Braided river delta deposition and deep reservoirs in the Bohai Bay Basin: A case study of the Paleogene Sha 1 Member in the southern area of Nanpu Sag

Abstract

Based on core description, conventional thin section, cast thin section and scanning electron microscope (SEM) analysis, combined with general physical analysis, the physical properties of the braided river delta sandstone reservoirs and the range of reservoir quality of the Sha 1 member have been analyzed in the southern Nanpu Sag of the Bohai Bay Basin. The characteristics and controlling factors of the southern provenance of Sha 1 reservoirs are examined to reveal the causes for the formation of deep, high-quality reservoirs more than 4000 m deep. Sha 1 reservoirs in the study area are made up of gravel rock, pebbled inequigranular sandstone, medium sand and gritrock. The lithology is mainly lithic feldspar sandstone, the contents of rigid particles like quartz and metamorphic rock cuttings are higher, the content of matrix is very low and the structure is supported by grains. The average porosity is 12.8%. The average permeability is 154.9×10−3μm2. The reservoir type is low porosity and high permeability. The reservoir space is mainly made up of primary and secondary pores. Parent rock type, sedimentary environment and the abnormal high pressure in the Sha 1 Member are the dominant factors controlling the reservoir properties. It is inferred that high quality sandstone reservoirs with high natural productivity of oil and gas are widely developed in the strata more than 4000 m deep in the Bohai Bay Basin, and so conventional sandstone reservoir exploration is expected to expand to these deeper intervals. Key words: Bohai Bay Basin, Nanpu Sag, Sha 1 Member, braided river delta, deep strata, reservoir characteristics