Acid-base alternation diagenesis and its influence on shale reservoirs in the Permian Lucaogou Formation, Jimusar Sag, Junggar Basin, NW China

Abstract

The diagenesis and diagenetic facies of shale reservoirs in Lucaogou Formation of Jimusar Sag were studied by means of microscopic observation and identification of ordinary thin sections and cast thin sections, X-ray diffraction, scanning electron microscope and electron probe tests. The results show that alkaline and acidic diagenetic processes occurred alternately during the deposition of Permian Lucaogou Formation in Jimusar Sag. The evolution of porosity in the shale reservoirs was influenced by compaction and alternate alkaline and acidic diagenetic processes jointly, and has gone through three stages, namely, stage of porosity reduction and increase caused by alkaline compaction, stage of porosity increase caused by acid dissolution, and stage of porosity increase and reduction caused by alkaline dissolution. Correspondingly, three secondary pore zones developed in Lucaogou Formation. The shale reservoirs are divided into three diagenetic facies: tuff residual intergranular pore-dissolution pore facies, tuff organic micrite dolomite mixed pore facies, and micrite alga-dolomite intercrystalline pore facies. With wide distribution, good pore structure and high oil content, the first two facies are diagenetic facies of favorable reservoirs in Lucaogou Formation. The research results provide a basis for better understanding and exploration and development of the Lucaogou Formation shale reservoirs.

Keywords

• acid-base alternation diagenesis
• porosity evolution
• shale oil
• shale reservoir
• Permian Lucaogou Formation
• Jimusar Sag