Organic matter types in the Taodonggou Group mudstone exhibit significant differences with depth. In order to understand the formation mechanism of this special phenomenon, we analyzed the mineralogy and geochemistry of the mudstone, as well as the source rocks, depositional environment, and depositional processes of the Taodonggou Group. Based on this, we have gained the following insights. (1) The Taodonggou Group mudstone was deposited in an intermediate-depth or deep, dysoxic, freshwater–brackish lake environment under warm and humid paleoclimatic conditions. The input of terrestrial debris was stable, but the sedimentation rate was slow. In addition, the sedimentation in the middle stage was influenced by hydrothermal activities, and the changes in the depositional environment corresponded to variations in organic matter types. (2) The source rocks of the Taodonggou Group mudstone are mainly andesitic and feldspathic volcanic rocks. Sediment sorting and recycling were weak, and hydrocarbon source information was well preserved. The tectonic background of the source area was a continental island arc and an oceanic island arc. Furthermore, changes in the provenance of the Taodonggou Group also had a significant impact on the variations in organic matter types. (3) The sedimentation of the Taodonggou Group involved both traction and gravity flows. The variations in source area, depositional environment, and depositional processes during different depositional periods led to changes in the organic matter types of the Taodonggou mudstone. (4) Based on the depositional environment, provenance, and depositional processes, the sedimentation of the Taodonggou Group can be divided into three stages. In the early stages, the sedimentation center was in the Bogda area. At this time, the Bogda Mountain region was not exposed, and the depositional processes inherited the characteristics of early Permian gravity flow sedimentation, resulting in the widespread deposition of a series of high-quality Type III source rocks in the basin. In the middle stage of the Taodonggou Group sedimentation, the sedimentation center gradually migrated to the Taibei Sag. During this period, the Bogda Mountain region experienced uplift and hydrothermal activity, and the depositional processes gradually transitioned to traction flows, resulting in the widespread deposition of a series of Type II source rocks in the basin. In the late stage of the Taodonggou Group, the uplift of the Bogda Mountain region ceased, and the sedimentation center completely shifted to the Taibei Sag. Meanwhile, under the influence of gravity flows, the organic matter types of the Taodonggou mudstone changed to Type III.

The Taodonggou group of Middle Permian is an important source rock in Taibei sag of Turpan-Hami basin. Due to its deep burial, drilling has only been revealed in recent years. Based on organic petrology and organic geochemistry experiments, this paper studies the organic petrology, organic geochemistry, sedimentary environment, and hydrocarbon generation potential of source rocks in Taibei sag, Turpan-Hami basin, and reveals the influence of the sedimentary environment on the organic matter abundance of source rocks. The results are as follows: (1) The organic matter of the Middle Permian source rocks in Taibei sag of Turpan-Hami basin is mainly sapropelite and exinite. The vitrinite is mainly vitrodetrinite, and the exinite is mainly lamalginiite. (2) The total organic carbon content value is 0.55–6.08 wt %, and the average value is 2.58 wt %. The PG value ranges from 0.78 mg HC/g to 30.86 mg HC/g, and the average value is 4.88 mg HC/g. Chloroform asphalt “A” is 0.046–0.8767 wt %, and the average value is 0.285 wt %. The types of organic matter are mainly III and II–III, and the R_o value is 0.628–1.49 wt % (average = 0.988 wt %). The T_max distribution is 329–465 °C. The average temperature is 434.7 °C, which is in the mature stage (oil window stage). The Middle Permian source rocks are mainly very good to excellent source rocks with a good hydrocarbon generation potential. (3) The source rocks are deposited in a semihumid and semiarid climate. Organic matter is input as a mixed source. The early and late stages is dominated by terrestrial higher plants. The middle stage is dominated by lower aquatic organisms, and the sedimentary environment consists of weak reduction and weak oxidation environments. (4) In the study area, the abundance of organic matter has a weak negative correlation with CPI and a positive correlation with Pr/Ph and ∑C₂₁–/∑C₂₂+. Under the coaction of paleoclimate, organic matter input, and redox environment, the enrichment model of organic matter with high productivity and weak oxidation environment characteristics can also form excellent source rocks. This study is of great significance and provides theoretical guidance for the exploration of deep oil and gas resources.

The analysis of the chemical characteristics of the Middle Permian formation water in 10 wells in the northwest of Sichuan Basin indicates that: the TDS of the Middle Permian formation water in Northwest Sichuan increases longitudinally with the burial depth of the formation, and the formation water is mainly of CaCl₂ type (Surin classification), Cl-Na-Ca type and Cl-Ca-Na type (Shukarev classification). The rNa⁺/rCl⁻ and rSO₄ ^2-*100/rCl⁻ of the formation water are 0.05-0.95 and 0-5 respectively, reflecting strong water-rock interaction and strong sealing conditions. The analysis of the chemical characteristics of formation water and the preservation conditions of oil and gas shows that the overall preservation conditions of the Middle Permian in Northwest Sichuan are good, most of which is located in the “preservation zone”; longitudinally, the preservation conditions of Qixia Formation are better than those of Maokou Formation; areally, the preservation conditions of Shuangyushi and Jiulongshan regions are good; the sealing property of the strata in Hewanchang region is poor.

The Middle Permian is an important basin-forming period in the Turpan-Hami Basin. Based on mineral characteristics and elemental geochemistry of the Taodonggou Group mudstone we analyze the parent rock type, source area location, sedimentary environment and source area tectonic background for this mudstone. On this basis we are able to reconstruct the source-sink system and lake basin evolution of the Taodonggou Group. We find the following: (1) Taodonggou Group mudstone minerals are mainly clay and quartz, and can be classified into four petrographic types according to mineral fraction. (2) The Taodonggou Group mudstone was deposited in a warm, humid and hot paleoclimate, with strong weathering. The parent rocks of the Taodonggou Group mudstone are two types of felsic volcanic rocks and andesites, with weak sedimentary sorting and recycling and with well-preserved source information. (3) The Taodonggou Group mudstone were deposited in dyoxic freshwater-brackish water in intermediate-depth or deep lakes with stable inputs of terrigenous debris but at slower deposition rates. Deposition of the middle of Taodonggou Group was influenced by hydrothermal activity; the tectonic setting of the Taodonggou Group source area was a continental island arc and an oceanic island arc. (4) The evolution of the Middle Permian Lake basin in the Turpan-Hami Basin can be divided into three stages: In the early part of the deposition of Taodonggou Group the depocenter was in the Bogda area. At this time the area that became Mt Bogda was not exposed and a succession of high-quality type-III source rocks was widely deposited in the basin. In the middle of the deposition of the Taodonggou Group the depocenter gradually migrated to the Taibei Sag. At this time the Mt Bogda area underwent uplift, and, together with hydrothermal activity, a succession of type-II source rocks was widely deposited in the basin. In the late part of the Taodonggou Group, uplift of the Mt Bogda area ceased and the depocenter transferred entirely to the Taibei Sag.

The Middle Permian is an important basin-forming period in the Turpan-Hami Basin. Based on mineral characteristics and elemental geochemistry of the Taodonggou Group mudstone we analyze the parent rock type, source area location, sedimentary environment and source area tectonic background for this mudstone. On this basis we are able to reconstruct the source-sink system and lake basin evolution of the Taodonggou Group. We find the following: (1) Taodonggou Group mudstone minerals are mainly clay and quartz, and can be classified into four petrographic types according to mineral fraction. (2) The Taodonggou Group mudstone was deposited in a warm, humid and hot paleoclimate, with strong weathering. The parent rocks of the Taodonggou Group mudstone are two types of felsic volcanic rocks and andesites, with weak sedimentary sorting and recycling and with well-preserved source information. (3) The Taodonggou Group mudstone were deposited in dyoxic freshwater-brackish water in intermediate-depth or deep lakes with stable inputs of terrigenous debris but at slower deposition rates. Deposition of the middle of Taodonggou Group was influenced by hydrothermal activity; the tectonic setting of the Taodonggou Group source area was a continental island arc and an oceanic island arc. (4) The evolution of the Middle Permian Lake basin in the Turpan-Hami Basin can be divided into three stages: In the early part of the deposition of Taodonggou Group the depocenter was in the Bogda area. At this time the area that became Mt Bogda was not exposed and a succession of high-quality type-III source rocks was widely deposited in the basin. In the middle of the deposition of the Taodonggou Group the depocenter gradually migrated to the Taibei Sag. At this time the Mt Bogda area underwent uplift, and, together with hydrothermal activity, a succession of type-II source rocks was widely deposited in the basin. In the late part of the Taodonggou Group, uplift of the Mt Bogda area ceased and the depocenter transferred entirely to the Taibei Sag.

The Middle Permian is an important basin-forming period in the Turpan-Hami Basin. Based on mineral characteristics and elemental geochemistry of the Taodonggou Group mudstone we analyze the parent rock type, source area location, sedimentary environment and source area tectonic background for this mudstone. On this basis we are able to reconstruct the source-sink system and lake basin evolution of the Taodonggou Group. We find the following: (1) Taodonggou Group mudstone minerals are mainly clay and quartz, and can be classified into four petrographic types according to mineral fraction. (2) The Taodonggou Group mudstone was deposited in a warm, humid and hot paleoclimate, with strong weathering. The parent rocks of the Taodonggou Group mudstone are two types of felsic volcanic rocks and andesites, with weak sedimentary sorting and recycling and with well-preserved source information. (3) The Taodonggou Group mudstone were deposited in dyoxic freshwater-brackish water in intermediate-depth or deep lakes with stable inputs of terrigenous debris but at slower deposition rates. Deposition of the middle of Taodonggou Group was influenced by hydrothermal activity; the tectonic setting of the Taodonggou Group source area was a continental island arc and an oceanic island arc. (4) The evolution of the Middle Permian Lake basin in the Turpan-Hami Basin can be divided into three stages: In the early part of the deposition of Taodonggou Group the depocenter was in the Bogda area. At this time the area that became Mt Bogda was not exposed and a succession of high-quality type-III source rocks was widely deposited in the basin. In the middle of the deposition of the Taodonggou Group the depocenter gradually migrated to the Taibei Sag. At this time the Mt Bogda area underwent uplift, and, together with hydrothermal activity, a succession of type-II source rocks was widely deposited in the basin. In the late part of the Taodonggou Group, uplift of the Mt Bogda area ceased and the depocenter transferred entirely to the Taibei Sag.