The interaction of recovery and environmental conditions: An analysis of the outer shelf edge of western North America during the early Triassic

Ultra-deeply buried (>5000 m) marine carbonate reservoirs have gradually become important exploration targets. This research focuses on providing an understanding of the basic elements of the ultra-deeply buried Middle Triassic Leikoupo marine carbonate petroleum system within the Western Sichuan Depression, China. Comprehensive analyses of organic geochemistry, natural gas, and C–H–He–Ne–Ar isotope compositions suggest that the reservoir is charged with compound gases from four source rock units including the Permian Longtan, Middle Triassic Leikoupo, Late Triassic Maantang and Xiaotangzi formations. Approximately a 50-m thick outcrop and 100-m length of drilling cores were examined in detail, and 108 samples were collected from six different exploration wells in order to conduct petrographic and petrophysical analyses. Thin-section and scanning electron microscope (SEM) observations, helium porosity and permeability measurements, mercury injection capillary pressure (MICP) analysis, and wire-line logging (5500–6900 m) indicate that the reservoir lithologies include argillaceous algal limestones, dolograinstones, crystalline dolostones, and microbially-derived stromatolitic and thrombolitic dolostones. Reservoir properties exhibit extreme heterogeneity due to different paleogeographic environmental controls and mutual interactions between constructive (e.g., epigenetic paleo-karstification, burial dissolution, structural movement, pressure-solution and dolomitization) and destructive (e.g., physical/chemical compaction, cementation, infilling, recrystallization, and replacement) diagenetic processes. An unconformity-related epigenetic karstification zone was identified in the uppermost fourth member of the Leikoupo Formation, which has developed secondary solution-enhanced pores, vugs, and holes that resulted in higher porosity (1.8–14.2%) and permeability (0.2–7.7 mD). The homogeneity and tightness of the reservoir increases with depth below the unconformity, and it is characterized by primary intergranular and intracrystalline pores, solution pores, fractures, stylolites, and micropores with a lower helium porosity (0.6–4.1%) and permeability (0.003–125.2 mD). Regional seals consist of the Late Triassic Xujiahe Formation, comprised of ∼300 m of mudstones that are overlain by ∼5000-m thick of Jurassic to Quaternary continental argillaceous overburden rocks. Effective traps are dominated by a combination of structural-stratigraphic types. Paleo-reservoir crude oil cracking, wet-gases, and dry-gases from three successive hydrocarbon generation processes supplied the sufficient hydrocarbon resources. The homogenization temperatures of the hydrocarbon-associated aqueous fluid inclusions range from 98 to 130 °C and 130–171 °C, which suggests hydrocarbon charging occurred between 220–170 Ma and 130–90 Ma, respectively. One-dimensional basin evolution models combined with structural geologic and seismic profiles across wells PZ1-XQS1-CK1-XCS1-TS1 show that hydrocarbon migration and entrapment mainly occurred via the unconformity and interconnected fault-fracture networks with migration and charging driven by formation overpressure, abnormal fluid flow pressure, and buoyancy forces during the Indosinian and Yanshanian orogenies, with experiencing additional transformation occurring during the Himalayan orogeny. The predicted estimated reserves reached ∼300 × 10⁹ m³. The results provide excellent scientific implications for similar sedimentary basin studies, it is believed that abundant analogous deeply buried marine carbonate hydrocarbon resources yet to be discovered in China and elsewhere worldwide in the near future.

Microbialites undergo a well – documented resurgence following the Permian – Triassic mass extinction. This comeback has been attributed to several causes, including the emptying of Early Triassic ecosystems by the severity of the extinction, a decline in burrowing activity and intensity, and the unusual chemistry of Early Triassic oceans. A laterally – continuous stromatolite horizon found within the Lower Triassic Virgin Limestone was examined at the Blue Diamond, NV U.S.A. locality in order to determine the factors that led to growth of the stromatolites, and by extension, the environmental conditions that led to the formation of widespread microbialites in the aftermath of the Permian-Triassic mass extinction. The Virgin Limestone at the study locality consists of ~6 m – thick cycles of shale and siltstone that pass upwards into bioturbated lime mudstone, and are capped by oolitic packstone or grainstone. Each ~6 m – thick cycle represents a rapid sea level rise that led to the deposition of fine – grained clastics below storm wave base, followed by deposition of coarsening – upwards carbonates that were deposited in progressively shallower environments. Analysis of redox-sensitive trace metals (U. V and Mo) from the shale and siltstone interbeds indicate that the fine – grained clastics were deposited under dysoxic waters, however, extensive bioturbation and macrofossils from the carbonate units indicates better aeration of shallower settings. The stromatolites occur within a single, laterally extensive horizon, and 4 distinct stromatolite morphologies are observed along a 3 km long transect, including: 1) aggregates of high-relief hemispherical domes and columnar stromatolites with well-developed laminae that are up to 0.75 m thick; 2) lozenge-shaped aggregates of intergrown columnar stromatolites with dimpled top surfaces; 3) isolated hemispheroids; and, 4) meandering ridges of intergrown domes that are small, low – relief (usually <7 cm), and have a clotted fabric. Isolated hemispheroids are found across the entire study area, while the large hemispherical and columnar aggregates, lozenge-shaped masses and the small intergrown domes are found in the southern, central and northern portion of the study area, respectively. Analysis of trace metals from shale directly underlying the stromatolites reveals an intensification of dysoxic conditions that may have initiated mound growth, while the occurrence of distinct burrows and a sparse fauna within the stromatolites indicates at least partially – oxygenated conditions as the stromatolites developed on the seafloor. Overall, stromatolite growth was controlled by a combination rapid calcification of the mounds related to the unusual carbonate chemistry of Early Triassic oceans, a microbial bloom driven by N – fixing diazotropic cyanobacteria, and possibly intermittent dysoxic conditions that limited grazing metazoans. Comparison of Lower Triassic stromatolites from the Blue Diamond locality to others from the western United States reveals the importance of localized conditions in initiating microbialite growth and determining the complexity of microbialite ecosystems.

In this paper we study the Lilang Super Group in the Spiti area, Indian Himalaya to understand environmental changes in the aftermath of the end-Permian mass extinction. We focus on the Mikin and Kaga Formations, which span the Induan to Ladinian stages of the Lower and Middle Triassic. These strata formed on the southern mid-palaeolatitude margin of East Gondwana and are interpreted as condensed, mixed carbonate – siliciclastic ramp deposits that deepened distally. Carbon isotope ratios of carbonate (δ¹³C_carb) show an increase from −2.6 to 0‰ from the Griesbachian substage to the Dienerian-Smithian transition, followed by a negative shift to −3‰ in the Smithian substage and a large positive excursion from −3.0 to 3.5‰ across the Smithian-Spathian boundary. A short negative shift to −1.0‰ occurred in the early Spathian substage, and is followed by a positive trend from ~−1.0 to 1.0‰ in the Middle Triassic, with several minor excursions occurring during the Aegean substage. Carbon isotope ratios of total organic carbon (δ¹³C_org) co-vary with δ¹³C_carb, suggesting that both proxies represent the original isotopic signatures. Sedimentary and palaeontological evidence, as well as trace metal geochemistry (Mo/Al ratio), indicate anoxic conditions developed from the late Griesbachian to the Dienerian substages, and dysoxic to probably fully oxic conditions from the late Dienerian substage onward. Anoxic conditions only very briefly reoccurred in the late Smithian substage. Ti/Ca and Zr/Al ratios suggest a consistently high terrestrial input in the first three substages of the Early Triassic, followed by a decrease across the Smithian–Spathian boundary. Thus, in the Spiti area, the positive δ¹³C_carb excursion across the Smithian–Spathian boundary is recorded in generally well‑oxygenated sediments and coincides with a decrease in terrestrial input. While evidence for enhanced weathering is lacking, observations are strongly at odds with studies postulating eutrophication as a universal kill mechanism for the Smithian–Spathian crisis. The strata in the Spiti area contain an Early Triassic gap in phosphorite deposition indicating a ~ 5 Myr waning of coastal upwelling in an otherwise persistent (>100 Myr) upwelling zone of the East Gondwana margin. The phosphorite gap suggests low P availability, low rates of organic matter degradation, and a reduction in nutrient exchange between deep and surface water masses. Altogether, the phosphorite gap, the paucity of sessile and filtering fauna in oxygenated waters, and low organic carbon burial rates indicate a collapse in marine productivity in the aftermath of the end-Permian mass extinction.