ABSTRACT

Pore-throat structure characterization is crucial in tight-oil sandstone reservoir studies. To better understand pore-throat size distribution and the effects on reservoir oil saturation, petrographic, scanning electron microscopy, quantitative grain fluorescence (QGF), nuclear magnetic resonance (NMR), and pressure-controlled mercury injection investigations were performed on a suite of tight sandstones from the fourth member of the Lower Cretaceous Quantou Formation (K1q4) in the southern Songliao Basin, China. The tight sandstone samples showed similar compositional and textural maturity but different reservoir qualities at different depth intervals. Two oily indicators, QGF index and QGF on extract spectra intensity, varied significantly with respect to burial depth, suggesting the existence of strong oil heterogeneities in the K1q4 tight sandstones. Oil emplacement occurred mainly in the primary pores associated with the absence of pore-filling authigenic minerals. Pore-throat size distributions had wider ranges in the oil-rich layers than in the samples lacking oil emplacement. The NMR pore-throat radii ranged from 0.01 to 10 μm in oil lacking samples but ranged from 0.01 to 100 μm in oil-rich layers. Oil signals were mainly detected in the pore throats with radii ranging from approximately 0.1 to 1.0 μm in oil-rich layers. The NMR porosity can be divided into three parts based on transverse relaxation time (T2) spectrum: (1) immovable fluid, (2) porosity of effective movable fluid (PEMF), and (3) porosity of bound fluid. Bivariate plots of the ratios of various porosity types have significant effects in deciphering the oil saturation of tight sandstone reservoirs. The PEMF reflected well the reservoir QGF index, among others. Therefore, it can be considered as an effective NMR parameter to characterize pore size distribution and helpful in oil reservoir prediction. A good relationship exists between PEMF and geometric mean of the T2 distribution (T2gm), indicating that T2gm could also reflect reservoir oil saturation. The PEMF is mainly contributed by large pore throats, and the poorly sorted pore-throat distribution is better for effective movable fluids occurrence in tight sandstones. The approach employed in this study provides a more comprehensive and accurate pore-throat structure characterization in tight sandstone oil reservoirs, generating effective parameters and convenient ways to evaluate reservoir oil saturation. Importantly, it could be applicable for improving tight sandstone oil exploration and development efficiencies.

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