An alkaline lake is a saline lake with high salinity and high content of total dissolved solid, and it usually has pH greater than 9 or even up to 11. Unlike freshwater lake, chloride-type salt lake, sulphate-type salt lake, and seawater, alkaline lake contains water dominated by Na⁺ and CO₃^2- (or HCO^3-)^[1], so it is also known as soda lake or carbonate salt lake. The famous modern alkaline lakes include the Mono Lake and Walker Lake in the United States, the Chahan Lake in the Inner Mongolia Plateau of China, and the Turkana Lake and Magadi Lake in Kenya^[2,3,4,5]. Alkaline lake comprises a large number of sodium carbonate minerals, and it is closely related to the occurrence of high-quality source rocks. For instance, in the Greater Green River Basin in the United States, the Tertiary Green River shale has the TOC as high as 20%- 30%^[6], and the lacustrine shale in the Green River Formation covering an area of about 6.5×10⁴ km² endows the original oil in place (OGIP) exceeding 2 000×10⁸ t^[7]. In the Biyang Sag of China, the Paleogene Hetaoyuan Formation is of typical alkaline lacustrine sediments with TOC of 1.4%-1.9%, smaller than that of the Green River Formation shale (4%-20%), but it is still a representative of “small and fertile” petroliferous basin, where multiple oilfields have been discovered^[8]. According to the analysis, high-quality source rocks can be formed in alkaline lake because of its diverse types of bacteria and archaea and high biological productivity^[2]. In 1974, Melack et al. investigated the bioavailability of several alkaline lakes near the Great Rift Valley in East Africa and found that the biological productivity of the alkaline lake was 2.0 to 12.6 times that of rivers and freshwater lakes^[3]. Additionally, the stratification of water in alkaline lake is favorable to the formation of a reducing environment at the lake bed, under which the oxidative decomposition of organic matter is reduced to some extent, thereby facilitating the preservation of organic matter. In 1990, Domagalski et al. conducted systematic sampling and analysis on the lake beds of the Mono Lake, Walker Lake and Great Salt Lake in the United States and they indicated that the alkaline lakes had higher TOC than ordinary salt lakes, and the alkaline lakes with stratification of water body had higher TOC that those without stratification of water body^{[2, 4]}.

Studies have shown that the formation of alkaline lakes requires special geological conditions. Firstly, its formation needs relatively closed catchment basin, where large evaporation can enable the salt to be continuously precipitated, and the large amount of water injected prevents the lake basin from premature drying up. Thus, sufficient HCO₃^- and CO₃^2- can be generated after the precipitation of Mg²⁺ and Ca²⁺ and then combine with Na⁺ to form sodium carbonate. Areas with such environment are usually arid climate zones^[9]. Secondly, the formation of alkaline lakes requires sufficient sources of solutes, including the solutes generated when rains, surface runoff, groundwater, and hydrothermal fluids take water-rock reaction with soil, bedrock or surrounding rock when they inject into the lake basin. According to the geographical distribution of modern alkaline lakes, volcanic rocks are widely observed within the alkaline lake basins^[10]. On one hand, the ultrabasic and medium-basic magmatic rocks, especially the vitreous in the corresponding eruptive rocks, are highly susceptible to weathering and hydrolysis, thereby providing abundant Na⁺ to the lake basin and increasing the alkalinity of water. On the other hand, the hydrothermal fluids bring abundant chemicals (including Na⁺, HCO₃^-, and CO₃^2-), volcanic gases (primarily CO₂), and water supply^[11]. Under this circumstance, the solutes have increasing HCO₃^-, CO₃^2- and Na+ but decreasing Mg²⁺ and Ca²⁺ in the process of complex chemical differentiation, ultimately giving rise to an alkaline lake.

In the Mahu sag of the Junggar Basin, NW China, the lacustrine organic-rich shale in the Lower Permian Fengcheng Formation is developed extensively and serves as the major oil supply interval in the giant Mahu Oilfield recently discovered in the slope zone^[12]. There is an arsenal of controversy in the academic community about the sedimentary environment of the Fengcheng Formation of the Mahu sag. Before 1980, it was mostly assigned as estuarine facies and residual marine deposits^[13]. With the increase of drilling, logging and seismic data, and the deepening of research on regional tectonic- sedimentary evolution, a consensus has been reached on the understanding that the Fengcheng Formation was deposited in a salified lake basin^[14]. Since 2010, a variety of minerals indicating alkaline environments and hydrothermal actions have been discovered in the Fengcheng Formation of the Mahu sag. This sedimentary signature is very similar to that of the alkaline lacustrine sediments in the Green River Formation and modern alkaline lacustrine sediments. Therefore, some scholars have successively proposed that the Fengcheng Formation of the Mahu sag is of alkaline lacustrine deposition^[15]. There are only few similar examples found in the world, but the Fengcheng Formation of the Mahu sag provides a good object for the study on deposition of paleo-alkaline lacustrine sediments. Nevertheless, the research on its petrological and mineralogical types and sedimentary environment is still in its infancy.

In order to deepen the understanding to the deposition of alkaline lacustrine sediments in the Fengcheng Formation of the Mahu sag and enhance the recognition on the genesis of alkaline lake basin, the research works (e.g. identification of alkaline minerals, lithofacies division, analysis of logging response characteristics, and paleoenvironmental reconstruction) are conducted through analysis of thin section, scanning electron microscope (SEM), and major and trace elements using the available drilling, logging and core data, to investigate the sedimentary evolution model of alkaline lacustrine sediments in the Mahu sag. The study results can provide sedimentary and fundamental references for deeper exploration of the alkaline lacustrine sediments.

The Mahu sag is located in northwestern Junggar Basin (Fig. 1a). It is the sub-order negative tectonic unit of the central depression, with an area of about 5 000 km². In 2017, the Triassic Baikouquan Formation was discovered with the 3P reserves up to 10×10⁸ t.

The Fengcheng Formation in the Lower Permian is underlied by the Jiamuhe Formation and overlaid the Xiazijie Formation. Upwardly, it is divided into Feng 1 Member (P₁f₁), Feng 2 Member (P₁f₂) and Feng 3 Member (P₁f₃), which show variable distribution of lithofacies (Fig. 1b). In the early stage of Feng 1 Member, the volcanic activity was frequent and the northeastern part of the Mahu sag was dominated by pyroclastic rock-sedimentary pyroclastic rock. Later, organic-rich mudstone, dolomite and dolomitic rock were developed successively. As the continual increase of water salinity in the lake basin, a large amount of alkaline minerals such as trona, wegscheiderite and shortite were developed in the central sag during the deposition of Feng 2 Member. In the stage of Feng 3 Member, the salinity of water in the lake basin reduced, and the sag was dominated by dolomitic rocks. At the top of Feng 3 Member, terrigenous clastic rocks were developed; the closer to the foothills of Zaire Mountains, the higher content of clastic rocks and the larger the grains. Longitudinally, the reedmergnerite-rich rocks were developed near the deep major fault in the sag^[16].

The Fengcheng Formation is mainly composed of quartz, feldspar, calcite and dolomite, as well as some unusual minerals such as reedmergnerite and northupite. It contains minor clay minerals (mostly smectite).

The quartz content is 3%-45%, or 27% on average. The feldspar is dominated by plagioclase (3%-27%, or 14% on average), and K-feldspar (3%-19%, or 6% on average). The existence of less K-feldspar reflects that the Fengcheng Formation is a sodium-rich and potassium-lean environment. Essentially, kaolinite is relatively stable under acidic conditions; with increasing pH, however, kaolinite can be converted into illite in a potassium-rich environment, or into smectite or chlorite in a sodium-, calcium- and magnesium-rich environment^[17]. The content of calcite is 2%-48%, or 15% on average, but it is rarely observed in the rocks rich in alkaline minerals. The content of dolomite differs vastly, ranging from 4% to 80%, with an average of 30%. In addition, there are reedmergnerite indicating the hydrothermal activity and eitelite representing alkaline environment. The clay content is less than 10%. High content of felsic and low content of clay minerals indicate that the Fengcheng Formation suffered mainly physical weathering, but no apparent chemical weathering. The content of pyrite is 2%-10% or 5% on average, which is mainly distributed in the Feng 2 Member, indicating the reducing environment.

In Well FN1, for example, the Feng 1 Member is dominated by volcanic rocks and tuffaceous clastic rocks, and contains mostly feldspar minerals (Fig. 2); the Feng 2 Member comprises mainly dolomitic rocks and argillaceous rocks, with high proportions of dolomite and calcite minerals; the Feng 3 Member is represented by conglomerate and siltstone, with felsic minerals in dominance. On the whole, the Fengcheng Formation is characterized by rich felsic minerals, lean clay minerals and abundant volcanic clasts.

The Fengcheng Formation of the Mahu sag is a typical fine-grained sedimentary system, with a small amount of coarse-grained clasts, typically represented by carbonate rocks, clastic rocks, and lava and pyroclastic rocks associated with volcanism. Based on previous studies, the Fengcheng Formation is divided into six lithofacies types through core observation and microscopic identification, including the pyroclastic rock-sedimentary pyroclastic rock, organic-rich mudstone, dolomite and dolomitic rock, rock with alkaline minerals, reedmergnerite-rich rock^[16] and terrigenous clastic rock (Fig. 3). The pyroclastic rock-sedimentary pyroclastic rock refers to the rocks with volcanic clasts content greater than 25% and sourced from in-situ parental rocks or close transportation. The terrigenous clastic rock contains volcanic clasts less than 25% and is usually sourced through transportation at a certain distance.

The lithofacies of pyroclastic rock-sedimentary pyroclastic rock is lithologically represented by basaltic rock, tuff and tuffaceous siltstone and tuffaceou glutenite transitioning to sedimentary rocks. Vertically, it is concentrated at the bottom of the Feng 1 Member. But in the northeastern part of the Mahu sag, the lithofacies almost overwhelms the Fengcheng Formation. In core samples, pyroclastic clasts are in scattered distribution or in the mudstone interlayer. The volcanic rocks of the Fengcheng Formation are somewhat alkaline, and a large amount of altered K-feldspar and soda feldspar are observed under microscope. It is speculated that alkaline volcanic rocks experienced hydrolytic dissolution and provided Na⁺ for water bodies^[1,2].

The lithofacies of organic-rich mudstone is widely distributed in the sag, and the development of white strips can be observed in core samples. According to the composition, it is divided into organic-rich dolomitic mudstone (the strips are dolomites) and organic-rich limy mudstone (the strips are calcites). The organic-rich limy mudstone shows much darker color than the organic-rich dolomitic mudstone. The measured average TOC value is 1.6% for the former and 1.0% for the latter^[16].

Dolomites and dolomitic rocks, which are extensively developed in the Fengcheng Formation, are mostly interbedded with argillaceous rocks. The well logging shows a large set of pure dolomite, such as the Feng 1 Member to Feng 2 Member in Well F7.

The lithofacies of rock with alkaline minerals is the most unique type in alkaline lakes. Through the analysis of core samples from Wells FN5, F26, and FN7, it is found that the Fengcheng Formation is rich in alkaline minerals including trona (Na₂CO₃·NaHCO₃), wegscheiderite (Na₅H₃(CO₃)₄), shortite (Ca₂Na₂(CO₃)₃), northupite (Na₃Mg(CO₃)₂Cl), and loughlinite (Na₄Mg₆(H₂O)₄ (Si₆O₁₅)₂(OH)_4.8H₂O), which are mainly distributed in the Feng 2 Member. The available drilling data show that the closer to the center of Mahu sag, more alkaline minerals developed, so it is inferred that alkaline minerals are the most developed in the center of the sag. The discovery of a large number of alkaline minerals has confirmed the Fengcheng Formation containing alkaline lacustrine sediments.

The coring wells such as FN5, FN7 and F26 close to the Wuxia Fault zone were analyzed. It is found that the lithofacies of reedmergnerite-rich rock is commonly observed in the vicinity of the fault, and often parental with alkaline minerals or interbedded with dolomitic rocks and argillaceous rocks. Its development is closely related to volcanic eruption and hydrothermal activity^[18].

In the late stage of the Feng 3 Member, the terrigenous clast was sufficient and mostly distributed in the piedmont of the Zaire Mountains. The lithology is fine-grained siltstone-fine sandstone and coarse-grained pebbly coarse sandstone-conglomerate.

There are fewer wells penetrating the Fengcheng Formation in the Mahu sag, and the coring in most of the wells is discontinuous. It is difficult to determine the lateral and longitudinal distributions of lithology by solely relying on core observation. Therefore, based on core identification, the log charts were prepared according to logging features of different lithologies, and the log data were used to predict the lithofacies distribution in the Fengcheng Formation.

2.3.1. Lithofacies of pyroclastic rock-sedimentary
pyroclastic rock

Taking Well MH5 as an example, the logging response of 20 m thick basaltic core was analyzed. It is found that the deep lateral resistivity (R_lld) and shallow lateral resistivity (R_lls) are box-shaped high values, which are significantly higher than logging values of overlying and underlying normal clastic rocks and tuffaceous clastic rocks, and the resistivity of flushed zone (R_XO) is separated from R_lld and R_lls. The natural gamma ray log (GR) reveals a box-shaped low value and the spontaneous potential log (SP) shows a negative offset (Fig. 4a). According to the drilling data, the tuffs with varying thicknesses can be observed at the bottom of multiple coring wells. Typically, continuous tuff about 20 m thick can be observed in Well FN14, and its R_lld, R_lls and R_XO are box-shaped high values, and GR value is medium to high (Fig. 4b). Due to the fact that the sedimentary provinces are close to the piedmont of the Hala Alat Mountain, there are the mixtures of terrigenous clast frequently observed with the tuffaceous siltstone and tuffaceous pebbly sandstone. The R_lld and R_lls of these tuffaceous clastic rocks are characterized by serrated medium-low values, which are significantly lower than that of basaltic rocks (Fig. 4a) and tuff (Fig. 4b), and the GR values are much lower than that of tuff (Fig. 4b). This is due to the fact that from basic rock to acidic rock, the content of metal element decreases but the content of silica increases, therefore the natural radioactivity gradually increases; reduced density lowers the tightness of rocks, resulting in decreased resistivity^[19].

2.3.2. Lithofacies of organic-rich mudstone

The organic-rich mudstone of the Fengcheng Formation is often associated with the lithofacies of rock with alkaline minerals (Fig. 4c) or interbedded with argillaceous dolomite (Fig. 4d). From the logging curve, it can be seen that the argillaceous rocks and dolomitic rocks of the Fengcheng Formation in the Mahu sag are basically the same as those in other areas. The organic-rich mudstone shows a relatively massive high value of R_XO in contrast to the lithofacies of rock with alkaline minerals (Fig. 4c), and lower R_lld, R_lls and R_XO than argillaceous dolomite; the GR value is not evident, and the SP value is basically near the baseline (Fig. 4d).

2.3.3. Lithofacies of dolomite and dolomitic rocks

According to logging data, there are about 60 m thick pure dolomite at the bottom of the Fengcheng Formation in Well F7, which however has not been confirmed by coring data. Argillaceous dolomite is often interbedded with the lithofacies of rock with alkaline minerals or organic-rich mudstone. For Well FN7, R_XO presents a peak-like high value, and GR shows a massive medium value in contrast to alkaline rocks; no expanding of well diameter is observed (Fig. 4c). For Well FN14, R_lld, R_lls and R_XO are higher than that of dolomitic mudstone (Fig. 4d).

2.3.4. Lithofacies of rock with alkaline minerals

The cores from Well FN7 reveal the rock with alkaline minerals interbedded with argillaceous dolomite, with a length of about 8 m. This lithofacies is often dissoluble, so its log responses show that the R_lld and R_lls logs present peak-like high values, the R_XO value decreases sharply, the GR has a funnel-like low value, and the expanding of well diameter is evident; the SP curves of high shale content are close to the baseline (Fig. 4c).

2.3.5. Lithofacies of terrigenous clastic rock

Compared with the lithofacies of pyroclastic rock-sedimentary pyroclastic rock (Fig. 4a), the lithofacies of terrigenous clastic rock shows apparent reduction of density, R_lld, R_lls, and R_XO. The coarser the grain, the higher the resistivity. And other logs represent no evident features.

2.3.6. Lithofacies of reedmergnerite-rich rocks

Reedmergnerite-rich rock is often parental with alkaline minerals or sandwiched in dolomitic rocks and argillaceous rocks, thereby showing variable logging features. It can be inferred from the available core data of drilling wells that the reedmergnerite-rich rocks are widely developed near the fault zone. The rocks are found in the coring wells such as F26, FN5 and FN7 closer to the fault zone, but not developed in Well FN4 which is far from the fault zone. However, due to the transportation of water flow, such rocks that have been transported and redeposited can also be seen in Well FN1 far from the fault zone^[16].

The drilling data of the piedmont of the Zaire Mountains (Wells MH7, B22, and BQ1), the low-lying zone of the sag (Wells FN7 and AK1), and the piedmont of Hala Alat Mountain (Wells FN4 and X202) were selected to plot the well-tie section parallel to the long axis of lake basin (Fig. 5). It can be seen that the maximum sedimentary thickness of the Fengcheng Formation is found near Well BQ1 in the piedmont of the Zaire Mountains, being inconsistent with normal sedimentary formation which has the maximum thickness in the center of the lake basin.

Wells MH7 and B22 are located in the western part of the sag. Well MH7 doesn’t penetrate the Fengcheng Formation. For Well MH7, the Feng 1 Member shows a medium-low resistivity, representing as the sediments of terrigenous clastic rocks. Compared with the Feng 1 Member, the Feng 2 Member reflects a resistivity curve not varying greatly at the bottom, representing the sediments of terrigenous clastic rocks in the second stage. Upwardly, the resistivity increases significantly, the R_XO curve is separated from the R_t and R_I curves, and the SP curve shows negative offset, suggesting the characteristics of basaltic rocks. The Feng 3 Member reflects a significantly reduced resistivity at the bottom, which is increasingly lower upwardly. It is speculated as the transition from the tuffaceous clastic rocks to terrigenous clastic rocks.

For Well B22, the Feng 1 Member shows a higher resistivity than that in Well MH7; the R_XO curve is separated from the R_t and R_I curves, and the GR curve shows a box-shaped low value, suggesting typical characteristics of basaltic rocks. The Feng 2 Member has log responses similar to that of Well MH7, suggesting as the transition from terrigenous clastic rocks to basaltic rocks. Upwardly, the resistivity reduces, in a similar behavior to the top of Feng 3 Member in Well MH7, with the development of terrigenous clastic rocks.

Well BQ1 is located on the west slope of the sag, adjacent to the Zaire Mountains. For the Feng 1 Member to the Feng 2 Member, the log doesn’t show great variation, and the GR curve is slightly serrated without obvious variation of amplitude. It is presumed to be sediments of lacustrine argillaceous rocks and dolomitic rocks. Occasionally, partial intervals show significant reduction of resistivity, which is possibly due to the supply of terrigenous clast. For the Feng 3 Member, the resistivity presents a tendency of gradual increase, which is the characteristic of transition from fine-grained to coarse- grained clasts.

Wells FN7 and AK1 are located in the sag, and both of them penetrate the Jiamuhe Formation, in which the resistivity shows box-shaped high values and the GR curve represents medium-high values, suggesting typical characteristics of tuff. For Well FN7, the resistivity of Feng 1 Member is lower at the bottom. Upwardly, the R_XO shows a peak-like high value, and it is presumably to be the sediments of argillaceous rocks in the early stage and dolomitic rocks in the late stage. The Feng 2 Member shows evident expansion of well diameter, sharp dropping of R_XO, and funnel-shaped low value of GR curve, indicating that alkaline minerals were deposited during this period. The Feng 3 Member shows sharp drop of resistivity, repeatedly oscillation in the medium-low values and normal well diameter, suggesting the characteristics of interbed of dolomitic rocks and argillaceous rocks. Unlike the log responses of Well FN7, Well AK1 is characterized by the logging characteristics of lithofacies of rock with alkaline minerals from the late stage of the Feng 1 Member to the late stage of the Feng 2 Member. Well AK1 is closer to the center of the lake basin than Well FN7. This indicates that the salinity and concentration of alkaline ion in the center of the lake basin were sufficient for the deposition of alkaline minerals during the deposition of the Feng 1 Member.

Well FN4 is located on the slope zone in the northeastern part of the sag and penetrates the Jiamuhe Formation. The Jiamuhe Formation is different from the Feng 1 Member in the fact that the Jiamuhe Formation shows low GR values and serrated medium-low resistivity values, while the bottom of the Feng 1 Member shows significantly increased GR and resistivity values. This indicates that the Jiamuhe Formation is the deposit of tuffaceous clastic rock and the bottom of the Feng 1 Member is the deposit of tuff. At the top of the Feng 1 Member, the resistivity decreases and the SP value is basically near the baseline, suggesting the typical characteristics of argillaceous rock. The resistivity of the Feng 2 Member is higher than that at the top of Feng 1 Member and the SP value oscillates back and forth around the baseline, representing the interbed of argillaceous rocks and dolomites. Upwardly, the resistivity and GR values are slightly lower, but each curve doesn’t vary greatly in the Feng 3 Member. It is presumably to be the deposits of argillaceous rocks.

Well X202 is located in the Xiazijie area in the northeast of the sag. As a whole, the logging curves are not variable greatly. There is no obvious difference in the resistivity between the Feng 1 Member and the Feng 2 Member, but a significant trough-shaped low value is found at the boundary; thus it is presumed to be two stages of tuff deposition. Upwardly, the resistivity reduces; combined with the general geological background of intense volcanic activities in the Xiazijie area, the Feng 3 Member is believed to have tuffaceous clastic rocks.

Based on the above correlation between logging response and lithology and the six sections established, the lithological distribution law of three members of the Fengcheng Formation in the Mahu sag was preliminarily determined. In the Feng 1 Member, organic-rich mudstone is developed in the center of the sag; dolomite and dolomitic rock is developed outward; tuff and tuffaceous clastic rocks are developed in the northeastern part, and basaltic rocks in the western part of the sag; there are mainly terrigenous clastic rocks in the piedmont of the Zaire Mountains (Fig. 6a). In the Feng 2 Member, the lithofacies of rock with alkaline minerals is developed in the center of the sag, and it transforms into dolomite and dolomitic rocks; the closer to the center, the higher the shale content; the northeastern part of the sag is dominated by tuffs and tuffaceous clastic rocks, with a scale much smaller than that in the Feng 1 Member; the distribution of reedmergnerite-rich rocks can be observed near the fault zone (Fig. 6b). In the Feng 3 Member, the scale of terrigenous clastic rocks increases, especially in the piedmont of the Zaire Mountains; the distribution of pyroclastic rock-sedimentary pyroclastic rock continues to shrink, with tuffaceous clastic rocks in dominance; the depositional range of lithofacies of rock with alkaline minerals shrinks or even disappears; the organic-rich mudstone, dolomite and dolomitic rocks are intercalated (Fig. 6c).

According to the available drilling and coring data, the wells that encounter the lithofacies of rock with alkaline minerals are mostly inclined to the west of the central sag. The logging curve also shows that the west of sag center contains a large area of lithofacies of rock with alkaline minerals. This may also be related to the distribution of volcanic rocks of different natures. The northeastern part is dominated by tuffaceous rocks (mainly felsic and acidic), and the western part by basaltic rocks (andesite). Hydrolysis of these rocks might provide alkaline ions (e.g. Na⁺ and Ca²⁺) to the lake base. Ca²⁺ precipitated to form dolomite before Na⁺. As the climate became dry, the input of freshwater was reduced; therefore, the basin was gradually alkalized.

It has been shown that the lake basin was generally occluded in the depositional period of the Fengcheng Formation. The lacustrine environment was mainly characterized by arid climate, shallow water, and high salinity^[20,21,22], which are similar to the formation environment of the source rocks in the Lucaogou Formation in the Jimusaer Sag^[23]. However, the paleoenvironmental evolution process in the depositional period of the Fengcheng Formation has not yet been determined. When the lake basin area was small, the influence of paleoenvironment could be reflected in the variation of elements, and such variation can’t be accurately identified by paleontological fossils^[24]. By the comprehensive analysis of element geochemical characteristics, this paper studied the subtle variations in the climate, salinity, water depth, and the likes in the deposition period of Fengcheng Formation.

Long and continuous cores from the Fengcheng Formation in Well FN1 in the Mahu sag were selected from 81 sampling points. The length of core was 63 m. The samples were taken intensively in intervals with great variation in lithology and in larger intervals with homogeneous lithology. The elemental abundance was systematically tested by using a portable X-ray fluorescence spectrometry (XRF) system, which can test more than 40 elements such as Ba, Sb, Sn, Zr, Rb, Fe, Mg, Ca, Sr, Al, Si, and V, under ore mode and soil mode. The ore mode is mainly to measure the major elements with a content of higher than 1%, generally in the unit of %, and the soil mode is mainly for trace elements with a content of less than 1%, generally in the unit of 10^-6. In the measurement process, in order to ensure the accuracy, three measurements were made at each point under each mode, and then the average was taken as the test result. The core was wiped before the test to remove the dust floating on the surface. Totally 81 sets of data were obtained in this test. Each set included constant elements such as Mg, Al, and Si, and trace elements such as Zr, V, and Cr. In this study, only elements with an error of less than 10% were selected for analysis, including Ba, Sb, Sn, Zr, Sr, Rb, As, Zn, Ni, Fe, Mn, Ca, K, Al, Si, Cl, S, and Mg.

In order to verify the test results, 35 samples were selected at the test point for elemental analysis, using the test instrument ELEMENT XR. Comparing the test results with the XRF results, it is found that the overall trend is consistent, notwithstanding some differences in the absolute values. Therefore, the test results can be used for paleoenvironment analysis.

According to the analysis of elemental abundance and its ratio, paleoclimate correlates well with paleo-bathymetry (Fig. 7). General, the larger ratios of Mg/Ca and Mg/Sr correspond to the drier and hotter climate, and the Mg/Ca in the alkaline-bearing interval may have a low value or even extremely low value^[25]. In the early stage of the Fengcheng Formation, the values of Mg/Ca and Mg/Sr show the change law of lower values → low values → higher values → continuous low values. Combined with variation of lithology, a thin layer of alkaline was deposited in the late stage of the Feng 2 Member deposition and the phenomenon of abnormally low values of Mg/Ca and Mg/Sr may even occur in arid environment. Therefore, the corresponding paleoclimate was semi-arid → humid → semi-arid → arid → semi-arid. The high values of Mn/Fe and Rb/K indicate shallow water body^[26]. The variation of Mn/Fe and Rb/K was consistent with the variation of water depth, almost experiencing a process of higher value → high value → medium low value → low value → higher value. And the corresponding water depth was deeper → deep → shallower → shallow → deeper. The change in paleo-bathymetry was almost identical to the change in paleoclimate.

Higher K/Na and Sr values reflect higher salinity of water^[27]. The ratios of elements reflecting paleo-salinity in the early stage of Feng 2 Member-Feng 3 Member are slightly higher, suggesting the high salinity environment. Sr roughly changes in a trend of medium-low value → low value → medium-high value → high value → medium-low value, and the salinity in a trend of slightly fresh → fresh → slightly saline → saline → slightly fresh. The measurement of freezing temperatures^[16] and quantitative calculation of boron (B) and clay content^[28] indicate that the Fengcheng Formation as a whole is of alkaline lacustrine deposits with high salinity. The contents of vanadium (V) and nickel (Ni) in the Fengcheng Formation are low, and they were not detected by the portable XRF system. The contents of V and Ni were obtained through whole rock analysis. The sampling points were largely spaced and the measurement results indicate that the value of V/(V+Ni) generally experienced a process of decreasing → increasing → decreasing, reflecting the redox of paleoenvironment of oxidation → reduction → oxidation^[29]. In addition, only three values of V/(V+Ni) among 27 samples that have been measured are less than 0.6 (Fig. 7), suggesting that the Fengcheng Formation was dominated by euxinic reduction environment.

The chondrite standardized values of rare earth elements in the Fengcheng Formation (the measured rare earth element content of the sample divided the rare earth element content of the standard chondrite) show that the light rare earth element (LREE) content is higher than the heavy rare earth element (HREE) content. Ce represents positive anomaly, and Eu shows negative anomaly (Fig. 8), indicating that the Fengcheng Formation as a whole was deposited in normal water bodies^[30]. Analysis of samples near the fault shows that LREE is significantly lower than HREE (Table 1). Combined with inclusion analysis, the homogeneous temperature is up to 154°C, indicating that hydrothermal activity occurred near the fault, and also confirming that the formation of reedmergnerite is closely related to hydrothermal activity. In the hydrothermal deposition zone, the Fe and Mn contents are quite high, and they are closely associated with each other. However, Fe and Mn are not associated in normal sedimentary rocks^[31]. The relative concentration of Al and Ti is mostly related to the intervention of terrigenous materials, and their contents are positively correlated with the content of fine- grained terrigenous materials. Therefore, the Al/(Al+Fe+ Mn) value is commonly used to determine whether it belongs to hydrothermal deposition. The abnormally small value indicates the occurrence of hydrothermal deposition^[32]. The abnormally low Al/(Al+Fe+Mn) value of the Fengcheng Formation appeared twice, indicating that it experienced two stages of hydrothermal activity (Fig. 7). The first stage (H₁) of hydrothermal activity occurred in the early state of Feng 2 Member and the second stage (H₂) of hydrothermal activity occurred in the middle-late stage of Feng 2 Member. Generally, the hydrothermal action promotes the deposition of alkaline minerals. According to the timing analysis of alkaline minerals, the first stage of hydrothermal activity did not promote the development of rock of this type, which might be resulted from the wetter and colder climate, deeper water body, and lower salinity than that in the second stage. As the climate became drier and hotter, the water body was characterized by shallower depth and higher salinity. After the second stage of hydrothermal activity, the lithofacies of rock with alkaline minerals was deposited.

The K₂O/Rb values of 35 samples from the Fengcheng Formation in the Mahu sag were analyzed. It is found that all K₂O/Rb values are high and fall in the provenance of pyroclastic clast-rich deposits (Fig. 9). Moreover, the lithofacies study reveals that the early deposition stage of Feng 1 Member was dominated by pyroclastic rock-sedimentary pyroclastic rock. For mineralogical composition, pyrite was abundant, feldspar content was high and content of clay minerals was low, indicating the anaerobic environment in the Fengcheng Formation under volcanic activities^[33].

The paleoenvironment reconstruction reveals that in the Fengcheng period the Mahu sag was in a highly-saline reduction environment, under the paleoclimate of semi-arid → humid → semi-arid → arid → semi-arid, where the paleo-bathymetry and paleo-salinity varied correspondingly, and were affected by at least two stages of hydrothermal activity (Fig. 7). Combined with the vertical association and plane distribution of lithofacies in the Fengcheng Formation, the sedimentary evolution model of the Fengcheng Formation in the Mahu sag was established. The sedimentary evolution is divided into five stages (Fig. 10), which are greatly sensitive to volcanic activities and paleo-climate, and characterized by frequent fluctuation of lacustrine level, restricted sedimentary ranges of lithofacies of rock with alkaline minerals and reedmergnerite-rich rocks, relatively small thickness of organic- rich mudstone with low purity, frequent intercalation with dolomitic rocks, and certain sedimentary sequences in the vertical direction. Due to the influence of volcanic activity, the pyroclastic rocks-sedimentary pyroclastic rocks show ebb and flow relationship with terrigenous clastic rocks.

The first stage corresponds to the early deposition stage of Feng 1 Member (Fig. 10a). The lacustrine level was high and the climate was semi-arid. The supply of terrigenous clast was minor, while the lithofacies of pyroclastic rock-sedimentary pyroclastic rock was developed, dominated by alkaline-sub- alkaline volcanic rocks. The content of sedimentary pyroclastic rock increased approaching the foothills of the Hala Alat Mountains. The volcanic ash interlayer can be observed far from the provenance. The western part of the sag was dominated by basaltic rocks, and the northeastern part was mainly volcanic rocks and tuffaceous clastic rocks. The water body was euxinic and the relatively thin and restricted organic-rich mudstones were deposited.

The second stage corresponds to the late deposition stage of Feng 2 Member (Fig. 10b). It was characterized by upgraded lacustrine level, humid climate, and weakened volcanic activities. But there was still supply of volcanic material in the gentle slope zone, and the sedimentary thickness of pyroclastic rock-sedimentary pyroclastic rock was increased. Abundant volcanic materials provided nutrients for aquatic organisms. The organic matter was abundant and the water body was strongly reducing in this period, which was favorable to the accumulation and preservation of organic-rich mudstone in the central lake basin.

The third stage was the early deposition stage of Feng 2 Member (Fig. 10c). The lake basin shrank and the climate became arid. The supply of terrigenous clast was still extremely small. The calcium and magnesium ions were supersaturated, and the calcite and dolomite were precipitated first, resulting in increased salinity of water and the beginning of stratification. The organic matter formed in this stage was well preserved. At the same time, the loose structure formed by the accumulation and decay of a large amount of algae resulted in the large-scale development of carbonate minerals. Besides, the lithofacies of dolomite and dolomitic rock alternated with the lithofacies of organic-rich mudstone in the Fengcheng Formation, or the lithofacies of dolomite and dolomitic rock overlaid the lithofacies of organic-rich mudstone. Moreover, due to the action of hydrothermal activity, the lithofacies of reedmergnerite-rich rock was deposited near the fault.

The fourth stage was the middle-late deposition stage of the Feng 2 Member (Fig. 10d). After the deposition of calcium and magnesium carbonate in the third stage, Ca²⁺ and Mg²⁺ in the water were exhausted, and the pH reached the highest value in the evolution process of the Fengcheng Formation. Due to the early stratification of water body, CO₂ generated by the biological respiration and organic matter decomposition enabled the CO₂ pressure of lacustrine water to increase. At the same time, due to the further shrinkage of lake basin, the climate was extremely dry and hot, the salinity was high, and the hydrothermal effect was evident. At this stage, the lithofacies of rock with alkaline minerals was deposited, and the deposits of reedmergnerite-rich rocks can be observed near the fault. The geochemical characteristics of crude oil show that the content of gammacerane was high in this stage, showing higher salinity and more evident stratification, which was more favorable to the preservation of organic matter^[35,36].

The fifth stage was the deposition stage of the Feng 3 Member (Fig. 10e). In the early stage of Feng 3 Member, the lacustrine level upgraded again, the climate became wet and cold, and the salinity gradually decreased. Argillaceous rocks and dolomitic rocks were deposited again. As the lacustrine level continued to fall, the climate became wetter and colder, the input of terrigenous clasts increased and the abundance of organic matter decreased. In the late stage of Feng 3 Member, terrigenous clasts were dominant.

The first four stages of the sedimentary evolution model represent the salinization and alkalization process of the lake basin. Even if there was abnormal fluctuation in the second stage, the trend of falling lacustrine level, hotter and drier climate, and increased salinity was observed generally. In this process, the deposition of the lithofacies of rock with alkaline minerals marked the completion of alkalization. Subsequently, in the fifth stage, the lake basin experienced transgression and developed towards desalination, when calcium-magnesium carbonate rocks were re-deposited and the sediments of terrigenous clasts increased.

The study reveals that the deposition of the Fengcheng Formation was mainly controlled by the frequently varied paleoclimate and volcanic activities. The paleoclimate determines the fluctuation of lacustrine level, which in turn controls the deposition of rock with alkaline minerals, organic- rich mudstone, dolomite and dolomitic rock, and terrigenous clastic rock. The arid climate promotes the saltation of the lake basin and the ultimate shaping of alkaline lake. The volcanic activities affect the distribution of pyroclastic rock-sedimentary pyroclastic rock, which grew alternatively with terrigeneous clastic rock. The hydrothermal activity associated with volcanic activity controls the deposition time and range of reedmergnerite-rich rocks.

There are six types of lithofacies in the Lower Permian Fengcheng Formation in the Mahu sag, Junggar Basin, which are pyroclastic rock-sedimentary pyroclastic rock, organic-rich mudstone, dolomite and dolomitic rock, rock with alkaline minerals, terrigenous clastic rock, and reedmergnerite-rich rock.

The lithofacies of pyroclastic rock-sedimentary pyroclastic rock was developed in the early stage of the Feng 1 Member and is mainly distributed in the northeastern part of the sag. The lithofacies of organic-rich mudstone was widely developed in the late stage of Feng 1 Member to the early stage of Feng 2 Member, and is often intercalated with dolomite and dolomitic rocks. The lithofacies of rock with alkaline minerals was developed in the late period of the Feng 2 Member, and is mainly distributed in the central sag. The lithofacies of terrigenous clastic rock was mainly developed in the Feng 3 Member. The lithofacies of reedmergnerite-rich rock was mainly developed near deep large fault.

The paleoenvironment of Fengcheng Formation was evolved in stages. Arid-semi-arid climate, high salinity, evident hydrothermal action, and provenance of alkaline-sub-alkaline pyroclastic rocks provided the geological conditions for alkaline lake formation. The paleoclimate was semi-arid → humid → semi-arid → arid → semi-arid, the paleo-depth is deeper → deep → shallower → shallow → deeper, and the paleo-oxidation and reduction process was oxidation → reduction → oxidation; the overall paleo-salinity was high, especially in the late stage of the Feng 2 Member, and the water body is layered obviously. During the deposition period, it was affected by two stages of hydrothermal activity. The lithofacies of rock with alkaline minerals was deposited in the second stage of hydrothermal activity.

The sedimentary evolution of the Fengcheng Formation was mainly controlled by paleoclimate and volcanic activities. It can be divided into five stages. In the first four stages, the lake basin gradually became salty and alkalized and showed the strongest alkalinity in the fourth stage. In the fifth stage, the lake basin began to be desalized. The paleoclimate controlled the variation of lacustrine level and the arid climate promoted the salinization of freshwater. The weathering hydrolysis of early alkaline-sub-alkaline pyroclastic rocks provided Na⁺ to the water body, making the lake basin evolve toward the water body with Na⁺ - CO₃^2- - Cl^- and eventually form an alkaline lake.

The study on the sedimentary model of the Fengcheng Formation in the Mahu sag helps to deepen the geological understanding to the alkaline lacustrine sediments. However, the relationship between the development of alkaline lake and the occurrence of high-quality source rocks in the Fengcheng Formation needs to be further investigated.