INFLUENCE OF NATURAL FRACTURES ON OIL MIGRATION AND CHARGING: A CASE STUDY ON THE PERMIAN LUCAOGOU FORMATION IN THE JIMSAR SAG, JUNGGAR BASIN, NW CHINA

The Lucaogou Formation is a typical tight reservoir. According to outcrops, cores, FMI, casting thin sections and SEM, we distinguish distinct kinds of natural fractures, including tectonic fractures, diagenetic fractures, and overpressure-related fractures. Among them, diagenetic can be categorized into bedding fracture, stylolite, intragranular fracture and shrinkage fracture. The calculation based on Monte Carlo method FMI shows that the tectonic fracture and diagenetic fracture enhance the reservoir permeability, both of which are the main fluid flow channel in tight sandstone. The formation of diagenetic fractures took place through the whole process of diagenesis, which is earlier than the formation of tectonic fractures, bedding fractures and stylolite dominating primary migration of oil. Intragranular fractures and shrinkage fractures increase the connectivity of tight sandstones by connecting micro-pores. Evidence from carbon and oxygen isotope of calcite fillings within tectonic fractures indicates that tectonic fractures formed in the Late Triassic, Mid-late Jurassic and Early Cretaceous. The first-phase tectonic fractures formed in the late Triassic coincides with the first-stage oil charging (235~210Ma) during which the permeability of reservoir was improved, providing a percolation channel of oil for tight sandstone reservoir. The second-phase tectonic fractures formed in the middle and late Jurassic coincides with the second-stage charging (180~150Ma), but this happened when the porosity and permeability of matrix decreased. Therefore, the tectonic fractures can not only improve the previous poor porosity and permeability, but also be used as the main seepage and important reservoir space of tight oil. The third tectonic fracture formed in the early Cretaceous, serving as the migration channel of oil to readjust the accumulation and distribution of tight oil. However, it may cause certain destruction to the preservation of tight oil. The overpressure-related fractures formed in the middle and late Cretaceous and were filled with calcite, which had little effect on tight reservoir. By discussing the characteristics, distribution law, formation time, validity for hydrocarbon charging and migration of the natural fracture, combining with oil production test, it is knew that the density and aperture of reservoir fractures together determine the productivity of tight oil. With equivalent degree of fractures development, the aperture of fractures affect oil productivity more significantly.