Diagenetic evolution of key minerals and its controls on reservoir quality of Upper Ordovician Wufeng-Lower Silurian Longmaxi shale of Sichuan Basin

Abstract

Based on core, thin section, scanning electron microscopy observations, X-ray diffraction analysis, as well as carbon and oxygen isotopes and energy spectrum analysis of carbonate rocks, the quartz, feldspar, pyrite, carbonate and clay minerals in shale of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations of Sichuan Basin were effectively characterised and classified, and the influences of their diagenetic evolution sequence on the development of shale reservoir were discussed. The results showed that good material basis and unique diagenetic sequence were the key factors for the formation of high-quality shale reservoirs. (1) Framboidal/euhedral pyrite, bio-quartz and microbial dolomite were mainly formed from the syngenetic stage to the A-substage of early diagenetic stage. They were both destructive and constructive for maintaining the original pores in shale, and the constructive supporting framework of which was critical for the formation of high-quality shale reservoir. The rigid framework formed by these early-formed minerals and terrigenous debris facilitated the maintenance of original pores and the reservoir stimulation of shale gas exploitation. (2) The co-evolution of hydrocarbon generation and diagenesis promoted the development of reservoir spaces. In the A-substage of middle diagenetic stage, the production and consumption of organic acids, the dissolution/alteration of unstable minerals (feldspar and carbonate minerals), clay mineral conversion and oil generation from kerogen were synchronic, which provided favorable space for the charging and retention of liquid hydrocarbons during the oil generation period. From the B-substage of middle diagenetic stage to the late diagenetic stage, the shale gas/organic pore generation and pressure increase of kerogen and retained hydrocarbon cracking promoted the development of organic pores and micro-fractures, which was conducive to the enrichment and high production of shale gas.

Keywords

• mineral type
• diagenetic evolution sequence
• shale gas
• wufeng formation
• longmaxi formation
• upper ordovician
• lower silurian
• sichuan basin