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The identification of geologic fluids and related fluid–rock interactions during diagenesis is the subject of much research in sedimentary petrology. Authigenic calcite potentially provides a record of geologic fluids and it occurs heterogeneously in the Upper Permian Wuerhe Formation (P3w) in the Shawan Sag, Junggar Basin, which has a complex history of geologic fluid activity. This provides an ideal opportunity to study the effectiveness of authigenic calcite in tracing fluids. We conducted optical, cathodoluminescence (CL), and scanning electron microscopic observations, as well as the major and trace element and stable carbon and oxygen isotopes of authigenic calcite. The results show that three generations of calcite were precipitated in the P3w Formation, and the diagenetic fluid was affected to varying degrees by paleo-meteoric water and hydrocarbon-bearing fluids. During early diagenesis, diagenetic fluid with low Mn contents precipitated the amorphous early-stage calcite (dark red in CL images, MnO <1.5%, δ13C = −8.6‰ to 2.1‰, VPDB). Its carbon source was mainly meteoric CO2. During mesodiagenesis, the limited hydrocarbon emplacement during the Middle Jurassic enriched the pore fluids in Mn and 13C-depleted organic derived CO2, subsequently precipitating the late-stage sparry calcite I (orange in CL images with MnO of 2.5%–4% and δ13C of −14.5‰ to −8.1‰). The carbon in this calcite came from the dissolution of early-stage calcite and CO2 generated by decarboxylation of organic acids. During the Early Cretaceous, large-scale hydrocarbon charging occurred and the pore fluids were further enriched in Mn and organic derived CO2, eventually precipitating the late-stage sparry calcite II (bright yellow in CL images with MnO of >4% and δ13C of −25.7‰ to −14.9‰). Its carbon source was mainly CO2 produced by the decarboxylation of organic acids. The precipitation of abundant late-stage sC-depleted calcite suggests that the hydrocarbons were oxidized to organic acids in the reservoir. The two periods of hydrocarbon charging caused the dissolution of laumontite and the early-stage calcite, forming secondary minerals and dissolution pores, which increased the porosity and permeability of the rock. Therefore, authigenic calcite is a useful tracer of fluid properties, fluid–rock interactions, and alteration processes in petroliferous basins.

Fused breccia tuff occurs globally, but its formation mechanism is very controversial. Volcanic reservoirs have developed at the bottom of the Permian Fengcheng Formation in the Wuxia region of the Junggar Basin, and here, the lithology is fused breccia tuff. The reservoir porosity is mainly vesicles, but the development and relative filling of the vesicles vary spatially, resulting in strong reservoir heterogeneity. Through core and thin section observations and structural analysis, and combined with reconstructions of the paleosedimentary environment, we discussed in detail the formation mechanism of the fused breccia tuff reservoir. Our conclusions are as follows. In the high-temperature and high-pressure environment of the deep crust, intermediate acidic lava containing volatile components rapidly rose to the earth’s surface along a fault. The volatile components in the lava foamed strongly and then exploded due to the sharp decline of pressure and temperature. A small part of the volcanic dust and pyroclastic material was erupted into the upper atmosphere. Most of the magma became magmatic pyroclast, vitric pyroclast, rock debris, dust, and other matter. This material was in a semimolten state and overflowed into a nearby low-lying lake. The extremely high-temperature pyroclastic flow quickly vaporized the water into high-pressure water vapor, which was squeezed into the pyroclastic flow and became mixed with other volatiles in the foam. On cooling, the pyroclastic material solidified into rock, and the vesicles were preserved. In a later period, due to strong tectonic movement, faults and fractures developed, surface water penetrated into the vesicles along the faults and fractures, and silica and other substances were deposited, filling the primary vesicles. To quantify the development and relative filling of vesicles, drilling parameters were used to establish different geologic models, and wave equation forward modeling was used to obtain a relationship between the development and filling of vesicles, and the seismic amplitude. The 3D seismic amplitude attributes were then extracted to predict the extent of the reservoir, yielding prediction results consistent with the drilling observations.

The coarse-grained glutenite rock mass of the proximal fan delta is characterized by the blocky texture, mixing of gravels with varied sizes, high mud content, and low porosity and permeability, leading to difficulties in assessment and exploration of oil and gas enrichment regularities of tight glutenite. The Permian Xiazijie Formation in the northwest margin of Junggar Basin has a set of tight glutenite reservoirs, and the reservoir quality is an important controlling factor for oil and gas enrichment. Based on three-dimensional earthquake, casting thin sections, rock physical property, geochemistry, the sedimentary facies division, petrologic features, physical property regularities, pore types and diagenesis of Xiazijie Formation were analyzed. This research develops the pore evolution model of the coarse-grained deposition via the quantitative analysis of porosity evolution. First, during the rapid compaction, intensive mechanical compaction results in reduction of the original porosity from 29.8% to 15.1%. Secondly, the cement formed during the eodiagenesis destructs the reservoir space and leads to an average porosity loss of 6.5%. Third, dissolution effectively improves reservoir quality. It mainly dissolves the zeolite cement, and the porosity grows to 12.1% from 8.6%. The dissolution occurs during the main hydrocarbon expulsion stage of the source rock, which is in favor of hydrocarbon emplacement. Fourth, during the telodiagenesis with the deepening burial and intensifying pressure solution, siliceous and carbonate cement precipitate, the reservoir physical property is degraded again, the porosity loss is about 3.4%. After a series of complex diagenetic processes, the current tight glutenite reservoir comes into being, with the porosity of about 8.7%. The research results provide theoretical reference for coarse-grained glutenite reservoir prediction.