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It is widely accepted that significant perturbations in ocean redox states are closely related to biological evolution during the early Cambrian. However, the host and geochemical cycle of redox-sensitive elements (V and Zn) of the Niutitang Formation shale in South China have not been well-constrained. This study reports lithofacies, mineralogy and geochemical data of high-resolution samples from slope and basin settings (Xa1 and Xb1 wells, Hunan Province), to evaluate the significance of redox condition and hydrothermal contribution to trace element accumulation during the early Cambrian. Our results reveal the Zn-rich host is primarily sphalerite, while the V-rich host includes organic matter, illite, and anatase minerals in the Niutitang shale. Illite and anatase minerals have significant V concentration (14.8% and 18.3%), whereas organic matter contains relatively lower V concentration (1.4%). Trace elements can be scavenged by dissolved organic matter in seawater, and then taken up by clay minerals or form sulfide during deposition/diagenesis of organic-rich shales. Although seawater restriction of the lower member (LM) shale was stronger than in the upper member (UM) shale, euxinic conditions were more conducive to V and Zn enrichment than ferruginous conditions. Specifically, dwindling concentrations of V and Zn in seawater existed in South China during deposition of the early LM shale. However, the ocean experienced episodic hydrothermal activity, which may have supplied abundant trace elements to supplement seawater content. Overall, we propose that the V and Zn geochemical cycles in the early Cambrian paleo-ocean were controlled by redox condition, organic matter, and the trace element inventory of seawater. • The hosts of V and Zn elements in organic-rich shale of Niutitang Formation have been determined. • Euxinic condition was more favorable for the enrichment of trace elements relative to ferruginous condition. • The seawater restriction of UM/LM shale of Niutitang Formation was investigated. • V and Zn inventory in the ocean was in a drawdown trend during the early Cambrian. • Hydrothermal activity may carry abundant trace elements into the ocean. [ABSTRACT FROM AUTHOR]

The Tuofutai Bulge in central Tabei of northern Tarim Basin provides insights into the complex geochemical processes of deep oil accumulations. Fifteen crude oil samples collected from deep Ordovician reservoirs were tested and analyzed with light hydrocarbons (LHs) and diamondoids to evaluate the oil family, secondary alteration, and mixing processes. Results show that the oil samples share similar distribution characteristics of LHs and diamondoids and contain low concentrations of methyldiamantanes, suggesting that they belong to one group and have not been significantly altered by thermal cracking. Geochemistry of LHs and diamondoids indicate that the thermochemical sulfate reduction is limited and negligible, whereas some samples are severely altered by biodegradation. The phase fractionation is mainly related to the leakage of light hydrocarbon caused by geological activities such as tectonic uplift and fault activity, and some samples show weak evaporative fractionation. The biodegradation causes the increase in the values of methyladamantane to adamantane (MAs/A) and the concentrations of (1 + 2)-methyladamantane and (3 + 4)-methyldiamantane. The thermal maturity of oil calculated on the basis of LHs and diamondoids is generally higher than that calculated using saturated and aromatic fractions. The difference of various maturity parameters related to saturated hydrocarbon, aromatic hydrocarbon, LHs and diamondoids records multiple oil charging and mixing processes. • Light hydrocarbons and diamondoids of the deep oils was revealed. • The deep oil suffered limited thermal cracking and weak evaporative fractionation. • Significant biodegradation was revealed in some oil samples. • Effect of biodegradation on diamondoid parameter was revealed. • Different molecular maturity indicators reflect multiple charging and mixing. [ABSTRACT FROM AUTHOR]

Pore structure plays an important role in controlling the oil migration and accumulation mechanism of subaqueous volcanic rocks (SVRs). The Santanghu Basin in western China is a unique region for providing insight into the pore structure characteristics of SVRs; however, little attention has been given to them. Thin-section analysis, scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), pressure-controlled porosimetry, and nuclear magnetic resonance (NMR) experiments are performed on SVRs in the Santanghu Basin to reveal their pore structure characteristics and pore development patterns. The results show that the SVRs are dominated by ink bottle-shaped pores and have the characteristics of rather low matrix permeabilities (the Swanson parameters are mainly <4.0%/MPa), fine pore throats (r apex mainly <0.5 μm) and high capillary pressures (P 50 > 14.0 MPa). An effective method, namely, integrating NMR and LTNA experiments, is proposed to estimate the full-range pore size distribution (PSD) of the SVRs. In the full-range PSD, three peaks are identified at pore sizes of <1 μm, 1–10 μm and >10 μm, mainly corresponding to intercrystalline pores (IPs), dissolved pores (DPs), and residual vesicles (RVs), respectively. The storage space of SVRs is dominated by IPs, while the contents of DPs and RVs are the key to SVRs becoming effective reservoirs. The content of RVs is mainly controlled by the depth of the water body during deposition and the distance from the volcanic strata boundary. The higher the content of K and Na in the original composition and the closer the distance to the source rock is, the higher the proportion of DPs. In accordance with the original composition and the depth of the water body during deposition, the SVRs are divided into four types. Moreover, we propose the pore development patterns of the four types of volcanic rocks. RVs mainly exist in type I and type II SVRs, while DPs are important storage spaces in type I and type III SVRs. Finally, type IV SVRs only develop IPs and a few nanoscale DPs. • This paper characterized the pore development characteristics of SVRs in detail. • Pore development in SVRs depends on the depth of the water body during deposition and the original composition. • Division of four types of SVRs based on the formation environment and original composition. • Various types of SVRs show distinct differences in pore development pattern. [ABSTRACT FROM AUTHOR]