Abstract

A grading system for tight sandstone reservoir quality is needed to predict tight oil enrichment areas and assess the resources. To explore the establishment of the grading system, a variety of research methods, such as rate-controlled mercury injection, conventional mercury injection, contact angle measurement, and the mechanical equilibrium principle, are integrated to determine the upper and lower limits of the porosity, permeability, and pore-throat radius of tight sandstones and to establish a quality grading system. Based on the porosity ϕ, permeability K, and pore-throat size r properties of the studied samples from the K1q4 Member, five sandstone classes have been identified. Three of these classes are tight sandstone reservoirs and include (1) high-quality tight sandstone reservoirs (ϕ=9%12%, K=0.11.0  mD, and r=0.0630.4  μm), (2) effective tight sandstone reservoirs (ϕ=6%9%, K=0.050.1  mD, and r=0.00630.063  μm), and (3) low-quality tight sandstone reservoirs (ϕ=4%6%, K=0.010.05  mD, and r=0.00040.0063  μm). Sandstones with ϕ, K, and r parameters higher than the high-quality tight reservoirs are deemed to be conventional reservoirs, whereas those with parameters lower than the low-quality tight sandstone reservoirs are considered as nonreservoir sandstones. It is also noted that oil saturation of the tight sandstone reservoirs correlates positively with the throat radius rather than with the pore size. High-quality tight sandstone reservoirs are usually developed in the distributary channel sand bodies near faults and/or fractures, and they are capable of producing more petroleum.

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