The Fengcheng Formation in the Mahu sag of the Junggar Basin was primarily composed of detritus, pyroclastic material, carbonates, and evaporites. In order to establish the diagenesis pathways of the Fengcheng Formation, some methods of polarized light microscope, SEM, CL, EPMA, LR, and fluid inclusion analysis were applied to discuss the diagenesis process. The results showed the following: (a) The formation of an alkaline lake was the result of the influence of a high concentration of sodium-rich sources, and it led to the preservation of alkaline minerals in the stratum. (b) After the sediments were buried, three mineral assemblages were formed in the Fengcheng Formation, which are carbonate mineral assemblages (i.e., calcite + ferreous dolomite), reedmergnerite and carbonate mineral assemblages (i.e., reedmergnerite + calcite + ferreous dolomite), and reedmergnerite and alkaline mineral assemblages (i.e., reedmergnerite + shortite + trona), respectively. (c) According to the homogenization temperature of reedmergnerite primary fluid inclusions, the alkaline diagenesis of Fengcheng Formation was divided into an early stage (≤100 °C) and a middle stage (>100 °C), respectively. The earlier stage is marked by the formation of ferrous saddle dolomite, quartz dissolution, and the agglutination of laumontite. These processes occurred under normal burial conditions. The latter is marked by the reedmergnerite’s appearance, which is correlated with the deep hydrothermal activity controlled by faults. (d) Based on sedimentary and diagenetic factors, including climate, provenance, diagenetic surroundings, and the action of subsurface fluid, the alkaline deposition-diagenesis model for shale series in four stages of the Fengcheng Formation was established.

The Fengcheng Formation in the Mahu Sag of the Junggar Basin was generally composed of detritus, pyroclastic material, carbonates, and evaporites. During the process of sedimentation, the climate was arid or semi-arid. Due to the effect of high concentration and sodium-rich source, an alkaline lake was formed, and alkaline minerals were also preserved in the stratum. After the sediments were buried, three mineral assemblages were formed in the Fengcheng Formation, which are carbonate mineral assemblages (i.e., calcite + ferreous dolomite), reedmergnerite and carbonate mineral assemblages (i.e., reedmergnerite + calcite + ferreous dolomite), and reedmergnerite and alkaline mineral assemblages (i.e., reedmergnerite + shortite + trona), respectively. According to the homogenization temperature of reedmergnerite primary fluid inclusions, alkaline diagenesis of Fengcheng Formation is divided into early stage（≤100℃） and middle stage（＞100℃）, respectively. The former is characterized by the generation of ferreous saddle dolomite, quartz dissolution, and agglutination of the laumontite, which occurred under normal burial conditions. The latter is marked by the reedmergnerite’s appearance, which is correlated to deep hydrothermal activity controlled by faults. Finally, based on sedimentary and diagenetic factors, including climate, provenance, diagenetic surroundings, and the action of subsurface fluid, the alkaline deposition-diagenesis model for shale series in four stages of the Fengcheng Formation was established.

The formation of oil-bearing pores in tight tuff has attracted considerable attention from petroleum geologists since the discovery of industrial oil.Devitrification may be an important cause for the formation of these pores; however, the relevant geological circumstance for devitrification still remains unclear. This study tries to decipher the formation of devitrification pores in the tight tuff in the Tiaohu Formation,the Santanghu Basin, Xinjiang, NW China. The result shows that the oil-bearing pore size in tuff is mainly in the range of micrometers to a few nanometers, and the porosity is mainly distributed between 0.10% and 26.71%; the permeability is mainly distributed between 0.17 and 1.20 mD. After high-temperature soaking, the oil-bearing tight tuff illustrated devitrification under both acidic and alkaline circumstances, with glassy tuff showing the greatest variation in porosity, followed by crystal pyroclast glassy tuff, while the mudstone tuff and silicified tuff show relatively small variations in porosity. The 140 °C threshold marks the optimal thermal window for devitrification-driven porosity: it coincides with the smectite-illite transition and the main hydrocarbon-generation stage in the Santanghu Basin. Porosity in all tuff varieties peaks at this temperature, recording a net gain of up to 16.31%; above 140 °C, porosity declines progressively. Devitrification proceeds in three successive stages: (1) neo-mineral nucleation, (2) metasomatic replacement, and (3) dissolution.