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A special type of sandstone in which carbonate rock fragments (CRFs) dominate the composition developed in the Upper Triassic Xujiahe Formation's fourth member (Xu4) in the western Sichuan Basin, known as calcareous sandstone. Calcareous sandstones are widely distributed in the western Sichuan and is the main production target of tight sandstone gas in the Sichuan Basin. In this study, thin sections, porosity–permeability testing, scanning electron microscopy, and X-ray diffraction are applied to examine the characteristics and control factors for high-quality reservoirs in the calcareous sandstones, with a view to providing guidance for natural gas exploration and development in calcareous sandstones. The results show that the calcareous sandstone belongs to litharenite, with an average framework grain composition of 30% quartz, 1% feldspar, and 69% rock fragments, while the Xu4 sandstone has a high quartz content (average content of 71%). Primary intergranular pores are the main storage space, and the reservoir quality is quite poor. Under the influence of different parent rock properties of sandstones, there are obvious differences in the composition of framework grains between the calcareous sandstone and the ordinary Xu4 sandstone, which in turn affects the reservoir storage space, diagenesis, and reservoir quality. High-energy depositional conditions, low content of late cements, and the development of fractures are the main controlling factors for the formation of high-quality reservoirs in Xu 4 calcareous sandstones. [ABSTRACT FROM AUTHOR]

Secondary porosity in the Upper Triassic Xujiahe tight sandstone of the western Sichuan Basin is mainly the product of feldspar dissolution. In the Xu-4 Member, the upper reservoir of the Xujiahe Formation, feldspars are dissolved to a significant extent and observations indicate that nearly all feldspars have been dissolved completely, with only 1.73% content of feldspar remaining. In the Xu-2 Member, the lower reservoir, feldspars are well preserved; the current content of feldspar is 12.54% on average, and the secondary porosity derived from feldspar dissolution is less than 1%. Kaolinite occurs almost exclusively in the Xu-4, but it is nearly absent in the Xu-2. The K+ content in the Xu-2 is 3.3 times higher than that in Xu-4. The K+/H+ ratio in the Xu-2 is also higher than that in the Xu-4. These differences between the two reservoirs can be attributed to their distinguishing fluid-rock systems. The low K+ content and relatively high δ18O in the Xu-4 formation water are the result of intensive fluid-rock interaction in an open fluid-rock system. The upper Xu-4 is close to the overlying coal-measures of the Xu-5 from which organic acid flowed into the Xu-4. Meanwhile, K+ contained in sandstone migrated out to the mudstones. The resulting low K+/H+ ratio in the formation water of the Xu-4 was responsible for almost all the feldspar dissolution and kaolinite formation. In contrast, due to the relatively closed fluid-rock system in the Xu-2, K+ did not migrate into adjacent rocks and acidic fluids did not invade, which led to K+-rich formation waters maintaining a high K+/H+ ratio. Hence, K-feldspar was well preserved and kaolinite was completely transformed into illite. Therefore, in contrast to the Xu-2 tight sandstone, the Xu-4 sandstone has relatively higher secondary porosity, which favours the formation of better quality reservoirs.

Dolostone-evaporite paragenesis systems (DEPS) have been widely found in Precambrian to Quaternary strata worldwide. However' despite its wide spatio-temporal distribution' the formation mechanism and controlling factors of DEPS remain poorly studied. By reference to copious literature from China and elsewhere combined with previous knowledge of DEPS in China' this study comprehensively summarizes the development characteristics' combination patterns and formation mechanism of DEPS. Firstly' its spatio-temporal distribution is clarified based on published studies: laterally' DEPS is known to be widely distributed in 51 regions globally' mainly in the northern hemisphere (mostly in Asia' followed by Europe and North America). Comparatively' it is found less often in Africa and is only scattered in South America and Oceania. Stratigraphically' although DEPS is found in many Precambrian to Quaternary strata' it appears mostly in the Permian. The combination patterns of evaporite and dolostone in DEPS include the interbedding of dolostone and evaporite' with thick dolostone overlying thick evaporite' thick evaporite overlying thick dolostone' thick dolostone mixed with thin evaporite' and thick evaporite mixed with thin dolostone. The formation process' distribution and main controlling factors of symbiotic differences as well as the particularity and universality of the paragenetic system development are discussed. Secondly' based on macro- to microscale characterization of mineral types in DEPS' six types of evaporites and five types of dolostones have been found and their genesis is discussed. Finally' the properties' characteristics' sources and evolution paths of diagenetic fluids in DEPS are summarized. DEPS studies provide new concepts for solving the "dolostone (or dolomite?) " question' and promote the understanding of the important basic geological problems associated with DEPS. Such studies also provide theoretical guidance for the exploration of oil and gas resources in the paragenetic system. [ABSTRACT FROM AUTHOR]