Weibing Shen

在确定华南板块中-新元古界石墨矿分布特征的基础上, 从地球表层环境演化的角度, 探讨了石墨矿成矿机制与有利区分布。结果表明, 华南中-新元古界石墨矿集中分布于古陆块边缘, 以区域变质-热接触变质型矿床为主, 碳源以有机成因占主导。华南板块中-新元古界石墨矿的形成是地球表层环境演化的综合响应: 中元古代早期与新元古代晚期岩石圈大陆裂解, 在板块边缘形成大量的裂谷盆地, 为石墨矿富碳质原岩的沉积提供了可容空间; 大陆裂解伴生的火山/热液活动促进了营养物质注入海洋, 导致生物的演化革新与繁盛; 中-新元古代气-水交换活跃的海洋表层水体为生物勃发提供了适宜的有氧环境与有机碳制造场所, 海洋深部广泛的缺氧还原水体为有机碳的保存、埋藏创造了良好的条件; 中元古代、新元古代转折期的克拉通化及新元古代-显生宙多期构造-岩浆过程伴生出的区域变质、热接触变质作用, 为石墨矿原岩中碳质石墨化成矿提供了持续的高温高压条件。根据地球表层环境演化作用下的石墨矿成矿模式, 华南古陆块内部裂谷沉积区的局限性分布导致中-新元古界规模性石墨矿不发育, 而大陆边缘富碳质原岩沉积区与构造高温高压变质带的叠合区为石墨矿有利成矿区, 其中扬子西缘、北缘岩浆热接触变质带为高品位晶质石墨矿有利勘探区。

Permian black shale, as a potential target for marine shale gas exploration in South China, is characterized by its great thickness and organic matter (OM) content. To understand the constraints on the heterogeneous accumulation of OM in Permian black shale, high-resolution geochemical data related to paleoenvironment variations are collected on the Gufeng and Dalong Formations of the Putaoling area, the Anhui province, and the Lower Yangtze area. The OM was heterogeneously enriched in the Permian shales, as shown by the highly organic-matter-rich Gufeng Formation and the moderately organic-matter-rich Dalong Formation. The distribution patterns of rare earth elements (REEs) indicate a stably high sedimentary rate throughout the shale deposition. Redox indexes, including Mo_EF, U_EF, V/Sc, and U/Th, indicate anoxic conditions for the deposition of the Gufeng and Dalong Formations, and that seawater oxygenation has occurred. The stratigraphic decreases in the (Fe+Mn)/Ti ratios, the index of chemical alteration (CIA), and the content of nutrient elements demonstrate the upward weakening patterns of hydrothermal activity and chemical weathering, which result in a reduction in the primary production. The redox state combined with the primary production jointly control the heterogeneous accumulation of OM in the Permian shales. Our paleoenvironmental evolution model for OM accumulation in the black shales indicates that the Gufeng Formation might be the priority object for the exploration of shale gases in the Permian strata within the Lower Yangtze area.

The distribution characteristics and main controlling factors of hydrocarbon phases in deep strata from petroliferous basins are important for the evaluation of oil–gas resources and decision-making regarding exploration. The distribution characteristics and controlling factors of the Ordovician hydrocarbon phases are systematically analyzed in the Tazhong I block, Tarim Basin, NW China. The results show that the Ordovician reservoirs in the Tazhong I block are characterized as multi-phase reservoirs with a lateral co-existence of condensates, normal oil reservoirs, and heavy oil reservoirs. From east to west, gas-rich in the fault belt and oil-rich in the platform area are presented. Meanwhile, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon, showing decreasing trends in the gas/oil ratio (GOR), wax contents, dryness coefficients, methane contents, and methane carbonate isotope values (δ¹³C₁) and an increasing trend in oil densities. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed in the Tazhong I block, the regular variations in the hydrocarbon phases and geochemical characteristics can be interpreted as records of gas invasion, biodegradation, multi-stage filling, thermal cracking, and thermochemical sulfate reduction (TSR) rather than controlled by the source rock organofacies. This indicated that different kinds of secondary processes for a diversity of the hydrocarbon phase can appear in one region. Our re-construction of the Ordovician hydrocarbon accumulation model in the Tazhong I block encourages future exploration to target gas reservoirs in the fault belt and oil reservoirs in the platform area.

To clarify the genetic mechanism for phase change of the hydrocarbon in the ultra-deep reservoirs, a case study from the Ordovician hydrocarbon in the Halahatang–Shunbei area (HSA), Tarim Basin, NW China, was conducted. The results show that the Ordovician reservoirs in the HSA are characterized as multi-phase reservoirs with a lateral co-existence of condensates, volatile-oil reservoirs, normal oil reservoirs, and heavy oil reservoirs. From north to south, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon in different blocks of the HSA, showing an increasing trend in GOR, dryness coefficients, methane contents, methane carbon isotope values, and ethane carbon isotope values, while a decreasing trend in oil densities and wax contents. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed and the kerogen-cracking gas is dominated in the HSA, the regular variations of the hydrocarbon phases and geochemical characteristics can be interpreted as records of biodegradation and multistage oil–gas filling rather than controlled by the source rock organofacies, oil cracking, and gas invasion. The formation mechanism of the Ordovician multi-phase reservoirs in the HSA suggests that the deep strata of the Tarim Basin hold potential for the exploration of natural gas resources.

The paleoenvironment during the Early Cambrian is closely related to the accumulation mechanism of organic matter (OM) from the Lower Cambrian black shales. However, paleoenvironment remains a controversial issue. Here, we reported a lot of detailed data of sedimentary stratigraphy and geochemistry of the Lower Cambrian Yuertusi Formation in the Aksu area, Tarim Basin. The Yuertusi Formation from the Yutixi outcrop consists mainly of silicalite at the base, two sets of black shales, and crystalline dolostone. Based on the redox conditions traced by U/Th, V/Cr, Ni/Co, and V/Sc, the hydrothermal activity traced by Ce/Ce*, Cr/Zr, U/Th, Fe/Ti, and (Fe + Mn)/Ti ratios, as well as paleo-productivity traced by Ba, Cu, Rb/Sr, and other parameters, variations were observed in the depositional environments of the Yuertusi Formation: 1) the silicalite at the base was deposited under an euxinic condition and intense hydrothermal activity. Mo-U co-variation analysis revealed that the north margin of Tarim Basin belonged to the unrestricted marine during the Early Cambrian, 2) the lower black shales were deposited under an oxygen-poor condition and weak hydrothermal activity, and 3) the upper black shales were deposited under oxygen-poor, sub-oxic conditions and almost no hydrothermal activity. Although the hydrothermal activity improved paleo-productivity, the TOC values were low on the whole, which may be due to the intense hydrothermal activity that damaged the formation of source rocks. Comprehensive studies showed a gradually oxidizing environment and weakening paleo-productivity during the Yuertusi Formation deposited. The anoxic conditions were conducive to the preservation of OM, and the high-quality source rocks represented by the black shales of the Yuertusi Formation were formed, especially the first set of black shales. However, the enrichment of OM may be affected by the intense hydrothermal activity.

The coseismic frictional temperature is increasingly recognised as an important factor for the transformation mechanism in shallow crustal faulting. We report changes in the mineralogical and geochemical compositions across the fault zone in the Shaba (SB) exposure of the 2008 Wenchuan earthquake (Mw 7.9), which contained the maximum value of vertical displacement and cut through shale-rich strata by the principal slip surface. Assuming Yb to be an immobile trace element, with respect to those in the adjacent damaged zone and wall rocks, Li, Rb, Cs and most light rare earth elements (LREE) exhibited marked enrichments of 143%, 247%, 475% and >60%, respectively; and a 40% depletion of Sr and <5% depletion of most heavy rare earth elements (HREE) was found, responding to mineralogical transformation such as alteration and dissolution of feldspar and formation of abundant clay minerals. In addition, heterogeneous adsorption of clay minerals could result in fractionation between and within LREE and HREE as well as negative Eu anomalies and positive Ce anomalies. The element enrichment/depletion model of the northern segment of the Yingxiu–Beichuan (YX–BC) fault zone was quite different from the middle/southern segments characterising faults in carbonate and granitic rocks. Through model calculation, the calculated trace element compositions (Li, Rb, Sr, Cs, Ba, La, Sm and Pb) of the fault gouge were obtained at 250 °C, 300 °C and 350 °C, and were produced by frictional heating. By comparing with the observed values, we suggest that high temperatures derived from frictional heating or due to buffering of the slip zone temperature by evaporation of pore water during the earthquake within the principal slip surface in the northern segment of the YX–BC fault zone may not have existed. Mechanical crushing and chemical alteration (not under high-temperature conditions) within the fault zone during the coseismic stage may result in mass removal and element enrichment/depletion.