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.