Abstract:

The significant oil and gas exploration breakthrough made in the middle-to-deep-layers in HTHP(high tempe-rature and high pressure)central diapir zone in DF area of Yinggehai Basin poses a big challenge to traditional oil and gas exploration theories.Lithology,diagenesis,physical properties and microscopic pore throats analyses were carried out on the reservoirs in the Huangliu Formation of the area to classify the types of the reservoirs and understand the genesis of high porosity and low permeability formation.The results indicate that the reservoirs are mainly siltstone experienced weak compaction and cementation.The physical properties of the reservoirs are dominated by medium porosity and medium-to-low permeability with great variance.Pore throat size distribution of the reservoirs is highly heterogeneous:reservoirs of different physical properties have similar pore size but greatly different average pore throat sizes.According to shale content,permeability,average throat size and displacement pressure,the reservoirs can be categorized into four types:slightly shaly reservoirs with medium porosity,medium permeability and small throat size;type Ⅰ and type Ⅱ moderately shaly reservoirs with low permeability and small throat size;shaly reservoirs with low porosity and extra-low permeability and fine-to-small throat.Evident shows that the formation of the reservoirs was closely linked to overpressure and active high-tempe-rature thermal fluid flow. However,the permeability of the reservoirs was mainly controlled by sedimentation.Fine grain size and high content of mud are considered to be the main reasons for the formation of the low permeability reservoirs.

Key words: pore throat structure, high temperature and high pressure, low permeability reservoir, Huangliu Formation, Yinggehai Basin