that the main tectonic framework of Longmen Shan consists of a series of imbricated thrust sheets that have accommodated strong crustal shortening. These suggest that the accumulated crustal shortening caused by imbricated thrusting and long‐term seismic activity along the YBF and deep concealed faults is an important process responsible for the rapid uplift of the Longmen Shan.