Around 2010, the hydrocarbon exploration in the Junggar Basin was faced with a series of issues. In the northwestern margin area, no substantial progress was made in hydrocarbon exploration; in the central part, the search for lithologic-stratigraphic reservoirs had been fruitless for a long time; and no major achievement had been made in basinwide gas discovery for many years. Therefore, from 2011 to 2013, when we were in charge of the risk exploration of CNPC, we proposed a series of new exploration concepts, such as “second-order fault step” and “hydrocarbon migrationward surface”, with regards to the key areas and directions for oil and gas exploration in the Junggar Basin. More than five years have passed by since then, and it now turns out that these new concepts have provided important guidance in the discovery and construction of the Permian and Triassic billion-ton-scale oil fields in the Mahu slope area, and in screening key target areas for hydrocarbon exploration in the central Junngar Basin. This paper summarizes the formative processes, connotations and application effects of these new concepts, hoping that readers in the petroleum industry and academia alike can benefit from it.

1.1.1. Proposal of the second-order fault step concept

From May 16, 2011 to March 20, 2012, China National Petroleum Corporation (CNPC) held seminars on “Risk Exploration Fields and Target Well Locations in the Tarim Basin and Junggar Basin” in Urumqi and Beijing, to collect opinions on the future major exploration fields and targets in the two basins.

At that time, the direction of hydrocarbon exploration in the Mahu sag of the Junggar Basin was unclear. Firstly, the exploration degree of the Kelamayi-Wuerhe fault zone in the northwestern margin was already quite high. As such, although there is still potential for discovery by deeper and detailed exploration in this mature area^[1], the chance of finding large oil and gas fields is relatively low. At that point, some researchers suggested looking for concealed structures under the Karamay thrust belt^[2]. However, due to the low degree of confirmed structures, unclear petroleum accumulation conditions and large burial depth, people were hesitant to take that suggestion. In addition, it seemed unclear at that time about the direction of searching lithologic-stratigraphic reservoirs in the slope area of the Mahu sag. Specifically, it was generally believed that the Mahu slope area lacked good reservoir facies belts^[3,4,5]. On top of that, this area has a potential sealing issue due to its juxtaposition with sediment source area updip, and lacks faults connected to oil source rocks. As a result, pessimistic views on hydrocarbon exploration in the Mahu sag predominated then and the focus of risk exploration was on tight gas in the Permian Jiamuhe Formation, with the deployment of Wells Zhongjia 1, Zhongjia 2 and Zhongjia 3.

Based on detailed analysis of 2D and 3D merged seismic data, especially the onlap, pinchout, and truncation nature of the Permian and Triassic, as well as variations in their internal reflection structures, we first proposed at the aforementioned meeting the existence of an ancient “early occurrence and early demise ” thrust belt in the Mahu slope area, northwest Junggar Basin. Given the existence of the Kelamayi-Wuerhe fault zone landward of the Mahu slope area, the newly discovered ancient thrust belt was named “second-order fault step” (Figs. 1 and 2). It was found that the ancient fault step could result in paleo-highlands and paleo-slope breaks, which controlled the development and distribution of the Permian and Triassic sedimentary facies and formation of lithologic- stratigraphic traps (and trap groups) in the slope area. It was also pointed out by us that it was necessary to strengthen research on depositional environments and sedimentary microfacies in regard to the "second-order fault step", in order to determine favorable facies belts. Furthermore, we also highlighted the need to evaluate lithologic-stratigraphic traps with the "four lines and three surfaces" principle, which should be the key to resolving the dilemma of hunting for lithologic- stratigraphic reservoirs in the Mahu slope area. The “four lines” refer to isopach, lithologic pinchout limit, stratigraphic truncation line or onlap limit, and structural contour of the top surface of main target intervals. In comparison, the “three surfaces” refer to fault plane, reservoir top and base, and large sedimentary interface.

The "second-order fault step" concept has two implications for hydrocarbon exploration in the western Mahu slope. Firstly, the paleo-tectonic setting related to the “second-order fault step” controls the development and distribution of sedimentary facies in the slope area, especially in the middle and lower slope, indicating that there should be good-quality reservoirs in this area. Secondly, as continental deposits vary greatly as a result of river sinuosity, bifurcation and abandonment, lithologic sealing conditions are prone to occur, and thus there should be no lack of lithologic traps in the slope area. At that meeting, it was explicitly requested that the Research Institute of Xinjiang Oilfield Company should work jointly with other relevant research institutes and universities to strengthen research on depositional environments and sedimentary facies, and on this basis, do a good job in the industrial evaluation of lithologic traps.

1.1.2. Geological characteristics of the second-order fault step

The Kelamayi-Wuerhe fault zone is classified as the first- order fault step, while the newly identified fault zone in the downdip direction is considered as the second-order fault step (Fig. 2). The second fault step developed mainly during the Permian Fengcheng Formation-Wuerhe Formation period, and underwent a phase of adjustment during the Triassic before entering the dormant stage. Although not large in fault displacement, it still played a role in controlling depositional environments and sedimentary facies. A number of large blind thrust faults developed on the second-order fault step, with fault displacement decreasing gradually from the Carboniferous to the Triassic, and finally disappearing in the Jurassic^[6-7] (Fig. 2).

1.1.2.1. Paleo-tectonic setting

Based on the structural section from the fault step zone to the localized slope zone, the second-order fault step has obvious characteristics indicative of early occurrence and early demise (Fig. 2). Two to four high-angle reverse faults occurred on the second-order fault step, resulting in a paleo-highland of tectonic origin. It is an ancient thrust belt developed mainly in the middle and late Permian with tectonic activity gradually decreasing until the Triassic. It extends from Wuerhe in the north through Karamay to Chepaizi in the south, with a tendency to decrease in intensity and scale from north to south^[7]. This Hercynian paleo-uplift shaped the development and distribution of the present tectonic flexure zone and exerted a significant control on stratigraphic distribution. The thinning of the Permian and Triassic towards the paleo-uplift, and the internal character of the Permian (e.g., truncation, toplap and progradation) (Fig. 3) reflect the tectonic activity of the second-order fault step and its control on sedimentation.

From the perspective of the plan-form distribution of paleo-structures, there were four main nosing structures in the Mahu slope area during the main development stage of the second-order fault step. From the south to the north, they are Manan, Maxi, Mabei-Mazhong and Xiayan-Dabasong nosing structures, respectively. They have clearly controlled or influenced the hydrocarbon accumulation conditions and distribution of relevant formations around the second-order fault step (Fig. 1). These include the depositional environments, facies belts and distribution of the strata that are involved in the paleo-uplift, the development of large lithologic-stratigraphic traps (or trap groups), and the main target area of hydrocarbon migration and accumulation. The oil and gas reservoirs discovered so far are mainly distributed in the ancient nosing stucture zones.

1.1.2.2. Control on stratigraphic development and depositional environments

The controlling effect of the “second-order fault step” on stratigraphic development is reflected by the frequent onlaps/ pinchouts or truncations of the Permian and Triassic towards the updip of the Mahu slope and surrounding area. In the Mabei and Xiayan-Dabasong slope, the Upper Wuerhe Formation (P₃w) is missing, the top of the Permian is eroded, and the top of the Lower Wuerhe Formation (P₂w₄) is truncated and pinched out towards the updip direction (Fig. 3). In the Maxi and Manan slope, the Upper Wuerhe Formation is present, while the Lower Wuerhe Formation is clearly eroded, with the upper part (P₂w₃ and P₂w₄) truncated and pinched out towards the updip direction. Meanwhile, low-stand onlapping deposits were developed above the sequence boundaries of the Permian and Triassic^[8,9]. Specifically, in the Manan-Maxi and Xiayan-Dabasong slope, low-stand fan deltas occur at the bottom of the Xiazijie, Lower Wuerhe, Upper Wuerhe and Baikouquan Formations, onlapping toward the updip direction and forming a large-scale stratigraphic onlap belt (Fig. 4b).

The second-order fault step developed several paleo-geomorphic slope breaks which exerted an obvious control on the distribution of sedimentary facies (Fig. 5). The slope-break belt of the second-order fault step consists of slope-break zones and terrace zones. The slopes of slope-break zones are relatively steep (with a gradient of 1.0°-2.5°), compared to the terrace zone (with a gradient of 0.15°-1.0°). This difference in ancient slopes could lead to facies belt differentiation^[12]. For one base level cycle, the slope-break zone was dominated by gravity flow deposition, forming fan-delta plain facies; whereas the terrace zone was dominated by traction flow deposition, forming fan delta front facies. Multiple slope breaks facilitated the long-distance transport of fan deltas in the Permian and Triassic and formed favorable facies belts of fan-delta fronts in terrace zones. In addition, in slope-break zones, fan-delta plain facies are characterized by sandy conglomerates of poor sorting, high mud content and overall poor physical properties, and are thus prone to form reservoir seals and baffles in the updip direction. In comparison, delta-front sandy conglomerate facies in terrace zones are characterized by relatively good sorting, low mud content and overall good physical properties, representing favorable areas for reservoir development. Multiple slope breaks controlled multiple couplets of seals/baffles updip and favorable reservoir belts downdip, resulting in multiple lithologic-stratigraphic trap belts in plan view (Fig. 5)^{[13,14,15,16]}.

1.1.3. Reservoir-controlling significance of the second-order fault step

The two fault step structure in the Mahu sag and surrounding area controls not only the stratigrapihc development but also the types and distribution of hydrocarbon accumulations (Figs. 1 and 5). The first-order fault step (fault-terrace zone) comprises many faults, and develops a series of fault-block, and fault-nose related structural traps, as well as a Jurassic-Cretaceous onlap pinchout zone. The oil reservoirs of structural origins predominate, in the first-order fault step with stratigraphic reservoirs blocked by heavy oil being subordinate, together forming more than 14.0×10⁸ tons of proven oil reserves. The second-order fault step is overall a slope setting characterized by relatively flat and gentle structures. It is dominated by lithologic-stratigraphic traps, with locally developed small anticline or fault nose traps (e.g. the anticline around Well Ma 2), making this area significant for searching for large lithologic-stratigraphic oil reservoirs.

The proposal of the second-order fault step concept made it clear that the slope area of the Mahu sag is a favorable site for seeking large stratigraphic-lithologic reservoirs, and made us more confident about hydrocarbon exploration in the middle and lower slope areas. This concept has effectively guided and promoted the overall research, evaluation and understanding of the slope area, and served as a theoretical basis for the discovery of the Permian and Triassic oil fields with 10×10⁸ t reserves in the Mahu sag. It has played an important role in driving the petroleum industry mindset shifting from structural reservoirs oriented to large lithologic-stratigraphic reservoirs oriented in the Mahu sag.

1.2.1. Proposal of the hydrocarbon migrationward surface concept

On August 11, 2012, China National Petroleum Corporation (CNPC) held another review meeting on “Risk Exploration Targets in Xinjiang Region” in Urumqi. The authors put forward the concept of six hydrocarbon-facing surfaces based on the fact that main source rocks of the Junggar Basin are below the main reservoir-seal pairs and the oil and gas migration direction is from bottom to top and from south to north. The authors also pointed out that selection of exploration targets should be around the six hydrocarbon-facing surfaces, and highlighted the need to examine hydrocarbon accumulation conditions in detail, to resolve specific problems, and to be confident in significant breakthroughs in hydrocarbon exploration from a long term perspective.

1.2.2. Connotation of the hydrocarbon migrationward surface concept

A large amount of oil-source correlation data has confirmed^[17,18,19] that in the Junggar Basin, the main source rocks are distributed in the Carboniferous-Permian and Jurassic, and are primarily concentrated in 6 large sags (Mahu, Well Pen-1 West, Shawan, Fukang, Dongdaohaizi and Sikeshu). In comparison, the high-quality reservoir-seal assemblages are mainly distributed in the Upper Permian, Triassic, Jurassic and Cretaceous. And the oil and gas reservoirs in the Junggar basin are mostly secondary “source below and reservoir above” type, with some being the “self-source and self-reservoir” type. It is worth mentioning that the shallowly-to-moderately buried Jurassic and the deeply buried Carboniferous-Permian are low in exploration degree and need to be studied further. The IV episode of the Yanshan movement and the Himalayan movement since the end of the Cretaceous caused the basin to tilt towards the south, so that the overall direction for oil and gas migration and accumulation direction is from south to north. Therefore, the northern slopes of the six major hydrocarbon-rich sags are the main target areas for hydrocarbon migration. With well-developed traps, there is a high probability of forming oil and gas reservoirs, so they are key targets for hydrocarbon exploration.

1.2.3. Significance of the concept of hydrocarbon migrationward surface

Based on the general trend of hydrocarbon migration and accumulation, as well as the “hydrocarbon migrationward surface” concept, the northern (west and east) slopes of the six hydrocarbon- rich sags have preferably been selected as key directions for oil and gas exploration, including: (1) the west-northeastern slopes of the Mahu sag and surrounding area; (2) the western slope of the Shawan sag; (3) the northern slope of the Sikeshu sag; (4) the northeastern ring zone of the Shawan-Well Pen-1 West sag; (5) the northern slope of the Dongdaohaizi-Wucaiwan sag; and (6) the north-northeastern ring zone of the Fukang sag (Fig. 6).

The “hydrocarbon migrationward surface” represents the main direction of hydrocarbon migration and accumulation. The proposal of this concept makes it clear that the northern (west and east) slopes of the six hydrocarbon-rich sags have the highest probability for hydrocarbon accumulation and discovery, and thus clarifies the direction for high-efficiency exploration. Recently, the encouraging results from some exploration wells deployed around the hydrocarbon migrationward surfaces indicate the merits of this concept.

Since 2011, the authors have pointed out at several meetings held by CNPC that the Junggar Basin is rich in both oil and gas resources. Although the reserves found so far are mostly oil, with a very low gas-oil ratio of only 0.06:1, this does not mean that the basin is deficient in natural gas resources. Such a conclusion is made mainly based on the following knowledge. Firstly, the source rock is deeply buried at the bottom of the sedimentary cover of the Junggar Basin, and its thermal maturity has entered the stage of large-scale gas generation. Secondly, the Carboniferous proto-basin is uplifted more significantly and shallower in burial depth in the east compared to the west. Noticeably, natural gas fields have been discovered in the Huoshaoshan-Liangliang area (the eastern Junggar), indicating that the Carboniferous produces largely natural gas and that the gas potential should be even better in the Mahu-Shawan area of the western Junggar, given the better preservation of the proto-basin and larger burial depth in this area. Thirdly, all the discovered reservoirs are secondary oil and gas reservoirs where some light components have been lost due to poor sealing conditions. This is the main reason why the present discovery comprises more oil than gas. With a deeper understanding of the hydrocarbon accumulation conditions and properties, the probability of discovering natural gas will increase significantly by searching the self-source and self-reservoir type of source-reservoir assemblages in the vicinity of hydrocarbon kitchens.

1.3.1. Natural gas potential

The judgement that at the Junggar Basin had rich natural gas resources is made on the following three bases. Firstly, the Carboniferous and Permian main source rocks are deeply buried (maximum depth more than 15 000 m) with high thermal maturity. Secondly, efficient exploration has revealed that there are three gas-bearing systems, under the control of the Carboniferous, Permian and Jurassic gas source rocks^[20]. The total natural gas resource is over 3.0×10¹² m³, of which the Carbonaceous and Jurassic had natural gas resources of about 1.0×10¹² m³ and 2.0×10¹² m³ respectively (PetroChina Xinjiang Oilfield Company, 2019). Thirdly, from the perspective of resource composition, the gas-oil ratio is 0.3:1.0, which is 4 to 5 times higher than the gas-oil ratio of the reserves that have been found.

1.3.2. Exploration significance of natural gas

The distribution of gas reservoirs discovered in the Junggar Basin has two major characteristics. Firstly, their distribution is controlled by the main gas source kitchen, concentrated near source rocks and around sags^[21]. For example, the Carboniferous natural gas fields such as Kelameili and Wucaiwan are controlled by the main gas source kitchens of Dishuiquan and Wucaiwan, and are distributed in the Diannan uplift zone. The Permian gas reservoirs in Mosuowan and Mobei are secondary gas reservoirs at intermediate to shallow depths, and are mainly distributed near the gas source faults in good configuration with the Permian high-mature gas source kitchen. The natural gas fields (reservoirs) sourced by the Jurassic source rock, such as the Hutubi and Mahe, are mainly distributed above the Jurassic gas source kitchen in the southern margin of the basin. Secondly, the gas reservoirs discovered are generally small in scale and have an obvious primary origin. This is mainly due to the dispersibility of natural gas and high requirements of caprocks. The formation of secondary gas reservoirs require both good migration conditions and sealing conditions, so it is difficult for them to occur. The above characteristics demonstrate that as long as the favorable areas with relatively suitable preservation and accumulation conditions are found in the vicinity of large-scale gas source kitchens, there is still a good chance to find large gas fields.

On May 16, 2011, in the seminar on risk exploration in the Tarim and Junggar Basins held in Urumqi, the authors arranged the overall research and evaluation of the Mahu sag based on the concept of “second-order fault step”. It mainly includes the following three respects: (1) to unify the Permian classification scheme and characterize the stratigraphic pinchout belts and distribution characteristics; (2) to study the segmentation of second-order fault step structure and deepen petroleum play evaluation; (3) to strengthen research on the sedimentary reservoirs of different fans to determine key risk prospecting targets. After that, on July 28 and September 15, 2011, two meetings were held to check the progress of the research. Through discussion, four major slope belts (Mabei-Mazhong, Maxi, Manan and Madong) were selected as the key target areas for risk exploration; multiple rounds of “risk exploration target demonstration meeting” were organized later on; and four risk exploration wells (Mahu 1, Maxi 1, Yanbei 1 and Jinyan 1) were deployed.

Well Mahu 1 was deployed in the southern part of the Kebai Slope in the western slope of the Mahu sag, close to the junction of the Mahu sag and the Zhongguai uplift. This well with a designed depth of 4680 m aimed to explore the hydrocarbon-bearing characteristics of the Middle Permian Xiazijie Formation and Wuerhe Formation, as well as the Triassic Baikouquan Formation. The well was spud in on May 21, 2012 and completed in the Lower Permian Fengcheng Formation on September 26, 2012, with a total drilling depth of 4486 m. The well had active oil and gas displays during drilling. In July 2012, the well revealed high-pressure oil and gas layers in the Baikouquan Formation. The well logging interpretation showed that there were 10 oil layers of 33.2 m in total; and the oil layers had a porosity of over 10%, and good physical and oil-bearing properties. The PetroChina Xinjiang Oilfield Company recommended that the drilling be completed in advance. After discussing with the relevant departments of the RIPED and the PetroChina Exploration & Production Company, the authors decided to protect the exposed Triassic Baikouquan Formation oil layers and then continue drilling to complete the geological purpose. On April 12, 2013, the interval of 3284.0-3310.0 m of the Baikouquan Formation was tested, and a high-yield industrial oil flow (daily oil production of 39.4 t and daily gas production of 2500 m³) was obtained through natural flow without fracturing. At the same time, the well also encountered an oil and gas layer of 38.0 m in the Wuerhe Formation of the Permian. The tested layer didn’t show good results at first, but in 2017, produced an industrial oil flow of 12.84 t per day, which prompted the major discovery in Wuerhe Formation, with controlled oil geological reserves of 9, 107×10⁴ t reported. This well had major breakthroughs in the Triassic Baikouquan Formation and the Permian Upper Wuerhe Formation.

Well Yanbei 1 was deployed on the Sangequan bugle of the Luliang uplift in the eastern slope of the Mahu sag. With a designed depth of 3950 m, this well was primarily to explore the Permian Wuerhe Formation (P₂w) as well as the Triassic Baikouquan Formation (T₁b). It was spud in on September 30, 2012 and completed on December 27, 2012 at the depth of 4100 m in the Lower Wuerhe Formation. It had good oil and gas displays during the drilling, and the well logging interpretation showed that there were 2 oil layers of 56.0 m thick combined. On October 25, 2013, the interval of 3784.0- 3797.0 m in the Triassic Baikouquan Formation was tested, obtaining a daily oil production of 5.18 m³ and daily gas production of 6.5 m³. This was the first breakthrough made in the Triassic Baikouquan Formation of the Madong Slope.

The success of Well Mahu 1 and Well Yanbei 1 promoted the overall breakthrough in the Mahu slope area. Well Mahu 1 encountered a thick oil layer in the Triassic Baikouquan Formation. Although the drilling was not finished in advance, this new discovery made the geologists realize the great potential of the Baikouquan Formation and to some extent changed the pessimistic view on the reservoirs in the slope area of the Mahu sag (Fig. 4). Since then, the PetroChina Xinjiang Oilfield Company put more efforts in re-examining old wells and deploying more wildcat wells and evaluation wells. Oil layers were discovered in the Baikouquan Formation in a group of old wells such as Fengnan 4, Xia 7202, Jin 201, Jin 202 and Xiayan 2. They tapped oil in a second test. The drilling of Well Ma 132, Ma 133 and other evaluation wells also showed good results. In May 2013, based on the drilling results and geological understanding of Well Mahu 1 and Well Maxi 1, Well Ma 18 drilled in the downdip direction of Well Maxi 1 obtained a high-yield industrial oil flow of 33.23 tons per day, marking a breakthrough in hydrocarbon exploration of the Baikouquan Formation in the Maxi Slope^[22]. Based on the drilling results and understanding of Well Yanbei 1 and Yanbei 2 in the upper part of the Madong Slope, in July 2015, Well Da13 was deployed in the downdip direction of the slope, which obtained an industrial oil flow of 15.06 t per day, realizing a substantial breakthrough in the Madong slope^[23]. In April 2016, Well Mazhong 4 was deployed to explore the Mazhong terrace and showed an industrial oil flow of 10.15 t per day, confirming the oil potential of the Mazhong terrace area. By the end of 2017, the Mabei, Manan, Maxi and Madong slopes and the Mazhong terrace area in the Mahu sag had seen all-round breakthroughs, revealing an encouraging situation of “whole sag-oil-bearing”. The sag had 5.2×10⁸ t of accumulated proven oil reserves, 12.4×10⁸ t of newly added third- order reserves, and its reserve scale was over 10×10⁸ t^[24].

In 2012, in the Risk Exploration Target Demonstration and Review Meeting in Xinjiang, the authors proposed to focus on the “hydrocarbon migrationward surfaces” based on the hydrocarbon accumulation conditions and the general trend of hydrocarbon migration and accumulation, to solve geological issues in a targeted manner and to made discoveries with strong determination. At that meeting, the authors listed the six “hydrocarbon migrationward surfaces” in the Junggar Basin and required geologists to further focus on the exploration target areas and evaluate targets on the “hydrocarbon migrationward surfaces”. The results of this phase effectively guided the deployment of Well Mahu 1 and Well Yanbei 1 in the west-northeaster slope area of the Mahu sag, and made breakthroughs. Since the authors were no longer in charge of the company's risk exploration work after August 2013 because of work transfer, the leaders and technicians of PetroChina Xinjiang Oilfield Company have been adhering to this concept when selecting targets. The deployment of some exploration wells reflects the concept of “hydrocarbon migrationward surface” and have achieved encouraging discoveries.

Following the idea that hydrocarbon is very likely to accumulate around the hydrocarbon migrationward surface in the slope areas of hydrocarbon-rich sags^[25,26], five risk exploration target areas were determined in the Permian-Triassic, Jurassic-Cretaceous lithologic-stratigraphic oil and gas accumulation areas, namely the western Shawan slope, the northwestern ring of the Shawan-Well Pen1 west sag, the north-northeastern ring of the Fukang sag, the northern slope of the Dongdaohaizi-Wucaiwan sag and the northern slope of the Sikeshu sag (Fig. 6). Recently, Wells Shatan 1, Pendong 1, Fangtan 1 have been deployed (Fig. 6). Among them, Well Pendong 1 and Well Shatan 1 have been completed. Well Fangtan 1 has been deployed. Among the two risk wells that have been drilled, Well Shatan 1 has seen a major breakthrough. A high-yield industrial oil flow of 30 m³ per day was tested in the interval of 5 344-5 375 m of the Permian Wuerhe Formation, unveiling the prelude to the exploration of large-scale lithologic-stratigraphic reservoirs in the slope area of the Shawan Sag, and realizing astrategic breakthrough in a new sag. The concept of “hydrocarbon migrationward surface” has effectively guided risk exploration of large-area lithologic reservoirs in the slope areas of the petroliferous sags, leading to major breakthroughs in the Mahu and Shawan slope areas. It will also play an important role in the exploration of hydrocarbon-rich sags such as Fukang and Dongdaohaizi in future.

Based on the re-examination of natural gas formation conditions and resource potential, the PetroChina Xinjiang Oilfield Company deployed Well Fu 26 and Well Quantan 1 around the main gas source kitchen of the Carboniferous in the eastern slope of the Fukang Sag in 2017. Well Fu 26 tested an industrial oil flow from the interval of 3440-3450 m in the Carboniferous. After fracturing, it had a daily oil production of 14.78 m³ and daily gas production of 14.379× 10⁴ m³ with a6 mm nozzle. Well Quantan 1 drilled in 2018 had good gas displays recently, confirming that the Fukang Sag and the peripheral slopes have huge exploration potential for natural gas. In January 2019, Well Gaotan 1 deployed near the Jurassic hydrocarbon-generation source rocks in the Sikeshu sag in the second-row tectonic belt of the southern margin saw a major breakthrough too. It tested a high-yield oil and gas flow of 1,213 m³ oil per day and 32.17×10⁴ m³ gas per day in the interval of 5768-5775 m of the Cretaceous Qingshuihe Formation (K₁q)^[27]. In particular, recently, around the Permian high-mature gas source kitchen, Well Qianshao 2 deployed in the northeastern ring of the Western Well Pen-1 Sag detected active gas displays when drilled to 3971-3989 m of the second member of the Sangonghe Formation in Jurassic (J_1s2¹). Comprehensive mud logging and well logging interpretation showed that the well had 4 gas layers of 14.3 m thick in total. On June 4, 2019, the well was tested at the interval of 3 972-

3 990 m, obtaining a high-yield oil and gas flow of 39.0 m³ oil per day and 20.359×10⁴m³ gas per day, an exciting result for us all.

In short, following the idea of searching around the main source kitchens, taking hydrocarbon migrationward surfaces as the main exploration target areas, and adhering to the principle of “exploring multiple target layers and evaluating all possible targets”, quite a few exploration wells had high-yield industrial oil flows recently, confirming that the Junggar Basin has a large potential for natural gas exploration, and the prelude to the large-scale discovery of natural gas is being opened.

In the northwestern margin of the Junggar Basin, there is a second-order fault step of “early occurrence and early demise” type, which controlled the depositional environments and facies belt distribution of the Permian and Triassic. It has the necessary conditions to form large lithologic-stratigraphic traps and represents a new area for hydrocarbon exploration. The concept of second-order fault step was proposed in 2011 and put into practice afterwards, which have driven the exploration of the Mahu slope area to shift from structural oil and gas reservoirs oriented to large-scale stratigraphic-lithologic reservoirs oriented.

The hydrocarbon accumulation conditions and nature of hydrocarbon migration and accumulation in the Junggar Basin determine the existence of six “hydrocarbon migrationward surfaces” in the periphery of hydrocarbon-rich sags. The six “hydrocarbon migrationward surfaces” are the west-northeastern slope of the Mahu sag, the western slope of the Shawan sag, the northern slope of the Sikeshu sag, the northeastern ring zone in the Shawan-Well Pen-1 west sag, the northern slope of the Dongdaohaizi-Wucaiwan sag and the north-northeastern ring zone of the Fukang Sag. They have long been on the way of hydrocarbon accumulation and migration, and are thus key oil and gas exploration targets in the basin. This concept was proposed in 2012, and has classified that the northern (west and east) slopes of the six hydrocarbon-rich sags are the target areas of highest probability in oil and gas accumulation and discovery. It has also pointed out directions for finding high-efficiency oil and gas reservoirs and has been proven by recent exploration.

There are three sets of gas source rocks (the Carboniferous, Permian and Jurassic) in the Junggar Basin. The Carboniferous and Jurassic coal source rocks, in particular, have great potential for gas generation, which makes the natural gas resources in the basin bound to be large in scale. Recently, several exploration wells had very encouraging displays of natural gas discoveries. We believe that more and more gas discoveries will be made in areas in the vicinity of main source kitchens. This will change the status quo of much oil but little gas discovered in the Junggar Basin, and a medium-sized gas zone can be expected soon.