Print this pageThe Basic Characteristic of Shale of Wufeng-Longmaxi Formation in Shilin-Nanchuan Section, Southeast of Sichuan Basin
Tong Lin1, Shugen Liu1,2, and Ziquan Yong1
1College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan, China
2State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (CDUT), Chengdu, Sichuan, China
Shales in Lower Cambrian Qiongzhusi Formation and Lower Silurian Longmaxi Formation are the two dominated source rocks in Sichuan Basin, and are the most prospective exploration area for shale gas.
Shilin-Nanchuan section locates at southeast of Sichuan basin. It exposed from Wufeng Formation Shale to Longmaxi Formation. The sedimentary environment is shelf. At the later period of the Ordovician, Silin section is situated in one of the three deep-water shelfs, among three ancient uplifts, Chuanzhong, Qianzhong and Jiangnan. The favorable black shale and the siliceous mudstone, are mainly deposited in this deep-water shelf from the Upper-Ordovician Wufeng Stage to the early stageof the Lower Silurian. In this section, it is rich in graptolite and framboidal pyrite in the shales. Then as the water shallowing and the hydrodynamic becoming stronger upward, the sand content increases and the graptolite and radiolarian decreases. It suggests the upper Longmaxi Formation was deposited in a shallow shelf. As the sedimentary environment changing, they are different in the main organic parameters of two black shales. For example, the total organic carbon(TOC) of the shales depositing in deep water are more than 2.0%, 3.81% in average, and the organic matter is overmature, with Ro more than 2.0% and dominated by Type II-kerogen. And the TOC of the top Longmaxi Formation, deposited in shallow shelf, is about 0.74% in average. Moreover, the mineral composition is also different. In the black shale of Wufeng-Longmaxi Formation in Shilin section are dominant with clay minerals, quartz, feldspar, pyrite, et al. At the bottom of Longmaxi Formation, the quartz composition is 54%, and decrease upward. The content of the brittle mineral (quartz, feldspar, carbonate) is conductive to the development of micro-pores and cracks. From SEM examination of the outcrop samples, it indicates that the measured average porosity is 2.52%. There are different pore types: intergranular pore, intragranular pore, fracture and organic pore, providing abundant surface area for shale gas. The Langmuir volume of the black shale is range from 1.56 to 2.61cm3/g, with an average of 2.15 cm3/g.
In the study area, with plenty of TOC and overmatured organic matter, the shales in Wufeng-Longmaxi Formation are the best target for shale gas exploration. Furthermore the content of organic carbon has positive correlation with quartz, residual hydrocarbon, Langmuir volume, and has negative correlation with clay. The TOC and quartz shows positive correlation with the porosity.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013