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Direct dating of vertebrate fossils is difficult due to complex postburial diagenetic processes and the often low and heterogeneous concentration of radioisotopes (e.g., U) in fossilized bone material. Here, we demonstrate a novel approach to dating vertebrate fossils via laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of early diagenetic calcite cements precipitated within bone cavities of a Jurassic sauropod from the Sichuan Basin, South China. Early diagenetic calcite yielded a U-Pb age of 165.3 ± 3.6/5.6 Ma, in agreement with a maximum depositional age of 165.8 ± 1.0 Ma from detrital zircon ages of the surrounding rocks, suggesting that diagenesis occurred shortly after the death of the sauropod. This new age demonstrates that the best-known Jurassic large sauropod faunas in South China are much older than those in North America and Africa, suggesting the geographical isolation of South China. Authigenic apatite U-Pb dating on the fibrolamellar bones from the same sauropod gave a distinctly younger age, indicating more complex U-Pb system behavior, possibly due to U uptake by residual organic matter and recrystallization of apatite after early diagenesis. Our findings demonstrate that U-Pb dating of calcite cements within bone cavities has significant potential for constraining the burial age of vertebrate fossils, which could aid in constructing a more robust temporal framework for the radiation and evolution of vertebrates. [ABSTRACT FROM AUTHOR]

Super-deep drilling in the central Sichuan Basin encountered volcanic rocks of the Suxiong Formation, which are overlain by multiple hydrothermal alterations within the upper section of the Ediacaran Dengying Formation. This provides an excellent research opportunity to understand the pre-Cambrian hydrothermal activity and geological evolution of the western margin of the Yangtze Craton. Observations revealed the development of a series of hydrothermal dolomite aggregates characterized by the presence of brown sphalerite within fractures and pores of the carbonate rock. Microscopically, the dolomite exhibited fibrous columnar crystallization, forming radial bands with a gradual decrease in crystallization intensity from the center to the periphery. Cathodoluminescence analysis revealed the presence of approximately eight dolomite bands within the aggregate. U–Pb dating from the inner to outer bands yielded isotopic ages of 781 ± 12 Ma for the second band, 683 ± 12 Ma for the fifth, 562 ± 12 Ma for the sixth, and 545.4 ± 6.9 Ma for the seventh. The in situ rare earth element (REE) distribution patterns of the 781 and 683 Ma dolomite bands exhibited similarities. They both showed low total REE content (∑REE), with significant fractionation between light and heavy REEs. Additionally, they exhibited negative anomalies in cerium (Ce) and europium (Eu), while heavy REEs were relatively enriched. The dolomite bands at 562 and 545 Ma also exhibited similar REE characteristics, with low ∑REE and weak fractionation between light and heavy REEs. They also displayed distinct negative anomalies in Ce and Eu, indicating similar distribution patterns. These findings suggested that formation of the banded hydrothermal dolostone occurred during different tectonic events, and the presence of heavy REE-enriched hydrothermal fluids suggested a deep-seated origin. This study has provided preliminary evidence that the Dengying Formation, previously considered to be of the Ediacaran age, has undergone multiple episodes of deep-seated fluid infiltration and alteration since the Mesoproterozoic Era. Importantly, these events coincide with the rifting of the Rodinia and Pannotia supercontinents, aligning with their respective timeframes. This finding raises questions regarding the stratigraphic division and correlation of the formations in the deeply buried core area of the basin. [ABSTRACT FROM AUTHOR]

The Middle Permian Maokou Formation in the Sichuan Basin has huge resources and is an important target for natural gas exploration. In recent years, significant exploration breakthroughs have been made in the dolomite field of member Mao-2 in central Sichuan, and the gas production of several wells has exceeded 1 × 106 m3/d, indicating promising prospects for exploration. However, the origin of the dolomite reservoir in member Mao-2 remains ambiguous, which restricts the accurate prediction of favorable reservoirs. This study focuses on drilling in the Hechuan area as its research object, by using a detailed description of the cores from member Mao-2 of seven wells; samples were selected for tests of the degree of dolomite cation ordering, stable carbon and oxygen isotopic compositions, strontium isotopic composition, rare earth elements, LA-ICP-MS element mapping and U-Pb dating. It is clarified that: (1) The crystalline dolomite of member Mao-2 in the Hechuan area is the main reservoir rock, and the heterogeneous vugs and fractures are the main reservoir space. The dolomite in member Mao-2 has been characterized by a low degree of cation ordering value (avg. 0.59), with values of δ13C (avg. 3.87‰), δ18O (avg. −7.15‰) and 87Sr/86Sr (avg. 0.707474) having similar geochemical characteristics to Middle Permian seawater; the REEs normalized distribution patterns have similar characteristics to limestone; and the U-Pb age (261.0~262.0 Ma) corresponds to the age in the Capitanian stage of the Permian Guadalupian Series. (2) Petrological studies show that member Mao-2 has vertical karstification zonation characteristics; syngenetic karstification controls the formation of a large-scale fracture-cave system in the phreatic zone; the dolomitization of sediment in the fracture-cave system occurred during the penecontemporaneous period with locally restricted seawater. (3) The main controlling factors of the reservoir were syngenetic karstification, early dolomitization and hydrothermal dissolution related to Emei taphrogenesis. The research results are of great significance for dolomite reservoir prediction; the highlands of paleogeomorphology with syndepositional faults are favorable areas for dolomite reservoirs. [ABSTRACT FROM AUTHOR]

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The extent of fracture development is associated with the degree of enrichment of a natural gas reservoir and its productivity. Based on numerical simulation results of the paleotectonic stress field, a set of evaluation methods for determining the degree of development of reservoir tectonic fractures were established using rock rupture criteria. Taking the fourth member of the Leikoupo Formation in the Pengzhou area of western Sichuan as an example, a finite element (FE) method was employed to simulate the paleo-tectonic stress field during the period of fracture development, and the degree of tectonic fracture development was further evaluated using the above methods. The results indicated that effective fractures were created in the Himalayan period. In this time, mainly NE–NEE and nearly E–W strike tectonic fractures were developed in the target layer. The fractures were mainly low-angle and oblique fractures, while the high-angle fractures were less developed. According to the integrative fracture index (F), five typical fracture development areas were determined: the fault zone, and the northern, eastern, southeastern, and central regions of the study area. The reliability of the fracture prediction results was verified using fracture distribution statistics and gas production test results. [ABSTRACT FROM AUTHOR]

The cumulative expression of multistage deformation is complex multiperiod fractures, which are commonly seen in tectonic zones. The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression, China, is a typical marine carbonate reservoir with natural fractures caused mainly by tectonic movements. According to outcrops, drill cores, image logging, and fluid inclusions, the fracture characteristics, types of natural fractures, and interactions of fractures are determined. In total, 419 natural fractures in 493.2 m of cores from 7 wells are investigated, which are mainly shear and tensile fractures with a small number of weathering generated fractures. Meanwhile, the results of the stable isotope analysis of δ13C and δ18O, as well as the flow fluid inclusion data, reveal four tectonic periods of fractures with different occurrences. Based on the history of regional tectonic evolution, indicating one period of weathering fractures ascribable to stratal uplift and three periods of structural fractures related to the sequential tectonic movements of the Longmenshan fault belt. By analyzing the interaction relationships of fractures, three types of fracture interaction relationships are observed: cutting, restraining, and overlapping. The four stages fractures are chronologically assigned to (1) the early Indosinian N-S trending compression, (2) the late Indosinian NW-SE compression, (3) the middle Yanshanian NE-SE compression, and (4) the early Himalayan E-W compression. The influence of natural fractures on gas migration and well production in marine carbonates is discussed, and indicates that tectonic fractures could provide seepage channels for gas migration and accumulation from near or distant hydrocarbon source rocks into the Middle Triassic Leikoupo Formation. This study utilizes a pragmatic approach for understanding the fracture genesis mechanism in oil and gas field with multiperiod fractures. [ABSTRACT FROM AUTHOR]

The microbial dolomite of the Ediacaran Dengying Fm., Sichuan Basin, SW China is endowed with abundant oil and gas resources. A complex diagenetic history greatly complicates the development of such microbial dolomite reservoirs, and has severely restrained hydrocarbon exploration. This study focused on key field profiles in Eastern Sichuan and aimed to decipher the origin and evolution of dolomites, using novel techniques, including elemental mapping, laser ablation U-Pb dating as well as carbon, oxygen, and strontium isotope analyses. Additionally, we compared the diagenesis and reservoir development model for the Dengying microbial dolomites of the Central Sichuan to other areas, with the aim to provide practical guidance for oil and gas exploration across the entire Sichuan Basin. Our results have shown that the Ediacaran microbial dolomite in Eastern Sichuan experienced four stages of diagenetic modification–specifically: (1) syn-sedimentary dolomitization; (2) penecontemporaneous cementation of fibrous dolomite cement; (3) bladed dolomite and brown-dark very fine crystalline dolomite cementation during the Ordovician; and (4) fine crystalline dolomite and medium-coarse crystalline dolomite cementation during the Silurian-Devonian. Petrology and geochemistry data imply that each diagenetic phase of dolomitization was mainly sourced from marine-dominated fluids. The influence of tectonically related fluids was rarely noted. Significantly, U-Pb ages of these dolomites reveal that the history of dolomite infill to pores within the Eastern Sichuan Basin clearly post-dated that in the Central Sichuan region, resulting in high porosities (6%–11%) that favored hydrocarbon accumulation. This study suggests that the platform margin at the eastern edge of the Xuanhan-Kaijiang paleo-uplift is favorable as the next key exploration target for microbial dolomites in the Sichuan Basin. [ABSTRACT FROM AUTHOR]