The Qiulitage structural belt is located in the junction area of frontal uplift belt and fold-and-thrust belt of Kuqa foreland basin in Tarim Basin. It is 300 km long from east to west and 25 km wide from south to north with totally exploration area 5200 km². The middle and west section of the structural belt has a low degree of exploration, and it is an important oil and gas exploration substitute area in the Kuqa area. Since the Qiulitage belt is complex in surface and underground geological structure, the seismic data of the area is poor in quality, it is difficult to confirm the traps, and the natural gas exploration in the central and western part of the structural belt has had no breakthrough for a long time. Through continuous research, it is found that Zhongqiu-Dongqiu section belongs to the same structural belt as subsalt fold-and-thrust structure of Kelasu structural belt, and has a thrust imbricate structure similar to the Keshen structural belt in the footwall of the fault (Zhongqiu section) and good accumulation conditions. On this basis, a risk exploration well, Zhongqiu 1, was deployed, which tested a high yield commercial hydrocarbon flow in the Cretaceous Bashkirchik Formation, turning the Qiulitage structural belt from a strategic replacement area into a practical area of increasing reserves and production^[1]. Previous researches on oil and gas source and accumulation in Qiulitage structural belt mainly focused on the east and west section of Qiulitage structural belt (hereafter referred to as Dongqiu section and Xiqiu Section)^[2,3,4], and didn’t cover the middle section of Qiulitage structural belt (hereafter referred to as Zhongqiu section). In this study, based on comparison of geochemical characteristics of natural gas in Qiulitage structural belt and its adjacent areas, a systematic correlation of oil and gas and source rocks of Well Zhongqiu 1 in Zhongqiu

Section is carried out to find out the oil and gas sources of Zhongqiu-1 structure and analyze the oil and gas accumulation process in Zhongqiu 1 trap, in the hope to provide a theoretical basis for the next oil and gas exploration and deployment in the Qiulitage structural belt.

The Qiulitage structural belt is located in the junction area of frontal uplift belt and fold-and-thrust belt of Kuqa foreland basin in Tarim Basin. The structural belt is divided into four sections, namely Dongqiu, Zhongqiu, Xiqiu and Jiamu (Fig. 1a). In the Qiulitage structural belt, different sections in the east and west are different in structural characteristics. The Zhongqiu and Dongqiu sections are similar to Kelasu structural belt in structural patterns, with thrust imbricate structure developed. While the Xiqiu and Jiamu sections have low amplitude echelon faults (Fig. 1b)^[1]. Well Zhongqiu 1 is located in Zhongqiu Section. In 2018, the well obtained commercial hydrocarbon flow from sandstone of Cretaceous Bashkirchik Formation (K₁bs). It tested a daily production of natural gas of 33.4×10⁴ m³ and crude oil of 21.4 m³ with 5 mm nozzle. The reservoir is a bottom-water, blocky, high temperature and ultrahigh pressure anticline condensate gas reservoir, with a formation pressure of 120.72 MPa, pressure coefficient of 2.0 and formation temperature of 146.35 °C.

The Jurassic of Kuqa depression mainly develops swamp facies coal-bearing sediments, and the Triassic mainly develops lacustrine mudstone. The Triassic-Jurassic source rocks have high abundance of organic matter, high maturity, large thickness and good continuity, and thus can provide good source rock conditions for Qiulitage structure^[5,6]. The main reservoir of Qiulitage structure is the Cretaceous Bashiqiki Formation, which is braided river delta sedimentary facies. In Jiamu-Xiqiu section, the reservoir is of medium-porosity and medium-permeability, in Zhongqiu section, the reservoir is of low-porosity and medium-permeability, in Dongqiu section the reservoir is of ultra-low-porosity and low-permeability. The Qiulitage structural belt has well developed caprocks, and the Paleogene and Neogene gypsum mudstone formations are the main caprocks. The Paleogene gypsum mudstone caprock, with thickness center close to Well Qiutan 1 of Xiqiu section, has a maximum thickness up to 4000 m, and thins gradually westward to Zhongqiu section and pinched out near Well Xiqiu 2. The Neogene Jidike gypsum-salt rock, with thickness center close to the Well Dongqiu 5, has a maximum thickness up to 3000 m, and thins gradually eastward to Zhongqiu section and thinning out near Well Dongqiu 8. Zhongqiu section is located in the area where these two sets of gypsum salt rock overlap, with a combined thickness of 60-200 m.

The Zhongqiu section of Qiulitage structural belt is low in exploration degree, with only Well Zhongqiu 1 drilled so far. Natural gas and crude oil samples were collected and systematical geochemical analyses were carried out by the authors. In addition, carbon isotope composition of n-alkanes in Jurassic and Triassic source rocks in the east of Kuqa Depression were analyzed, and related data of natural gas in the adjacent oil and gas fields (e.g. Dina, Dibei, Dabei, Kela 2, Keshen, Yaha, Bozi) was collected and sorted out.

The natural gas found in Qiulitage structural belt is mainly distributed in Cretaceous and Paleogene. The natural gas samples have methane contents of 81.6%-92.3%, ethane contents of 3.8%-10.6%, drying coefficients (C₁/C_1—6) of 0.83- 0.95, and are all wet gas. The non-hydrocarbon gases, low in content, mainly include N₂ and a small amount of CO₂, with a total content of N₂ and CO₂ of less than 5%. The natural gas from Well Zhongqiu 1 has a methane content of 92.3% and ethane content of 4.58%, which is similar to that in Dongqiu section and higher than that in Xiqiu section. The natural gas from Well Zhongqiu 1 has a drying coefficient of 0.94, which is equivalent to that in Dibei gas field, lower than that in Kela, Keshen, Dabei gas fields, and significantly higher than that in Yaha, Bozi, Dina gas fields (Table 1). The difference of natural gas drying coefficient in Kuqa depression is related to thermal evolution extent of natural gas, the type of source rock parent material, and reformation of oil and gas reservoir etc^[7,8].

Carbon isotopic composition of methane from Well Zhongqiu 1 (δ¹³C₁) is -32.6‰, that of ethane (δ¹³C₂) is relatively high, reaching -22.5‰, that of propane (δ¹³C₃) is -20.7‰, and that of butane (δ¹³C₄) is -20.6‰. The carbon isotopic composition series is in positive sequence (δ¹³C₁<δ¹³C₂<δ¹³C₃< δ¹³C₄ ) (Table 1). According to the identification standards of natural gas genetic types proposed by Dai Jinxing et al. and Song Yan et al.^[9,10], the natural gas samples from Well Zhongqiu 1 and gas fields nearby should be classified as humic coal-derived gas (Fig. 2).

In Well Zhongqiu 1, Dina Dibei, Bozi gas fields, the natural gas carbon isotopic composition series are in positive sequence. In Yaha gas field the series are partially reversed, with δ¹³C₃>δ¹³C₄, and in Dabei, Kela 2, Keshen gas fields, carbon isotopic series are reversed to a larger extent, with δ¹³C₂> δ¹³C₃>δ¹³C₄ (Table 1). In general, the factors that may lead to carbon isotopic composition series reversal of natural gas include thermal evolution degree, source of parent material and mixing of gases etc^[11]. Compared with source of parent material and mixing of natural gases, thermal evolution degree is the key factor causing natural gas carbon isotopic composition series reversal in Kuqa depression. According to the empirical formula for calculating natural gas maturity by means of methane carbon isotopic composition proposed by Dai Jinxing et al.^[12], the maturities of natural gases in Bozi, Dina, Zhongqiu 1, Dibei, Yaha, Dabei, Kela 2, Keshen gas fields were calculated at 0.8%, 1.0%-1.1%, 1.3%, 1.1%-1.4%, 1.2%- 1.4%, 1.8%-6.1%, 2.8%-3.4%, and 2.7%-5.3% respectively. It is found that when R_o is greater than 1.4%, the carbon isotopic composition series shows slightly partial reversal, and when R_o is greater than 2.0%, the reversal will greatly increase in degree, becoming δ¹³C₂>δ¹³C₃>δ¹³C₄. The maturity of natural gas from Well Zhongqiu 1 is about 1.3%, and the carbon isotopic composition series are in positive sequence, indicating that the gas is moderate in thermal evolution degree and hasn’t reached the maturity degree of reversal.

Light hydrocarbons in natural gas from Well Zhongqiu 1 and from Dina, Yaha, Keshen and Kela 2 gas fields all contain higher contents of aromatics and cycloalkanes (Figs. 3a and 4), which indicates source characteristics of humic parent materials^[13,14,15]. The natural gas light hydrocarbons from Well Zhongqiu 1 have much lower content of aromatics than those from Dina, Dabei, Kela 2 gas fields, equivalent to that from Yaha gas field, and higher than that from Bozi gas field (Fig. 4). In general, the aromatics content in light hydrocarbon increases with the maturity of natural gas. In Ahe Formation gas reservoir of Dibe gas field, the C_6—7 light hydrocarbons contain higher content of N-alkanes and lower content of benzene and toluene, showing the source characteristics of sapropelic parent material (Fig. 3d), which is related to its main source of Triassic lacustrine mudstone^[16].

The crude oil from Well Zhongqiu 1 has a density of 0.806 7 g/cm³ (20 °C), wax content of 6%, and Kinematic viscosity (50 °C) of 1.128 mm²/s. The crude oil belongs to light oil, with a saturated hydrocarbon content of 73.83%, aromatics content of 17.87%, saturated hydrocarbons/aromatics ratio of 4.13, and lower contents of asphaltene and non-hydrocarbon of 1.49% and 0.11% respectively.

C_6—7 light hydrocarbon spectrogram (Fig. 3b) of crude oil from Well Zhongqiu 1 shows that benzene content is the highest, followed by the contents of toluene, methylcyclohexane, n-hexane, n-heptane and cyclohexane successively. It should be noted that, the abundance of toluene and n-octane etc in crude oil from Well Zhongqiu 1 is much higher than that in natural gas from this well (Fig. 3a), which indicates that light hydrocarbon components with higher carbon number are more likely to concentrate in crude oil. Generally speaking, the distribution characteristics of light hydrocarbon in crude oil from Well Zhongqiu 1 are similar to that in natural gas from this well, both of them contain higher contents of aromatics, which indicates that the light hydrocarbons of both crude oil and natural oil have the characteristics of humic parent material source.

In gas chromatography of saturated hydrocarbons of Well Zhongqiu 1, the distribution of n-alkanes has the main peak at C18, CPI (carbon preference index) and OEP (odd-even predominance of organic matter) of 1.09, 1.03 respectively, nC₂₁-/nC₂₂+ of 3.72, and low carbon n-alkanes predominance, indicating the oil is from aquatic organisms. The oil has a Pr/Ph of 1.1, Pr/nC₁₇ of 15, and Ph/nC₁₈ of 0.13, indicating that the organic matter is type II and deposited in slightly reducing environment. In crude oil of Well Zhongqiu 1, content of tricyclic terpene series compounds is relatively high, and content of pentacyclic terpane is significantly low, which is speculated to be related to high thermal evolution extent of the crude oil. The crude oil from Well Zhongqiu 1 has higher content of tricyclic terpanes and lower content of pentacyclic terpanes, which is inferred to be related to the higher thermal evolution degree of the oil. The tricyclic terpanes have the main peak at C₂₃, and are in generally normal distribution. Regular steranes of C₂₇, C₂₈, C₂₉ are in V-shape distribution. The ratio of C₂₇ sterane to C₂₉ sterane is 1.01, showing C₂₇ and C₂₉ steranes are similar in content (Fig. 5). The ratio of gammacerane to C₃₀ hopane is 0.18, which reflects the characteristics of low salinity lake aquatic organisms.

In crude oil of Well Zhongqiu 1, the content of phenanthrene is much higher than that of methylphenanthrene (Fig. 6), with a phenanthrene / methylphenanthrene ratio of 2.24, which is speculated to be related to the high thermal evolution stage. In high thermal evolution stage, due to the enhancement of thermal effect, methyl phenimethyl would have obvious demethylation, leading to a sharp increase of the relative abundance of phenanthrene. According to the methyl phenanthrene index (MPI₁) formula proposed by Radke et al.^[17] and the two-stage empirical estimation formula between MPI1 and vitrinite reflectance^[18], the maturity (R_c) of crude oil from Well Zhongqiu 1 was estimated. If R_o value is 0.65%-1.35%, the calculated R_c is 0.49% which is in immature stage, if R_o is 1.35%-2.00%, the calculated R_c is 2.22%, which is in over mature state. The two calculation results are quite different, which indicates that it is difficult to accurately determine the maturity of crude oil from Well Zhongqiu 1 by using methylphenanthrene index.

Through observing the distribution characteristics of four methylphenanthrene compounds in the aromatic hydrocarbons in crude oil from Well Zhongqiu 1, it is found that the relative contents of 9-methylphenanthrene and 1-methylphenanthrene are higher than those of 3-methylphenanthrene and 2-methylphenanthrene, and the content of 9-methylphenanthrene is abnormally high (Fig. 6). Comparison of distribution characteristics of methylphenanthrene in different sedimentary environments and source rocks show freshwater swampy-lacustrine crude oil is more abundant in 2-methylphenanthrene and 3-methylphenanthrene than brackish-salt lakes and marine crude oil^[19,20,21]. Therefore, the abnormally high content of 9-Methylphenanthrene in aromatic hydrocarbon of crude oil from Well Zhongqiu 1 is related to sedimentary environment and the source of parent material, indicating that the parent material came from aquatic organisms in brackish water -salt water sedimentary environment, which is consistent with the source characteristics indicated by saturated hydrocarbon biomarkers (Fig. 5).

The formation of adamantane compounds is generally not affected by the input of organic matter and the sedimentary environment of source rock. These compounds, which are stable in chemical nature, and not easily affected by thermal, biodegradation and chromatography during migration process, can be used as an effective indicators to distinguish pyrolysis products of high mature crude oil^{[22,23,24,25]}. In the crude oil of Well Zhongqiu 1, adamantane and diamantane series compounds have been detected, the adamantane maturity (MAI) is 62.6% and diamantane maturity (MDI) is 40.6%. The maturity of crude oil from Well Zhongqiu 1 was calculated at about 1.3% according to the relationship between adamantine index and maturity (Fig. 7), which is similar to the maturity of natural gas and is in high maturity stage.

4.1.1. Identification of crude oil source by biomarkers

The Jurassic in Kuqa depression consists of largely coal bearing sediments of swamp facies. Among the biomarkers of tricyclic terpanes, the main peak is C₁₉, tricyclic terpanes of C₂₀, C₂₁, C₂₃ are in descending order, steranes of C₂₇, C₂₈, C₂₉ are in inverted L-shaped distribution. Pr/Ph value is 1.48-4.83, which indicates oxidation environment. The gammacerane/C₃₀ hopane ratio ranges between 0.08 and 0.09, indicating freshwater sedimentary environment (Fig. 8a). Triassic is mainly lacustrine mudstone. Among the biomarkers, tricyclic terpanes are in normal distribution, steranes, C₂₇, C₂₈, C₂₉ are in V-shaped distribution, indicating a high proportion of aquatic organism input. The mudstone has a low content of diasterane and rich gammacerane, with a gammacerane/C₃₀ hopane ratio range of 0.16-0.24, indicating brackish water environment (Fig. 8b)^[16]. The Pr/Ph ratio ranges between 0.86 and 2.01, indicating slightly reducing environment.

As mentioned previously, in crude oil of Well Zhongqiu 1, the tricyclic terpanes have the main peak at C₂₃ and are in normal distribution roughly. Regular steranes of C₂₇, C₂₈ and C₂₉ are in V-shape distribution (Fig. 5). The gammacerane/C₃₀ hopane is 0.18, and Pr/Ph value is 1.10, which are consistent with the biomarker characteristics of the Triassic lacustrine mudstone. The crude oil of Well Zhongqiu 1 has high content of 9-Methylphenanthrene and 1-Methylphenanthrene, also suggesting the oil is from aquatic organisms in brackish water and salt water.

4.1.2. Identification of the source of crude oil with carbon isotopic composition of n-alkanes

The source of crude oil of Well Zhongqiu1 can be identified by comparing carbon isotopic composition of n-alkanes of crude oil from this well with those of Triassic-Jurassic source rocks in Kuqa depression. As shown in Fig. 9, carbon isotopic composition of n-alkanes in Jurassic source rock of Kuqa depression falls between -27.2‰--23.5‰, that in Triassic source rock is significantly lighter and ranges between -31.6‰ and -28.2‰, and that in crude oil of Well Zhongqiu 1 ranges between -31.6‰ and -29.8‰, which is similar to that of the Triassic source rock, indicating that the main source of crude oil in Well Zhongqiu 1 is Triassic lacustrine mudstone.

In conclusion, the biomarker characteristics and stable carbon isotopic composition characteristics of crude oil from Well Zhongqiu 1 match with those of Triassic lacustrine mudstone in Kuqa depression. It should be noted that the crude oil of Well Zhongqiu 1 has high aromatics content of C_6—7 light hydrocarbon, which indicates the contribution of coal measures. Therefore, it is speculated that the crude oil of Well Zhongqiu 1 mainly comes from Triassic lacustrine mudstone and has some light hydrocarbon from Jurassic coal measures mixed in.

The source of natural gas in Well Zhongqiu 1 can be identified by comparing characteristics of light hydrocarbons from rock pyrolysis and in natural gas^{[14, 16]}. In Kuqa depression, Jurassic mainly develops coal measures, while Triassic source rock is mostly lacustrine mudstone, and the light hydrocarbons in pyrolysis products of the source rocks are quite different (Fig. 10). When the Jurassic mudstone and coal samples are heated to 300 °C and 500 °C, the light hydrocarbons from their pyrolysis are characterized by high content of C_6—7 aromatics (more than 30%) and low content of C_6—7 branched alkanes (less than 10%). When the Triassic mudstone sample is heated to 300 °C and 500 °C, the light hydrocarbons of pyrolysis are characterized by relatively low content of C_6—7 aromatics (less than 50%) and relatively high content of C_6—7 alkane (more than 15%)^[16]. Therefore, gas-source correlation can be conducted.

In this study, the relative proportion of C_6—7 aromatic hydrocarbon and C_6—7 alkane in light hydrocarbon are used to identify the source of natural gas. Natural gas with C_6—7 aromatic hydrocarbon content of more than 30% and C_6—7 alkane content of less than 10% is identified as coming from Jurassic coal measures, and natural gas with C_6—7 aromatic hydrocarbon content of less than 50% and C_6—7 alkane content of more than 15% is identified as coming from Triassic lacustrine mudstone. The natural gas of Well Zhongqiu 1, with a C_6—7 aromatic hydrocarbon content of 36.1% and C_6—7 alkane content of more than 11.9%, falls in the transitional area of Jurassic coal measures and Triassic lacustrine mudstone (Fig. 10). Therefore, natural gas in Well Zhongqiu 1 is identified as a mixture of gases from both Jurassic and Triassic source rocks.

Putting data of light hydrocarbons of crude oil samples from Well Zhongqiu1 into the Fig. 10, it can be seen that the crude oil samples have much higher C_6—7 aromatic hydrocarbon contents than the natural gas samples, indicating certain contribution of Jurassic coal measures. It has been pointed out previously that the oil in Well Zhongqiu 1 mainly comes from Triassic lacustrine mudstone, but the light hydrocarbons of crude oil contain relatively high content of C_6—7 aromatic hydrocarbons which usually come from coal measures and obviously don’t come from Triassic lacustrine mudstone.

The high content of C_6—7 aromatic hydrocarbon in Well Zhongqiu 1 crude oil should mainly come from Jurassic coal measures. First, Jurassic coal measures in Kuqa depression are large in scale, with a thickness of 400-700 m. Secondly, according to the matching relationship between natural gas maturity and source rock maturity, natural gas in Well Zhongqiu 1 is in high maturity stage. Since the sedimentary period of Neogene Kuqa Formation, thermal evolution degree of Triassic source rock has been basically greater than 2.0%, in over maturity stage; while thermal evolution degree of Jurassic source rock has been basically greater than 1.5%, in high-over maturity stage^[2,5-6]. At present, the natural gas maturity in the gas reservoir is about 1.3%, which matches with Jurassic source rock. Since the depositional stage of Neogene Kangcun Formation, the Jurassic coal measures has generated a large amount of condensate gas and dry gas with high aromatic hydrocarbon content, which has massively charged into the oil reservoir supplied by Triassic source rock. The aromatic hydrocarbons in Jurassic natural gas dissolve in the Triassic crude oil, resulting in high content of C_6—7 aromatic hydrocarbon in the present crude oil. At same time, this also indirectly indicates that most of the natural gas in Well Zhongqiu 1 trap comes from Jurassic coal measures.

Natural gas of Well Zhongqiu 1 mainly comes from Jurassic coal measures of Kuqa depression, while the crude oil mainly originates from Triassic lacustrine mudstone. The reservoir is sandstone of Cretaceous Bashkirchik Formation, and the direct caprocks are mudstone and gypsum mudstone of Paleogene Kumglimu Group. The oil and gas accumulate features “early oil and late gas”.

Qiulitage structural belt is bordered by the south margin of Baicheng depression in the west and Yangxia depression in the east. Zhongqiu Section where Well Zhongqiu 1 is located is in between Baicheng sag and Yangxia sag which have widespread Jurassic and Triassic source rock. According to the well log and seismic data of Kuqa depression, in Zhongqiu section the Jurassic source rock is up to 200 m thick and the Triassic source rock is up to 100 m thick, featuring large thickness and stable distribution. Jurassic-Triassic source rocks have high abundance organic matter, mainly type III and some type II organic matters, maturity or high-maturity^[2,5-6], and total gas generation intensity of (50-100)×10⁸ m³/km², suggesting superior source rock conditions.

In Zhongqiu 1 trap, there are two sets of reservoirs vertically, sandstone at Paleogene Jidike Formation bottom and Cretaceous Bashkirchik Formation (Fig. 11). The reservoir at Jidike Formation bottom is shore-shallow lake facies, with a total thickness of 273 m. In this section, sandstone, gypsum and mudstone interbed, sandstone layers are 1-2 m thick each and 113 m thick in total, with a ratio of sand to formation of about 41%. The reservoir in Bashkirchik Formation is braided pro-delta sub-facies sandstone mainly, with a thickness of 248 m and net sand to formation ratio of up to 86%. The reservoir is stably distributed laterally, and has superimposed sand bodies vertically. The reservoir is mainly composed of feldspathic lithic sandstone and lithic sandstone, and contains mainly intergranular dissolved pores, secondarily intergranular pores, and a small amount of micropores and intragranular dissolved pores. The reservoir has a porosity range of 8%-18%, and an average porosity of 12.8%, a permeability range of mainly (1.4-3.8)×10^-3 μm², and an average permeability of 2.06 × 10^-3 μm², representing low porosity and medium permeability reservoir^[1]. By comparing the sandstone thickness and porosity and permeability conditions of these two sets of reservoirs, it is concluded that sandstone of the Cretaceous Bashkirchik Formation is the main reservoir.

There are two sets of regional caprocks in Zhongqiu 1 trap, Neogene Jidike Formation gypsum mudstone, and Paleogene mudstone and gypsum mudstone. The Paleogene gypsum mudstone is not only the regional caprock, but also the direct caprock. According to the analysis of logging and seismic data, the gypsum mudstone in the middle and upper part of the Neogene Jidike Formation has a trend of thinning from Zhongqiu section to Xiqiu section, with a thickness of 967 m in Well Zhongqiu 1. In contrast, the Paleogene gypsum mudstone thins from Xiqiu section to Zhongqiu section, with a thickness of 138m in Well Zhongqiu 1 (Fig. 11). As no core has been taken from Well Zhongqiu 1, the physical properties of the gypsum mudstone in the middle and upper part of the Neogene Jidique Formation refer to the data of Well Dongqiu 5 nearby. The gypsum mudstone in the Neogene Jidique Formation of Well Dongqiu 5 has an average porosity of 3.1% and average permeability of 0.07×10^-3 μm², showing good caprock quality. The Paleogene gypsum mudstone is located under the Neogene Jidike Formation gypsum mudstone, and must be more highly compacted. Therefore the Paleogene gypsum mudstone is the main caprock with stronger oil and gas sealing capacity.

According to the analysis of reservoir and caprock development characteristics in Well Zhongqiu 1, there are two sets of reservoir-caprock assemblages in this area. One is Neogene Jidike Formation gypsum mudstone and glutenite section at the bottom of Jidike Formation - Suweiyi Formation salt sandstone reservoir. The other is Paleogene Kumglimu Formation gypsum mudstone and Cretaceous Bashkiqike Formation reservoir. As Jidike-Suweiyi Formation reservoir has relatively low ratio of sand to formation, poor physical conditions, and it is difficult for oil and gas to migrate through the caprock along active faults to this set of reservoir, it is not a favorable reservoir-caprock assemblage. In comparison, the gypsum mudstone of Paleogene Kumgeliemu Formation and the reservoir in Cretaceous Bashkirchik Formation constitutes a high-quality reservoir-caprock assemblage. As it is difficult to break the plastic gypsum mudstone by active faults, the gypsum mudstone caprock can effectively inhibit the vertical dissipation of oil and gas in Bashkirchik Formation. In addition, the Paleogene gypsum salt layer can play a role of detachment layer, which can absorb the structural stress under the salt, and offset the influence of some structural deformation, so the imbricate faulted anticline traps in the Cretaceous Bashkirchik Formation can be well preserved, providing good trap conditions for oil and gas accumulation.

Based on the study of the Zhongqiu 1 structural evolution history, combined with the source of oil and gas in Zhongqiu 1 trap and the evolution characteristics of hydrocarbon generation of source rock^[2,5-6], the oil and gas accumulation process in Zhongqiu 1 trap has been analyzed.

(1) In the early depositional stage of Jidike Formation, Triassic and Jurassic source rocks were less than 0.7% and 0.5% in R_o value respectively and in immature to low mature stage, and generated a small amount of light oil of low maturity, but the Cretaceous trap of Zhongqiu section had not yet been formed at this point. In the middle to late depositional period of Jidike Formation, with the influence of continuous sedimentation and tectonic movement, the Jurassic source rock was still in a low mature stage, producing a small amount of light oil, while the Triassic source rock reached the R₀ values of 0.7%-1.3% and started to generate light oil and condensate oil massively^[26], and the oil generated migrated along active faults towards Cretaceous traps formed in the same period to accumulate.

(2) In the depositional period of Kangcun Formation, the Triassic source rock, reaching the R_o values of 1.3%-2.0%, started to generate condensate and dry gas, and the Jurassic source rock, reaching the R_o values of 0.7%-1.3%, started to generate a large amount of light oil and condensate. At the same time, the Cretaceous trap increased in amplitude because of the tectonic compression, and the oil and gas generated by the source rocks moved along the active faults to Cretaceous Zhongqiu 1 trap, the footwall trap of Qiulitage structural belt, and Kela-Keshen Cretaceous trap.

(3) During the deposition of Kuqa Formation, under the action of strong tectonic compressional movement, the Cretaceous traps increased in amplitude and almost finalized in shape. Meanwhile, both Triassic and Jurassic source rocks reached the R_o values of more than 1.3%, entering the stage of massive condensate gas and dry gas generation. Along the active faults, the generated natural gas migrated to and accumulated in Zhongqiu 1 trap, the footwall trap of Cretaceous, and Cretaceous Kela-Keshen trap, accompanied by large-scale gas invasion to early accumulated oil reservoirs. The above analysis shows that the formation period of Zhongqiu 1 trap is earlier than or in the same period of hydrocarbon generation and expulsion of the source rocks, and the active faults connected the source rocks and the traps, providing channels for hydrocarbon migration.

The oil and gas accumulation in the Zhongqiu 1 trap is characterized by "early oil and late gas". In the early stage of massive oil generation (middle-late depositional period of Jidike Formation, depositional period of Kangcun Formation) and the late stage of massive natural gas generation (depositional period of Kuqa Formation), the oil and gas generated by the source rocks migrated along the active faults to the faulted anticline traps under the gypsum salt caprock and accumulated. This is summarized as coupling reservoir control model of "source-fault-trap-cap" four elements, which made it possible for oil and gas to migrate and accumulate in Zhongqiu 1 trap. There are a series of structural traps in the footwall of Cretaceous faults in the Zhongqiu structural belt, which have similar reservoir accumulation elements as those of Zhongqiu 1 trap. Therefore, they will be the next favorable exploration targets in the Zhongqiu structural belt (Fig. 12).

The natural gas and crude oil of Well Zhongqiu 1 are both in high mature stage. The natural gas is coal-derived gas, mainly from Jurassic coal measures, and the crude oil is mainly from Triassic lacustrine mudstone.

The Zhongqiu 1 reservoir has the characteristics of "early oil and late gas". The crude oil charged mainly from depositional period of Jidike Formation to Kangcun Formation, and the Triassic lacustrine mudstone contributed most of oil. The natural gas charged largely in the depositional period of Kuqa Formation, the Jurassic coal measures made major contribution to the gas, and the coal-derived gas generated by the Jurassic source rock later caused gas invasion to the early oil reservoir. The formation period of Zhongqiu 1 trap is earlier than or in the same period of hydrocarbon generation and expulsion of the source rocks. The active faults connected the source rocks and the traps, providing hydrocarbon migration condition, and the "source, fault, trap, cap" matched well in space and time. Fault footwall traps in Zhongqiu structure have the similar reservoir accumulation conditions with Zhongqiu 1 trap, and are the next exploration targets.