Lithofacies and depositional characteristics of gas shales in the western area of the Lower Yangtze, China

Abstract

The Lower Yangtze is considered as a potentially productive shale gas area in China, but only limited research has thus far been carried out there. On the basis of a detailed investigation of fourteen outcrops, eight lithofacies in the Hetang Formation (510-541 Ma), the Gaojiabian Formation (430-443.4 Ma), the Gufeng Formation (270-281 Ma) and the Dalong Formation (252-263 Ma) have been identified: silicalite, siliceous noncalcareous mudstone, siliceous shale, carbonaceous shale, calcareous mudstone, silty-shaly interlaminated mudstone, siltstone and limestone. Three types of fossils were also found in the four formations: sponge spicules, radiolarians and graptolites. Moreover, four key outcrops of the fourteen were suitable for more additional detailed analyses and interpretation of their depositional environments and plausible sequence of the stratigraphic system tracts. As a result, the four potential shale gas formations were all divided into third-order sea-level sequences. Every third-order sequence was further divided into four system tracts, corresponding to an early transgressive system tract (ETST), a late transgressive system tract (LTST), an early highstand system tract (EHST) and a late highstand system tract (LHST). On the basis of the interpretation of the lithofacies and depositional environments in the western area of the Lower Yangtze, sediments can be related to hydrothermal and biological processes, terrigenous clastic input and calcium compensation. These processes were interpreted to occur in a sequential order that we divide into three stages. The ETST period was the first stage, in which hydrothermal upwelling from the open ocean produced a siliceous lithological combination. During the second stage corresponding to the LTST + EHST, our results suggest that some biological communities in the stable deep water provided plenty of organic-rich matter. The LHST period was the third stage, in which terrigenous clastic material was deposited close to the land, and some carbonate sediments began appearing near the platform.

Lithofacies characteristics and its effect on gas storage of the Silurian Longmaxi marine shale in the southeast Sichuan Basin, China

“…For instance, shale was classified into categories including carbonaceous shale, siliceous shale, silty shale, and calcareous shale and described extensively (Du et al, 2014;Tan et al, 2014;Chen et al, 2015). These studies are of great significance for understanding the sedimentary process and setting of shale and characterizing shale reservoirs.…”

Shale gas potential of the Lower Permian Gufeng Formation in the western area of the Lower Yangtze Platform, China

“…In addition, based on the detailed analyses of the outcrops representing the typical characteristics of gas shales, the Gufeng Formation was deposited during transgression/regression cycles in a partially silled basin and can be subdivided into four stratigraphic sequence units (Tong and Yin, 1998;Du et al, 2014) (Fig. 2B).…”

The Lower Yangtze area: A next shale gas block for China? Preliminary potential assessment from some geology and organic geochemistry information

“…The Lower Yangtze Platform, which is located in the eastern part of South China and along the Lower Yangtze Valley, is a large sedimentary basin covering a total area of approximately 36 × 10 4 km 2 (Du, Zhang, et al, ; Pan et al, ; Wang et al, ). It is bounded by the Tan‐lu Fault Zone in the north‐west and the Xuefeng–Jiangnan Fault Zone in the south‐east, and four tectonic units have been identified, namely, Qiantang Depression, Jiangnan Uplift, Nanjing Depression, and Subei Slope (Du, Song, et al, ; Figure a).…”

“…Since 2011, China Petroleum & Chemical Corporation (SINOPEC), China National Offshore Oil Corporation (CNOOC), and China Geological Survey (CGC) have organized several preliminary investigation projects in the Lower Yangtze area. Geologists have conducted assessments on shale properties and gas potential in the western part of the Lower Yangtze area (Cai, Huang, Xia, & Xiang, ; Du, Zhang, Lu, Chen, & Lu, ; Wang et al, ). In December 2016, the success of the Well Gangdi‐1 in the Ningguo area confirmed the potential of shale gas in this area.…”