DIAGENESIS OF ANALCIME-BEARING RESERVOIR SANDSTONES - THE UPPER PERMIAN PINGDIQUAN FORMATION, JUNGGAR BASIN, NORTHWEST CHINA

The Junggar Basin is one of the largest and most important oil-produci ng basins in China, in which Upper Permian lacustrine oil shales are a mong the thickest and richest source rocks in the world. The Upper Per mian Pingdiquan Formation was deposited predominantly in fan-delta seq uences within a lacustrine setting, In the northeastern Junggar Basin, the Pingdiquan Formation sandstones (volcanic and feldspathic lithare nites) constitute the principal oil reservoirs, whereas the interbedde d black shales are the predominant oil source rocks, The early diagene tic minerals in the sandstones include siderite, pyrite, analcime, alb ite, calcite, and trace amounts of halite, Late diagenetic minerals in clude K-feldspar, ankerite, and minor mixed-layer clay minerals. A sim ilar diagenetic sequence was recognized in the intercalated mudrocks, which contain abundant analcime, albite, and microcrystalline dolomite . Early authigenic mineral formation (e.g., calcite versus analcime/al bite) was controlled by alternating periods of lower and higher salini ty in the lacustrine environment. Cementation by siderite, analcime, c alcite, and albite substantially occluded sandstone porosity early in diagenesis, However, extensive dissolution of analcime cement and detr ital feldspars during burial produced significant secondary porosity i n the sandstones, improving their reservoir potential. Organic acids g enerated during oil-shale maturation may be responsible for secondary porosity in the interbedded sandstones.