查看更多>>摘要：Alluvial fans, typically characterized by a mixture of fine-and coarse-grained materials, high heterogeneity, and weakly structured organization, are formed through various depositional processes. Fan deposits are considered valuable reservoir exploration targets in the hydrocarbon field worldwide. A series of Quaternary fans in northwestern and northern China, subjected to secondary geomorphological processes of varying degrees, were analyzed. Primary geomorphic processes, characterized by lobe or sheet elements of debris flow, hyper-concentrated flow and sheet floods deposits, generally represent rapid fan aggradation and dominate the sedimentary succession of these fans. Deposition of secondary processes is mainly developed by streamflows reworking with identifiable features, such as sheet-like lag, braided/point bar, and channel elements. A lack of sediment input to the fan and the redistribution and remodification of the primary deposits characterize this process. Synsedimentary reservoir quality is significantly improved by reorganizing sorting, support styles, and bedding structures. The proportion of primary and secondary architectural elements is proposed to evaluate the relative intensity of secondary processes experienced by alluvial fans. The dominant drivers of strong secondary processes are attributed to frequent streamflow events in response to climate changes on millennial timescales. Physical properties and connectivity of alluvial fan reservoirs related to secondary processes are commonly improved compared to reservoirs directly originating from primary processes, as a deep-time example from the Karamay oilfield. The results of this study provide new insights into constructing alluvial fan models in deep-time terrestrial basins and contribute to predicting favorable reservoir distribution and adjusting development strategies for alluvial fan reservoirs worldwide.
查看更多>>摘要：The Tortonian Pena Adrian Formation represents the youngest depositional unit of the Miranda-Trebino basin (Basque-Cantabrian Pyrenees), which developed on the Southern Basque-Cantabrian Pyrenees from late Eocene to Late Miocene times. The formation is a 50-160 m thick succession of alluvial detrital grading to lacustrine carbonates that contain rich and varied calcareous fossil biota (gastropods, ostracods and charophytes). The fossil association characterizes warm temperate, shallow lakes with vegetated bottoms and well-oxygenated and alkaline fresh waters. Integration of stratigraphic, sedimentological, paleontological and C-O stable isotopic data allows the differentiation of a wide range of sedimentary facies, the construction of a depositional model and the definition of distinct evolutionary phases and relation to allogenic processes. Up to 3 metre-thick facies sequences record repetitive water-level changes, likely reflecting short-term climate changes. Overall, the succession outlines an asymmetric cycle of gradual expansion and faster contraction of a shallow ramp-like lake system evolving under oscillating climatic conditions. C and O stable isotopes are consistent with decreasing salinity and increasing precipitation/evaporation balance trough time. Excellent preservation of aragonitic and bimineralic gastropods characterizes the open lacustrine deposits, whereas shell dissolution and neomorphism are distinct in the palustrine ones. This contrasting degree of preservation of calcareous biota clearly reflects changes in the physico-chemical conditions that prevailed during sedimentation and early burial. The findings add to the knowledge of carbonate lake basins, help discern the factors that controlled their evolution and highlight specific depositional and preservation conditions for gastropod-rich carbonate records.
查看更多>>摘要：The Early Triassic was a crucial period for biotic recovery immediately after the end-Permian mass extinction, with widespread development of ooid deposits around the world. However, two competing hypotheses regarding the inorganic or organic origins of ooids remain under debate. Here, we focus on the Lower Triassic Yelang Formation in the Yunfeng section of Guizhou Province and conduct sedimentological and geochemical analyses, combined with statistical analysis of ooid size and morphology, to reveal the origin of ooids and explore their formation in relation to extreme environmental changes. Our results show that hydrodynamic conditions and microbial activities are key factors influencing ooid formation. The evolution of sedimentary facies, changes in ooid size and types, and variations in ooid morphology and sorting all suggest that stronger hydrodynamic conditions favor ooid formation, leading to more abundant and larger ooid deposits. Low delta 15N values (average +1.51 parts per thousand) suggest flourishing microbes dominated by photoautotrophic nitrogen-fixing cyanobacteria, while the C/N vs. Delta 13Ccarb-org crossplot indicates a significant contribution from eukaryotic algae. Furthermore, micro-structural evidence of microbial remains and borings in ooids indicates both constructive and destructive roles of these microbes in their formation. Abundant filamentous microbial fossils and nanograin aggregates observed in ooids also provide direct evidence of biogenic signatures intrinsically related to organomineralization. A comprehensive biochemical model of ooid formation is proposed here, involving microbial-induced mineralization and biomediated carbonate precipitation. In addition, we suggest that the combined effects of exacerbated marine anoxia and extreme hyperthermal climates contributed to the widespread development of ooids during the Early Triassic.
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Elsevier