Correlation between formation pressure and hydrocarbon enrichment in Triassic Xujiahe Formation, Western Sichuan Depression

The Upper Triassic Xujiahe Formation in the Xinchang structural belt of the Western Sichuan Depression is an important exploration target for tight sandstone hydrocarbon in the Sichuan Basin. Several studies are carried out in this paper to improve the effectiveness of exploration and development including the characteristics of oil and gas movement with a view of dynamic point combining with fluid dynamics, determine the distribution characteristics of oil and gas in combination with static research results, and study the relationship between hydrocarbon enrichment and formation pressure. Overpressure is developed in the study area, and the degree of overpressure in the third member of Xujiahe Formation is the highest. Whilst the formation pressure of the main gas producing layer, the second member of Xujiahe Formation, is generally between 60-80 MPa with the maximum residual pressure reaches 40 MPa. The residual pressure gradients in most areas distribute between 0-3 MPa/km and up to 10 MPa/km as the highest. According to dynamic hydrocarbon enrichment theory and the relevant research on residual pressure and residual pressure gradient, it is considered that it has an important relationship with faults, fluids, hydrocarbon distribution and production performance. The area with higher residual pressure gradient (1-2 MPa/km) both improves fluid migration capacity and provides favorable reservoir, which is conducive to long-term high-yield and stable production

Detection of trends in precipitation extremes in Zhejiang, east China

Extreme weather exerts a huge impact on human beings and it is of vital importance to study the regular pattern of meteorological and hydrological factors. In this paper, a selection of seven extreme indices is used to analyze the trend of precipitation extremes of 18 meteorological stations located in Zhejiang Province, east China using the Mann–Kendall test. Then the precipitation trends in the plum season (from May to July) and typhoon season (from August to October) are studied separately. The results show that the precipitation trend varies from east to west. There is a positive trend in the east and a negative one in the west. The largest part of Zhejiang Province shows a positive trend in heavy precipitation and the most significant upward trend is detected in Dinghai with 3.4 mm/year for precipitation on very wet days. Although the upward trend of extreme precipitation is not prevailing, the range of increase in specific areas is apparent, like Dinghai with 1.3 mm/year. Precipitation intensity exhibits an upward trend in most areas and a typical upward trend can be found in Dachendao, Tianmushan, and Yuhuan with 0.04, 0.02, and 0.05 mm/year respectively. Precipitation intensity in both plum and typhoon seasons has increased too, especially for the coastal station

The Effect of Size Distribution on the Geochemistry and Mineralogy of Tropical River Sediments and Its Implications regarding Chemical Weathering and Fractionation of Alkali Elements

International audienceThe mineralogical and geochemical compositions of the sediments deposited by rivers have been used extensively to evaluate past chemical weathering and the physical erosion history of drainage basins at different time scales. However, the role of sediment sorting in the mineralogical and geochemical compositions of river sediment needs to be better constrained because it could significantly modify proxies used to evaluate weathering intensity in the past. In this study, major and trace element concentrations and mineralogical compositions were determined on seven different grain-size fractions of riverbed samples taken from the Pahang and Kelantan Rivers in the Malay Peninsula. The main aims are to assess chemical weathering and the fractionation of alkali elements during sediment transportation under tropical climate conditions. Fine sediments (from <2 μm to 4-8 μm) were characterized by higher concentrations of Al 2 O 3 , P 2 O 5 , V, Sc, and Cs than coarse sediments (from 8-16 μm to 32-63 μm). The presence of heavy minerals, such as zircon and monazite, was found to have critical effects on the concentration of several elements (e.g., Zr and Th), leading to extremely high elemental concentrations in the coarse-sediment fractions. Variations in elemental concentrations with particle sizes are mainly attributed to changes in the mineralogical composition during mineral sorting and weathering. In both rivers, the intensity of chemical weathering increased with decreasing grain sizes. However, the relationship between grain size and chemical weathering trends in the Pahang and Kelantan River basins was different in each case due to differences in plagioclase and K-feldspar concentrations, especially in the coarse-sediment fractions (16-32 μm and 32-63 μm). Kaolinite percentage, illite chemistry index, and illite crystallinity were directly proportional to the chemical index of alteration (CIA), suggesting that these mineralogical parameters are suitable proxies for determining chemical weathering intensity in sediments that have undergone significant changes due to mineral sorting during transportation. The comparison of Rb/K and Cs/K ratios and the CIA suggested that K and Rb mainly derive from primary minerals during the process of chemical weathering, whereas Cs is mainly present in fine weathered particles due to the reabsorption of Cs on clay minerals. Overall, our results highlight the relationship of grain size and mineral assemblages to different states of chemical weathering (and alkali fractionation) in tropical river basin sediments. These relationships need to be considered when using mineralogical and geochemical compositions to reconstruct weathering history in a "source to sink" approach

Ecological risk and contamination history of heavy metals in the Andong tidal flat, Hangzhou Bay, China

<p>In this study, we collected two sediment cores (C1 and C2) from the Andong tidal flat, Hangzhou Bay, and studied the temporal variations of heavy metals in the cores. Vertical distributions of heavy metals were almost unchanged in both the cores before 2000. After 2000, however, the heavy metal concentrations increased dramatically, suggesting that the sediments have been affected by enhanced human pollution in the recent decade. In the core C1, the sediments were severely polluted by Pb, moderately to considerably polluted by Cr and Zn, and low to moderately polluted by other heavy metals. The core C2 was relatively unpolluted before 2000 and low to moderately polluted after 2000. Multi-statistical analyses indicated that the core C1 was additionally contaminated by local human activities such as wastewater discharge and the Hangzhou Bay Bridge. The heavy metals in the core C2, however, were largely contributed by the Yangtze River and controlled by sedimentation process. The calculated sedimentary flux (4–8 g m⁻² a⁻¹) of heavy metals generally increased with time. It was closely related to the wastewater discharge in adjacent areas. This study reconstructed the local heavy metal pollution history and provides important information for environmental protection and policy making.</p