Characteristics and controlling factors of tight sandstone reservoir of the Jurassic strata in the Lenghu region, Northern Qaidam Basin

The Jurassic strata in Lenghu area are rich in oil and gas resources and have a high degree of reservoir densification. In order to find out the genetic mechanism of porous and low permeability reservoirs and to analyze the controlling factors of reservoir development, a variety of experimental methods are used in this work, such as analyses of normal thin-section, cast thin-section and scanning electron microscope, X-ray diffraction of clay minerals and nuclear magnetic resonance. Based on the description of petrology characteristics of reservoir pore structures characterization and physical characteristics, the characteristics of reservoir diagenesis are investigated. The results show the following: (1) The rock types of the Jurassic reservoir rocks in the study area are mainly feldspathic lithic sandstones and lithic sandstones. The main pore types of sandstones are mainly secondary solution pore and the proportion of micro pores are relatively high. The average porosity of sandstone is 9.96% and the average permeability of sandstone is 2.26×10⁻³ μm², belonging to special low porosity-ultra-low permeability reservoirs. (2) The types of diagenesis are mainly compaction, cementation and dissolution. The diagenetic evolution mainly takes place during the mesodiagenetic phase A. The diagenetic sequence can be summarized as follows: compaction - a small amount of calcite cement is formed - feldspar and rock debris are slightly corroded - secondary enlargement of quartz - feldspar and rock debris are strongly corroded due to the formation of a large number of organic acids - clay minerals appear widely - secondary enlarged edges appear again in quartz particles - feldspar, rock debris and clay minerals are gradually transformed into illite - a small amount of iron-bearing calcite cement is formed. (3) Various types of diagenesis had once played important roles in the formation of ultra-low permeability tight sandstone reservoirs. The coal measure strata were rich in the remains of aquatic and terrestrial animals and plants. After the sediment was buried, the organic matter can decompose and produce humic acid, and then form an acidic environment. There were little calcite cements in the clastic reservoir because of the acidic water medium condition of the coal measures strata in the early diagenetic stage. The strong compaction greatly reduced the primary porosity. The widespread clay minerals occupy the pore space and divide the macropores into countless micro pores. The cementation of clay minerals further intensifies the reservoir densification, which intensifies the process of reservoir densification. The continuous dissolution of feldspar and rock debris components by organic acid fluids which came from thermal evolution of organic matter played a key role in improving reservoir physical properties. This study can deepen the understanding of diagenetic evolution of tight sandstone reservoir in the northern margin of Qaidam Basin, which has a guiding significance for further oil and gas exploration.