Mineralogical and elemental geochemical characteristics of Taodonggou Group mudstone in the Taibei Sag, Turpan–Hami Basin: implication for its formation mechanism

Abstract

Abstract. 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.

Characteristics and Genetic Mechanism of Effective Seepage Channels in the Tight Reservoir of the Fuyu Oil Layer in the Northern Part of the Songliao Basin

“…can be used as a basis for judging redox conditions. In combination with the criteria for determining redox conditions, , the depositional environments of different types of effective seepage channels can be analyzed.…”

Pore Structure and Migration Ability of Deep Shale Reservoirs in the Southern Sichuan Basin

“…Additionally, the mineral composition of shale plays a crucial role in determining the reservoir alteration difficulty, often represented by the brittleness index. This index is determined by the proportion of brittle minerals in the shale [48]. The brittleness index of Longmaxi Formation shale in the study area ranges from 21.23 to 72.7, with an average value of 44.53.…”

Analysis of geological factors affecting propagation behavior of fracture during hydraulic fracturing shale formation

“…In addition, scholars have investigated the effect of the widely distributed bedding structure in shale on hydraulic fracture propagation, pointing out that the bedding structure is the key to the generation of a complex network of seams by hydraulic fractures (Tan et al 2018;Suo et al 2020). Besides discontinuities, researchers have also investigated the effects of shale mineral composition and connections between mineral grains on hydraulic fracture propagation behaviors, pointing out that mineral composition indirectly affects hydraulic fracture propagation paths by influencing shale characteristics such as inhomogeneity, anisotropy, mechanical properties, and pore structure (Bakhshi et al 2020;Chen 2022a;Li et al 2020;Miao et al 2021Miao et al , 2023Sun et al 2021;Xiong et al 2021;Zeng et al 2019;Liang et al 2019). Then, scholars also analyzed the stress state of the subsurface shale and found that the length of hydraulic main cracks becomes larger with the increase of the main stress difference (Blanton 1982(Blanton , 1986Renshaw and Pollard 1995;Gu and Weng 2010;Gu et al 2012).…”