Characteristics and Formation Mechanism of a High-quality Reservoir in the Deep Strata of the Guaizihu Depression, Yin-E Basin

Key words: 
• deep-burial clastic rock /
• reservoir characteristics /
• gravity flow deposit /
• Bayingebi Formation /
• Guaizihu Depression

Abstract: A set of deep-water gravity flow deposits was developed in the Bayingebi Formation in the Guaizihu Depres-sion. Although the buried depth is more than 3 400 m, the reservoir exhibits good physical properties. In order to illus-trate the characteristics of this high-quality reservoir, petrological features, reservoir properties, and pore structure characteristic were analyzed based on the integrated application of thin section observations, scanning electron micros-copy (SEM), mercury penetration, and petrophysical properties, combined with the burial history features. We then discuss the formation mechanism. The results show that the sand bodies are mainly composed of feldspathic litharen-ite with low compositional maturity, low porosity, and extra-low permeability (the average porosity is 11.4%; the aver-age permeability is 6.093×10^-3 μm²). The types of reservoir space are dominated by residual intergranular pores, fol-lowed by dissolved intergranular and intragranular pores, and a small amount of microfractures. The throat of the reser-voir is small, and the pore/throat ratio is relatively large. This reservoir is mainly characterized by medium-large pores and a medium-thin throat. The primary textural and mineralogical compositions of the sediments were the fundamental conditions for the formation of the high-quality reservoir in the Bayingebi Formation. Good initial physical properties developed due to strong hydrodynamic conditions, coarse-grained sediments, medium-sorting, and low matrix content. In addition, high hard particle contents gives the reservoir a stronger ability to resist compaction, and some feldspar and rock debris provide material basis for later dissolution. Early rapid burial, the later stage of tectonic uplift, and the hydrocarbon generation of organic matter played an important role in the formation of overpressure and dissolution. The combination of abnormal high pressure, dissolution by organic acid, and hydrocarbon filling were the key factors in forming the high-quality reservoir in the deep-burial Bayingebi Formation.