To explain the strong spatial heterogeneity of volcanic reservoirs porosity in the Songliao Basin and provide new ideas for predicting good volcanic reservoirs in other similar basins, the relationship between the pore evolution process and lithology of volcanic reservoirs has been described in this article. With the description and interpretation of core, thin section, scanning electron microscope, and the results of mercury injection experiment, this article clarifies the lithology, pore types, and pore structure features of the volcanic reservoirs in the Songliao Basin. The rocks of volcanic reservoirs in study area contain pyroclastic rock and volcanic lavas. The most common lithologies are rhyolite, volcanic breccia, and volcanic tuff. The pore size, morphology, and structure vary greatly between these three lithologies, the reason of which we think is the different volcanic eruption process as well as rock composition and its structure. The digenetic evolution of rhyolite includes gas dissipation of magmatic condensation; vesicles fulfilling by hydrothermal fluid; kaolinization and sericitization of feldspar phenocrysts; carbonation, devitrification, and recrystallization of felsic matrix; and finally, the dissolution of feldspar phenocrysts and felsic matrix. As for volcanic breccia, it usually go through the compaction, quartz and calcite filling the original pores between volcanic breccias, and dissolution of mineral debris together with tuff matrix. Similar with the rhyolite, volcanic tuff also undergoes the carbonation and kaolinization of felsic matrix, the dissolution of feldspar and felsic matrix, and compaction. Due to these comprehensive processes, a comprehensive analysis of volcanic rock lithology, which can indicate lithology distribution vertically and horizontally, is very necessary during volcanic reservoirs evaluation and prediction. These detailed analyses will help explorers to find potential reservoirs by distinguishing the diagenetic evolution and pore characteristic of volcanic reservoirs.

The diagenetic environment is vital in controlling reservoir quality by influencing diagenetic processes, especially porosity evolution. The Zhuhai Formation is a critical exploration target in the Huizhou Sag. However, there are considerable differences between the reservoirs in the west (Huixi) and east (Huidong) Huizhou Sag with only a 400 m difference in burial depth. To investigate causes of these differences, scanning electron microscopy (SEM), cathodoluminescence analysis, fluorescence analysis, and fluid inclusion analysis are employed for the petrological and diagenetic characterization of the target formation. Moreover, image analysis software is used to qualitatively evaluate the impacts of diagenesis on porosity. The results show that the digenetic environments in Huixi and Huidong are different. Huixi is characterized by a high content of feldspar with a stronger dissolution phenomenon than that observed in Huidong. The main cement types in Huixi are siliceous cement and kaolinite. Moreover, Huixi had a low palaeo-temperature, low palaeo-salinity, and low palaeo-pH diagenetic environment. In contrast, Huidong is accessible via deep faults and rich in lithic fragments which show visible deformation. Moreover, cements in this area include carbonate, gypsum, quartz overgrowth, kaolinite, and chlorite. This evidence confirms that the diagenetic environment in Huidong was a high palaeo-temperature and high palaeo-salinity environment with high palaeo-pH. Additionally, the palaeo-pH evolved following the sequence of weakly alkaline, acidic, alkaline and acidic. Microscopy evidence indicates that the Dongsha Uplift was a likely source of Huixi sediments during the deposition of the Zhuhai Formation. The determination of the porosity evolution shows that compaction and lithic fragment content are the main controlling factors on the reservoir quality of the Zhuhai Formation.

The ultra-deep tight sandstone reservoirs of the Lower Cretaceous Bashijiqike Formation in the Kelasu Structural Belt, Kuqa Depression, have been significantly influenced by intense lateral tectonic compression since the late Himalayan period (ca. 5 Ma), resulting in the formation of fractured tight sandstone reservoirs. Deciphering the control of the stress-strain state on reservoir quality is crucial for establishing a predictive model. This study systematically investigates the impact of stress state and strain degree on reservoir quality through thin-section identification, analysis of rock mechanical parameters, triaxial stress tests, and rock stress-strain experiments on samples from typical structural positions. The results show that: (1) Sandstones in the hanging wall of the Kelasu Fault experienced weak tectonic compression, characterized by low paleo-stress and present-day stress, minimal strain, and relatively high porosity (10%-14%). (2) Footwall sandstones exhibit higher stress and strain than those in the hanging wall, with notable spatial variations: proximal footwall areas display low vertical stress due to thrusting from the hanging wall, forming collapse structures with weak compression, good physical properties (porosity ~ 10%), and underdeveloped fractures; distal footwall areas show concentrated stress accumulation and the strongest relative/effective compressive stress. These areas develop intensely compressed imbricated and pop-up structures with high strain, low porosity (< 5%), and well-developed fractures. (3) The eastern Keshen area in the footwall has long been subjected to deep burial and intense compression, exhibiting higher strain, relatively lower porosity, and more developed fractures than the western Dabei and Bozi areas at similar structural positions. These findings reveal how stress-strain variations, controlled by structural positioning, govern reservoir quality. This supports a new genetic model for reservoir formation in the Kelasu Structural Belt and provides key geological insights for reservoir prediction in future natural gas exploration and development.

Although buried over eight thousand meters, there is still developing high production from tight gas reservoir with low porosity and ultra-low permeability in the sandstones of Bashijiqike Formation of Cretaceous in Kelasu area of Kuqa depression. It is an interesting issue for the origin of the ultra-deep reservoirs. This paper aims to present the characteristics and origin of the nano-pore and nano fissures of that ultra-deep reservoirs using field emission scanning electron microscope, laser scanning confocal microscope and other high resolution measure experiments. There are developed two kinds of nano-pores with pore diameters between 40 nanometers to 1000 nanometers, and many nano fissures with width between 10 and 600 nanometers in this sandstone reservoir. Based on their petrophysical properties and microstructure characteristics, the nano pores are connected by those nano fissures that are also as valid pore throats. So, there is valid permeability in sandstones with even very low porosity. The effective combination between nano pores and pore throats is the key factors for high production in this tight gas sandstones. This point can help us to predict gas reservoirs with effective and recoverable reserve in this kind of ultra-deep gas field, also maybe in other similar gas field.

Xianzhang YANG, Dakang ZHONG Lu ZHOU, Xiaojuan HUANG, Zhenping XU, Yani XIE, Guowei ZHANG, Xueqi ZHANG, Haoyu LUO & Haitao Sun - THE ORIGIN AND CONTROLLING FACTORS OF SWEET SPOTS IN JURASSIC TIGHT SANDSTONES OF KUQA FORELAND DEPRESSION, TARIM BASIN, NW CHINA, Carpathian Journal of Earth and Environmental Sciences, August 2021, Vol. 16, No. 2, p. 373 – 381; DOI:10.26471/cjees/2021/016/182