The expansion of petroleum exploration and exploitation from conventional to unconventional oil and gas is an important trend in the development of the petroleum industry. The conventional and unconventional petroleum resources obviously differ in oil and gas types, geological characteristics and accumulation mechanisms^[1,2]. More and more exploration and researches have revealed that conventional and unconventional oil and gas often orderly coexist within the same petroleum system^[2,3], forming a total petroleum system^[4,5]. Hence, generally, the discovery of a conventional reservoir indicates the coexistence of unconventional petroleum resources in the direction of hydrocarbon supply, while the discovery of unconventional oil and gas implies that there may be associated conventional resources in the outer space. Therefore, researches on the orderly coexistence of conventional and unconventional hydrocarbons and the accumulation mechanisms of the total petroleum system have great theoretical values and exploration significance, and are hot spots of petroleum geology at present.

The Junggar Basin is a large superimposed petroliferous basin in western China. According to the analysis of the geological background, the high-quality alkaline lacustrine source rocks of medium-high maturity in the Lower Permian Fengcheng Formation of the Mahu sag are most likely to form in-source and off-source hydro-carbon accumulations and total petroleum system^[6,7,8]. Based on this reasoning, PetroChina Xinjiang Oilfield Company successsively deployed and drilled wells FC1, BQ1, AK1, and so on, around the Fengcheng Formation in the Mahu sag. The Fengcheng Formation has not only argillaceous source rocks but also multiple types of reservoir rocks, such as sandy conglomerate, volcanic rock and fine-grained diamictite, forming shale oil^[9], tight oil, and conventional oil reservoirs with united source-reservoir type^[10]. All of these show that the Fengcheng Formation has the characteristics of a total petroleum system with coexistence of conventional and unconventional hydrocarbons. In addition, around the Fengcheng Formation source rocks in the Mahu sag, several sets of oil-bearing series from the Triassic to Cretaceous have been found outside the source, especially the Lower Triassic Baikouquan Formation in the sag, the Middle Triassic Karamay Formation in the fault zone and the Jurassic-Cretaceous, forming two giant oil (gas) provinces of over one hundred kilometers, which are Kebai-Wuxia and Mahu West Slope^{[6, 8]}. The northwest margin of the Junggar Basin is a typical example of the total petroleum system based on high-quality alkaline source rocks.

The current understanding on the total petroleum system is not deep enough. In particular, the study on oil and gas accumulation within the source strata of the Fengcheng Formation is weaker than that outside. Under the background of the global oil and gas exploration pushing into source gradually, this problem needs to be fixed urgently to promote the exploration process. The types, forming conditions, spatial distribution and accumulation models of hydrocarbons in the Fengcheng Formation directly affect the selection of exploration targets and target areas. Given this, based on the new drilling information of the Fengcheng Formation in the Mahu sag since 2007, especially the systematic coring data of well MY1, as well as high precision merged three-dimension seismic data, the petroleum types, forming conditions, controlling factors and accumulation models were examined systematically in this work, to reach an overall understanding on the conventional and unconventional petroleum orderly coexistence and total petroleum system in the Fengcheng alkaline deposits of the Mahu sag, which will guide the integrated exploration of conventional and unconventional oil and gas in the Junggar Basin, and enrich and develop the petroleum geological theory of hydrocarbon-rich sags in continental basins of China.

The Mahu sag is located in the west of the central depression of the Junggar Basin. In the sag, the Wuxia fault zone, Kebai fault zone, and Zhongguai uplift (from north to south) are in the west, and the Shiyingtan uplift, Yingxi sag, Sangequan uplift, Xiayan uplift and Dabasong uplift (from north to south) are in the east (Fig. 1). Influenced by the strong collision and compression between the West Junggar Ocean and Kazakhstan Plate, especially the collision during the Middle-Late Carboniferous to Early Permian, a large nappe structure came up in the northwest of the basin, while a foreland depression was formed in the area from the Mahu to Penyijingxi sag^[11]. This period is the most important formation period of the Mahu sag. The development of foreland basins is often accompanied by deposition of high-quality source rocks^[12]. Particularly, the sedimentary period of the Lower Permian Fengcheng Formation was the drastic development period of the western foreland basin, and the most important set of oil source rock in the Junggar Basin was formed^[13,14]. To date, Ke-Wu and Mahu West Slope, two giant oil (gas) provinces of over one hundred kilometers related to the Fengcheng Formation source rocks have been discovered, with cumulative proved reserves of 17.9×10⁸ t, raising this area to one of the world's famous large oil regions^[15,16].

The Lower Permian Fengcheng Formation in the Mahu sag is mainly composed of multi-source mixed fine- grained deposit under the background of semi-deep to deep alkaline lake^[9], including endogenous chemical deposit caused by the hot and arid climate, volcanic materials provided by peripheral volcanic activities during the development of foreland basin (distributed in relatively limited marginal regions of the sag), and proximal rapid build-up fan delta terrigenous clast from denudation of the western edge of the nappe (with sediment particles fining from gravel to silt grade towards the lake center, and distribution limited in the west margin of the foreland basin). Vertically, under the influence of the lake basin water body changes and different source systems, the Fengcheng Formation can be divided into the first member (P₁f₁), the second member (P₁f₂), and the third member (P₁f₃) from bottom to top, with obvious differences in sediments^[17] (Fig. 2).

Recent exploration results show that the mixed deposition of terrigenous clasts, volcanic materials, and endogenous carbonates in the Fengcheng Formation has formed various rock types such as conglomerate, sandstone, dolomite, mudstone, tuff, and salt rock and multiple transitional lithologies, with frequent interbeds of various rock types vertically^[18]. Specifically, P₁f₁ is composed of pyroclastic sediments and volcanic rocks in the lower part, and transgressive organic mudstone and dolomite in the upper part. Depositing in a strong evaporative environment with high salinity and limited exogenous source material, P₁f₂ has relatively coarse clastic particles in limited distribution, but mainly organic-rich dolomite and mudstone, and typical alkaline minerals in the center of the sag. During the depositional period of P₁f₃, with the increase of exogenous source material, the salinity decreased and the sediments were similar to those at the top of P₁f₁^[17]. On the plane, controlled by the supply of external detrital sources, the lithology changes from gravel at the edge to fine-grained sediments such as dolomitic silt-fine sandstone, dolomitic mudstone, mudstone and saline rock in the center of the basin gradually. Wells drilled have detected oil and gas shows in formations of all the lithologies, reflecting a good prospect for exploration (Fig. 2). Well BQ1, located in the center of the foreland depression encountered hugely thick conglomerate of fan delta facies. Wells MH28 and MH33 toward the sag encountered thick dolomitic silt-fine sandstone with thin shale interbeds, which had hydrocarbon shows detected in the whole well section and high yield industrial oil flows tested. Wells MY1, X72 and X87 in the north of the depression revealed high-quality volcanic rock reservoirs in P₁f₁, and had oil and gas shows detected in the dolomitic fine-grained rocks of entire P₁f₂ and P₁f₃. Wells FC1 and AK1 in the middle of the region encountered frequently interbedded dolomite and mudstone and had active oil and gas shows detected in the Fengcheng Formation. Well FC1 had high yield industrial oil flow after small-scale fracturing. All these indicate that rock layers of different lithologies of the Fengcheng Formation have formed reservoirs, but the hydrocarbon types are complex, so it is urgent to carry out systematic research.

As mentioned above, the total petroleum system of the Fengcheng Formation consists of two subsystems, i.e., the internal and external source rock ones, and this paper focuses on the former one. The Fengcheng total petroleum system has characteristics consistent with the classic total petroleum system^[4,5], including “orderly coexistence” of conventional and unconventional hydrocarbons^[2,3]. In other words, the hydrocarbons can transport to conventional conglomerate and volcanic reservoirs (with traps) out of source rocks, and also can be generated and stored inside source rocks (with no trap), showing "source-reservoir coupling and orderly accumulation"^[5]. The basic formation conditions described below focus on source and reservoir.

Source rocks in the Fengcheng Formation now are mainly oil-prone with high abundance, good types of organic matter, and medium-high maturity^[19,20]. Rapid trace element test on core samples taken from well MY1 show the samples have a highest dolomite content of (magnesium calcium carbonate) 30%, and lime (calcium carbonate) content of less than 20% generally (Fig. 3), suggesting that the Fengcheng source rocks are a set of carbonate-bearing shale^[21]. The Fengcheng source rocks have organic matter abundance of fair to good quality source rocks^[19,20].

The geochemical comprehensive evaluation of the Fengcheng Formation shows that the organic matter abundance generally increases from bottom to top (Fig. 3b). Besides argillaceous rocks, dolomitic rocks, and tuffs that have high organic matter abundance, more than 96% of the siltstone samples have residual organic carbon content greater than 0.6% and hydrocarbon generation potential greater than 2 mg/g, that is to say the siltstone also has considerable hydrocarbon generation ability (Table 1), which is conducive to oil and gas accumulation inside or near source. In addition, the samples have generally low maturity (with pyrolysis peak temperature (T_max) of less than 430 °C generally). However, the maturity of the measured samples cannot fully represent the thermal evolution degree of source rock in the main part of Mahu sag, and basin modeling predicted the source rocks reached mature to high mature stage on the whole (Fig. 4).

In terms of hydrocarbon index (S₁/TOC), most samples of the Fengcheng Formation from the depth range of 3200-6000 m have HI values higher than 100 mg/g^[22], indicating that the Fengcheng Formation has retained hydrocarbons in this depth range (Fig. 3b). The peak value of S₁ and content of chloroform bitumen “A”, which are widely used in China and abroad, were used to estimate the amount of mobile hydrocarbons in the Fengcheng Formation in this study^[23,24]. The S₁ values of the Fengcheng Formation obtained from the Rock-Eval pyrolysis range from 0.01 mg/g to 9.35 mg/g, with an average value of 1.27 mg/g. 25% of the tested samples have S₁ values higher than 2 mg/g, and some of them have S₁ values of more than 4 mg/g, proving the source rock contains retained hydrocarbons. In particular, the cores from the whole well section of Fengcheng Formation in well MY1 have oil and gas shows, suggesting the whole section is oil-bearing and appears as a typical shale oil system.

Although the Fengcheng Formation has shale sections with low organic matter abundance and S₁ values vertically, sections with rich microfractures filling with free hydrocarbons can form large-scale shale oil (tight oil) reservoirs. The Fengcheng Formation is comparable with classic marine shale source rocks, such as the Eagle Ford, Barnett and Bakken abroad, and even better than the marine shale formations with low organic matter abundance such as Monterey, Mowry, etc.^[25], indicating that the Fengcheng Formation has the source rock conditions for forming shale oil (tight oil) reservoirs.

In addition to the quality of source rocks, the quantity of generated hydrocarbon is crucial for large-scale hydrocarbon accumulation. The source rocks of the Fengcheng Formation are widely distributed and thick (Fig. 4a). The source rocks with TOC larger than 0.5% are 233.63 m thick on average, and the source rocks with TOC larger than 1.0% are 196.70 m thick on average. High-quality source rocks with TOC larger than 2% and maturity reached 0.7% cover the whole Mahu sag. The total amount of oil generated reaches 143×10⁸ t, especially in the central area of the sag, the oil-generation intensity reaches up to 800×10⁴ t/km². The total amount of oil expelled is 83×10⁸ t, and the amount of retained oil is nearly 60×10⁸ t. Apart from the lost oil and discovered conventional oil derived from the Fengcheng Formation, some of the expelled hydrocarbons can form free oil in matrix pores and fractures, which together with the retained oil constitutes the resource base of shale oil (tight oil) in the Fengcheng Formation.

There are multiple types of reservoir rocks in the multi- source mixed deposits of the Fengcheng Formation forming in alkaline lacustrine environment. Exploration has proved that argillaceous rock, dolomitic rock, siltstone, sand-conglomerate rock, and volcanic rock of the Fengcheng Formation all can act as reservoirs. Besides the sand-conglomerate rock and volcanic rock reservoirs with frequent oil and gas shows, the dolomitic rocks in the fine-grained sediments have large accumulation thickness of oil immered and oil spot cores, and higher oil contents, especially the dolomitic siltstone. With the air permeability of 1×10^-3 μm² and pore diameter of 1 μm as dividing points^[26], the reservoirs in the Fengcheng Formation can be classified into two types, conventional and unconventional ones. The conventional reservoirs include two main types of rocks: conglomerate and sandstone (clastic rocks), ignimbrite and basalt (volcanic rocks). The unconventional fine-grained diamictite reservoirs with air permeability generally less than 1×10^-3 μm² include dolomitic sandstone, dolomitic siltstone, dolomite, argillaceous dolomite and dolomitic mudstone (Fig. 5).

Statistics on reservoir physical properties show that the samples from the Fengcheng Formation have a porosity range of 0.1% to 13.0%, with an average of 2.89%. Among them, samples with porosity greater than 5% only account for 18.2%, and samples with permeability less than 0.1×10^-3 μm² account for 67.99%, showing characteristics of low porosity and tightness. Early researches on the Fengcheng Formation focused more on its role of source rocks, while paid less attention to its potential as oil and gas reservoirs. The sections with good oil and gas shows in the Fengcheng Formation of wells FN14 and FN1 in the northern part of the Mahu sag obtained oil flows. And the reservoirs are mainly fine-grained dolomitic siltstone and mudstone, with poor physical properties. No large-scale breakthrough has been made, leading to slow research progress.

The drilling of Well MY1 provides sufficient data for the detailed study of reservoir rock types. In this well, cores of 365.38 m long in total were taken from the Fengcheng Formation systematically, of which 6.12 m reaches the oil-immersion grade, 175.03 m of the oil-spot grade, and 184.23 m of the oil-trace grade. The cores have a maximum oil-bearing area proportion of up to 54% (Figs. 3 and 6). P₁f₁ has two sections of sedimentary rocks and volcanic rocks sandwiched in between them, the two set of sedimentary rocks and tuff sections in the middle part reach oil-immersion grade and have a cumulative thickness of more than 60 m. Through observation under microscope, the parent source materials of the clastic particles in the two sets of sedimentary rocks (Fig. 5g) are consistent in characteristics with the volcanic rocks below (Fig. 5h), so it is preliminarily considered that the tuffaceous conglomerate-bearing sandstones, and sandstones are formed by the proximal deposition of denuded highland volcanic rocks. The commingled test of the two sets of sedimentary rocks and the middle tuff layer achieved an industrial breakthrough daily output of 16.3 t. The measurement of downhole liquid profile confirmed that the two sections of sedimentary rocks contributed more than 80% of the oil produced, and had stronger oil production capacity than the tuff layer in the middle. The exploration potential of the sedimentary rocks above and below the tuff layer wasn’t aware in the previous exploration. This section has a porosity of 2.4%-12.4%, on average 8.7%, and a maximum permeability of 0.511×10^-3 μm², and permeability generally less than 0.1×10^-3 μm². The core fluorescence scanning results show that this section is characterized by universal oil-bearing of matrix pores, distinct bedding features of sedimentary rocks, and less developed micro-fractures (Fig. 6i).

The upper part of P₁f₁ to P₁f₃ is made up of interbeds of dolomitic rocks and shale, with a single-layer thickness of less than 0.5 m (Fig. 6). The section has obvious fine-grained laminar structure indicating lacustrine deposits, drainage structures, collapse deformation structures (Fig. 6f), channels formed by the hydrothermal jet later filled with calcite at bottom etc. in local parts. The rapid analysis shows the reservoirs are generally less than 10% in porosity, on average 5.79%, and generally less than 0.1×10^-3 μm² in permeability. However, a few samples have a porosity of more than 10% and a permeability of more than 0.1×10^-3 μm². Core scanning and fluorescence scanning results show these samples have micro-fractures developed. For example, the fourth barrel of core taken from the burial depth of 4612.31 m (Fig. 6b), is dolomitic mudstone, which has a porosity of up to 17.7%, and permeability of 0.036×10^-3 μm², many bedding fractures and high angle fractures, and obvious oil-bearing characteristic. Fluorescence scanning shows the fractures are about 3-4 fractures/m in density, less than 1 mm in width, small in length, and concentrated in some sections. In comparison, vertical erect fractures are larger in scale, with vertical spans in meter order (Fig. 6c), but are few in number. In addition, the porosity significantly decreases and permeability increases somewhat with the increase of the carbonate content (Fig. 6g, h). This rule is related to the brittleness of the rock affected by the carbonate content. As the carbonate content increases, the brittleness of rock increases, and micro-fractures are more likely to occur to improve the physical properties of the reservoir. In addition to the obvious oil-bearing characteristics of fractures, the lime-bearing dolomitic mudstone has oil-bearing dissolved pores (Fig. 5b, k, j), and the dolomitic siltstone is characterized by oil-bearing matrix pores (Figs. 6g and 5l). There are nanoscale and microscale pores with poor connectivity, and these pores can only turn out to be effective storage space when there are fractures (natural or artificial) connecting them. Moreover, the content of dolomite can greatly improve their brittleness^[27,28]. Samples from well MY1 contain relatively high carbonate content, particularly, the salinization of alkaline lake environment of the Fengcheng Formation also made it easier to form carbonate chemical deposits, which could include tight dolomitic reservoirs with better fracability than the Lucaogou Formation in the Jimusar sag^[9]. A breakthrough of 50.8 m³/d was achieved by large-scale volumetric fracturing with long and vertical well in the fine-grained section with hydrocarbon shows of Well MY1, confirming the exploration potential of the fine-grained dolomitic tight reservoir in the Mahu sag.

On the plane, among the conventional reservoirs, the clastic rock ones are distributed at the edge of the lake basin at shallower buried depths (2800-3600 m); conglomerate ones are distributed in the alluvial fan and fan delta plain, and a small amount in the fan delta front, mainly including small conglomerate, fine-grained conglomerate, glutenite, sandy conglomerate, and a small amount of medium conglomerate, and are large in thickness and have no mudstone interlayers^[10]. Sandstone reservoirs, accounting for a small proportion in the Fengcheng Formation, are mainly distributed in the fan delta front, are gray or gray green, medium to coarse in particle size, and small in thickness, often contain gravels, and are tight sandstone reservoirs. The volcanic reservoirs with 15-50 m thick are distributed in a limited area of Well X72 to Well FC1 in the northern Mahu sag, and the area of Well K81 in the southern Mahu sag. Fine-grained dolomitic deposits are widely distributed across the sag, which results in the wider enrichment range of shale oil (tight oil) reservoirs.

For the total petroleum system of the Fengcheng Formation, the thermal evolution, hydrocarbon generation and expulsion, and properties of hydrocarbons generated by the source rocks, the diagenetic evolution of reservoir space, and the evolutionary coupling of source rocks and reservoirs are the keys to the formation of different types of reservoirs. The thermal evolution simulation (Fig. 7) shows that under the influence of the basin basement subsidence and nappe of the orogenic belt in the northwest margin, the early sedimentary center of the alkaline lake of Fengcheng Formation entered the stage of low maturity first. And the main body of the Fengcheng Formation basically reached the hydrocarbon generation threshold at the end of Permian. At this stage, the Fengcheng Formation generated and expelled a small amount of low mature oil. In the Triassic, tectonic subsidence and sedimentary changes were relatively slow, and the source rocks increased in maturity with the increase of burial depth, and largely entered low mature-mature stage. Particularly during the Middle Triassic, the source rocks generated a large amount of mature oil. With lower overburden pressure, the Fengcheng Formation was weaker in compaction and larger in volume of primary pores, thus, the generated oil was discharged in large quantities. To the Early Jurassic, affected by the see-saw movement of southern basin falling and northern basin rising, the source rocks in the Mahu sag changed little in buried depth and increased slowly in maturity, but had entered the mature stage, and generated mainly mature oil and some low maturity oil around the margin of the sag due to shallow burial depth. It is noteworthy that since the Middle to Late Triassic, due to the long-term hydrocarbon generation and expulsion of source rocks, and the continuous sedimentation, the Fengcheng Formation began to have residual pressure under the effect of hydrocarbon generation and pressurization. The occurrence of the residual pressure accelerated the expulsion of hydrocarbons from the source, resulting in the mature oil discharge peak from the Late Triassic to the Early Jurassic. Later, with the rapid subsidence of the basement during the Middle Jurassic to Cretaceous, the source rocks matured rapidly, and to the end of the Cretaceous, reached the maturity basically the same as that at present. On the plane, the Fengcheng Formation source rocks have the largest buried depth and highest maturity of up to 2.0% in the central area of the Mahu sag, and gradually decrease in maturity towards the slope of the sag (Fig. 4b). At the end of Jurassic, due to the rising of Chemogu uplift, the subsidence and sedimentation of the Mahu sag stagnated, and a long hydrocarbon expulsion period came along. Then the hydrocarbon generation and expulsion started again in the early Cretaceous.

Overall, since the Middle Jurassic, the source rocks in the depocenter of the Fengcheng Formation has reached high mature evolution stage, and begun to generate a large amount of high mature oil. The high mature oil was discharged together with the low mature-mature crude oil trapped in the source rocks in the early stage, leading to the characteristics of "early hydrocarbon generation, early hydrocarbon expulsion, two stages, and long time sequence". The characteristic of two-stage continuous hydrocarbon generation provides good conditions for the formation of the Fengcheng total petroleum system. With the long hydrocarbon generation window, the crude oil found in the Mahu sag gradually become lighter from the periphery to the central sag, also from shallow to deep buried depths, while the gas to oil ratios increases.

In a word, the continuous thermal evolution of the Fengcheng Formation organic matter provides abundant oil source, and makes it possible to form various types of oil and gas. The expelled hydrocarbons can form conventional reservoirs outside the source rocks, and the crude oil charging into the tight coarse clastic reservoirs during different thermal evolution stages can give rise to continuously distributed tight oil. While the hydrocarbons retained in the fine-grained tight reservoirs in the middle and high mature stages can form large-scale shale oil. The overpressure environment formed by oil and gas, determines the production capacity of different types of hydrocarbons in the later period.

Limited by the burial depth, the exploration of the Fengcheng Formation in Mahu was centered around the areas relatively shallow in buried depth in the early stage, including the north, west and south of the Mahu sag, and mainly focused on conventional structural-lithologic reservoirs. In recent years, guided by the theory of orderly accumulation of conventional-unconventional oil and gas^{[2, 29]}, the exploration has expanded into the field of unconventional oil and gas, and has achieved great success. Wells MH28 and MY1 have successively tapped industrial tight oil and shale oil flow respectively. The hydrocarbon source studies^{[6-7, 9-10, 19]} confirmed that three types of oil and gas found in the Fengcheng Formation (conventional oil, tight oil and shale oil) are all products of the Fengcheng Formation source rocks. They are related to each other, but still have their own unique characteristics. The conventional oil was first discovered in wells represented by BQ1. In this well, the Fengcheng Formation is 1752 m thick, and the section with fluorescence shows is 1430 m thick; the P₁f₃ is mottled tight sandy conglomerate of plain faces, which can act as good cap rocks; the P₁f₂ is grey sandy conglomerate, acting as low-permeability reservoirs, and obtained oil flow through well testing. The oil, with a density of 0.8422 g/cm³ and a freezing point of 7.25 °C, belongs to mature oil. On the plane, restricted by the thrust faults in the northwest margin and the distribution of sand-conglomerate reservoirs, conventional oil reservoirs are distributed near the fault zone. At present, two types of targets, i.e. the structural-lithologic reservoirs in JW3 well area and stratigraphic-lithologic reservoirs in B253 well area of southern Mahu sag have obtained proven reserves. In the area of Well F5 to Well FC1 in northern Mahu sag affected by fault activities, structural-lithologic reservoirs have been confirmed too. For example, in FN4 well area, the Fengcheng Formation is at the buried depth of 4388-4402 m, the crude oil has a density of 0.909 4 g/cm³, freezing point of -12 °C, δ¹³C value of -30.04‰, and Pr/Ph value of 0.79, and biomarker showing the characteristics of the mature oil^[6]. In the P₁f₁ volcanic reservoir at 4808-4862 m of well X72 in northeastern Mahu sag, the crude oil has a density of 0.839 1 g/cm³, freezing point of 5 °C, δ¹³C value of -30.32‰, and Pr/Ph value of 0.86. The carbon isotope composition (δ¹³C) and Pr/Ph values of the oils from the two different kinds of reservoirs show that both of them are derived from the Fengcheng Formation but the high mature oil has been slightly degraded. Take the well documented well FC1 as an example, the density and biomarker characteristics of the crude oil (with isomerization index of steranes and Ts/Tm as the main references) vertically reflect the presence of products formed from low mature to mature, and high mature evolutionary stages (Fig. 8). The oil from the burial depth of 3119-3143 m has C₂₉ααα20S/(20S+20R) and C₂₉αββ/ (αββ+ααα) values of 0.37 and 0.41, indicating the oil is low mature oil. The oil from the burial depth of 3960- 3976 m has C₂₉ααα20S/(20S+20R) and C₂₉αββ/ (αββ+ ααα) values of 0.47 and 0.53, representing mature oil. The crude oil from the burial depth of 4193.93-4272.18 m has C₂₉ααα20S/(20S+20R) and C₂₉αββ/(αββ+ααα) values of 0.53 and 0.55, representing high mature oil. All of them are originated from the Fengcheng Formation, but affected by the thermal evolution and difference of bio-precursors, they have some differences in biomarker characteristics. Overall, oil and gas has been charging continuously at different thermal evolution stages over time, which fully reflects the characteristic of multi-stage continuous hydrocarbon generation of the high-quality alkaline lake source rocks of the Fengcheng Formation^[6].

In comparison, the spatial coexistence of different types of oil and gas emphasizes the macroscopic distribution relationship. Different from the complex and orderly accumulation described above, the spatial locations of reservoirs determine the final accumulation of hydrocarbons. The spatial coexistence relationship between different types of oil and gas in the Fengcheng Formation mainly depends on the sedimentary construction.

This study followed the idea of combining the lithology with well logs and calibrating seismic data with well logs. Based on the detailed description and experimental analysis of cores from well MY1, petrophysical analysis was conducted to establish the relationship of different lithologic logs and seismic sensitive parameters. On this basis, seismic attributes were extracted, and then in combination with the facies division in wells, the spatial distribution relationship of different lithofacies belts was predicted. For example, the dolomitic rocks with higher oil content are dominated by dolomitic siltstone. Their electrical logs are characterized by low acoustic time difference, low neutron porosity, high density, and wave impedance ranging from 11.5×10⁶ kg/(m²·s) to 15.5×10⁶ kg/(m²·s). They feature layered continuous reflection at low to medium frequencies. By using the method mentioned above, several typical facies profiles in the whole region were established, and then the spatial distribution of different types of lithofacies in the whole region was predicted (Fig. 9). Hence, the spatial coexistence relationship of three types of reservoirs has been figured out.

The results show that there were four provenance systems near the Hala’alate Mountains and Zhayier Mountains, Xiazijie fan, Huangyangquan fan, Baqu fan, Zhongguai fan, forming the proximal fan delta deposits under the control of the foreland depression. And near the western margin of thrust belt, there was sufficient accommodation space, thus a hugely thick short-axis restricted fan turned up with stable supply of source materials. The sandy conglomerate with relatively coarse clasts in the fan delta plain and nappe faults constitute the fault-lithologic reservoir zone under the conventional stratigraphic background. To the direction of the slope, under the influence of source materials, water salinity of lake basin, and paleoclimate, terrigenous clasts, endogenous chemical deposits and volcanic mixed deposit formed the dolomitic sandstone in the delta front. This belt has relatively coarse grain size of clastic particles, and is mainly fine-medium sandstone. However, with stronger diagenesis, the reservoirs in this belt are tight, laterally connected to the oil source area towards the sag, forming of lithologic tight reservoir with large area near source. Further towards the sag, relatively fine-grained clastic sediments mixed with the endogenous chemical sediments, under the effect of intermittent lake-level fluctuations, giving rise to thin interbeds of dolomitic slit-fine sandstone, dolomitic mudstone and argillaceous rock over a large range of the sag and slope, which is a typical shale oil belt with integrated source and reservoir. In the main body of the sag, lamellar dolomitic shale and mud shale deposited, forming a favorable area of pure shale oil with a large thickness. According to statistics on the thickness of fine-grained deposits in wells and the trend of predicted attributes, the mud shale layers in the sag are 100-1500 m thick combined and widely distributed laterally (Fig. 1), so they can form shale oil reservoirs with huge reserves, with the alkaline lacustrine source rocks as the source kitchen. It should be noted that in the forming process of shale oil reservoir, the existence of fractures can greatly improve the property of the reservoir. In some local structural belts, influenced by dolomite content and tectonic stresses, the shale layers of the Fengcheng Formation form fractured structural oil and gas reservoirs with unified oil-water boundary as oil, gas and water in the structural traps differentiate. This kind of oil reservoir is mainly distributed around the peripheral of Well FC1-Well FN14. Towards the center of the sag, with no fractures developed, hydrocarbons occur in the form of shale oil.

Vertically, influenced by the variations of lake level from deep to shallow to deep again, the terrigenous clastic sediments present reverse cycling characteristic. Correspondingly, from P₁f₁ to P₁f₃, conventional sand-conglomerate reservoirs and tight oil reservoirs are most extensive in P₁f₂, but limited in P₁f₁ and P₁f₃. On the contrary, the shale oil is the largest in range in P₁f₃. In addition, there was a certain range of volcanic activities in the north and south of Mahu sag near the deep fault zone, so volcanic rocks and pyroclastic rocks deposited in the early stage of P₁f₁. Adjacent to the oil source, the volcanic rock and pyroclastic rock could form near-source volcanic rock or tight oil reservoirs similar to the Lucaogou Formation in the Santanghu Basin^[30].

In general, controlled by the thermal evolution of the regional source rocks (Fig. 4b), structures (Fig. 1) and lithofacies (Fig. 9), the conventional and unconventional oil and gas in the Fengcheng Formation are distributed orderly and continuously. They appear as three zones as follows: (1) the mature conventional oil zone, controlled by structures and lithology, is mainly distributed in the fault zone around the sag; (2) the medium-high mature tight oil zone in strip shape, controlled by the reservoir distribution, is distributed in striped shape in downdip direction of the conventional oil; and (3) the medium-high mature shale oil zone is widely distributed in the structurally stable sag under the control of the dolomitic rocks. From the fault zone to the sag, mature conventional oil, medium-high mature tight oil, and medium-high mature shale oil occur orderly. Vertically, around the fault zone, medium-high mature shale oil, medium-high mature tight oil, and mature conventional oil occur orderly from bottom to up. And in the area of the slope to sag, medium-high mature conventional oil, medium-high mature tight oil, and medium-high mature shale oil are present orderly from bottom to up.

As mentioned above, the oil and gas types of the Fengcheng Formation in the Mahu sag, Junggar Basin are relatively complicated. In order to better reflect the hydrocarbon accumulation model of the Fengcheng Formation, considering the integration of the source rocks and reservoirs, proceeding from the spatial relationships (position and vertical combination) between the source rocks and reservoirs, and the connotation of each resource type, the source-reservoir structural relationships, lithology and spatial distribution of reservoirs, characteristics of hydrocarbon migration, and types of oil and gas were combed in this study, and three types of accumulation models were sorted out: integrated source-reservoir, source-reservoir in close contact, and separated source-reservoir (Table 2 and Fig. 10a).

This type of reservoirs features in-situ oil generation and storage within the source rock. The reservoirs include fine-grained dolomitic siltstone-sandstone and argillaceous rocks, with obvious laminae. Driven by the over pressure caused by hydrocarbon generation, the crude oil is accumulated under the action of pressure difference between the source and reservoir, and distributed continuously in a large area in the reservoirs of argillaceous shale with dolomitic siltstone interlayers. For example, Well MY1, the fine-grained argillaceous rock reservoirs in the well have dissolved pores and micro-nano pores (Fig. 5i), and observation by a field emission scanning electron microscope revealed that the mudstone has oil films occurring in adsorbed state in the pores on the surface of mineral particles (Fig. 5l). In contrast, in the tight reservoirs of coarse dolomitic siltstone, the crude oil occurs in matrix pores and micro-fractures, but the layers are usually less than 0.5 m thick each. In addition to retained hydrocarbons of adsorbed state in the source rocks, some free state hydrocarbons in the dolomitic siltstone reservoirs were generated by the upper and lower argillaceous source rocks and then migrated a very short distance to the dolomitic siltstone reservoir, forming the shale oil accumulation model characterized by integrated thin interbedded source-reservoir, in-situ generation and accumulation.

This type of reservoirs highlights reservoirs adjacent to the source rock (in lateral contact or vertical adjacent), and hydrocarbon accumulation near source rock. In this kind of reservoir, the hydrocarbons generated from the source rocks usually experienced primary or short-distance secondary migration, the reservoir space is mainly intergranular matrix pore, and the hydrocarbons are in free state. Moreover, this kind of oil reservoir has higher requirements on physical conditions of reservoirs, which are often tight reservoirs. For example, the interbedded dolomitic silt-fine sandstone and argillaceous rock developed in the fan delta outer front and pro-fan delta, and the oil and gas in the reservoirs, mostly generated by adjacent source rocks, accumulate in the reservoir after a certain distance of migration. Although the sediment is small in grain size and rich in laminae, and the dolomitic siltstone in the reservoirs may have some hydrocarbon generation capability (Table 1). Each layer of sediment is usually more than 1 m thick, meeting the features of tight oil reservoir. This type has been found in the area of dolomitic sandstone, which has been confirmed by the successful breakthrough of Well MH28. Shale oil and tight oil are continuously distributed over a large area, and the whole section contains oil.

This kind of reservoirs is represented by sand-conglomerate reservoirs in Wells X72 and BQ1 shallow in burial depth located in the area with strong tectonic activities. The reservoir rocks are complex and diverse, and poorer in physical property generally, with a permeability smaller than 0.1×10^-3 μm². The reservoirs are usually not in direct contact with the source rocks, and the hydrocarbons often accumulate outside the source after secondary migration. The conventional reservoirs are characterized by oil displacing water under buoyancy effect, with trap as the unit. They have all the accumulation elements of conventional reservoir from source to trap, for example, the sand-conglomerate fault-lithologic reservoirs in Well B25^[10].

The oil and gas reservoirs in the Fengcheng Formation of the Mahu sag are complex and diverse, covering all kinds of conventional and unconventional types. The accumulation models determine that the vertical and horizontal distribution of the reservoirs is controlled by the spatial-temporal relationships of source and reservoir as well as the structure and lithofacies, presenting an orderly pattern (Fig. 10b, c). The orderly coexistence of multi-types of oil and gas in the Fengcheng Formation is a proof example of the total petroleum system. The study on Fengcheng Formation from hydrocarbon generation and expulsion, to different types of reservoirs (including sedimentary rocks, volcanic rocks and carbonate rocks), and the coexistence of conventional and unconventional oil and gas, means that the oil and gas research of the Fengcheng Formation has shifted from “source rocks to traps” to “source-reservoir coupling and orderly accumulation” of the total petroleum system. The Fengcheng Formation is a successful case of exploration from "outside source" to "inside source". It should be noted that although the Fengcheng Formation has reached the mature and high mature gas-generation stage, no large-scale gas reservoirs have been found yet. Considering the gas to oil ratios and formation pressure changes, it is possible that there are large-scale condensate shale oil reservoirs and condensate gas reservoirs in the center of the sag. Moreover, the Fengcheng shale layers more than 4500 m deep is a wide domain for deep-buried shale oil and gas of alkaline lake type uncharted in China or abroad. In fact, all kinds of oil reservoirs discovered there so far have various levels of gas shows, indicating that the formation of gas reservoir cannot be absolutely ruled out.

In the Fengcheng Formation of the Mahu sag in the Junggar Basin, an alkaline lacustrine sedimentary system developed in the foreland depression. Due to the external source supply of the orogenic belt in the northwest margin and climate changes, dolomitic diamictites composed of mixed terrigenous clasts, endogenous chemical deposits and volcanic materials were formed. The fine-grained dolomitic and argillaceous rocks act as source rocks with high abundance and good types of organic matter, which have generated hydrocarbons continuously and efficiently in two stages. Under the effects of microfractures and the dissolution, some of them can become effective reservoirs under the tight background. Conventional reservoirs of sand-conglomerate and volcanic rocks also occur under the influence of source supply and volcanic activities.

Reservoirs of the Fengcheng Formation are controlled by structures and lithofacies. There are many types of reservoirs, such as conventional, tight oil and shale oil ones. Due to the orderly evolution of source rocks and reservoir conditions, they are orderly distributed in space. On the plane, from the fault zone to the sag, mature conventional oil, medium-high mature tight oil, and medium-high mature shale oil transition gradually in turn. Vertically, from bottom to the top, medium-high mature shale oil, medium-high mature tight oil, and mature conventional oil appear in turn.

According to the relationships between source and reservoir, lithofacies and spatial distribution, characteristic of hydrocarbon migration, and hydrocarbon type, the Fengcheng Formation has three types of accumulation models: integrated source-reservoir model, source- reservoir in close contact model, and separated source- reservoir model, corresponding to shale oil, tight oil, and conventional structural-lithologic reservoirs respectively.

It is suggested to carry out integrated deployment around the total petroleum system of the Fengcheng Formation in the Mahu sag, which should focus on conventional oil first, and then unconventional tight oil and shale oil. Through the risk well exploration and technology research, we hope to make an overall breakthrough in the unconventional fields of deep-burial shale oil and tight oil, which can be a new field of large-scale reserve expansion after the early Permian-Triassic conglomerate oil region above source in the Mahu sag. To this end, the deployment of deep-ultra deep risk wells targeting shale oil and tight oil of the Fengcheng Formation in the Mahu sag, as well as the basic researches and technological studies should be speeded up. The basic researches and technological studies should cover the phase state of deep buried hydrocarbons, the coupling mechanism of overpressure and thermal evolution, the coupling mechanism of hydrocarbon generation and expulsion, diagenesis and hydrocarbon charge, the hydrocarbon generation and expulsion features of different bio-precursors, the forming mechanism and recognition of sweet spots, the mechanism of crack extension, the causes and features of brittleness, and the fracturing technology etc. We should explore the no-go zone more than 4500 m deep for shale oil and tight oil, and pioneer even further to the formations more than 5000 m deep in the center of the sag, to deepen the theoretical understanding on the continental unconventional oil and gas theory, and to enrich and develop the theoretical researches of the total petroleum system.

HI—hydrogen index, mg/g;

S₁—the content of free hydrocarbons, mg/g;

S₂—the content of pyrolytic hydrocarbons, mg/g;

T_max—peak pyrolysis temperature, °C.