Search

The Carnian Pluvial Episode (CPE) fingerprints global environmental perturbations and biological extinction on land and oceans and is potentially linked to the Wrangellia Large Igneous Province (LIP). However, the correlation between terrestrial environmental changes and Wrangellia volcanism in the Ordos Basin during the CPE remains poorly understood. Records of negative carbon isotopic excursions (NCIEs), mercury (Hg), Hg/TOC, and Hg enrichment factor (HgEF) from oil shales in a large‐scale terrestrial Ordos Basin in the Eastern Tethys were correlated with marine and other terrestrial successions. The three significant NCIEs in the study section were consistently correlated with those in the CPE successions of Europe, the UK, and South and North China. The U‐Pb geochronology indicates a Ladinian–Carnian age for the Chang 7 Member. A comprehensive overview of the geochronology, NCIE correlation, and previous bio‐ and chronostratigraphic frameworks shows that the Ladinian–Carnian boundary is located in the lower part of Chang 7 in the Yishicun section. HgEF may be a more reliable proxy for tracing volcanic eruptions than the Hg/TOC ratio because the accumulation rates of TOC content largely vary in terrestrial and marine successions. The records of Hg, Hg/TOC, HgEF, and NCIEs in the Ordos Basin aligned with Carnian successions worldwide and were marked by similar anomalies, indicating a global response to the Wrangellia LIP during the CPE. Anoxia, a warm‐humid climate, enhancement of detrital input, and NCIEs are synchronous with the CPE interval in the Ordos Basin, which suggests that the CPE combined with the regional Qinling Orogeny should dominate the enhanced rate of terrigenous input and paleoenvironmental evolution in the Ordos Basin. [ABSTRACT FROM AUTHOR]

To explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle‐shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge‐like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite. [ABSTRACT FROM AUTHOR]

Pore distribution and micro pore‐throat structure characteristics are significant for tight oil reservoir evaluation, but their relationship remains unclear. This paper selects the tight sandstone reservoir of the Chang 7 member of the Xin'anbian Block in the Ordos Basin as the research object and analyzes the pore size distribution and micro pore‐throat structure using field emission scanning electron microscopy (FE‐SEM), high‐pressure mercury injection (HPMI), high‐pressure mercury injection, and nuclear magnetic resonance (NMR) analyses. The study finds that: (1) Based on the pore size distribution, the tight sandstone reservoir is characterized by three main patterns with different peak amplitudes. The former peak corresponds to the nanopore scale, and the latter peak corresponds to the micropore scale. Then, the tight sandstone reservoir is categorized into three types: type 1 reservoir contains more nanopores with a nanopore‐to‐micropore volume ratio of 82:18; type 2 reservoir has a nanopore‐to‐micropore volume ratio of 47:53; and type 3 reservoir contains more micropores with a nanopore‐to‐micropore volume ratio of 35:65. (2) Affected by the pore size distribution, the throat radius distributions of different reservoir types are notably offset. The type 1 reservoir throat radius distribution curve is weakly unimodal, with a relatively dispersed distribution and peak ranging from 0.01 μm to 0.025 μm. The type 2 reservoir's throat radius distribution curve is single‐peaked with a wide distribution range and peak from 0.1 μm to 0.25 μm. The type 3 reservoir's throat radius distribution curve is single‐peaked with a relatively narrow distribution and peak from 0.1 μm to 0.25 μm. With increasing micropore volume, pore‐throat structure characteristics gradually improve. (3) The correlation between micropore permeability and porosity exceeds that of nanopores, indicating that the development of micropores notably influences the seepage capacity. In the type 1 reservoir, only the mean radius and effective porosity have suitable correlations with the nanopore and micropore porosities. The pore‐throat structure parameters of the type 2 and 3 reservoirs have reasonable correlations with the nanopore and micropore porosities, indicating that the development of these types of reservoirs is affected by the pore size distribution. This study is of great significance for evaluating lacustrine tight sandstone reservoirs in China. The research results can provide guidance for evaluating tight sandstone reservoirs in other regions based on pore size distribution. [ABSTRACT FROM AUTHOR]

The Ordos Basin, as the second largest petroliferous basin of China, contains abundant oil and gas resources, oil shale, and sandstone‐type uranium mineral resources. Chang 7 shale is not only the major source rock of the Mesozoic petroliferous system of the Basin, but is also crucial in determining the space‐time distribution relationship of the shale section for the effective exploration and development of the Basin's oil and gas resources. To obtain a highly precise age of the shale development section, we collected tuff samples from the top and bottom profile of the Chang 7 Member, Yishi Village, Yaoqu Town, Tongchuan District, on the southern margin of the Ordos Basin and performed high‐precision chemical abrasion (CA)–isotope dilution (ID)–thermal ionization mass spectrometry (TIMS) zircon U‐Pb dating on the basis of extensive laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb dating data. Our results show the precise ages of the top and bottom zircon in the Chang 7 shale to be 241.06±0.12 Ma and 241.558±0.093 Ma, respectively. We first obtained Chang 7 age data with Grade 0.1‐Ma precision and then determined the age of the shale development in the Chang 7 Member to be the early‐Middle Triassic Ladinian. This result is supported by paleontological evidence. The deposition duration of the Chang 7 shale is 0.5Ma with an average deposition rate of the shale section being 5.3 cm/ka. Our research results provide time scale and basic data for further investigation of the basin–mountain coupling relation of the shale section, the sedimentary environment and volcanic ash and organic‐matter‐rich shale development relation, and the organism break‐out and organic‐matter enrichment mechanism. [ABSTRACT FROM AUTHOR]