Characteristics and assessment of the Chang-7₁ reservoirs from the Yanchang Formation in the Maling region, northern Shaanxi

The petrographic, physical and pore throat characteristics and heterogeneity of the Chang-7₁ reservoirs from the Yanchang Formation in the Maling region, northern Shaanxi are discussed on the basis of core observation, cast section, scanning electronic microscope, X-ray diffraction analysis, grain-size analysis and mercury-injection curves. The Chang-7₁ reservoirs in the Maling region are characterized by low to extralow porosity, ultralow to extralow permeability and intense heterogeneity. The main factors influencing the reservoir capacity include sedimentary environments, nature of sketchetal grains and diagenesis. The sedimentary environments, paulopost diagenesis and cementation are responsible for poor physical properties of the reservoirs, and dissolution led to the modification of the reservoirs. In the light of the physical and porosity parameters, the reservoirs in the study area may be classified into four types, and the Chang-7₁ reservoirs in the Maling region are dominated by the III type.