Shiyou shiyan dizhi (Jul 2021)
Characteristics and constrains of low-permeability reservoirs in the first member of Eocene Liushagang Formation, Weixinan Sag, Beibuwan Basin
Abstract
Reservoir analysis data such as core, thin-section, physical property and scanning electron microscope (SEM) were applied in this paper to study the reservoir characteristics and main constrains of the first member of Eocene Liushagang Formation in the Weixinan Sag, Beibuwan Basin. Reservoir features, physical properties, sedimentation, diagenesis, and the coupling of tectonics as well as hydrocarbon generation and accumulation were analyzed. Results showed that the reservoirs of the first member of Eocene Liushagang Formation in the area of this study were mainly feldspathic debris quartz sandstones and lithic quartz sandstones with low compositional and structural maturities. The reservoir space was mainly consisted of primary intergranular residual pores and secondary dissolved pores. In the northwestern and western source areas, the reservoirs mainly had moderate porosity and medium-high permeability, while those in the eastern source area had relative low porosity and low permeability. Sedimentation caused variations in grain size, sorting and argillaceous complexes. Diagenesis such as compaction, carbonate cementation and organic acid dissolution were the main controls for the physical properties of low-permeability reservoirs. The coupling of tectonics and hydrocarbon generation and accumulation controlled early hydrocarbon charging, while abnormal high pressure protected primary pores, which, combined with fractures derived from faults, improved the physical properties of low-permeability reservoirs. In the northwestern and western source areas, the shallow-buried reservoirs in the first member of Liushagang Formation had relatively strong hydrodynamic force, coarser grain size and better sorting, and showed better properties in the area with less mud. In the eastern source area close to the No. 2 fault around the main hydrocarbon-generating depression, there exist early hydrocarbon filling, strong dissolution of organic acids and partial high-pressure protection. Secondary pores and high-pressure pores were developed at 2 500-3 500 m deep, and favorable deep-burial reservoirs were formed.
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