Characteristics of abnormal pressure and its influence on deep and ultra-deep tight reservoirs in the Junggar Basin

In recent years，discoveries and breakthroughs have been made successively in exploration of the deep and ultra-deep sequences in the Junggar Basin. The reservoirs found in the sequences are mostly tight with abnormally high pressure. In order to clarify the development characteristics of abnormally high pressure and its impact on reservoir quality，statistical analysis is carried out by using the pressure data obtained through systematic production tests in the key wells to determine the characteristics and types of pressure distribution in deep and ultra-deep layers of the basin，and its impact on the quality of tight reservoirs in combination with diagenesis. The results show that abnormally high pressure is widely developed in the deep and ultra-deep layers where residual pressure and pressure coefficient are high，vertical and lateral distribution of the abnormal pressure are broad. Four abnormally high pressure zones are recognized，i. e. ，the fairly high，moderately high，very high and super high pressure zones. Blocks in the western and eastern parts are located in the fairly high or moderately high pressure zones，blocks in the central part are located in the fairly high or very high pressure zones，and blocks in the southern part have fairly high，moderately high，very high or super high pressure. The vertical evolution of pressure coefficients can be described as gradual，abrupt and changeable due to different circumstances. The influence of abnormal high pressure on deep and ultra-deep tight reservoirs is mainly reflected in enhanced pressure bearing capacity of pore fluid，which in turn would reduce the pressure of rock skeleton particles，inhibit compaction，and contribute to the preservation of a large number of primary intergranular pores in the reservoirs. The abnormal high pressure also inhibits the formation and precipitation of cement and thus decreases continuously the volume fraction of cement with increasing burial depth，restrains the evolution of organic matter to extend hydrocarbon generation and expulsion period as well as the time and scope of organic acid action，enhances dissolution to form secondary pores，and promotes the accumulation of pore fluid pressure to facilitate the formation of non-structural fractures when the pore fluid pressure exceeds 60 % of overburden pressure. Abnormally high pressure plays a key role in the improvement of the quality of deep and ultra-deep tight reservoirs in the basin，and serves as one of the control factors for reservoirs to become effective. © 2023 Editorial Department of Oil and Gas Geology. All rights reserved.