The southern margin of the Junggar Basin (hereinafter shortened as the southern margin) is located in the piedmont of the North Tianshan Mountains. It is a rejuvenated large- scale foreland basin formed between the Neogene Period and Quaternary Period^[1,2,3] (Fig. 1). With a long oil and gas exploration history, the southern margin is one of the earliest regions for oil and gas exploration in China. In the 1930s, the Dushanzi oilfield was discovered, representing the start of the modern petroleum industry in the Junggar Basin^[4]. In the southern margin, the following three sets of regionally distributed mudstone (gypsiferous mudstone) cap rocks are developed: the Neogene Taxihe Formation, Paleogene Anjihaihe Formation, and Cretaceous Tugulu Group, which form the upper, middle, and lower assemblages accordingly. The upper assemblage contains reservoirs in the Neogene Dushanzi Formation and Taxihe Formation and gray and grayish-green mudstone cap rocks in the Neogene Taxihe Formation. The middle assemblage contains reservoirs in the Anjihaihe Formation and Ziniquanzi Formation and regional cap rocks in the Anjihaihe Formation. The lower assemblage contains reservoirs in the Badaowan Formation, Sangonghe Formation, Xishanyao Formation, and Toutunhe Formation in the Jurassic and the Qingshuihe Formation in the Cretaceous as well as cap rocks in the Cretaceous Tugulu Group. For a long time, the southern margin has been developed mainly in the middle and upper assemblages and the following have been discovered: 3 oilfields (Dushanzi, Qigu, and Kayindike), 2 gas fields (Hutubi and Mahe), and some oil and gas structures, such as Tugulu, Khorgos, and Anjihai. Cumulatively, 27.195 million tons of oil and 32.96 billion cubic meters of gas have been proved in place. In general, the middle and upper assemblages have favorable conditions for forming oil and gas reservoirs. However, their structures are cracked, reservoirs are transversally variable, and the filling degree of traps is low, which make medium- and small-sized oil and gas fields. Therefore, the lower assemblage has become the major exploration direction for discovering large oil and gas fields in the southern margin. Since 2008, PetroChina has been ceaselessly exploring the favorable traps in the lower assemblage and has drilled the following wells: Xihu 1, Dushan 1, and Dafeng 1, where good oil and gas shows have been observed. Though the traps or engineering activities have failed, the drilling operations show that the lower assemblage has favorable conditions for forming large oil and gas fields. Finer geological studies have been carried out to solve challenging technical problems and evaluate exploration objectives. The high-risk exploratory well, Gaotan 1, was deployed in 2018 and tested on January 1, 2019. During the testing, 12.13 million cubic meters of oil and 321.7 thousand cubic meters of gas were produced daily in the Cretaceous Qingshuihe Formation, which indicates the initial breakthrough of oil and gas exploration in the lower assemblage of the southern margin. This breakthrough will inspire the analysis on the oil and gas forming conditions in the lower assemblage and guide the faster exploration of the lower assemblage in the southern margin which is still less developed.

Over the past century, rich oil and gas seeps have escaped to the surface in the southern margin of the Junggar Basin in the north of the Tianshan Mountains. Rows of large tectonic zones and multiple sets of developed source rocks have attracted great concerns and become promising. Several generations have been dreaming of discovering large oil and gas fields in the southern margin. In summary, the exploration in the southern margin includes the following two phases:

Exploration of the middle and upper assemblages before 2008: This phase includes several periods of drilling activities with different targets, such as shallow wells from 1936 to 1960, Neogene zones from 1979 to 1994, and large structures in middle assemblage from 1996 to 2008. The exploration in each period has delivered increasing geological understandings and technical advancements. For example, the understanding obtained at early stages that shallow structures map with deep structures drives us to start studying the relatively complete structures below the detachment layer. However, due to the limited seismic technologies, the high-spot deviation positions could not be effectively determined. In addition, due to the limited drilling technologies, the exploration of complex, high-pressure, and high-dip structures was carried out only in the foreland hanging wall, and small oil fields, such as the Dushanzi Oilfield and Qigu Oilfield, were discovered. Another understanding that the Ziniquanzi Formation in the middle assemblage in the Khorgos-Manasi-Tugulu anticlines has relatively-complete structures and moderate depth helps make this formation the major target zone for exploration. Improving the mountainous 2D seismic prospecting technologies can effectively facilitate the determination of the targets and high-spot positions of the middle assemblage. The capability of drilling the foreland complex structures has brought the breakthrough in the drilling of abnormally-high- pressure zones and high-dip zones. These have enabled the exploration in the middle assemblage to deliver breakthrough results: two medium-sized gas fields (Hutubi and Manasi) have been discovered^[5]. The two gas fields have high formation pressure and high single-well production which make them high-efficiency gas fields. According to the exploration practices in this phase, the reservoirs in the middle assemblage are relatively thin and have a large area of structural trap. However, pools are formed at high spots and cannot form large gas fields. During the exploration in this phase, the lower assemblage was also investigated. In 2003, Well Gaoquan 1 was drilled in the Gaoquan anticline in the Sikeshu sag but failed to drill into the target zone in the Jurassic after three attempts of sidetracks due to limited drilling technologies.

Exploration of the lower assemblage since 2008: This phase aims at discovering large oil and gas fields in the lower assemblage through dedicated geological studies, technical challenge tackling, and drilling operations as follows: (1) Studied the key reservoir forming factors of the lower assemblage, identified the conditions of hydrocarbon source rocks, reservoirs, and traps, and determined a favorable trap area up to 2 140 km². (2) Intensified the tackling of seismic issues and started the processing of 2D seismic wide lines, high-density 3D and complex structure pre-stack depth migration (PSDM), which delivered good results and boosted the fulfillment of the goal. (3) Carried out experiments for tackling technical challenges in drilling complex structures, multiple sets of pressure systems, and super-thick pelinites, which improved the capabilities of drilling deep wells and complex wells. (4) Deployed exploratory wells in the lower assemblage, including Xihu 1, Dushan 1, and Dafeng 1. Due to trap causes or engineering causes, no breakthrough was obtained, but it was found in the drilling that large-scale reservoirs were developed in the lower assemblage and good oil and gas shows were observed, which further facilitated the strong faith of exploration. (5) Deployed Well Gaotan 1 based on reasonable decisions and deeper geological understanding and obtained significant breakthroughs.

After Xihu 1, Dushan 1, and Dafeng 1 were drilled, determining the key points for breakthrough became the major job in the lower assemblage of the southern margin. Based on the continuous study from 2016 to 2018, the Sikeshu sag in the west section of the southern margin was selected as the preferred region for breakthrough in the lower assemblage with the Gaoquan anticline selected as the target zone, and the location of Well Gaotan 1 was determined based on the following results:

(1) The Sikeshu sag has potentials of producing hydrocarbons. Major source rocks are coal deposits in the Jurassic Badaowan Formation, Sangonghe Formation, and Xishanyao Formation which have great potentials of producing hydrocarbons with an effective source rock area of about 3 500 km². The Sikeshu sag is one of the two major hydrocarbon-producing sags. It is developed with Indochina-Yanshanian paleo-structures and fossil fractures, controlling the distribution of the Gaoquan and Aika structural zones from northwest to southeast. In later stages, the sag became stable. It had weak Himalayan tectonic movements. In the testing of Well Xihu 1, oil and gas show flows were achieved, which means that the sag is capable of forming reservoirs. The Sikeshu sag is successively developed with the Yanshanian paleo-structures and the lower assemblage has a moderate depth and is shallower than the middle section. The top of the Jurassic is between 4000 m and 6 000 m, where drilling is not that challengeable.

(2) The Cretaceous Qingshuihe Formation in the Sikeshu sag is developed with a southern provenance system and has favorable reserving conditions. Outcrops, drilling data, and seismic data were used to study the sedimentary system and predict the reservoir. For the first time, it was confirmed that the southern provenance system developed in the Cretaceous Qingshuihe Formation is braided-river delta sediment with frontier facies sand bodies distributed in large scales and controlled by the paleogeomorphy (two sags and one uplift). In addition, the sag developed the Jurassic Toutunhe reservoirs which are sediment developed in the shrinking period of lake basin. The reservoirs have large thickness and are another important target zone for exploration.

(3) The Gaoquan anticline has the paleo-structure background and is successively developed. It has the uplift-in-sag characteristics which indicate a trap. The Gaoquan structure is located in the south of the sag and developed with two structural zones, one deep and the other shallow, controlled by the detachment fault. The deep lower assemblage is successively developed with the Yanshanian transpressional thrust anticline and stable later-stage structures. The velocity modeling method through integration, layering, and 2D-3D fitting of well and seismic data is used together with the variable velocity mapping and forward modeling methods based on structure superposition. This eliminates the impact of the transversal changes of the overlying super-thick high-velocity conglomerates and gypsum rocks. It is confirmed that the structural area of the Jurassic top in the Gaoquan anticline is 71.4 km² and the trap morphology and high spots are reliable (Fig. 2).

(4) According to a comprehensive evaluation, the Gaoquan anticline is adjacent to the hydrocarbon-producing center and is a favorable area for hydrocarbon migration and accumulation. It is developed with Jurassic-Cretaceous reservoirs and has a large area of structural traps. Therefore, it is the first- choice key point for exploring the lower assemblage. Based on fine calibrations, Well Gaotan 1 is spotted and drilled as a high-risk exploratory well.

In Well Gaotan 1, a total of 103.4 m thick oil layers in the Cretaceous Qingshuihe Formation and Jurassic Toutunhe Formation are interpreted. The 103.4 m includes 11.5 m in the Qingshuihe Formation and the porosity is 18% according to the interpretation result. On January 6, 2019, Well Gaotan 1 flew through a 13 mm choke from the Qingshuihe Formation, and 12.13 million cubic meters of oil and 321.7 thousand cubic meters of gas were produced daily, which indicated high pilot production and sufficient energy. As of February 19, 2019, Well Gaotan 1 has delivered a daily oil production of 194 m³, a daily gas production of 70 000 m³, and a cumulative oil production of over 10 000 m³ through a 4 mm choke with the tubing pressure at 88.25 MPa. The crude oil recovered from Well Gaotan 1 has a density of 0.824 4 g/cm³, a viscosity of 4.2 mPa•s at 20 °C, a water saturation of 0.373%, a freezing point of -4.7 °C, a paraffin content of 7.42%, and a wax precipitation point of 19.2 °C. The relative density of the gas recovered is 0.736. Table 1 describes the components of gas sampled in the interval between 5 768 m and 5 775 m.

Well Gaotan 1 is the first high-yield well explored in the lower assemblage of the southern margin and the exploratory well with the highest production in continental clastic rocks in China. It is a significant milestone in the exploration of the southern margin.

Well Gaotan 1 has proved the reality of the major source kitchen of the Jurassic and facilitated the faith of exploration in the lower assemblage of the southern margin. According to a comparison between hydrocarbon sources, the saturated hydrocarbon gas chromatography does not show any β-dancanes and light hydrocarbons but humic kerogens. Oil and gas are sourced from coal-derived source rocks in the Jurassic. The high yield and stable pilot production of Well Gaotan 1 indicate that the Jurassic source kitchen has sufficient hydrocarbon sources and strong hydrocarbon supplying capability, providing a material basis for forming a large oil and gas field. The Jurassic source kitchen is rich of oil with richer gas and is highly thermally evolved in the middle section of the southern margin. The middle section is developed with gas while the west section is developed with oil.

It is discovered that the lower assemblage has favorable geological conditions for forming large-scale favorable reservoirs. According to the drilling results in Well Gaotan 1, major rocks in the Cretaceous Qingshuihe Formation are fine sandstones at a depth of 5 770 m and have a porosity of up to 18% according to mud log interpretations. Such porosity at this depth indicates that favorable reservoirs may also be developed in the lower assemblage. In general, the Qingshuihe Formation in the southern margin has good physical properties, including a reservoir thickness between 60 m and 100 m and a porosity between 15% and 20%, so it is an important target zone. Multiple sets of thick reservoirs are developed in the Jurassic, such as the Khalza Formation and Toutunhe Formation with a porosity between 6% and 10% or even between 13% and 14%. They have great potentials for exploration.

Multiple sets of overpressured thick mudstones are developed in the southern margin and have good sealing conditions. There are three sets of regional mudstone cap rocks in total. The Neogene Taxihe Formation is developed with gypsiferous mudstones while the Paleogene Anjihaihe Formation and the Cretaceous Tugulu Group are developed with overpressured mudstones with a pressure coefficient between 1.5 and 2.2 gradually increasing from shallow zones to deep zones. According to the drilling results of Well Gaotan 1, the Cretaceous Tugulu Group is a set of overpressured superthick mudstone with a thickness between 500 m and 2000 m and a pressure coefficient of 2.2, which indicates good sealing capability.

In the southern margin, multiple sets of hydrocarbon source rocks are developed, including the Paleogene, Cretaceous, Jurassic, Triassic, and Permian^[6,7]. Among them, the Jurassic and Permian are the major source rocks. They provide a resource basis for forming large-scale oil and gas accumulations in the southern margin.

The Jurassic hydrocarbon source rock is considered as a major source rock in the southern margin and is mainly located in the middle and lower Jurassic where three groups (two categories) of hydrocarbon source rocks are developed. These source rocks are mainly distributed in the Badaowan Formation, Sangonghe Formation, and Xishanyao Formation and are dark mudstones, carbonaceous mudstones, and coals. Major categories are argillaceous hydrocarbon source rocks and coals. The center for generating hydrocarbons is located in the Fukang sag, Shawan sag, foreland thrust zones, and Sikeshu sag^[8,9,10]. The source rocks are 600-800 m thick. The dark mudstone in the Badaowan Formation has the largest thickness and the widest distribution, and the area of hydrocarbon source rocks thicker than 100 m is about 46 000 km² (Fig. 3). According to the thermal evolution, the Jurassic hydrocarbon source rocks reach their peak time of producing oil in the end of the Cretaceous and reach their peak time of producing gas in the end of the Neogene. There are about 14 000 km² of dark mudstones with a thermal maturity (R_o) greater than 1.3% in the Badaowan Formation. In general, the Jurassic hydrocarbon source rock now mainly produces gas. Its west section has a low maturity and therefore mainly produces oil. The crude oil of Well Gaotan 1 has a Pr/Ph value of 3.16, a Pr/nC17 value of 0.22, a Ph/nC18 value of 0.07, and a carbon isotope composition value of -26.83‰. According to the geochemical characteristics of the crude oil, the kerogen is humic in an oxidation environment and has the typical characteristics of Jurassic hydrocarbon source rocks. According to the gas component analysis, the content of methane is between 72.39% and 75.14% with a dry coefficient between 0.75 and 0.78. The carbon isotope composition value is -40.49‰ for CH₄, -29.14‰ for C₂H₆ and -26.90‰ for C₃H₈. These isotope compositions are light hydrocarbons, which indicate that the gas is formed in the matured phase of source rocks.

The Permian source rock is developed in the Permian series and mainly distributed in the central east of the southern margin from Qigu to the east of Urumchi, even to the Fukang fault, and favorable oil shale outcrops are observed in the foreland of the Bogda Mountain. It has not been confirmed about whether the Permian source rock is also distributed in the west of the southern margin. The thickness center of the source rock is mainly located in the Fukang-Urumchi region with a general thickness between 50 m and 250 m and a total organic carbon (TOC) between 1.08% and 26.66% (mostly greater than 2.00%, average 7.6%). The Permian source rock enters the oil generating phase in the early Jurassic, reaches the peak time for generating oil in the middle Jurassic, and starts generating a large amount of gas in the Early Cretaceous. However, how much the Permian source rock contributes to the formation of reservoir needs to be further studied.

Intensive evaluations are required to study the oil and gas resource potentials in the southern margin. For now, the volumes of generated hydrocarbons and drained hydrocarbons in the Jurassic source rock have been evaluated and the following results have been obtained: A total of 397.3 billion tons of hydrocarbons have been generated in the Jurassic with 140.3 billion tons of hydrocarbons (including 38.9 billion tons of oil and 127 trillion cubic meters of gas) drained. In addition to the contribution of potential source rocks in the Triassic and Permian, the resource potentials and total volume of hydrocarbons generated in the southern margin will further increase.

According to the outcrop profile and drilling data, three sets of large-scale effective reservoirs (the Cretaceous Qingshuihe Formation, Jurassic Toutunhe Formation, and Khalza Formation) are developed in the lower assemblage of the southern margin^[11,12]. Major reservoirs in the Qingshuihe Formation have medium-to-low porosity and medium-to-high permeability while those in the Toutunhe Formation have low porosities and low permeability and those in the Khalza Formation have medium-to-low porosity and medium-to-low permeability (Table 2).

Reservoirs in the Cretaceous Qingshuihe Formation are mainly developed in section 1 and the thickness ranges from 20 to 100 m. Major rocks are braided-river delta and sector delta frontier sand bodies with about 15 000 km² of favorable facies. The southern and northern provenance systems are developed in the Qingshuihe Formation, controlling the distribution of sand bodies (Fig. 4). The middle section of the southern margin is mainly the southern provenance system while the east and west sections are affected by both the southern and northern provenance systems. At the intersection of the southern and northern sedimentary systems, the sand body thickness is larger, for example, the sandstone in the Qingshuihe Formation in Well Dong is over 100 m thick. Reservoirs in the Qingshuihe Formation have good physical properties with the porosity ranging from 9.0% to 18.6% (average 15%-18%) and the permeability ranging from 0.097 75 μm² to 0.186 μm². According to the log interpretation, the porosity ranges from 13.4% to 18.4% in the 5 767.5-5 774.7 m interval of Well Gaotan 1 in the Qingshuihe Formation and is 16.2% in the 5 792-5 812 m interval of Well Fangcao 1 in the Qingshuihe Formation. Major pores are primary residual inter-granular pores with good connectivity. In addition, reticular fractures are observed in the samples of the Jurassic-Cretaceous cuttings taken from Well Gaotan 1. It is estimated that these fractures are formed due to the crush of high stress and can effectively improve the reservoir permeability.

Major reservoirs in the Jurassic Toutunhe Formation are braided-river delta frontier sand bodies developed with the southern and northern provenance systems with over 15 000 km² of frontier zones (Fig. 5). The middle section of the southern margin is mainly the southern provenance system with large-scale sand bodies about 60-384 m thick. The west section of the southern margin is mainly the northern provenance system with a small scale of southern provenance, and the sandy conglomerates are 100-236 m thick. The east section of the southern margin is mainly the southern provenance system, where rocks under the impact of the northeast provenance are small slope-type braided-river delta deposits and thin interbed of sandstones and mudstones in a large area of superimposed distribution. Major rocks in the Toutunhe Formation are sandy conglomerates, conglomerate-bearing inequigranular sandstones, siltstone, and fine sandstones among which fine sandstones have the best physical properties. The average porosity of downhole samples ranges from 7% to 12% or even from 13% to 14%. A large amount of superfine reticular fractures are observed in the cuttings from the Toutunhe Formation in Well Gaotan 1. These fractures can improve the physical properties of deep reservoirs.

Reservoirs in the Jurassic Khalza Formation are mainly distributed in the middle and east sections of the southern margin in a limited scope where the southern provenance system is the major system and large-scale alluvial fans and braided-river delta groups are developed. In general, rocks are a set of superthick massive sandy conglomerate and sandstone deposits. In the outcrop area, the sandy conglomerate thickness is above 150 m which reaches to the maximum, 860 m, in the Khalza region. According to the drilling results, the sandstone is 210-450 m thick and covers an area of 10 000 km² (Fig. 6). Reservoirs in the Khalza Formation have medium-to-low porosity and medium-to-low permeability with greatly variable physical properties and are favorable in some areas. According to the core analysis, Well Changshan 1 has a maximum porosity of 23.4% with a permeability of 0.022 μm² and Well Dafeng 1 has a porosity of 19.1%.

The Cretaceous Tugulu Group is the most regional cap rock of the lower assemblage in the southern margin. From bottom to top, the Tugulu Group is divided into the Qingshuihe Formation, Hutubi Formation, Shengjinkou Formation, and Lianmuqin Formation in 500-2 000 m thick mudstones, developed under overpressure with good sealing capabilities. In the Qingshuihe Formation, major rocks are grayish-green and brownish-red mudstones sandwiched with thin argillaceous siltstones and a set of thick basal conglomerates are developed on the bottom with a mudstone-formation percentage of 50%- 100%. According to the drilling results, the cumulative mudstone thickness is 188 m and the maximum single-layer thickness is 78 m. Major rocks in the Hutubi Formation are grayish-green and brownish-red mudstones with a mudstone-formation percentage of 80%-95% and the mudstone thickness is 300-700 m with a maximum single-layer thickness of 138 m. Major rocks in the Shengjinkou Formation are thin interbeds of mudstones, silty mudstones, and argillaceous siltstones with a mudstone-formation percentage of 30%-100% and a mudstone thickness of 57 m. Major rocks in the Lianmuqin Formation are thin interbeds of mudstones, silty mudstones, and argillaceous siltstones with a mudstone-formation percentage of 40%-100% and the mudstone thickness is 157 m with a maximum single-layer thickness of 124 m^[12,13]. According to the analysis results of previous experiments^[14,15], the diagenesis, physical properties, temperature, and pressure of both argillaceous rocks and gypsum-salt cap rocks change with the depth and these rocks experience the brittle deformation, brittle-ductile deformation, and ductile deformation processes. When the confining pressure of a mudstone reaches 70 MPa, brittle deformation turns to be semi-plastic deformation and cap rocks are not subject to cracking, which helps effectively seal and cap the oil and gas. Most mudstones in the Cretaceous Tugulu Group are deeper than 4000 m where the formation pressure is higher than 70 MPa, which makes them effective cap rocks. In addition, thick mudstones in the Cretaceous Tugulu Group have abnormally high pressure with a coefficient above 1.8 which reaches 2.2 in Well Gaotan 1. Overpressure improves the sealing capability^[16,17,18] which, however, makes it challenging to drill the lower assemblage of the southern margin (Fig. 7).

In the southern margin, the foreland thrust is about 400 km long from the east to west and about 40 km wide from the south to north, which makes the southern margin a region with the most developed large anticlines in the Junggar Basin. Due to intensive compression and thrust, rows and belts of large structural traps are formed. In addition, truncation and onlap occur above and below the unconformable surface between the Jurassic and Cretaceous in the northern clival region which is a potential region developed with large stratigraphic- lithologic traps. At present, 40 structural targets have been identified in the piedmont thrust zone. 21 of the 40 have been discovered with a trap area of 2 486 km², an estimated trapped oil reserve of 1.577 billion tons, and an estimated gas reserve of 242 million m³. Four traps have been drilled where large oil and gas fields may be discovered (Tables 3 and 4).

The thrust zone in the southern margin can be divided into the following three types based on the structural evolution, structural deformation, and structural composite superimposition: (1) The Gaoquan structure with the occurrence of paleo- uplifts in the early period (Yanshanian) and successive superimposed structures by compression in the late period (Himalayan): Similar structures include the Dushanzi and Dunan anticlines which develop successively in the early and late period and have favorable conditions for charging oil and gas as well as advantages from the time perspective. (2) The Huo-Ma-Tu structure that is formed and finalized in the late period, mainly in the late Himalayan period: Similar structures include the Huo-Ma-Tu structural zone in the second row, Anjihai structural zone and Hutubi structural zone in the third row, and Dongwan structural zone between the first and second rows. These structures are usually in large scale and intact in the center of the highly-matured Jurassic source kitchen (Fig. 8). (3) The Qigu structure at the basin-range joint where the earliest thrust occurs: This structure is also known as the structural zone in the first row, including the Khalza structure in the east and the Tuositai structural groups in the west. Due to poor seismic imaging quality, only low- level structural characterization and observation have been carried out for wedge structures and delta zones in the foot wall of the master thrust fault, so there is still a large probability of discovering new traps.

From the static perspective, the large-scale reservoir in the large structural target of the lower assemblage is close to the Jurassic source rock. The 1 000-2 000 m thick Lower Cretaceous overpressured mudstone is overlaid, which enables good mapping of source rocks, reservoirs, and cap rocks vertically^[19]. Large-scale structural traps in the lower assemblage are located in the highly matured Jurassic source kitchen or adjacent hydrocarbon-producing center, and the traps are well matched with the source rocks from the space perspective. Thrust faults form networks for connecting oil and gas between the source rock and reservoir of the lower assemblage. Therefore, the static reservoir forming elements are well configured in the lower assemblage of the southern margin.

From the dynamic perspective, the forming time of the structural zone in each row of the southern margin varies, but the overall structure forming time is roughly 24.0-5.5 Ma before present. In the late period, the structure is continuously developed and then finalized^[3,20]. The thrust zone has experienced three evolution phases: transformation from a deep fault to a fold in the early period, reverse fault development in the middle sections, and shallow fault thrust reconstruction in the late period, and the structural deformation occurred early in the lower assemblage. Therefore, the structural deformation and thrust in the late period strongly advance and damage the middle and upper assemblages but slightly advance and damage the lower assemblage. According to the thermal evolution simulation of the source rock, the Jurassic source rock entered into the phase of producing and draining a large volume of hydrocarbons in 12 Ma before present and the hydrocarbon conversion rate is 90%. In general, the structure forming period of the lower assemblage is well matched with the peak time for producing and draining hydrocarbons of the major source rock, which is favorable for forming large oil and gas fields.

According to the preceding analysis, the lower assemblage of the southern margin is more likely to be a source rock with good conditions for forming reservoir-cap rock assemblages and it is more structurally intact with favorable geological conditions for forming large oil and gas fields. Based on the geological conditions, target preparation levels, and exploration reality, the following three favorable exploration directions are presented for the lower assemblage of the southern margin:

The Sikeshu sag in the west section of the southern margin: This exploration direction belongs to the practical field for reserve improvement. It has been proved by the drilling results of Well Gaotan 1 that the lower assemblage has the conditions of rich accumulation and high production and belongs to the recent practical exploration field. So far, six traps have been discovered in the Gaoquan structure. In addition, there may be a batch of favorable traps in the Xihu and Dushanzi structures and the piedmont overlap zones, which has not been confirmed yet but is still worthy of faster explorations. Major challenges for exploration lie in that most seismic data of this region is based on two-dimensional seismic surveys, requiring better 3D seismic solutions to accurately determine structural morphology and high spots for the identification of faults. The next-step key exploration jobs include the following: (1) Evaluating the Cretaceous reserve of the Gaotan 1 block in a faster manner to quickly set up the productivity. (2) Determining the hydrocarbon bearing potential of the Jurassic Toutunhe reservoir and carrying out preliminary exploration in new formations in the Jurassic Sangonghe Formation and Badaowan Formation, providing significant basis for deploying the overall exploration in the southern margin.

Large structures in the middle and east sections: This exploration direction belongs to a key breakthrough field. These structures are located in the main body of the piedmont depression in the major hydrocarbon producing center of the Jurassic source rock and cover an area of about 20 000 km², which make them important fields for strategic breakthroughs. The Huo-Ma-Tu structural zone in the middle section is developed with en-echelon large structures in rows. So far, 8 anticline traps have been discovered and the trap area is 918.5 km². Piedmont buried structures are also developed, bringing more drillable targets. Well Dafeng 1 is the only well that has been drilled so far. Though it has been abandoned from the engineering perspective, good oil and gas shows observed in both the Cretaceous and Jurassic have proved its reservoir forming conditions. The Fukang fault zone in the east section is the Bogda piedmont zone and has large-scale buried structures in the foot wall, which has been proved by oil and gas shows observed in Well Jiuyun 1. Most seismic data is 2D seismic data and partially regular 3D seismic data, and the east section is deeper than the west section, making the drilling more challenging and requiring high-density wide-azimuth 3D seismic data and PSDM solutions. In addition, multi-information structure modeling is required to effectively guide the interpretation of structures, and technical solutions are required to drilling complex ultra-deep wells. The next- step key exploration jobs include the following: (1) Selecting favorable traps for risk exploration to achieve high production. (2) Intensifying preliminary explorations based on strategic breakthroughs, refining the strategies, and determining the reserves of the major structures in the middle and east sections.

Stratigraphic-lithologic reservoirs in the clival formations in the north: This exploration direction belongs to the potential replacement field. The clival region in the north with an area of 23 400 km² is a large clival zone formed by the northward uplift of the piedmont chasm. It is adjacent to the highly matured Jurassic source kitchen and functions as a favorable zone for migrating oil and gas northwards. Large stratigraphic-lithologic reservoirs may also be developed in the clival region and major target zones are the Cretaceous, Paleogene, and Neogene, which makes the clival region in the north a replacement of the southern margin for exploration. The exploration challenge lies in that the stratigraphic-lithologic targets are highly concealed and make characterization and prediction more challenging. The next-step key exploration jobs include the following: (1) Deploying 2D seismic grid lines, tackling technical problems, and intensifying geological evaluations to determine favorable target zones. (2) Evaluating the selected favorable targets and drilling risk exploration wells or preliminary exploration wells.

The 1 000 m³ daily oil production in the Cretaceous in Well Gaotan 1 is a significant milestone and has proved the major Jurassic source kitchen. It has revealed that large-scale favorable reservoirs are developed in the deep lower assemblage and the Cretaceous overpressured mudstone has good sealing capabilities.

The lower assemblage of the southern margin has favorable reservoir forming conditions for forming large oil fields: (1) Multiple sets of hydrocarbon source rocks are developed. The Jurassic and Permian are the major source rocks with a large- scale source kitchen. (2) Multiple sets of large-scale effective reservoirs are developed, including the Cretaceous Qingshuihe Formation and the Jurassic Toutunhe Formation and Khalza Formation. (3) The Cretaceous Tugulu Group regional superthick mudstone cap rock is developed in overpressure and has good sealing capabilities. (4) Rows and belts of large anticline structures. The clival region in the north has conditions for forming large-scale stratigraphic-lithologic traps, providing good trap conditions for forming large oil and gas fields. (5) Static elements of source rocks, reservoirs, and cap rocks are well matched, and the dynamic evolution is favorable for accumulating oil and gas in large scale.

The lower assemblage of the southern margin has the following three favorable exploration directions: (1) The Sikeshu sag in the west section: This exploration direction belongs to the practical field for reserve improvement. Reserves for scaled benefits are expected to be quickly determined through intensified preliminary exploration. (2) Large structures in the middle and east sections: This exploration direction belongs to a key breakthrough field and is the key area for discovering large oil and gas fields. New breakthroughs are expected to be achieved through intensified risk explorations. (3) Stratigraphic-lithologic reservoirs in the clival formations in the north: This exploration direction belongs to the potential replacement field. Targets are highly concealed and therefore require intensified preparation and exploration for locating backup resources.

J₁b—Badaowan Formation;

J₁s—Sangonghe Formation;

J₂x—Xishanyao Formation;

J₂t—Toutunhe Formation;

K—Cretaceous;

E_1-2z—Ziniquanzi Formation;

E_2-3a—Anjihaihe Formation;

N₂s—Shawan Formation;

N₁t—Taxihe Formation;

K₂d—Donggou Formation;

K₁tg—Tugulu Group;

N₂d—Dushanzi Formaiton;

J—Jurassic;

GR—natural gamma, API;

R_t—electrical resistivity, Ω·m;

SP—natural potential, mV;

Δt—acoustic interval transit time, μs/m.