Depositional Environment and Sedimentary Model of the Fengcheng Formation in Mahu sag, Junggar Basin

doi:10.19657/j.geoscience.1000-8527.2023.061

Abstract

Abstract:

The Mahu sag is one of the most prolific oil-gas sags in the Junggar Basin.The Lower Permian Fengcheng Formation is the main high-quality lacustrine source rocks, and is considered as the key exploration target in the Mahu sag.The lithofacies distribution and sedimentary environment of the Fengcheng Formation are of great significance in studying the tectonic evolution and further oil-gas exploration of the Mahu sag.Based on the latest logging data, we established a comprehensive stratigraphic column for the Fengcheng Formation from well Maye 1, and clarified the lithofacies distribution of the Fengcheng Formation.The tectonic sedimentary environment and sedimentary model of the Fengcheng Formation are constructed subsequently.The Fengcheng Formation is mainly composed of dark fine-grained sedimentary rocks in well Maye 1, with a large number of soft-sediment deformation structures.The middle member of the Fengcheng Formation was likely deposited in (semi-)deep lacustrine facies.According to the lithofacies profile in the Fengcheng Formation, the cyclic characteristics of progradation-retrogradation-progradation developed from bottom to top, and the sedimentary facies changed from fan delta, lacustrine to river delta facies from southwest to northeast.Volcanism likely played a key role in the lower member of the Fengcheng Formation of shallow lacustrine facies in the northern part of Mahu sag.In the middle member of the Fengcheng Formation, the southern part of Mahu sag developed fan-delta deposits, which is transformed into delta-shore shallow lacustrine deposit to the west of Mahu sag, and semi-deep to deep lacustrine deposits are developed in the northern part.The middle member of (semi-)deep lacustrine facies is characterized with extensive distribution and abundant alkaline minerals in the depocenter.The chemical precipitation in the middle member of the Fengcheng Formation is the most widely distributed, indicating the maximum flooding surface.The progradation in the upper member of the Fengcheng Formation may have resulted from shrinking of the sag.Fan delta deposits are developed in the southern and western parts of the Mahu sag, and semi-deep lacustrine to shore-shallow lacustrine facies are developed in the northern part.Combined with its sedimentary evolution process, the Fengcheng Formation is likely deposited and multiple-sourced, including terrigenous clastic deposits, endogenous chemical precipitates and pyroclastic material.

Key words: Junggar Basin, Mahu sag, Fengcheng Formation, soft-sediment deformation structure, depositional environment, sedimentary model

CLC Number:

NI Minjie, ZHU Hexuan, HE Wenjun, YANG Sen, ZOU Yang, ZHANG Yuanyuan. Depositional Environment and Sedimentary Model of the Fengcheng Formation in Mahu sag, Junggar Basin[J]. Geoscience, 2023, 37(05): 1194-1207.

Figures/Tables 8

Fig.1 Sketch map (a) of the structural location of Mahu sag, Junggar basin, showing the well location (b)[39]

Fig.2 Comprehensive stratigraphic column of the Fengcheng Formation from well Maye 1

Fig.3 Major types of lamination developed in the Fengcheng Formation in well Maye 1

Fig.4 Core photos of the Fengcheng Formation from well Maye 1, showing typical soft-sediment deformation structures

Fig.5 Lithofacies distribution map of the northeast-Southwest trending well-section in Mahu Sag (profile location seeingFig.1)

Fig.6 Distribution of sedimentary facies of the members of the Fengcheng Formation in Mahu sag (modified from ref.[24])

Fig.7 Profle of sedimentary facies of Fengcheng F ormation in the Mahu sag (profile location seeing Fig.6)

Fig.8 Sedimentary architectural model of the Fengcheng Formation, Mahu sag (after ref.[24])

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