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Mercury injection capillary pressure (MICP) tests, nuclear magnetic resonance (NMR), (fluorescence) thin section, X‐ray diffraction (XRD), and scanning electron microscope (SEM) analyses were used to describe the size and distribution of entire pore‐throat structures in the sandstones of the E1f3 (the third member of Paleogene Funing Formation) in the Subei Basin. The lithologies of E1f3 in the Subei Basin are mainly dark‐gray, very fine‐grained sandstones and siltstones, interbedded with dark mudstones. The pore systems predominantly feature secondary intergranular and intragranular dissolution pores, micropores coexisting with minor amounts of intergranular pores, and microfractures. The high threshold pressure and bulk volume of irreducible fluids values and the significant variation in the NMR and MICP parameters indicate that the E1f3 reservoirs are characterized by complex and heterogeneous microscopic pore structures. Microscopic pore‐throat parameters are linked with macroscopic properties through the reservoir quality index (RQI). The NMR T2 (transverse time relaxation) spectrum is unimodal or bimodal but with weak right peaks, indicating the rarity of large intergranular pores. However, large‐scale pore throats, though only account for a minor part of the total pore volume, significantly contribute to the total permeability. The abundance of small‐scale pore‐throat systems (short T2 components) results in high irreducible water content. Therefore, the oil saturation in E1f3 sandstones is low, and the pore structure, especially the number of micropores, determines the oil‐bearing property. Oil primarily occurs in the intragranular dissolution pores with minor amounts occurring in the large intergranular pores. Most of the micropores are bound by capillary water. The sandstones with chlorite clay minerals tend to be oil‐wet and have high oil‐bearing potential, while the abundance of detrital clay or illite contributes to a low oil‐bearing grade. The combination of core and microscopic observations and the MICP and NMR analyses have allowed the determination of the pore structure characteristics and their coupling effects on oil‐bearing property. [ABSTRACT FROM AUTHOR]

Laboratory measurements include porosity, permeability, high pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR) measurements and microscopic analysis of thin sections and scanning electron microscope (SEM) were used to gain insight into the nature of the pore throat structure including pore geometry, pore size distribution and pore connectivity of Bashijiqike tight gas sandstones in Keshen 5, Keshen 6, Keshen 8, Keshen 9 and Keshen 13 Wellblock of Kuqa depression. The relationships between microscopic pore structure parameters and macroscopic petrophysical behaviors were investigated by regression analysis. The results show that various types of pores with wide ranges of pore radius are observed. The pore throats are very small (commonly <1 μm), and large pores are generally connected by tiny pore throats. Pore throats larger than r apex (pore radius at the apex of the Pittman's hyperbola) accounts for a small fraction of the pore volume, but make a significant contribution to permeability. The porosity connected by large pore throats (> r apex ) controls permeability in tight gas sandstones. The NMR pore size distribution is broader than the HPMI pore throat distribution. In the NMR analysis, only the pore systems connected by the relatively large pore throats should be accounted as movable, and some portion of large pores, which are not connected by effective large pore throats, are not effectively movable. The effective movable porosity is calculated by excluding those pores restricted by the tiny pore throats. The content of effective movable porosity from NMR measurements shows good correlation relationship with the porosity connected by large pore throats in HPMI analysis. Three typical types of pore structures are recognized through thin section, SEM analysis combined with the capillary curve and NMR T 2 spectrum, and the microscopic and macroscopic characteristics of the three pore structures are investigated. The pore structure were comprehensively evaluated and characterized by linking NMR T 2 spectrum with HPMI analysis. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs typically contain a wide pore throat sizes ranging from the nano-scale to micro-scale, and have complex pore geometry and pore throat structure. Microscopic pore throat structures are the most important factors affecting the macroscopic reservoir quality and fluid flow in tight sandstones. Evaluation and characterization quantitatively the microscopic pore structures, including pore geometry, pore size distribution, and pore connectivity, are of great importance for maintaining and enhancing petroleum recovery. This paper critically reviews the pore throat structures of tight sandstones, as assessed from peer reviewed papers in the literature as well as from the authors' personal experiences, in the particular contexts of comprehensive characterization and description of the entire pore throat structure using various complementary techniques. The depositional controls and diagenetic imprints on reservoir quality and pore structure are firstly discussed. The pore systems including pore throat type, pore geometry, pore size and connectivity, which are related to the depositional attributes and diagenetic modifications, are summarized. Then the theories and procedures of various testing techniques commonly used for pore structure characterization of tight sandstones are reviewed. Additionally, the pore throat structure characteristics in tight sandstones are obtained from various techniques such as MICP, NMR, N 2 GA and XCT. Pore throat distribution and capillary parameters of tight sandstones are examined, and the relationship between pore throat size distribution and permeability is overviewed. The pore size distribution and 3D pore connectivity are evaluated from NMR and XCT analysis. The NMR spectrum is also linked to the macroscopic performance, and the pore network is determined from N 2 GA. Then fractal theory is introduced to explain the irregularity and heterogeneity of pore throat structure characteristics, and the models for fractal dimension calculation through various techniques are summarized. Lastly the integration of various techniques is encouraged to fully characterize the entire pore size spectrum in tight sandstones by considering the complex pore structures and limitations of a single experiment in pore throat structure evaluation. This review will provide important insights into the microscopic pore structure characteristics of tight sandstones, and address the gap in comprehensive and quantitative evaluation of the heterogeneity in tight sandstones with complex microscopic pore structures. [ABSTRACT FROM AUTHOR]

Fractal geometry provides an effective method for characterization of the complex and irregular pore structure of Eocene Shahejie low permeability sandstones in the Raoyang Sag, the Bohai Bay Basin, China. Laboratory measurements including porosity, permeability, scanning electron microscope (SEM), thin sections, nuclear magnetic resonance measurements (NMR) and X-ray computed tomography (CT) technology are used to provide insights into the fractal characteristics of pore structure in sandstones of Eocene Shahejie Formation in the Bohai Bay Basin, China. Quantitative CT analysis reveals the pore radius is not always linear to T 2 (transverse relaxation time) value obtained from the NMR tests, but instead power function of T 2. Fractal analysis was performed on the T 2 distribution using various fractal models, and the related fractal dimensions are calculated. The fractal dimensions calculated using various fractal models are correlated with NMR parameters and permeability. The fractal curves break into two segments at the T 2cutoff (T 2 separating the immovable and movable fluids) value or smaller when using fractal model Ⅰ and fractal model Ⅱ, and only the large-scale pore networks can be described by the fractal geometry. Mostly the entire pore size distributions (micro-pores to large-scale pore networks) can be described by the fractal model Ⅲ, and the calculated fractal dimensions are in accordance with the CT scanning and thin section data, and are strongly correlated with the T 2gm (geometric mean of T 2), permeability and BVI (bulk volume of immovable fluids). The fractal behaviors of pore size distributions from NMR analysis have implications for pore structure evaluation in low permeability sandstones with similar geological settings. • Fractal analysis was performed on the NMR T 2 spectrum. • Fractal dimensions are calculated using various fractal models. • Fractal dimensions are related with the NMR parameter and permeability. [ABSTRACT FROM AUTHOR]