Reservoir quality prediction of deeply buried tight sandstones in extensively faulted region: A case from the middle-Upper Jurassic Shishugou Group in central Junggar Basin, NW China

Abstract

The reservoir quality of tight sandstones is generally determined by the depositional environment, burial depth and diagenesis. However, the relationship between reservoir quality of tight sandstones and the controlling factors are very complicated. Deeply buried Middle-Upper Jurassic Shishugou Group sandstones are important tight oil sandstone reservoirs in the central Junggar Basin of NW china. The individual sandstones are thin (2–20m, av.7.8m), overlain by an unconformity and directly connected to the source rock by a well-developed fault system. The diagenetic system and reservoir quality of the Shishugou Group sandstones in the Central Junggar Basin was investigated using petrology, mineralogy, pore characteristics, sedimentary facies, burial and thermal history, knowledge of hydrocarbon charging, fault/fracture system, diagenetic reactions and sequence, controlling factors of reservoir quality, paleo-porosity reconstruction and formation mechanism of high-quality reservoir. The thin-section observation, scanning electron microscope (SEM and BSD), granularity analysis, Xray diffraction (XRD) and fluorescence microscope, 3D seismic and well logging analysis were used in this research. The top unconformity and faults system were the pathways for water and hydrocarbon migration. Organic acids and meteoric water with high concentrations of CO2 were the main fluids that caused dissolution of volcanic rock fragments, feldspar, analcime cement and early calcite cement. The paleo-porosity variation of Shishugou Group sandstones was analyzed quantitatively by the inversion and back stripping with the constrain of diagenetic evolution history and theoretical compaction models. The comprehensive researches including the origin of pore waters, origin of acidic fluids, fault/fracture system and hydrocarbon charging history, show that near-surface or meteoric diagenetic reactions of Shishugou Group sandstones closely beneath top unconformities were in open (geochemical) diagenetic systems, and that diagenetic (geochemical) systems were relatively open, especially in the faults-developed zone during burial process. The central sandstone body of the point bar, underwater distributary channel and distributary channel, which were cut through by the oil source fault, retained high-quality reservoir. The reservoir quality can be mostly predicted as a function of sedimentary environment (sedimentary facies and provenance), diagenetic reactions and burial history with the constrain of source rocks, diagenetic system and fault system.