Research Advances
Sedimentary environment of organic-rich shale in the Upper Permian Longtan Formation in Qinglong area, western Guizhou, China
Rong Chen *, Kun Yuan, Zi-ya Zhang, Qiu-feng Xu
Oil and Gas Survey , China Geological Survey , Ministry of Natural Resources , Beijing 100083, China
1. Objective
The Qinglong area is located in the Eastern Yunnan-Western Guizhou uplift tectonic belt, west of the Zishui fault zone and north of the Youjiang basin. There are many sets of organic-rich shale in Longtan Formation in Qinglong area.Many predecessors did a great deal of research on the geological features of Permian coal fields in Western Guizhou (Mei MX et al., 2004; Dou XZ et al., 2012), but lack of the research on sedimentary background and distribution characteristics of organic-rich shale in Longtan Formation.Therefore, this study focuses on the analysis of the sedimentary environment of the Upper Permian Longtan Formation in Qinglong area.2. Methods
The distribution characteristics and sedimentary environment of shale are the important basis for shale gas exploration (Zhai GY et al., 2018). Based on the analysis of sedimentary facies of typical profiles and wells, the sedimentary facies were analyzed and six sections (wells) in Qinglong and its surrounding area have been contrasted according to rock type, mudstone color and biodistribution.The sedimentary environment of Qinglong area in the late Permian is discussed.3. Results
According to the lateral comparison, the depositional thickness of the Upper Permian Longtan Formation in the northern part of Qinglong County is the thickest, and in the central part is the thinnest. The sedimentation thickness of the
Longtan Formation increases from south to north. The Longtan Formation in Qinglong and its peripheral area can be divided into three sequences and five system fields (Fig. 1).Sequence I developed transgressive systems tract and highstand system tract (Fig. 1). There are one or more biogenic limestone layers in the transgressive system tract.The biological fossils in sequence I shows that sedimentary facies were tidal flat and lagoon facies. In the high system tract, there are mostly mudstone and siltstone with one or more sets of coal seam, showing the deepening of water bodies. In Qinglong area, the sedimentary facies of Sequence I were mainly shunt bay, swamp, tidal flat and lagoon facies (Fig. 2a). During this period, the Qinglong area was still in a relatively deep-water environment.
In sequence II, the transgressive system tract and the highstand system tract are developed. The northern part of Qinglong was the delta front facies with deltas developed, and gradually transformed into the former delta facies and the tidal flat-lagoon facies (Fig. 2b). During this period, the water was shallower from south to north. The delta appearing in the north shows that the source may come from the north.
Sequence III developed transgressive system tract. The wells in the northern area are mainly composed of siltstone and thin-bedded mudstone. In the southern area, sequence III developed siltstone, mudstone, coal seam and argillaceous limestone, the layers of Limestone are more than the northern area. The sedimentary facies of northwestern Qinglong convert to shunt bay of the delta plain (Fig. 2c). The southwestern Qinglong remained deep water basin but compare to sequence II the depth of water was much shallower.4. Conclusion
The Longtan Formation of the Upper Permian in the Qinglong area can be divided into three second-order sequences and five system tracts. The early depositional
* Corresponding author: E-mail address : akihisa@https://www.doczj.com/doc/fb5263656.html, (Rong Chen).
doi:10.31035/cg2018086
2096-5192/? 2019 China Geology Editorial Office.
China Geology 1 (2019) 108?109
China Geology
Journal homepage: https://www.doczj.com/doc/fb5263656.html,