Lacustrine shale oil reservoirs of the Fengcheng Formation in the Mahu Sag, Junggar Bain, NW China contain abundant oil resources, with the highest daily oil production of a single well reaching over 700 barrels. Based on core and thin section observation along with borehole image interpretation, this study integrated paleotectonic stress regime, fluid inclusion analysis, and calcite U-Pb dating to clarify fracturing periods and discuss the influence of fractures on shale oil accumulation. Results show that natural opening-mode fractures are wide-spread in the Fengcheng Formation, which play an important role in oil accumulation. Fractures are developed in several stages and filled by three minerals: reedmergnerite, quartz and calcite. Reedmergnerite and quartz fillings show similar fluid inclusions and petrographic features, with numerous hydrocarbon inclusions compared to aqueous ones, yet, such inclusions were less observed in calcite fillings. Fractures in the study area were formed in two separate stages. In the first period, due to the NW-SE compression of the Hercynian movement, the WNW-ESE and NNW-SSE striking fractures were formed in the Late Permian, with Th values of 95-115 degrees C. The first stage of fractures coincided with hydrocarbon generation, providing migration pathways for the oil. In the second period, due to the nearly S-N compression of the Indosinian movement, the NNW-SSE and NNE-SSW striking fractures were formed in the late Triassic, with Th values of 120-140 degrees C. The second stage of frac-tures was formed subsequent to major hydrocarbon generation and expulsion period, hence their contribution to hydrocarbon migration and accumulation was found relatively minor. Moreover, fractures with the nearly E-W, ENE-WSW and WNW-ESE strikes intersect with the present-day maximum horizontal principal stress at a small angle, with large apertures and good connectivity, which should have a positive impact on shale oil accumulation.

Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear. Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections, scanning electron microscope (SEM), electron microprobe (EMP), C, O and Sr isotopes analysis, and other techniques. (1) Dolomites were mainly precipitated in three stages: penecontemporaneous–shallow burial stage (early stage of the Middle Permian), middle burial stage (middle stage of the Middle Permian), and middle–deep burial stage, with the former two stages in dominance. (2) Dolomitization fluid was high-salinity brine originating from alkaline lake. In the penecontemporaneous–shallow burial stage, Mg2+ was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass. In the middle burial stage, Mg2+ mainly came from the transformation of clay minerals, devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar. (3) Regular changes of Mg, Mn, Fe, Sr, Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid, and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial. (4) In the penecontemporaneous stage, induced by alkaline-lake microorganisms, the micritic–microcrystalline dolomites were formed by primary precipitation, replacement of aragonite and high-Mg calcite, and other processes; in the shallow burial stage, the silt-sized dolomites were formed by continuous growth of micritic–microcrystalline dolomite and replacement of calcites, tuffs and other substances; in the middle burial stage, the dolomites, mainly silt- and fine-sized, were formed by replacement of volcanic materials. The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.

With the great discovery of unconventional oil in the Fengcheng Formation in the Mahu sag, the Wuxia fault belt, which shows similar lithological characteristics and lithofacies, is considered the most favorable area for future petroleum exploration. However, the complicated structural patterns remain unclear and restrict the petroleum exploration and development. In this study, combined with new seismic data and some borehole data, we conclude the structural styles in the Wuxia fault belt and analyze their distribution characteristics, and further investigate their implications for tectonic evolution and hydrocarbon accumulation. Five typical seismic sections are captured from the west to the east of the fault belt. Decollement folds and fold accommodation faults superimpose on the underlying basement fault related folds. Structure patterns also show a zonation in S–N direction and a segmentation in E–W direction. The balanced section reveals that the present-day structure features were fundamentally formed by Late Permian. The structural deformation shows distinctive features of a foreland basin which may develop in Early Permian and continue until the Late Permian. The oil reservoirs in the Fengcheng Formation in deeper detachment fold and the autochthonous Fengcheng Formation in fault propagation folds are the most favorable regions for further unconventional petroleum exploration.

Shale reservoir of Fengcheng Formation in Mahu Sag, Junggar Basin is another important discovery after Lucaogou Formation in Jimusar Sag. During the sedimentary period of Fengcheng Formation, Mahu sag is characterized by near–terrigenous source supply, frequent volcanic activity and arid and hot climate, which creates conditions for the unique saline and alkaline lake sedimentary environment and is also an important reason for the complex and changeable lithology of Fengcheng Formation. Volcanic rock, internal rock, terrigenous clastic rock and multi–source mixed rock are frequently stacked longitudinally, forming shale oil reservoirs of meter to centimeter size. Based on the characteristics of lithologic changes, the lithology and lithofacies of shale oil reservoirs in Fengcheng Formation in Mahu Sag, Junggar Basin were analyzed by means of sub-ion polishing electron microscopy, energy spectrum testing and fluorescence analysis, with the structural style as the breakthrough point, cm–scale core observation as the basis and high–frequency sedimentary structure analysis as the auxiliary. The shale oil reservoirs of Fengcheng Formation in Mahu sag can be divided into 4 types and 8types. The reservoir properties of glutenite, volcanic rock, mixed rock and internal rock are gradually weakened.Due to the differences in structure and composition, the mixed mudstone reservoir can be divided into mixed shale, laminate–laminate–like mixed mudstone, root–reticular mixed mudstone, snowflake–star–point mixed mudstone. Foliation development, dolomite content and its occurrence pattern are the important controlling factors for the difference of reservoir of reservoir physical properties of mixed rock reservoirs.

The Lower Permian Fengcheng Formation in the Mahu Sag develops a set of organic-rich alkaline lacustrine shale strata, which is a key area for shale oil exploration and development. As an important storage space and seepage channel for shale reservoirs, natural fractures have an impact on shale oil enrichment, production and development effect. In this study, the types and characteristics of natural fractures were first analyzed using core, thin section and imaging logging data. On this basis, combined with the distribution of fractures in single wells, the vertical distribution law of fractures is discussed. Finally, the planar distribution of fractures is evaluated using different seismic attributes such as coherence, curvature, likelihood, and AVAz. The results showed that three types of fractures are existed, including transformational shear fractures, intraformational open fractures and bed-parallel shear fractures, with intraformational open fractures being the most developed. The development degree of fractures in different layers has obvious differences, mainly controlled by lithology and brittle mineral content. The basalt and tuff are developed in the Feng 1 Member, with low carbonate mineral content, resulting in a relatively low degree of fracture development. The dolomite and argillaceous dolomite are developed in the Feng 2 Member and the Feng 3 Member, with high carbonate mineral content and brittleness, resulting in a high degree of fracture development. Additionally, the closer to the fault, the higher the degree of fracture development. On the plane, the fracture zone develops near the main and secondary faults, with the trend mainly oriented in the E-W direction and approximately parallel to the direction of the faults. The width of the fracture zone is largest in the central and southern part of the study area. These fractures are fault-related and are caused by regional stress fields resulting from the activity of the main-secondary faults.

The sedimentary period of the Early Permian Fengcheng Formation in Mahu Depression of NW Junggar Basin is the key period for the formation of lacustrine organic-rich shale. The main objective of this paper is to use various spectral analysis methods to construct a tuned floating astronomical chronological time scale for the fine-grained sedimentary rocks of Fengcheng Formation. In this study, we provide astronomical constraints for the relationship between the fine-grained deposition of Fengcheng Formation and regional geological events in Junggar basin. High-resolution stratigraphic frameworks are critical for understanding the nature and patterns of major geological events. A detailed time series analysis of fine-grained sedimentary rocks of the Fengcheng Formation dominated by deep lacustrine shales was carried out in Borehole MY01. The results show that in the early Permian, driven by the long-eccentricity (413 kyr), short-eccentricity (100 kyr), obliquity (43.9 kyr and 34.9 kyr), precession (21.0 kyr and 17.6 kyr), the periodic variation wavelengths are 62.5 m, 14.71 m, 6.41 m, 5.24 m, 3.22 m and 2.75 m respectively. According to the published biostratigraphic division and U-Pb zircon age constraints, the stable 413 kyr tuned floating astrochronology time scale in the Early Permian shows that the deposition duration of the Fengcheng Formation shales is about 3.34 myr and the sedimentation rate is about 15 cm/kyr. In the future work, the systematic study of the absolute age of volcanic rocks will further help to more accurately constrain the sedimentary age of the Fengcheng Formation in Mahu Depression, NW Junggar Basin.

Maximum burial depth of the Permian Fengcheng Formation, which contains significant oil resources in the Mahu Sag, exceeds 4500 m constituting a deep reservoir. Fractures are widespread in the deep lacustrine shale, providing important storage space and fluid conduits. Based on cores, borehole image logs, and petrographic analysis, we identified and analyzed the types and characteristics of fractures and conducted X-ray diffraction (XRD) and Cathodoluminescence (CL) tests to investigate the factors influencing fracture development. We found faults and three types of opening-mode fractures: bed-bounded fractures, unbounded fractures and bed-parallel fractures. Mechanical stratigraphy (depositional rock types modified by diagenesis) and proximity to faults, are main controls of opening-mode fracture occurrence and attributes. Specifically, Dolomitic mudstone with a high carbonate content shows the highest fracture density of 5.24 m(-1). Bed-bounded fractures are mainly developed within brittle layers, with fracture density lower in thicker beds. On the other hand, unbounded (tall) fractures primarily occur near reverse faults (0-1500 m), with orientations parallel to extension directions implied by the kinematics of nearby faults. Additionally, the effectiveness of early-formed fractures as fluid conduits is likely modified (reduced) by cementation and enhanced by dissolution. Fractures have a positive influence on oil production, corresponding to high fracture density and aperture in the vertical direction.

The alkaline lake source rocks of the Fengcheng Formation are developed in the Mahu Sag of the Junggar Basin. Different from traditional continental fresh water and saltwater lake source rocks, alkaline lake source rocks lack targeted evaluation criteria, and it is unknown whether their hydrocarbon generation models are consistent with traditional models. Therefore, in the present study, evaluation standards and hydrocarbon generation models of alkaline lake source rocks are discussed based on geological and organic geochemical data and a systematic summary of the geochemical characteristics of the Fengcheng Formation source rocks. The Fengcheng Formation source rocks are mainly diamictite with mixed argillaceous rock and dolomite; most total organic carbon (TOC) values range from 0.2–1.4%; and the kerogen is primarily oil-prone type II, reaching low- to high-maturity stages. Based on the types of organic matter in source rocks and the relationships between organic matter abundance parameters, the evaluation standard of alkaline lake source rocks is proposed. The Fengcheng Formation is mainly composed of good to excellent source rocks (55.5%) with high hydrocarbon generation potential. The single-peak hydrocarbon generation model of the Fengcheng Formation is similar to that of traditional freshwater or saltwater lakes, with a high hydrocarbon generation rate, two to five times that of the traditional model; its main particularity is in the formation of naphthenic crude oil from the kerogen of bacteria and algae. A new understanding of the hydrocarbon generation potential and model of alkaline lake source rocks in the Fengcheng Formation can provide support for tight oil and shale oil exploration in the Mahu Sag.