Formation environment and hydrocarbon-generating potential of Carboniferous source rocks in the Gucheng Sag, Junggar Basin

doi:10.13745/j.esf.sf.2025.7.5

Abstract

Abstract:

The Carboniferous petroleum system is one of the most significant gas exploration domains in the Junggar Basin. In recent years, substantial source rocks and natural gas have been discovered in the Upper Carboniferous Bashan Formation in the eastern Fukang Sag of the southeastern Junggar Basin. The Gucheng Sag, also located in the southeastern Junggar Basin, shares a similar Carboniferous tectonic and paleogeographic background with the eastern Fukang Sag. However, the Carboniferous source rocks in the Gucheng Sag are poorly understood. Based on stratigraphic chronology, sedimentary petrology, organic geochemistry, and basin simulation, we determined the age of the Carboniferous source rocks in the Gucheng Sag. We analyzed their sedimentary environment and hydrocarbon generation potential, and delineated the distribution of large-scale effective source rocks. The results indicate that the Carboniferous source rocks in both the Gucheng Sag and eastern Fukang Sag were deposited during the early Late Carboniferous (Moscovian-Bashkirian Stage), corresponding to the Bashan Formation. The Bashan Formation in both sags is characterized by a fan delta succession. However, the Gucheng Sag exhibits steep gradients and narrowly spaced sedimentary sub-environments, with swamps rarely developed on the delta plain and sedimentary gravity flow deposits commonly occurring at the delta front. The Bashan source rocks in the Gucheng Sag are dominated by Type III kerogen with medium organic matter abundance. They reached the hydrocarbon generation threshold in the late Late Permian, and the area currently within the main oil-generating window covers 727 km². In contrast, the Bashan source rocks in the eastern Fukang Sag show significant variations in organic type, abundance, and maturity. Those in the Gucheng Sag were deposited in a reducing brackish environment with organic matter input dominated by bacterial and algal sources, whereas the eastern Fukang Sag samples display greater variations in redox conditions, salinity, and biogenesis, dominated by terrestrial organic sources. The Bashan source rocks in the Gucheng Sag generally exceed 50 m in thickness, with thickness centers (>100 m) in the north and south. Their hydrocarbon generation intensity typically exceeds 2.0 million tons/km², covering an area of 890 km². These results enhance the understanding of the Carboniferous petroleum system in the Junggar Basin and provide a foundation for accelerating petroleum exploration in the study area.

Key words: Junggar Basin, Gucheng Sag, Carboniferous, sedimentary environment, hydrocarbon generation potential, biogenesis characteristics, source kitchen

CLC Number:

TE122.23

GONG Deyu, HAN Yan, YANG Bin, ZHANG Hua, ZHOU Chuanmin, WANG Shaoqing, WANG Ruiju, MIAO Yihao. Formation environment and hydrocarbon-generating potential of Carboniferous source rocks in the Gucheng Sag, Junggar Basin[J]. Earth Science Frontiers, 2025, 32(6): 438-456.

Figures/Tables 16

Fig.1 Structural units of the Junggar Basin (a), structure diagram of the Carboniferous top surface in the Gucheng Sag (b) and the comprehensive stratigraphic column in the Gucheng Sag (c)

Fig.2 Sporopollen identification diagram of the Carboniferous source rocks in Well Cheng 1 in the Gucheng Sag, Junggar Basin

Fig.3 U-Pb age harmonic diagram of the Carboniferous andesite core (3 940.3 m) from Well Cheng 1 in the Gucheng Sag, Junggar Basin

Fig.4 Microscopic characteristics of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Fig.5 Sedimentary environments and sedimentary facies of the Upper Carboniferous Bashan Formation in the Gucheng Sag (a) and eastern Fukang Sag (b)

Fig.6 Conceptual depositional model of the Carboniferous Bashan Formation in the eastern Junggar Basin

Fig.7 Crossplot of HI vs. Tmax (a) and Tr vs. Tmax (b) of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Table 1 Evaluation parameters of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Fig.8 Relative percentage of organic macerals in the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Fig.9 Characteristics of whole rock organic macerals of Carboniferous source rock in the Gucheng Sag and eastern Fukang Sag

Fig.10 Crossplot of TOC and S1+S2 of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Fig.11 Contour maps of Ro at the top of the Carboniferous (a), the thickness of the Bashan source rocks (b), and the hydrocarbon-generating intensity of the Bashan source rocks (c) in the Gucheng Sag

Fig.12 Burial history and thermal evolution history of Well Cheng1 in the Gucheng Sag

Fig.13 Biomarker spectrum of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Fig.14 Geochemical characteristics of biomarker compounds in the extracts of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

Fig.15 The relative content of C27-C29 regular steranes in the extracts of the Upper Carboniferous Bashan source rocks in the Gucheng Sag and eastern Fukang Sag

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