Characteristics and controlling factors of shale reservoir in Wufeng Formation-Longmaxi Formation of the Zheng'an area

Abstract

The complex tectonic deformation zone in the east of the Sichuan Basin has a great potential of shale-gas resources, but has not been commercially exploited yet.This paper utilizes the mineralogical, organic chemical, and petrologic data of the Wufeng Formation-Longmaxi Formation shale cores, which was deposited in the transition zone of deep-shallow continental shelf and is currently located on the Zheng'an area in the complex tectonic deformation zone, to uncover the characteristics and main controlling factors of gas-shale reservoir in the tectonic deformation zone.The Wufeng Formation-Longmaxi Formation shale samples in the Zheng'an area have a high quartz component, which ranges from 60% to 80% and contributes a high brittleness index.Quartz in the Wufeng Formation-Longmaxi Formation shale is dominant by inorganic origins, including the primary clastic quartz deposition and the secondary quartz from clay diagenesis.The cracking of primary clastic quartz grains or bending of laminated clay under tectonic compaction increases pore space; whereas the filling of secondary quartz into the pore space between mineral grains decreases the bulk porosity.The Wufeng Formation-Longmaxi Formation shale samples in the Zheng'an area have a total organic carbon(TOC) content up to 5.8%, a content of amorphous organic matter more than 80%, and a bitumen reflectance of 2.6%-3.1%.The cracking of organic matter generated number of organic pores, which improved the bulk porosity first and then was compacted and collapsed to be long and narrow polygons.

Keywords

• organic pore
• inorganic pore
• complex tectonic deformation zone
• wufeng formation-longmaxi formation shale