Pore structure and connectivity of tight sandstone reservoirs in petroleum basins: A review and application of new methodologies to the Late Triassic Ordos Basin, China

摘要

Pore structure and connectivity are key factors that determine the quality of tight sandstone reservoirs, which are a frontier field for exploration worldwide in the 21st century but are quite challenging to characterize. Here we explored novel methodologies based on a case study of the well-known Chang-7 reservoir in the Upper Triassic Yanchang Formation, Ordos Basin, China. Pore size distribution (PSD) can be quantitatively characterized by nuclear magnetic resonance cryoporometry (NMRC), and the connectivity of the reservoir can be described by using a combination of Wood's metal (WM) impregnation, environmental scanning electron microscopy (ESEM), and nano-computed tomography (CT). The PSD of tight sandstones with different oil-bearing levels exhibit obvious differences. The complexity of pore boundaries and orientations are key factors that affect reservoir physical properties and oiliness. The pore fracturing in tight sandstones can be divided into three types: poor, medium, and good connectivity. The ball-and-stick model is suitable for the study of reservoir connectivity, and the effect of tight sandstone heterogeneity on connectivity analysis can be reduced significantly. Our data suggest that multi-scale characterization is reliable for describing the pore structure and connectivity of tight reservoirs. It is found for the first time that the quality and oiliness of tight sandstone reservoirs are not determined solely by physical properties, but are also controlled by PSD and pore morphology and connectivity. This may lead to underestimation of the effective reservoir and potential resources as previously thought.