Abstract
The pore-throat size determines the oil and gas occurrence and storage properties of sandstones and is a vital parameter to evaluate reservoir quality. Casting thin sections, field emission scanning electron microscopy, high-pressure mercury injection and rate-controlled mercury injection are used to qualitatively and quantitatively investigate the pore-throat structure characteristics of tight sandstone reservoirs of Xiaoheba Formation in the southeastern Sichuan Basin. The results show that the pore types include intergranular pores, intragranular dissolved pores, matrix pores, intercrystalline pores in clay minerals, and microfractures, and the pore-throat sizes range from the nanoscale to the microscale. The high-pressure mercury injection testing indicates that the pore-throat radius is in range of 0.004–11.017 µm, and the pore-throats with a radius >1 µm account for less than 15%. The rate-controlled mercury injection technique reveals that the tight sandstones with different physical properties have a similar pore size distribution (80–220 µm), but the throat radius and pore throat radius ratio distribution curves exhibit remarkable differences separately. The combination of the high-pressure mercury injection and rate-controlled mercury injection testing used in this work effectively reveals the total pore-throat size distribution in the Xiaoheba sandstones (0.004–260 µm). Moreover, the radius of the pore and the throat is respectively in range of 50–260 µm and 0.004–50 µm. The permeability of the tight sandstones is mostly affected by the small fraction (<40%) of relatively wide pore-throats. For the tight sandstones with good permeability (>0.1 mD), the larger micropores and mesopores exert a great influence on the permeability. In contrast, the permeability is mainly influenced by the larger nanopores. Furthermore, the proportion of narrow pore-throats in tight sandstones increases with reducing permeability. Although the large number of narrow pore-throat (<100 nm) makes a certain contribution to both reservoir porosity and permeability, they have contribution to the former is far more than to the latter.
Highlights
With the increasing depletion of conventional oil and gas reservoirs, tight oil and gas reservoirs are becoming an area of increased research interest in the current geological community (Camp, 2011; Ghanizadeh et al, 2015a, 2015b; Yin and Gao, 2019)
A set of typical marine tight sandstones sandwiched between the overlying thick mudstones of the Hanjiadian Formation and the underlying shales of the Longmaxi Formation is developed in the Xiaoheba Formation of the Lower Silurian system, southeastern Sichuan Basin, thereby forming a good source–reservoir–caprock association
Two hundred ninety samples stained with blue epoxy resin under vacuum were processed into thin sections with a thickness of 0.03 mm, and all thin sections were polished on both sides to perform petrographic, mineralogical, and pore type analyses with the polarizing light microscopy instrument (Leica DM4500P) (Leica Microsystems Inc, Wetzlar, Germany)
Summary
With the increasing depletion of conventional oil and gas reservoirs, tight oil and gas reservoirs are becoming an area of increased research interest in the current geological community (Camp, 2011; Ghanizadeh et al, 2015a, 2015b; Yin and Gao, 2019). A set of typical marine tight sandstones sandwiched between the overlying thick mudstones of the Hanjiadian Formation and the underlying shales of the Longmaxi Formation is developed in the Xiaoheba Formation of the Lower Silurian system, southeastern Sichuan Basin, thereby forming a good source–reservoir–caprock association. In this work, taking the tight sandstones in the Lower Silurian Xiaoheba Formation, southeastern Sichuan Basin, PLM, FE-SEM, HPMI, and RCMI are conducted to investigate the pore-throat systems. As a valuable stratum for petroleum exploration in the southeastern Sichuan Basin, the Xiaoheba Formation, vertically, is sandwiched between the black carbonaceous shale of the Longmaxi Formation and the grayish-green thick mudstone of the Hanjiadian Formation, which has formed excellent source–reservoir–caprock combinations with both sufficient source rocks and a large mudstone caprock thickness. Porosity and permeability tests were performed first, and these core plugs were divided into four parts for thin-section observation, FE-SEM, HPMI, and RCMI experiments
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