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Compaction is an important part of diagenesis affecting porosity in conjunction with burial depth and time. However, the relation between initial porosity and sandstone compaction remains unclear. This paper aims to clarify the effect of initial porosity on the compaction of consolidated sandstone in the microdeformation stage via experiments, mathematical modeling, and statistical analyses. According to the physical experiments in the Ordos Basin, sandstone porosity decreases with increasing overburden load, and the porosity-decrease trends of two similar samples with different initial porosities converge gradually. Based on the Bingham visco-elasto-plastic stress–strain model, a theoretical model for the vertical uniaxial compaction of consolidated porous rocks was constructed. The equivalent viscosity coefficient (EVC) of consolidated sandstones was approximately estimated by a contrastive analysis and an iterative computation on the basis of the geologic statistical model proposed in this study. The results in Hangjin Qi show that the Young’s modulus and the EVC of low-porosity compaction trend (LPCT) are exponential and power-law functions of the corresponding porosity, respectively. The derivation of theoretical model indicates that the compressibility and the initial porosity of porous sandstone during microdeformation are positively correlated, in accordance with simulation experiments. The initial porosity affects the elastic modulus and the viscosity coefficient and consequently, the sandstone compaction during microdeformation stage. Such an effect can contribute to the improvements of both the compaction degree analyses of different burial processes in sandstone porosity evolution and reservoir quality evaluation. [ABSTRACT FROM AUTHOR]

Considering the complex factors controlling volcanic reservoirs, the Carboniferous strata in the eastern slope area of the Mahu Sag (ESMS) in the northwestern Junggar Basin (NJB) were investigated using rock cores, thin sections, scanning electron microscope (SEM), physical properties, major elements, X-ray fluorescence (XRF), well logging, and seismic data. The volcanic rocks revealed by drilling are mostly weathering crust reservoirs (WCRs), the formation of which in and around the study area is significantly controlled by weathering and leaching (WL). Most types of volcanic rock can be improved by long-term weathering. Favorable reservoirs in the ESMS are often developed within 150 m below the tectonic unconformity boundary at the top of the Carboniferous. The longer the weathering duration, the better are the overall quality of the WCRs. Weathering duration of about 40 Ma is probably an important threshold in the NJB. Ultra-long leaching of atmospheric water and strong late dissolution of acidic fluids before oil and gas accumulations are important for reservoir development and petroleum accumulation in volcanic strata filled with authigenic minerals, especially calcite. The early regional tectonic movement affected the volcanic eruption and controlled the lithofacies distribution. The linear density of fractures was negatively correlated with the distance from the main controlling fault. Owing to the relatively weak filling, high-angle fractures contribute significantly to the reservoir permeability. The physical properties of volcanic breccia are better than those of tuff, and the porosity, permeability, and fracture density of andesite are higher than those of basalt. The physical properties of near-source facies belts of a volcanic edifice are better than those of far-source facies ones. Favorable exploration areas are the structural highs and fault zones where the duration of WL is more than 40 Ma, explosive facies and effusive facies near the crater are developed, or the inherited ancient buried hills transformed by faults and fractures near excellent source rocks, where the dissolution of atmospheric water and organic acidic fluids are strong. • Weathering duration controls degree of weathering, affects volcanic reservoir quality. • Most of favorable reservoirs developed within 150 m below tectonic unconformity. • Weathering duration of ∼40 Ma is probably an important threshold in NW Junggar Basin. • Coupling relationship between weathering-leaching and calcite filling plays a key role. [ABSTRACT FROM AUTHOR]