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The high-pressure mercury intrusion (HPMI) experiment is widely used to assess the pore architecture of tight sandstone reservoirs. However, the conventional analysis of the high-pressure mercury intrusion has always focused on the mercury injection curves themselves, neglecting the important geological information conveyed by the mercury ejection curves. This paper quantitatively describes the fractal characteristics of ejection curves by using four fractal models, i.e., Menger model, Thermodynamic model, Sierpinski model, and multi-fractal model. In comparison with mercury injection curves, we explore the fractal significance of mercury ejection curves and define the applicability of different fractal models in characterizing pore architectures. Investigated tight sandstone samples can be divided into four types (Types A, B, C and D) based on porosity, permeability, and mercury removal efficiency. Type D samples are unique in that they have higher permeability (>0.6 mD) but lower mercury removal efficiency (

Biodegradation always alters the original characteristics of biomarkers, thereby limiting the identification of heavy oil sources. The eastern Chepaizi Uplift is a typical heavy oil-producing unit in northwestern China, most of heavy oils were discovered in Lower Neogene Shawan Formation, Lower Cretaceous Tugulu Group, and Upper Carboniferous Xibeikulasi Formation, the source of heavy oils is a controversial issue. This study adopted a biomarker recovery method based on the quality conservation principle that uses the sum of the mass of the residual biomarkers and their corresponding biodegradation products to obtain the mass of the original biomarkers. The 25–norhopane and the 17–nortricyclic terpane homologs, which were detected in severely to extremely biodegraded oils in the Chepaizi Uplift, while the corresponding hopanes (H) and tricyclic terpanes (TT) were eliminated, which are therefore an excellent option for biomarker recovery. A total of 13 relevant parameters were selected for recovery, the results indicated that the effect is particularly notable: the standard deviations of the tricyclic terpenes-related parameters (for example, (C20+C24) TT/(C23+C24) TT) and the hopanes-related parameters (for example, C30H/G (Gammacerane)) were 0.258–0.268, 0.400–0.540 before the parameter recovery, then they changed to 0.174–0.260, 0.176–0.273 after the parameter recovery, which indicates the dispersion of those parameter sets are significantly reduced and them numerical characteristics are more clearly highlighted. And the recovery result shows that the characteristics of 13 parameters are more consistent with the Carboniferous source rocks in this region and the Permian source rocks in Shawan Sag, indicating genetic affinity of heavy oil with these source kitchens. The reliability of oil source determination was verified using the stepwise discriminant analysis method and stable carbon isotope discrimination. Therefore, we believe that the biomarker recovery method is an effective method for oil–source correlation of heavy oil and can help deep oil exploration in biodegradation regions.

Analysis of reservoir sensitivity to velocity, water, salt, acid, alkali and stress is critical for reservoir protection. To study the tight sandstone reservoir sensitivity at different formation depths (effective stress) and formation water conditions (pH, salinity, and fluid velocity), a series of dynamic core flow tests under different pH, salinity, acid, and effective stress conditions were performed on samples from tight sandstone reservoirs of the Upper Triassic Yanchang 8 (T3y8) Member and conventional reservoirs of the Middle-Lower Jurassic Yan'an 9 (J1-2y9) Member in the Ordos Basin. The results indicate that, compared with the conventional reservoirs, the tight sandstone reservoirs are more sensitive to velocity and stress, less sensitive to water, alkali and salinity, and respond better to acid fracturing. In addition, the critical conditions (salinity, velocity, pH, and stress) for pumping drilling, completion, and fracturing fluids into tight sandstone reservoirs were investigated. A combination of scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS), cathodoluminescence (CL), casting thin section (CTS) and nuclear magnetic resonance (NMR) images, high-pressure mercury injection capillary pressure (MICP) measurements as well as X-ray fluorescence spectral (XRF) analyses were employed to analyze the damage mechanisms of the conventional reservoirs (J1-2y9) and tight sandstone reservoirs (T3y8) caused by fluid invasion. The results suggest that reservoir sensitivity is primarily conditioned by the composition of detrital components and interstitial fillings, petrophysical properties, pore-throat structure, and diagenetic facies. All these factors control the sensitivity types and extent of the reservoirs. Our results indicate that the poorer the reservoir physical properties, the stronger the reservoir heterogeneity and sensitivity, implying that tight sandstone reservoirs are more susceptible to changes in fluids than conventional reservoirs. This study offers insights into the reservoir damage types and helps to improve the design and implementation of protection measures for tight sandstone reservoir exploration.