Heterogeneity of alkaline lacustrine source rocks of the lower Permian Fengcheng Formation in the Mahu Sag,Junggar Basin,northwestern China

Abstract

The alkaline lacustrine source rocks in the lower Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China, exhibit significant heterogeneity. However, most previous studies have focused on the vertical variability of the source rocks, and the lateral variability is poorly understood. This study investigated recently drilled samples of the Fengcheng Formation in the Manan area to assess the lateral variability of the source rocks. The geochemical characteristics of the Fengcheng Formation source rocks in the Manan area are variable, with low–high abundances of organic matter, which comprises types I–III kerogen that is mature to high-mature. These features differ from the typical Fengcheng Formation source rocks in the Wuxia–Fengcheng area, which have a high abundance of organic matter with substantial hydrocarbon and shale oil generation potential. Biomarker data show that the organic matter in the Fengcheng Formation source rocks in the Manan area is a mixture of higher plants, algae, and bacteria, which differs from the Wuxia–Fengcheng area where algae and bacteria are dominant. The Fengcheng Formation source rocks in the Manan area were deposited in an oxic to anoxic environment characterized by large variations in water salinity and stratification. Mudstones were deposited rather than carbonate rocks. This differs from the strongly reducing, hypersaline, and water-stratified depositional environment of the Fengcheng Formation source rocks in the Wuxia–Fengcheng area. These two areas are separated by the Dazhuluogou Fault, and there are no alkaline lacustrine deposits in the Manan area. The distribution of the alkaline lacustrine source rocks was controlled by the paleogeography and sedimentary environment, which was characterized by significant spatial heterogeneity.

Keywords

Hydrocarbon source rocks Mahu Sag Fengcheng Formation alkaline lake hydrocarbon-generating precursors sedimentary environment shale oil

Introduction

Alkaline lakes have waters with a pH > 9, alkali metal (Na⁺ and K⁺) contents that are much larger than those of alkali earth metals (Ca²⁺ and Mg²⁺), and high HCO₃⁻ and CO₃^2– contents (Cangemi et al., 2016; Pecoraino et al., 2015). The primary productivity of alkaline lakes is extremely high (10 g cm^–2 d^–1), which is >16 times higher than the global average primary productivity of lakes (0.6 g cm^–2 d^–1) (Jones et al., 1998; Pecoraino et al., 2015). In theory, alkaline lakes can form hydrocarbon source rocks, which represent an important end-member type for studies of hydrocarbon generation (Cao et al., 2020; Tang et al., 2021; Tissot et al., 1978; Xia et al., 2021, 2022). Case studies have shown that the depositional environment in alkaline lacustrine settings exhibits significant heterogeneity. For example, the Eocene Green River Formation in the United States contains alkaline lacustrine source rocks, but the main part of the Green River Basin deposited the trona, whereas the main part of the Piceance Creek Basin deposited the nahcolite (Dyni, 2003). Most of the high-salinity, alkaline lacustrine source rocks were deposited in the Parachute Creek Member, whereas the sedimentary environment of the lower part of the Green River Formation was characterized by less saline water (Birgenheier et al., 2019; French et al., 2020). Another example is the Eocene Hetaoyuan Formation in the Biyang Sag, Nanxiang Basin, China, which is the largest trona deposit in China, but alkali minerals are limited to the Anpeng area in the Biyang Sag (Chen, 2010; Yang et al., 2014). This depositional heterogeneity in alkaline lakes has implications for oil and gas exploration and risk, but has not been systematically investigated.

The Junggar Basin is a large-scale, superimposed, hydrocarbon-bearing basin in northwestern China. The Mahu Sag is located at the northwestern margin of the basin and is hydrocarbon-rich. In particular, the lower Permian Fengcheng Formation contains a globally unique series of Paleozoic alkaline lacustrine source rocks (Figure 1) (Cao et al., 2015; Kuang et al., 2012; Wang, 2013). Previous studies have focused mainly on the Wuxia Fault Zone and Fengcheng area in the northern Mahu Sag. The Fengcheng Formation contains high quality source rocks deposited in a high pH and saline lacustrine environment (Cao et al., 2020; Chen et al., 2022; Yu et al., 2018). The water pH was >9.25, based on stable N isotope data, the presence alkali minerals such as wegscheiderite, abundant spherical bacterial fossils, and low clay mineral contents. The lake waters also had a high salinity and were strongly reducing, based on organic and inorganic geochemical data, and were affected by hydrothermal fluids (Cao et al., 2015, 2020; Wang et al., 2020). The alkaline lake in which the Fengcheng Formation was deposited underwent a complete sedimentary evolutionary cycle (Cao et al., 2020; Qin et al., 2016; Xia et al., 2020a, 2020b). Zhi et al. (2016) and Wang et al. (2018) showed that the high-quality source rocks of the Fengcheng Formation in the Wuxia–Fengcheng area have high organic matter abundances, which are of a good kerogen type (i.e. oil > gas) and have a high hydrocarbon generation potential. The source rocks are characterized by a high hydrocarbon production rate and continuous hydrocarbon generation, and have formed high quality light oil. Unconventional oil and gas resources (e.g. shale oil) occur in the Fengcheng Formation and the shale oil sweet spots and source rocks are closely associated in the strata (Kuang et al., 2012; Zhi et al., 2019, 2021). The Fengcheng Formation in the piedmont of the Hala'slat Mountains also developed similar high-quality alkaline lacustrine source rocks (Zhang et al., 2018).

Figure 1.

Structural location of the Mahu Sag in the Junggar Basin, northwestern China, which shows the well locations in the Manan area. The yellow line from A to A’ corresponds to the section shown in Figure 2. The base map of this figure refers to the Permian. Maidong = Eastern Mahu Sag, Mainan = Southern Mahu Sag, Maixi = Western Mahu Sag, Maibei = Northern Mahu Sag.

Recently, Fengcheng Formation source rock samples have been obtained by drilling in the southern Mahu Sag (i.e. the Manan area), which provide a rare opportunity to study the lateral heterogeneity of alkaline lacustrine source rocks. You et al. (2021) conducted a preliminary study of these rocks, and proposed that the second and third members of the Fengcheng Formation are the main source rocks in the Manan area. The formation and distribution of source rocks in alkaline lakes is significant for hydrocarbon exploration and exploitation. After discoveries in the Mabei and Maxi areas, it is important to assess whether the adjacent Madong and Manan areas are similar, and whether they contain alkaline lacustrine oil and gas systems.

In this study, we investigated the geochemistry and origins of the source rocks of the Fengcheng Formation in the Manan area, Mahu Sag. The organic matter sources and sedimentary depositional environments are compared with the typical alkaline lacustrine source rocks of the Fengcheng Formation in the Wuxia–Fengcheng area in Mabei area. Our results identified significant heterogeneity in alkaline lacustrine sediment deposition, which can guide regional oil and gas exploration and resource evaluation of source rocks in other alkaline and saline lacustrine basins worldwide.

Geological setting

The Junggar Basin is located in the northern Xinjiang Uygur Autonomous Region, China. It is surrounded by mountains, and is nearly triangular in shape and wide in the south and narrow in the north. The basin is divided into six primary structural units and thirty-four secondary structural units (Figure 1; Carroll, 1998; Yang et al., 2004). The Mahu Sag is located on the northwestern margin of the Junggar Basin (Figure 1; Yang et al., 2004; Wang, 2013; Zhi et al., 2021). During the Carboniferous, a marine foreland basin developed in the northwestern Junggar Basin, which was in a littoral–neritic setting in which marine–terrestrial mudstones, carbonaceous mudstones, and volcanic rocks were developed (Chen et al., 2016; Zhi et al., 2019). From the end-Carboniferous to Permian, the Mahu Sag became a continental setting and the Junggar Basin formed. A single, inland sedimentary basin had formed by the Triassic (Wang, 2013). The alkaline lacustrine source rocks of the lower Permian Fengcheng Formation (P₁f) occur mainly in the Mahu Sag (Figures 1 and 2; Cao et al., 2015). The Fengcheng Formation was deposited in a Permian, post-orogenic, fault depression developed on Paleozoic basement, which comprised a fan delta–lacustrine sedimentary system (Zhang et al., 2018). The Fengcheng Formation is divided into three members. The first member is mainly dark gray tuff and welded rhyolitic breccia; the second member is mainly dark gray, thin, argillaceous dolomite interbedded with dolomitic mudstone, with locally developed siltstone with alkaline minerals; the third member consists mainly of dark gray to dark brown mudstone and sandy mudstone with irregular bed thicknesses (Cao et al., 2020; Wang et al., 2020).

Figure 2.

Cross-section of the Wuxia–Fengcheng–Manan area in the Mahu Sag, Junggar Basin. K = Cretaceous, J = Jurassic, T₃b = Baijiantan Formation in the Upper Triassic, T₁b = Baikouquan Formation in the Lower Triassic, P₃w = Upper-Wuerhe Formation in the Upper Permian, P₂w = Lower-Wuerhe Formation in the Middle Permian, P₂x = Xiazijie Formation in the Middle Permian, P₁f = Fengcheng Formation in the Lower Permian, P₁j = Jiamuhe Formation in the Lower Permian, C = Carboniferous.

The Fengcheng Formation in the Manan area is located between the Southwestern Uplift and Central Depression of the Mahu Sag. This region comprises an uplift (Zhongguai Uplift), a fault zone (Kebai Fault Zone), and a sag (Mahu Sag) with a complex tectonic framework (Figures 1 and 2; Wang et al., 2021; You et al., 2021). The Manan area is located between the Kebai Fault Zone and Zhongguai Uplift, and the two sediment provenance regions were the Zhongguai and Baijiantan fans (Figure 1; Wang et al., 2021).

Samples and methods

Samples

A total of 130 source rock samples of the Fengcheng Formation in the Manan area were collected for analysis, including 6 samples from the Well JL34, 9 samples from the Well JL48, 5 samples from the Well JL49, 3 samples from the Well K204, 28 samples from the Well K207, 11 samples from the Well MH025, 32 samples from the Well MH28, and 36 samples from the Well MH39. The number of samples from different regions and stratigraphic levels varies due to availability of drill core material.

Methods

We undertook total organic carbon (TOC), pyrolysis, organic carbon isotope, and biomarker analysis in Research Institute of Experiment and Detection, Xinjiang Oilfield Company, CNPC, Karamay, Xinjiang. The samples were powdered to <100 mesh before TOC analysis. The powder was treated with dilute HCl at 60 °C and then centrifuged and dried at 50 °C to remove inorganic carbon. TOC analysis was conducted with a LECO-CS-200 C–S analyzer.

Prior to rock pyrolysis analysis, the samples were powdered to <100 mesh. The powdered samples were heated with a Rock–Eval VI pyrolysis instrument. Samples were first heated to 300 °C for 3 min to obtain the free hydrocarbon contents (S₁). The temperature was then increased to 600 °C to obtain the cracked hydrocarbon contents (S₂). The hydrogen index was calculated as HI = S₂/TOC × 100.

Prior to chloroform extraction of bitumen, the powdered samples were reacted with concentrated HCl and heated in HCl–HF to remove carbonates and silicates, respectively. The treated residue was centrifuged and washed, and soluble organic matter was extracted with chloroform to obtain bitumen. The δ¹³C values (i.e. organic carbon isotope values) were determined with a MAT-253 mass spectrometer, with an accuracy of <±0.1‰ relative to the Vienna-Pee Dee Belemnite standard.

The extracted saturated hydrocarbons were further analyzed for biomarker compounds by gas chromatography (GC) and GC mass spectrometry (GC-MS). GC analysis was undertaken with a HP-6890 GC instrument equipped with a HP-5 elastic quartz capillary column (thickness of 0.25 μm) using N₂ as the carrier gas. The initial temperature was 80 °C for 5 min, which was then increased to 290 °C at 4 °C/min and finally held at 290 °C for 30 min. GC-MS analysis was conducted with an Agilent 5973N chromatograph using He as the carrier gas. The initial temperature was 60 °C for 5 min, which was increased to 120 °C at 8 °C/min, 290 °C at 2 °C/min, and then held at 290 °C for 30 min.

Results and discussion

Basic geochemical characteristics of the source rocks

Organic matter abundance

The organic matter abundance is a first-order control on the ability of source rocks to generate hydrocarbons, and is particularly important in the evaluation of source rock quality and resource potential (Maier et al., 2011; Pei et al., 2016; Zhang et al., 2002). In this study, the TOC contents, S₁ contents, and hydrocarbon generation potential (PG = S₁ + S₂) were used to evaluate the organic matter abundance of the studied samples, and compare with source rocks in the Wuxia–Fengcheng area.

The TOC, S₁, and PG values indicate that most of the studied samples are non-source to good quality source rocks, but some are of very good to excellent quality source rocks. In comparison, most samples of the Fengcheng Formation in the Wuxia–Fengcheng area are source rocks of fair to very good quality (using the evaluation criteria of Peters and Cassa, 1994) (Figure 3).

Figure 3.

The organic matter abundance of source rocks in the Fengcheng Formation from the Manan area in the Mahu Sag. (a) S₁ versus TOC contents; (b) PG versus TOC contents. The evaluation criteria are from Peters and Cassa (1994). Data for the Fengcheng Formation from the Wuxia–Fengcheng area are from Xia et al. (2020a). NS = Non-source rocks, F = Fair source rocks, G = Good source rocks, VG = Very good source rocks, E = Excellent source rocks.

For the source rocks of the Fengcheng Formation in the Manan area, the TOC contents decrease in the order Wells JL48 and JL49 (good to excellent quality) > Wells MH28 and MH39 (non-source to very good quality) > Well JL34, K204, K207 and MH025 (non-source to good quality) (Figure 3). The S₁, and PG values both decrease in the order Wells L48 and JL49 ≈ Wells MH28 and MH39 > Wells JL34, K204, K207, and MH025 (Figure 3).

The TOC contents of the Fengcheng Formation in the Manan area are not significantly different, or perhaps slightly lower, than those in the Wuxia–Fengcheng area. However, the S₁ and PG values of the Fengcheng Formation in the Wuxia–Fengcheng area are relatively high compared with those of the studied samples (Figure 3). In general, the TOC, S₁, and PG values of Wells JL48 and JL49 are the highest of the studied samples, and represent good to excellent quality source rocks; Wells MH28 and MH39 have intermediate values, relatively high S₁ contents, and are mostly fair to very good quality source rocks; Wells JL34, K204, K207, and MH025 have the lowest TOC, S₁, and PG values, and are mostly non-source to fair quality source rocks.

Organic matter types

The type of organic matter determines whether source rocks generate oil or gas (Cheng et al., 2008; Kostyreva and Sotnich, 2017; Makeen et al., 2015). Geochemical parameters used to determine the type of organic matter in source rocks include organic C isotopes (δ¹³C_org) and a plot of HI–T_max. Based on δ¹³C_org values and a plot of HI–T_max (Figure 4), most of the organic matter in source rocks from the Wells JL48 and JL49 is type I–II kerogen, whereas Wells MH28, MH39, JL34, K204, K207 and MH025 contain type II–III kerogen (the evaluation criteria are from Espitalié et al., 1977).

Figure 4.

Organic matter types in the Fengcheng Formation source rocks from the Manan area, Mahu Sag. (a) δ¹³C_org versus TOC contents; (b) T_max versus HI. The evaluation criteria of the organic matter types are from Espitalié et al. (1977). Data for the Fengcheng Formation from the Wuxia–Fengcheng area are from Xia et al. (2020a).

Samples from Wells JL48 and JL49 have δ¹³C_org = −32.7‰ to −30.3‰, indicative of type I kerogen sourced mainly from aquatic bacteria and algae, which generates mainly oil. The HI–T_max plot shows that the main kerogen types in Wells JL48 and JL49 are I–II, and will generate mainly oil and possibly a small amount of gas (Figure 4).

The organic matter in samples from Wells JL34 and MH28 has δ¹³C_org = −26.1‰ to −24.3‰, indicative of type II₂–III kerogen sourced from an increasing proportion of higher plants, which can generate oil and gas (Espitalié et al., 1977). The HI–T_max plot shows that the organic matter in Wells JL34 and MH28 is type II kerogen, which has the potential to generate both oil and gas at the same time (Figure 4).

The organic matter in the source rocks from Wells K204, K207, MH025, and MH39 have δ¹³C_org = −27.4‰ to −20.9‰, indicative of type II₂–III kerogen derived from a complex/mixed source with a higher plant input, which can generate both oil and gas (Espitalié et al., 1977). The HI–T_max plot also shows that the organic matter in these wells is type II–III kerogen. Compared with the samples from the Wells JL34 and MH28, the gas generation ratio is higher for the samples from the Wells K204, K207, MH025, and MH39 (Figure 4).

In general, the organic matter types of samples from the Fengcheng Formation in Wells JL48 and JL49 in the Manan area are similar to those of the Wuxia–Fengcheng area, indicating that these source rocks generate mainly oil. The organic matter in the samples from Wells MH28, MH39, JL34, K204, K207 and MH025 in the Manan area contains more input from higher plants, which can generate oil and gas. However, the type of organic matter in Wells JL34 and MH28 is relatively better than the other wells in this group.

Organic matter maturity

Hydrocarbon generation also depends on the organic matter maturity. Source rocks can only generate hydrocarbons when a certain maturity range is reached (Harouna et al., 2017; Li and Jin, 2000; Wang et al., 2003). It is difficult to find vitrinite in samples of the Fengcheng Formation (Wang, 2013). Therefore, we used indirect geochemical proxies to assess the organic matter maturity in the source rocks, including T_max, C₂₉ sterane ααα20S/(20R + 20S), C₂₉ sterane αββ/(ααα + αββ), CPI, OEP, C₃₁ hopane 22S/(22S + 22R), and Ts/Tm values (Peters et al., 2005).

T_max values indicate that the Fengcheng Formation source rocks in the Manan area are immature to high-mature, and have slightly higher maturity than those in the Wuxia–Fengcheng area, which are immature–mature (Figure 5(a)). T_max values indicate that most samples in Wells JL48 and JL49 are mature to high-mature, samples in Wells JL34 and K204, and K207 are immature, most samples in Well MH025 are low-mature to mature, and samples in Wells MH28 and MH39 are immature to mature and immature to high-mature, respectively.

Figure 5.

Maturity parameters for the Fengcheng Formation source rocks in the Manan area of the Mahu Sag. (a) T_max versus C₂₉ sterane ααα20S/(S + R); (b) C₂₉ sterane ααα20S/(S + R) versus C₂₉ sterane αββ/(αββ + ααα); (c) OEP versus CPI; (d) C₃₁ hopane 22S/(S + R) versus T_s/T_m. The evaluation criteria for the thermal maturity were taken from Peters et al. (2005). CPI = [(C₂₅ + C₂₇ + C₂₉ + C₃₁ + C₃₃)/(C₂₄ + C₂₆ + C₂₈ + C₃₀ + C₃₂) + (C₂₅ + C₂₇ + C₂₉ + C₃₁ + C₃₃)/(C₂₆ + C₂₈ + C₃₀ + C₃₂ + C₃₄)]/2 (Bray and Evans, 1961); OEP = [(C_i + 6C_i+2 + C_i+4)/(4C_i+1 + 4C_i+3)]^(–1)i+1, where i + 2 is the main n-alkane peak (Scalan and Smith, 1970). Data for the Fengcheng Formation from the Wuxia–Fengcheng area are from Xia et al. (2020a).

The biomarker maturity indicators each reveal similar results (Figure 5). Peters et al. (2005) showed that C₂₉ sterane ααα20S/(20R + 20S) and C₂₉ sterane αββ/(ααα + αββ) ratios exhibit a significant positive correlation. These two ratios show that most of the studied samples are mature (>0.4), although a few samples are high-mature (close to the equilibrium values of 0.55 and 0.70; Figure 5(b)). The C₂₉ sterane maturity index value for source rocks from the Wuxia–Fengcheng area indicate that most are mature (>0.4), a few are low-mature (0.2–0.4), and only a few samples have C₂₉ sterane ααα20S/(20R + 20S) close to the equilibrium value of 0.55. As such, only a few samples of the Fengcheng Formation in the Wuxia–Fengcheng area are of high maturity, and the overall maturity is slightly lower than in the Manan area.

The CPI and OEP parameters also exhibit an obvious positive correlation, and both decrease to ∼1 with increasing maturity (Figure 5(c)). CPI and OEP values of most studied samples are 0.8–1.5, and only a few samples have values >1.5, indicating that the samples are generally mature. The CPI and OEP values of the Fengcheng Formation source rocks in the Manan area are closer to 1 than those in the Wuxia–Fengcheng area, indicating that their maturity is relatively high. This is consistent with the inferences made from the C₂₉ sterane-related maturity parameters (Figure 5(b)).

With increasing maturity, C₃₁ hopane 22S/(22S + 22R) and Ts/Tm ratios increase gradually, and Tm is all converted to Ts in the late oil-generating window to reach the equilibrium value (Farrimond et al., 2004). Ts/Tm ratios of the studied samples are not very high (Figure 5(d)), indicating that the Fengcheng Formation source rocks from the Manan area have not reached the over-mature stage. Moreover, the proportion of mud in the source rocks from the Manan area is relatively high, which precludes the possibility that the clay mineral contents were too low to inhibit the conversion of Tm to Ts (McKirdy et al., 1984). Compared with the source rocks from the Wuxia–Fengcheng area, the Ts/Tm ratios of samples from Wells JL48 and JL49 and some samples from Well MH39 in the Manan area are significantly higher, which may reflect the higher maturity of samples from the Manan area. However, the samples from the Fengcheng Formation in the Manan area have a higher shale ratio, which also enhances conversion of Tm to Ts.

In summary, the T_max values indicate that the Fengcheng Formation source rocks in the Manan area are immature to high-mature and, in the Wuxia–Fengcheng area, are immature to mature. The biomarker maturity indexes indicate that the Fengcheng Formation samples in the Manan area (mostly mature, with a few samples being high-mature) have slightly higher maturity than in the Wuxia–Fengcheng area (mainly mature, with a few samples being high-mature). The maturity acquired from different parameters has certain variation; for example, the values based on the T_max values are significantly larger than those based on the biomarkers. This represents a common feature for alkaline lacustrine source rocks. The influencing factors are complex, e.g., the high chloroform-extracted bitumens retained in source rocks resulting in abnormally low T_max values (Zhang et al., 2006).

Hydrocarbon generation potential

The hydrocarbon generation potential of source rocks depends on the quantity and quality of organic matter in the source rocks. The quantity is mainly controlled by the abundance of organic matter and is partly affected by the thermal maturity. The hydrocarbon generation potential depends on the type and source of organic matter (Erik et al., 2005; Hakimi and Abdullah, 2013; Tissot and Welte, 1984). We used the PG/TOC and S₁/TOC ratios to assess the hydrocarbon generation potential of the source rocks.

The hydrocarbon generation potential of the source rocks from the Manan area are significantly lower than that of source rocks in the Wuxia–Fengcheng area (Figure 6). The source rocks from the Wuxia–Fengcheng area have a high hydrocarbon generation potential. The samples from Wells JL34, JL48, JL49, and MH28 in the Manan area have a relatively high hydrocarbon generation potential, whereas those from Wells K204, K207, MH025, and MH39 are relatively low (Figure 6).

Figure 6.

Organic matter types in the Fengcheng Formation source rocks from the Manan area of the Mahu Sag. (a) PG/TOC versus S₂/TOC; (b) PG/TOC versus S₁/TOC. The data for the Fengcheng Formation from the Wuxia–Fengcheng area are from Xia et al. (2020a).

Shale oil is a liquid petroleum resource retained in a shale system after it has undergone hydrocarbon generation and expulsion (Clarkson and Pedersen, 2010; Lu et al., 2012; Sonnenberg and Pramudito, 2009). Shale oil has become an important oil and gas resource in China (Jin et al., 2019; Zhao et al., 2020; Zou et al., 2014). The Mahu Sag contains abundant shale oil resources in the Fengcheng Formation (Kuang et al., 2012; Zhi et al., 2021). We now evaluate whether the Fengcheng Formation source rocks in the Manan area have shale oil potential by Oil shale index (OSI) values (S₁/TOC). When OSI > 100, the source rocks have shale oil potential. When OSI < 100, the source rocks have no shale oil potential (Jarvie, 2012). Like some Fengcheng Formation source rocks in the Wuxia–Fengcheng area with OSI values of >100, some samples from the Manan area also have shale oil potential (Figure 6(b)). Most samples from Well MH28 have OSI > 100, which indicates a high shale oil potential. Some samples from Well K207 also have OSI > 100, but the TOC contents are 1 wt.%, and the shale oil potential is not high. OSI values of samples from the Wells JL48, JL49, K204, MH025, and MH39 are <100, and these have no shale oil potential. There are too few samples from Well JL34 to evaluate its shale oil potential.

In general, the source rocks in Wells JL34, JL48, JL49, and MH28 have a high hydrocarbon generation potential, but this is slightly lower than for those in the Wuxia–Fengcheng area (Cao et al., 2015; Wang et al., 2018; Zhi et al., 2016). Source rocks in Wells K204, K207, MH025, and MH39 have a relatively low hydrocarbon generation potential. In addition, Well MH28 in the Manan area also has considerable potential for shale oil development.

The organic matter abundance and type of the studied samples are shown in Figure 7. In the Manan area, source rocks in Wells JL48 and JL49 located in the southern part of the area have the highest organic matter abundance, good organic matter type, and a high hydrocarbon generation potential, and are ideal for conventional oil exploration and development. Samples from Well JL34 are too few to assess its prospectivity. Samples from Wells K204, K207, and MH025 located in the north of the area have the lowest organic matter abundance, a poor organic matter type, and low hydrocarbon-generation potential. Samples from Well MH28 in the middle of the area have a higher organic matter abundance, intermediate organic matter type, good hydrocarbon generation potential, and excellent shale oil potential. Samples from Well MH39 have a high organic matter abundance, poor organic matter type, low hydrocarbon generation potential, and some gas generation potential.

Figure 7.

Vertical changes in the geochemical characteristics of source rocks in the Fengcheng Formation from the Manan area of the Mahu Sag. PG is in units of mg/g rock; HI, PG/TOC, and S₁/TOC are in units of mg HC/g TOC.

Source rock biomarker characteristics

Biomarker characteristics related to the organic matter sources

Stable C isotope ratios of organic matter can reflect its source and are less affected by the thermal evolution than biomarkers. δ¹³C_org values of the Fengcheng Formation source rocks increase in the order Wells JL48 and JL49 < Wuxia–Fengcheng area < Wells JL34 and MH28 < Wells K204, K207, MH025, and MH39 (Figure 4(a)). This shows that the hydrocarbon-generating material in Wells L48 and JL49and in the Wuxia–Fengcheng area are mainly aquatic bacteria and algae, and also includes higher plants in Wells K204, K207, MH025, and MH39. The proportion of higher plants is greater in Wells JL34 and MH28.

The distribution of n-alkanes can also be used to assess the source of organic matter, but it is affected by thermal evolution and needs to be evaluated in combination with other indicators (Peters et al., 2005). In general, n-alkanes represent the properties of lipid compounds in the original organic matter. For marine, terrestrial, and mixed sources of organic matter, the main carbon peaks of the n-alkanes are generally <C₂₁, >C₂₇, and C_21–27, respectively (Hong, 1983; Hu, 2021; Zhou et al., 1999). The main peak of n-alkanes in the Fengcheng Formation in the Manan area is C_17–25, reflecting a mixed source of aquatic bacteria, algae, and terrestrial higher plants. The main peaks of n-alkanes in the Fengcheng Formation in the Wuxia–Fengcheng area are C₁₇, C₂₀, and C₂₃, reflecting aquatic algae and bacteria sources (Hong, 1983; Hu, 2021; Zhou et al., 1999). The contribution of terrestrial higher plants to the Fengcheng Formation in the Manan area was significantly higher than in the Wuxia–Fengcheng area (Figure 8).

Figure 8.

Typical chromatograms of bitumen extracts from source rocks in the Fengcheng Formation from the Manan area of the Mahu Sag. The chromatogram for the Fengcheng Formation (Well F20; 3,248 m) from the Wuxia–Fengcheng area is from Xia et al. (2020a).

A plot of Pr/n-C₁₇ versus Ph/n-C₁₈ can reflect the redox state of the depositional environment of source rocks and also the organic matter source (Shanmugam, 1985). Compared with samples from the Fengcheng Formation that plot in the marine/saline lake organic matter field, most Fengcheng Formation samples from the Manan area plot in the mixed marine/saline lake organic matter field (Figure 9(a)), which is consistent with the n-alkane distribution. This indicates that the organic matter in the Fengcheng Formation source rocks from the Manan area was derived from higher plants, bacteria, and algae (Figure 9(a)), and that the terrigenous input was high. More samples from Wells JL34, K207, and MH28 plot in the marine/saline lake organic matter field, and a small number of samples from Well MH39 also plot in the marine/saline lake organic matter field.

Figure 9.

Biomarker characteristics related to the organic matter sources in the source rocks of the Fengcheng Formation in the Manan area of the Mahu Sag. (a) Ph/n-C₁₈ versus Pr/n-C₁₇; (b) Ph/n-C₁₈ versus sterane/hopane; (c) C₂₈/C_27–29 sterane versus C₂₈/C₂₉ sterane; (d) (C₁₉ + C₂₀)/C₂₃ tricyclic terpenes versus C₂₁/C₂₃ tricyclic terpenes. Data for the Fengcheng Formation from the Wuxia–Fengcheng area are from Xia et al. (2020a).

Steranes are mainly derived from algae and other eukaryotes, whereas hopanes are mainly derived from a wide range of aerobic bacteria, including cyanobacteria (Rohmer et al., 1984; Volkman, 2003). The sterane/hopane ratio can reflect the contribution of eukaryotes (mainly algae) relative to prokaryotes (mainly bacteria) in source rocks, and used to approximate the relative contributions of algae and cyanobacteria (Bobrovskiy et al., 2020; Peters et al., 2005; Rohrssen et al., 2013). In Phanerozoic sediments that were deposited when algae were the main primary producers, about 70% of samples have sterane/hopane ratios of 0.2–2.0 (Bobrovskiy et al., 2020; Brocks et al., 2017). Most samples of the Fengcheng Formation from the Wuxia–Fengcheng area have sterane/hopane ratios that are higher than 0.2–2.0. Samples from the Fengcheng Formation in the Manan area have significantly lower sterane/hopane ratios (Figure 9(b)). This shows that the algae/bacteria ratio in the Fengcheng Formation source rocks in the Manan area was significantly lower than in the Wuxia–Fengcheng area where algae was dominant (Xia et al., 2020a). Apart from some samples from Wells K207, MH28, and MH39 with sterane/hopane ratios of >2 (Figure 9(b)), most have values within or lower than the range for Phanerozoic sediments (0.2–2.0), indicating that the ratio of algae/bacteria in most samples was similar or lower than in the Phanerozoic.

The relative contents of the regular steranes C₂₇–₂₉ can be used to assess the organic matter source (Volkman, 2003). It is generally considered that C₂₇ sterane represents input of lower algae, and C₂₉ sterane represents input of higher plants or green algae (Kodner et al., 2008; Peters et al., 2005; Volkman, 2003). The relative content of C₂₈ steranes is controlled by the sediment age. Prior to the Triassic, C₂₈ steranes were mainly derived from a specific species of green algae and, after the Triassic, C₂₈ steranes were derived from chlorophyl a + c algae in dinoflagellates, diatoms, and coccolithophores (Falkowski et al., 2004; Kodner et al., 2008; Schwark and Empt, 2006). Therefore, the C₂₈ steranes in the samples of the Fengcheng Formation in the Mahu Sag were all derived from green algae rather than chlorophyl a + c algae. Samples from the Fengcheng Formation in the Wuxia–Fengcheng and Manan areas all have C₂₉ > C₂₈ > C₂₇ regular steranes, and the C₂₇ sterane/regular sterane ratio is low. This shows that the studied samples contained green algae or mixed green algae and higher plants. The C₂₈/C₂₉ regular steranes and C₂₈/total steranes ratios of the studied samples are significantly lower than those of source rocks from the Wuxia–Fengcheng area (Figure 9(c)), indicating the two areas had different sources of organic matter.

A plot of C₂₁/C₂₃ tricyclic terpenes versus (C₁₉ + C₂₀)/C₂₃ tricyclic terpenes can reflect the distribution and organic matter source of the tricyclic terpenes (Peters et al., 2005; Tao et al., 2019). The distribution of tricyclic terpenes in the studied samples in the Manan area is complex (Figure 9(d)), with rising, descending, and mountain peak types at the same time, and a small number of samples are valley type, which is obviously different form the Fengcheng Formation in the Wuxia–Fengcheng area, with the main distribution of tricyclic terpenes is rising type (Cao et al., 2015). This reflects differences in the organic matter sources and sedimentary depositional environments of the Fengcheng Formation in the Manan and Wuxia–Fengcheng areas.

Based on the above discussion, the organic matter in the Fengcheng Formation in the Manan area was a mixture of terrestrial higher plants, and aquatic algae and bacteria, and the input of higher plants was higher than in the Wuxia–Fengcheng area. The contribution of algae was greater than that of bacteria in most samples from the Manan area, but in a small number of samples bacteria were dominant, and the algae/bacteria ratio was significantly lower than in the Wuxia–Fengcheng area where algae was dominant. The samples of the Fengcheng Formation from the Manan area all contained green algae mixed with higher plant material, but the specific organic matter sources were significantly different from those in the Wuxia–Fengcheng area.

Biomarker characteristics related to the sedimentary depositional environment

Numerous biomarker indicators can be used to infer the sedimentary depositional environment. In the following, we discuss mainly the pristane (Pr)/phytane (Ph) ratio, β-carotene index [(β-+λ-carotene)/n-C_{main peak}], gammacerane index (gammacerane/C₃₀αβhopane), C₃₅S/C₃₄S hopane–C₂₉αβ/C₃₀αβ hopane and C₂₂/C₂₁–C_24/C₂₃ tricyclic terpene plots.

In general, Pr/Ph < 1 and Pr/Ph > 1 are indicative of hypoxic and oxic environments, respectively (Didyk et al., 1978; Peters et al., 2005). Pr/Ph ratios of samples from the Fengcheng Formation in the Manan area vary widely, unlike the Fengcheng Formation in the Wuxia–Fengcheng area where Pr/Ph < 1 indicates deposition in a strongly reducing environment (Figure 10(a); Cao et al., 2020). For the Manan area, the Pr/Ph ratios decrease in the order Well K207 > Wells JL34, K204, MH025, MH28, and MH39 > Wells JL48 and JL49. This and the Pr/nC₁₇–Ph/nC₁₈ plot (Shanmugam, 1985) were used to classify the depositional environment of the organic matter (Figure 9(a)). Source rocks in Well K207 formed in a marine/saline lacustrine depositional environment; source rocks in Wells JL34, MH025, MH28, and MH39 formed mainly in a mixed marine/saline lake facies sedimentary environment; source rocks in Wells K204, JL48 and JL49 formed mainly in mixed facies, with a sedimentary environment that gradually transitioned to oxic conditions.

Figure 10.

Biomarker characteristics related to the sedimentary depositional environment of the source rocks in the Fengcheng Formation from the Manan area of the Mahu Sag. (a) Pr/Ph versus (β- + λ-carotene)/n-C_{main peak}; (b) Pr/Ph versus gammacerane/C₃₀ αβ hopane; (c) C₃₅S/C₃₄S hopane versus C₂₉αβ/C₃₀αβ hopane; (d) C₂₂/C₂₁ tricyclic terpenes versus C₂₄/C₂₃ tricyclic terpenes. Data for the Fengcheng Formation from the Wuxia–Fengcheng area are from Xia et al. (2020a).

A high β-carotene index reflects a reducing and high salinity depositional environment, because salt-tolerant photosynthetic organisms (e.g. Dunaliella) can accumulate >14% of β-carotene relative to their dry cell weight (Ding et al., 2020; Francavilla et al., 2010; Tao et al., 2019). Unlike the Fengcheng Formation in the Wuxia–Fengcheng area with β-carotene index values of >1 (Figure 10(a); Xia et al., 2020a), the β-carotene index values of the Fengcheng Formation in the Manan area vary widely and can be <1 and >1, which indicates the water salinity was significantly lower than in the Wuxia–Fengcheng area (Figures 9 and 10(a)). This may also reflect the less reducing depositional conditions of the Fengcheng Formation in the Manan area, because β-carotene is readily degraded in an oxic sedimentary environment (Peters et al., 2005). The β-carotene index values decrease in the order of Wells K207 and JL34 > Wells MH025, MH28, and MH39 > Wells K204, JL48, and JL49.

Gammacerane is considered to be the diagenetic product of tetrahymenin, which is produced by protozoan ciliates living at the redox interface of stratified water. As such, the gammacerane index is used to infer the degree of water stratification (Sinninghe Damsté et al., 1995). Unlike most samples of the Fengcheng Formation in the Wuxia–Fengcheng area that have gammacerane index values of >0.2, the samples from the Fengcheng Formation in the Manan area have values that vary widely, which indicate that the water stratification was less than in the Wuxia–Fengcheng area (Figure 10(b)). The gammacerane index values of the Fengcheng Formation in the Manan area are consistent with the β-carotene results.

Plots of C₃₅S/C₃₄S hopane–C₂₉αβ/C₃₀αβ hopane and C₂₂/C₂₁–C₂₄/C₂₃ tricyclic terpenes can distinguish carbonate- and argillaceous-derived crude oils (Peters et al., 2005; Tao et al., 2019). Most samples of the Fengcheng Formation from the Manan area plot in the marine/lacustrine mudstone area, and only one sample plots in the carbonate area, which is much different from the Fengcheng Formation from the Wuxia–Fengcheng area with some samples plotting in the marine/lacustrine mudstone area and some samples in carbonate area (Figure 10(c)). This indicates that the Fengcheng Formation in the Manan area was deposited in a weakly evaporatic environment, which did not reach the salinity conditions necessary for carbonate formation. This is consistent with the freshwater–saltwater environment inferred from the β-carotene and gammacerane indexes. The Fengcheng Formation in the Wuxia–Fengcheng area has high C₂₂/C₂₁ tricyclic terpenes and low C₂₄/C₂₃ tricyclic terpenes ratios (Figure 10(d)), indicative of a higher carbonate/mudstone ratio, whereas the Fengcheng Formation in the Manan area exhibits the opposite characteristics, indicative of a higher proportion of mudstones. This is consistent with the C₃₅S/C₃₄S hopane–C₂₉αβ/C₃₀αβ hopane plot.

To test the main environmental contributor for the organic matter abundance in the Manan area, the correlation analysis was performed between TOC and biomarker proxies of the redox condition, salinity and lithology above. The results show that there is no significant correlation between TOC and any of these biomarker proxies, implying that the organic matter abundance of hydrocarbon source rocks here is controlled by a combination of various depositional environmental factors rather than any single factor alone.

Heterogeneity of alkaline lacustrine source rocks

Based on the above results and discussion, the depositional environment of samples from the Fengcheng Formation in the Manan and Wuxia–Fengcheng areas were different. The latter was deposited under strongly reducing conditions, with a high water salinity and stratification, which resulted in a high carbonate/mudstone ratio (Cao et al., 2015, 2020). The latter were generally deposited under oxic to anoxic conditions, with highly variable water salinity and stratification, which resulted in a predominance of mudstone over carbonate rocks. The sedimentary depositional environment of Wells K207 and JL34 was the most reduced and had the highest salinity and strong water stratification; the sedimentary depositional environment of Wells K204, JL48, and JL49 was the most oxidizing and had a low salinity and no water stratification; the sedimentary depositional environment of Wells MH025, MH28, and MH39 was intermediate between these two types. This indicates that the Mahu Sag may have been bounded by the Dazhuluogou Fault, and that the Fengcheng Formation in the northwestern Wuxia–Fengcheng areacomprises alkaline lacustrine deposits. However, in the Manan area, there are no alkaline lacustrine deposits in the Fengcheng Formation. As such, the distribution of the alkaline lacustrine source rocks may have been controlled by the lithofacies and paleogeography.

Accordingly, the alkaline lakes of the Fengcheng Formation in the Mahu Sag of the Junggar Basin developed in the area north of the Dajuluogou Fault. Vertically, there are alkaline lacustrine deposits in the first to the third Members of the Fengcheng Foramation, and the strong alkali-forming stage is reached in the second Member (Cao et al., 2020; Yu et al., 2018). Taking Well FN7 in the Wuxia-Fengcheng area as an example, the base to the lower part of the first Member of the Fengcheng Formation is dominated by mudstones, and the upper part consists of carbonate-bearing rocks, with a small amount of transitional Na carbonate minerals. The second Member is characterized by the appearance of evaporites and a large amount of Na carbonate minerals, such as trona, reflecting the peak of the alkaline lake conditions. The third Member is characterized by a gradual decrease in the abundance of dolomitic rocks and alkali minerals, reflecting the gradual end of the alkaline lake conditions (Cao et al., 2020; Yu et al., 2018).

The lateral heterogeneity of the Fengcheng Formation is also evident from the Eocene Hetaoyuan Formation in the Biyang Sag, Nanxiang Basin, China. For example, its alkali deposits only occur near the Anpeng area in the southeastern part of the sag, and the Hetaoyuan Formation in many other areas in the sag contain with no alkali mineral developed (Chen, 2010; Yang et al., 2014).The stratigraphic heterogeneity of the Fengcheng Formation is also evident in the Eocene Green River Formation in the USA, where the high-salinity, alkaline lake deposited the Parachute Creek section in the upper part of the Green River Formation, and the lower part of the Green River Formation was deposited in a freshwater to brackish lacustrine–deltaic setting (Birgenheier et al., 2019; French et al., 2020). This temporal and spatial heterogeneity is a common feature of alkaline lacustrine source rocks.

Conclusions

We undertook an organic geochemical study of Fengcheng Formation source rocks from the Manan area, Mahu Sag, Junggar Basin, and compared this with the typical alkaline lacustrine source rocks of the Fengcheng Formation in the Wuxia–Fengcheng area. Our main conclusions are as follows:

Unlike the Fengcheng Formation in the Wuxia–Fengcheng area, which has high organic matter abundance, good organic matter type, and excellent hydrocarbon generation and shale oil potential, the characteristics of the Fengcheng Formation source rocks in the Manan area vary widely. The order of the organic matter abundance (from high to low), organic matter type (from oil-prone to gas-prone), and hydrocarbon generation potential (from high to low) follow the order: Wells JL48 and JL49 > Wells MH28 and MH39 > Wells JL34, K204, K207, and MH025. Well MH28 has excellent shale oil potential. The source rocks of the Fengcheng Formation in the Manan area are mainly mature, with a few samples being high-mature.

Biomarkers indicate that the organic matter in the Fengcheng Formation from the Manan area is a mixture of terrestrial higher plants, and aquatic algae and bacteria, which differs from the Fengcheng Formation in the Wuxia–Fengcheng area that is dominated by algae and bacteria. The contribution of algae is greater than that of bacteria in most samples of the Fengcheng Formation in the Manan area.

The Fengcheng Formation in the Manan area was deposited in a setting with large variations in redox conditions (oxic to anoxic) and water salinity and stratification, which resulted in a high mudstone/carbonate ratio. The Fengcheng Formation in the Wuxia–Fengcheng area was deposited under reducing conditions with a high water salinity and stratification, which resulted in a high carbonate/mudstone ratio. The distribution of alkaline lacustrine deposits in the Fengcheng Formation is laterally heterogeneous. These deposits developed in the area north of the Dazhuluogou Fault. Alkaline lacustrine deposits occur in the first to third Members of the Fengcheng Formation north of the fault, and the second Member records the strongest alkaline conditions.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Wanyun Ma

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