Abstract:

It is important to elucidate the reservoir occurrence and its microscopic characteristics under microfacies control, which is of great significance to evaluate the oil enrichment differences. To clarify the characteristics and constraints meandering river reservoirs, Yan 9 reservoir (Hujianshan oilfield) in the Ordos Basin is taken as an example. Based on core logging and sampling, grain size analysis, thin section observation, scanning electron microscopy, mercury injection method, clay mineral X-ray diffraction and conventional physical property analysis, and logging data verification, our work shows that meandering river facies are developed in Yan 9, and include mainly channel (river bed retention sediment), point bar, natural levee, and floodplain microfacies. The reservoir lithology comprises mainly feldspathic litharenite and lithic arkose sandstone, whilst the reservoir open-space is mainly composed of residual intergranular pores and feldspar solution pores. Three major types of throats were identified, i.e., medium-fine throat, fine throat and micro-throat. The porosity is mainly concentrated in 11.51%-18.87%, and the permeability is mainly in (2.08-79.86)×10^-3 μm². It can be classified as medium-low porosity, medium-low permeability, extra low permeability, medium large porosity and fine throat reservoir. Cementation and mechanical compaction of clay minerals, siliceous and calcareous materials are the main cause for forming the tight reservoir. Dissolution of feldspar particles and some cement, and the inhibition of chlorite film on compaction and cementation are conducive to the primary pore preservation. Nonetheless, when the chlorite content exceeds 0.3%, the physical property change is unobvious.

Key words: meandering river, reservoir characteristic, controlling factor, Yan 9, Hujianshan oilfield