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.
In order to find out the enrichment mechanism and forming type of deep shale gas, taking the Longmaxi Formation shale in the Desheng–Yunjin Syncline area of Sichuan Basin as an example, we determined the mineralogy, organic geochemistry, physical property analysis, gas and water content, and the influence of three factors, namely sedimentation, structural conditions, and hydrogeological conditions, on the enrichment of shale gas. The results show that Longmaxi Formation shale in Desheng–Yunjin Syncline area is a good hydrocarbon source rock that is in the over-mature stage and has the characteristics of high porosity, low permeability, and high-water saturation. The contents of clay and quartz are high, and the brittleness index is quite different. According to the mineral composition, nine types of lithofacies can be found. The development characteristics of Longmaxi Formation shale and the sealing property of the roof have no obvious influence on the enrichment of shale gas, but the tectonic activities and hydrodynamic conditions have obvious influence on the enrichment of shale gas. The main control factors for shale gas enrichment in different regions are different. According to the main control factors, the gas accumulation in the study area can be divided into three types: fault-controlled gas, anticline-controlled gas, and hydrodynamic-controlled gas. The fault-controlled gas type is distributed in the north of the Desheng syncline and the north of the Yunjin syncline, the anticline-controlled gas type is distributed in the south of the Desheng syncline and the south of the Yunjin syncline, and the hydrodynamic-controlled gas type is distributed in the middle of the Baozang syncline. This result is of great significance for deep shale gas exploration.
Shale gas exploration and development is regarded as one of the vital areas in which breakthroughs are urgently needed. The Longmaxi Formation shale in the Weiyuan Block was considered a case study. Based on the analysis of the physical evolution, burial history, thermal history, as well as hydrocarbon generation history of the Longmaxi Formation shales, the impact of uplift amplitude on the shale gas reservoirs, the shale gas transport pattern and direction in the adjustment stage, and the shale gas enrichment pattern are discussed in this study. The results from this study showed that (1) the Weiyuan Block could reach the maximum burial depth in the Late Middle Cretaceous, and then there was uplifting and erosion of the strata, with an erosion amount of 2000–4200 m. (2) The migration of shale gas is mostly contributed by Darcy flow and slippage flow. In addition, Darcy flow contributed to larger migration, which is about 100 times that of slippage flow. Darcy flow occurs in about 13.7% of the pores in the Longmaxi Formation shale. (3) The enrichment of shale gas can be significantly impacted by the uplift amplitude. The gas reservoirs with uplift amplitudes higher than 3000 m and lower than 3000 m differ significantly in terms of their physical characteristics, pressure coefficients, and gas concentrations. (4) The Longmaxi Formation shale gas reservoirs can be separated into five categories based on the uplift amplitudes, migration characteristics, and pressure coefficients: a lost gas reservoir, an enriched gas reservoir, a gas reservoir first enrichment and then loss, a gas reservoir first loss and then enrichment, and a supply gas reservoir. Although the supply gas reservoir occupies the largest area, the enriched gas reservoir is the key zone for analysis, as well as the development of the Longmaxi Formation shale gas in the Weiyuan Block.
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