Classification and control factors of pore-throat systems in hybrid sedimentary rocks of Jimusar Sag, Junggar Basin, NW China

Aiming at the complicated problem of the genesis of high-quality hybrid sedimentary rocks, the pore-throat systems, controlling factors and fluid mobility of hybrid sedimentary rocks in the Permian Lucaogou Formation in Jimusar Sag were examined. The results show that the hybrid sedimentary rocks contain 5 types of pore-throat system, intergranular (Type A), mixed intergranular-dissolved-intercrystalline (Type B), dissolved (Type C), mixed dissolved-intercrystalline (Type D) and intercrystalline (Type E) ones. The pore-throat systems are controlled by 3 major factors, the component content and arrangement (CCA) of hybrid sedimentary rocks, sedimentary environment and diagenesis. CCA controls the matrix support mode of hybrid sedimentary rocks, and therefore controls the types and changes of pore-throat system. The sedimentary environment mainly controls the macroscopic distribution of pore-throat system, i.e., hybrid sedimentary rocks deposited in the near source and high-energy environment are characterized by high content of coarse-grained component, granular/interbedded-support mode, and development of Type A and Type B pore-throat systems. Hybrid sedimentary rocks deposited in the medium-energy environment far from source are characterized by dolomitic/mud support mode and Type C and Type D pore-throat systems. Hybrid sedimentary rocks deposited in low-energy environment far from source have mainly Type E and Type D pore-throat systems. Diagenetic processes such as compaction and calcite cementation make the proportions of Type A and Type C pore-throat systems decrease further. In the hybrid sedimentary process of sandy-mud, pore-throat system types show a change of “A→B→C→D”, in that of dolomite-sand, pore-throat system types show a change of “A→C→D→E” or “B→D→E”, and in that of dolomite-mud, pore-throat system types show a change of “D→E”, which are affected in details by the contents of coarse-grain component, feldspar and dolomite. The reservoir with Type A pore-throats has the best physical properties and fluid mobility, and the reservoirs with Type D and Type E pore-throats have the poorest. The movable fluid distribution is related to the matrix support mode, and the larger pores in hybrid sedimentary rocks of dolomite/mud support mode have no obvious advantage in fluid mobility. The findings of this study provide a geological basis for evaluating and building reasonable interpretation model of hybrid sedimentary rocks sweet spot.