Pore Characteristics and Their Controls on Movable Fluid Distribution of Low-Permeability Reservoirs in the Chang 2 Member, Ordos Basin, Northwest China

Pore structure controls the movable fluid distribution in low-permeability reservoirs. The microscopic characteristics of the pores and the movable fluid in the reservoir in the Chang 2 member in the Ordos Basin were studied with thin sections, scanning electron microscopy, mercury intrusion, and nuclear magnetic resonance. The pore type is mainly intergranular pores, followed by dissolution pores. The pore-throat ratio was between 21.11 and 269.66. The coordination number of the pore throat was mainly 1 or 2, the bound water saturation distribution was between 7.36% and 71.43%, the two-phase seepage area was small, the initial oil phase permeability decreased rapidly, the NMR T2 spectrum was mostly bimodal, the main NMR peak in the reservoir with high connectivity was located on the right side of the relaxation time of the T2 spectrum and the frequency amplitude was relatively large. The large pores were developed, the coordination number of the pore throat was high, the pore-throat ratio was low, the reservoir fluid was easy to flow, and the water flooding effect was better. The main NMR peak of the low-connectivity reservoir is located on the left side of the relaxation time of the T2 spectrum and the frequency amplitude is relatively large. The micropores are developed and the coordination number of pore throats is low, which indicates that it is easy to form dead pores and that water flooding is difficult.