Shale gas accumulation mechanism in a syncline setting based on multiple geological factors: An example of southern Sichuan and the Xiuwu Basin in the Yangtze Region

Song

et al. 2019

Open Geosciences

Self Cite

It is a common method to use sequence stratigraphic theory to identify favourable intervals in hydrocarbon exploration. The Lower Cambrian shale of Well Jiangye-1 in Yangtze Region in Xiuwu Basin was chosen as the research object. The content of excess silicon of siliceous minerals in shale was calculated quantitatively, and the concentration distribution of Al, Fe, Mn showed that the excess silicon is of hydrothermally origin and the shale deposited in an environment with hydrothermal activity. Using U/Th values in the study, combined with lithology and logging data, in order to divide sequences of the Lower Cambrian shale in Yangtze Region in Xiuwu Basin. The result shows that the shale of the Lower Cambrian shale is recognized as 1 2nd sequence (TST-RST, TST = Transgressive systems tract; RST = Regressive systems tract) and then further subdivided into 5 3rd sequences (SQ1-SQ5). During the deposition of SQ2 and SQ3, hydrothermal activity was active, and their excess silicon content was generally above 20%-30%. Rising sea level and active hydrothermal activity were beneficial for the enrichment of siliceous minerals and organic matter. Based on the comparison of the reservoir parameters, it tells that SQ2 and SQ3 have relatively higher content of TOC, higher content of brittle minerals (such as siliceous minerals, carbonate minerals and so on), larger effective porosity and higher content of gas, which make it as the most favourable intervals of the Lower Cambrian in Xiuwu Basin.

Section: Discussionmentioning

confidence: 99%

Division of shale sequences and prediction of the favorable shale gas intervals: an example of the Lower Cambrian of Yangtze Region in Xiuwu Basin

Song

et al. 2019

Open Geosciences

Self Cite

“…To accurately determine whether there is hydrothermal activity in the study area, excess siliceous mineral content was used for analysis in this study Figure 1 for the well location. 5 Geofluids [59][60][61][62][63]. Excess siliceous mineral content (abbreviated Si ex ) refers to all siliceous minerals except those from terrestrial clastic deposition and can be calculated by the following formula:…”

Section: Elemental Analysismentioning

confidence: 99%

“…As targets of geological explorations of oil and gas change from conventional reservoirs to unconventional reservoirs, the exploration and development of unconventional oil and gas (including shale gas, shale oil, and tight gas) have achieved great success in North America, and this makes the United States essentially self-sufficient in oil and gas, thus achieving "energy independence" [1][2][3][4][5][6]. There are also unconventional oil and gas resources with great potential in South China, among which are two sets of marine shales, the Upper Ordovician-Lower Silurian shales and the Lower Cambrian shales, which are the main exploration target series.…”

Section: Introductionmentioning

confidence: 99%

Study on the Mechanism of Organic Matter Enrichment in Early Cambrian Marine Shales in the Lower Yangtze Area, South China: An Example Using Well JXY1

Cheng

et al. 2020

Geofluids

Self Cite

The abundance of organic matter in shales, which has a direct effect on the hydrocarbon generation potential of shales, is an important organic geochemical parameter for evaluating shale gas reservoirs. The total organic carbon content (TOC content) in shale is controlled by the abundance of original sedimentary organic matter. Therefore, it is very important to study the mechanism of organic matter enrichment in shale. In this paper, the Lower Cambrian marine shales from the Lower Yangtze region are selected as the research subject, most of which originate from a typical area well called Well JXY1. The degree of pyritization (DOP) is used to characterize the redox environment of the water body, while the P/Al ratio is used to analyze the biological productivity of paleoseawater. The paleosalinity of seawater is calculated via carbon and oxygen isotope analysis. In addition, the early Cambrian hydrothermal activities were studied by using core description; Si, Al, Fe, and Mn elemental analysis; and oxygen isotope calculations. The results show that during the early Cambrian Wangyinpu sedimentary period, the seawater was an anaerobic water body with H2S, and the oxygen concentration was approximately 0 mL/L. In the middle stages of the Wangyinpu sedimentary period, the water body had the strongest reducibility and the highest biological productivity. Moreover, the paleoocean in this period between the Yangtze plate and the Cathaysian plate was greatly affected by hydrothermal activities, with temperatures ranging from 90°C to 120°C. Active hydrothermal activities promoted high biological productivity and an anaerobic environment, both of which were conducive to the preservation and enrichment of organic matter, resulting in extremely high TOC content in the Wangyinpu shales (from 6.5% to approximately 16%).

“…In the deep sea where the sedimentary environment is relatively low-energy, shale with horizontal bedding and massive bedding will develop [45][46][47][48]. If hydrothermal activity exists in the seabed, due to the fast flow rate and large flow volume, it will be easy to form special sedimentary struc-tures in the relatively soft seabed sediments, thus providing a basis for identifying hydrothermal activity.…”

Section: Special Sedimentary Structuresmentioning

confidence: 99%

Source Analysis of Silicon and Uranium in uranium-rich shale in the Xiuwu Basin, Southern China

Yan

et al. 2019

Open Geosciences

Self Cite

Uranium deposits are crucial resources for the development of the nuclear energy. Among known sources of uranium, the uranium-rich shales have recently obtained significance. In this paper, the Lower Cambrian Wangyinpu Formation shale in the Xiuwu Basin, southern China, has been studied using a combination of techniques including element analysis (Al, Fe, and Mn), δ30Si silicon isotopic analysis, δ18O oxygen isotopic analysis, study of core samples. It has been observed that significant hydrothermal activity occurred in the Xiuwu Basin during the Early Cambrian period. The results show that 20%–40% of the silicon in most of the sections of the Lower Cambrian Wangyinpu Formation were inherited from the hydrothermal fluids, with temperatures ranging between 75∘C and 102∘C. It is concluded that more than 90% of the uranium in most of the sections of the Lower Cambrian Wangyinpu shale was derived from submarine hydrothermal fluids, while less than 10% from the terrigenous detritus. The enrichment of uranium in the basin was observed in the Middle-Upper part of the Wangyinpu Formation and the geological resources estimated to a tune of ~4.9×103 t. In this paper, we proposed a model for silicon and uranium enrichment in the Lower Cambrian shale controlled by hydrothermal activity in the Xiuwu Basin. This model also provides a scientific rationale for uranium further exploration and exploitation of the uranium resource.