Characteristics, genetic mechanism and oil & gas exploration significance of high-quality sandstone reservoirs deeper than 7000 m: A case study of the Bashijiqike Formation of Lower Cretaceous in the Kuqa Depression, NW China

Abstract

Ultra-deep clastic reservoirs generally have poor physical properties and low single-well productivity, but Well BZ9, which is newly drilled in the Kuqa Depression of Tarim Basin, NW China, encounters a thick high-quality reservoir in the Bashijiqike Formation of Lower Cretaceous deeper than 7600 m and produces a high-yield industrial gas flow. In order to reveal the characteristics and genesis of the Bashijiqike Formation reservoir and reduce the exploration risk of ultra-deep oil and gas layers, we discussed its characteristics, genetic mechanism and oil & gas exploration significance based on cores, well logging and experimental analysis, combined with regional temperature–pressure conditions and burial evolution history. The following research results were obtained. First, the rock types of ultra-deep reservoir of the Bashijiqike Formation in this area are medium- and fine-grained feldspathic litharenite and lithic arkose with point-line contact between grains, and its reservoir space is dominated by primary intergranular pores. At present, it is still at its middle diagenetic stage. Second, different from other ultra-deep fractured low-porosity sandstone reservoirs, this set of ultra-deep reservoir is a pore-type reservoir with porosity of 4–13%, permeability of 0.1–50.0 mD and better porosity and permeability correlation. Third, during the sedimentation of the Bashijiqike Formation, thick sand bodies of delta front were widely developed, medium- and fine-grained sandstones accounted for more than 85% and grains had strong compressive capacity. After that, this reservoir experienced long-term shallow burial during the early–medium stage and rapid deep burial during the late stage, and the burial compaction effect was weaker. During the late stage, a canopy structure was formed from the overlying Paleogene thick gypsum salt bed due to thrust and compression, and it further suppressed vertical compaction. In the meantime, the study area was far from the orogenic belt and the structural transition zone, so the lateral compressive stress was weak. Therefore, the primary intergranular pores were preserved in large quantities. It is concluded that coarse lithology, weak compaction and low tectonic stress are the key factors to the development of this ultra-deep high-quality reservoir. In addition, the development of large-scale effective reservoirs deeper than 7000 m provides favorable material conditions for the high-abundance enrichment of natural gas and the reserves of trillion cubic meters in the Kuqa Depression, and the oil and gas exploration potential is huge.