Diagenesis effects on the conglomerate reservoir quality of the Baikouquan Formation, Junggar Basin, China

Large accumulations of oil and gas have been found in fan delta conglomerate reservoirs in the upper Triassic Baikouquan Formation in the Mahu Sag at the northwestern margin of the Junggar Basin. To effectively explore strongly heterogeneous, high-quality, conglomerate reservoirs, their deposition, diagenesis, and genetic mechanisms must first be thoroughly studied. Analyses of cores, thin sections, fluid inclusions, oxygen isotopes, logging, physical properties data, X-ray diffraction, scanning electron microscopy, and cathodoluminescence confirm that the conglomerates have high feldspar contents as well as volcanic fragments that comprise a variety of micro-lithofacies. The heterogenous reservoir properties include permeability values of less than 1 mD, 1–10 mD, and higher than 10 mD, which correspond to 44.19%, 37.06%, and 18.75% of the reservoir, respectively. The tight reservoirs, mostly developed in the fan delta plain, have experienced extensive compaction and/or calcite cementation and consist of only a few secondary pores. Reservoirs with high porosity and permeability developed in subaqueous channels and experienced relatively weak compaction. These reservoirs contain large numbers of secondary pores that were mainly formed by the dissolution of K-feldspars and volcanic rock fragments. Comprehensive studies of sedimentation and diagenesis in the Baikouquan Formation indicate that these high porosities originated from the chronological coupling of the following three important geological processes: (1) Conglomerate reservoirs with better sorting and roundness and lower mud content are mainly developed in subaqueous distributary channels and have higher porosity and permeability than reservoirs with poorer sorting and roundness. (2) Strong dissolution of K-feldspar and volcanic debris occurred in reservoirs with good original physical properties, forming large numbers of secondary pores. Acidic fluids from source rocks in the Fengcheng Formation led to dissolution. (3) The byproducts of dissolution, which consist mostly of mixed-layer illite/smectite and illite, reduced porosity and permeability. In reservoirs with good original physical properties and fault development, dissolution plays an obvious role in improving physical properties in such relatively open diagenetic systems.