The Hetao Basin is a Mesozoic and Cenozoic depression- graben basin on the Paleoproterozoic-Paleoarchean metamorphic crystalline basement of the Yimeng old craton^[1,2,3]. Long from east to west, narrow from the north to south, it appears as a narrow arc on plane distributed between the Yinshan Fold Belt and the Ordos Basin, with an area of ​​about 4×10⁴ km². Shallow in the north and deep in the south, it takes on an asymmetrical dustbin shape. In the basin, the sedimentary rocks are 3 000-6 000 m thick in general, and up to 14 000 m thick in the northern deep sag. According to uplift amplitude of basement, thickness of sedimentary rock, distribution of primary faults, the basin is divided into five first-order structural units from west to east: the Linhe Depression, Wulashan Uplift, Wuqian Depression, Baotou Uplift and Huhe Depression (Fig. 1). Among them, the Linhe Depression has the largest area of ​​2.43×10⁴ km², and is the major sedimentary depression and hydrocarbon-bearing area. There are three arc-shaped fault systems in this area. The Langshan Fault in the north is a basin-controlling fault, which has been controlling the development and evolution of the basin. In the history of sedimentary filling, the central Hangwu Fault belt is a sag-controlling zone between the depression and the slope. The southern Huanghe Arc Fault is a boundary fault between the Hetao Basin and the Ordos Basin. Taking the abovementioned fault systems as the boundary, the Linhe Depression is further subdivided into six secondary structures: the northern deep sag, southern fault step, eastern slope, Jixi uplift, Jilantai shallow sag, and Jidong Slope according to the geological features of different blocks, showing the structure and sedimentary characteristics of typical faulted basin. The depocenter and subsiding center are close to the Langshan Fault which controls the basin^[4], and the favorable hydrocarbon-generat-ing center in the northern deep sag appears as a narrow trough extending along the Langshan Fault, with a width of only 20 km and a length of 300 km, and an area of about 5 000 km². The basement of the basin is the metamorphic rock series in the Paleoproterozoic-Paleoarchean Wulashan Group. Upwardly, the sedimentary formations are the Lower Cretaceous Guyang Formation, Upper Cretaceous Bikeqi Formation, Paleogene Eocene Urad Formation, Oligocene Linhe Formation, Neogene Miocene Wuyuan Formation, Pliocene Wulate Formation, and Quaternary Hetao Formation. Among them, the Lower Cretaceous Guyang Formation and the Oligocene Linhe Formation, with lacustrine source rock, are the major hydrocarbon-bearing strata^[5,6]. The shallow lacustrine mudstone in the Wuyuan Formation among them is the regional caprock. There are multiple types of reservoir-cap assemblages, including source and reservoir in one, source below reservoir, and new source and old reservoir ones (Fig. 2).

The large-scale oil and gas exploration in the Hetao Basin has a history of 40 years since started in 1978 firstly. Successively, 28 exploration wells have been drilled, 870 km of continuous electromagnetic profile (CEMP), and 6 514 km of 2D seismic line have been surveyed. Preliminary exploration showed that the sedimentary sag in the Wuqian Depression was small, with an area of only 770 km², and the target formation there was shallow in burial depth, low in thermal evolution degree, and thus poor in exploration prospect. The sedimentary sag in the Huhe Depression has a large area of ​​2 700 km². Well Bitan 1 and Hucan 1 drilled in the area have confirmed that the Oligocene dark-colored mudstone is not developed, and the shallow Quaternary humus and lacustrine mudstone are developed, providing the basic conditions for the formation of biogas sources^[7,8]. In 2004-2006, exploration for shallow biogas was carried out in the area and six wells were drilled. The wells all had gas shows, and had a methane natural gas flow of 3.4-121.2 m³/d with some water tested. It was concluded from analysis that the large-scale gas reservoir was difficult to form due to the lack of effective traps. The sedimentary sag of the Linhe Depression is large, and the area of northern deep sag is about 7 000 km². Many wells in the slope of the southern flank of the sag revealed the development of lacustrine mudstone in the Guyang Formation and Linhe Formation, which have certain hydrocarbon-generating potential and have been key areas for exploration. However, due to the large settlement magnitude in the deep sags in the north, the burial depth of the target formation there is generally greater than 5 000 m, so for a long time, the drilling activities were mainly concentrated in the southern fault step zone, where the source rock is thin in single layer (0.5- 1.8 m), poor in longitudinal continuity, and thus has limited hydrocarbon-generating capacity. If the source rocks in the northern deep depression is similar to that in the southern fault step zone, and if there are source rocks with big thickness and good quality are the key factors restricting exploration and development of the northern deep depression. In order to answer these questions, Changqing Oilfield has adopted a series of new measures since 2010. (1) Low-frequency controllable vibroseis intensive acquisition and pre-stack migration processing technology have been developed through research, which can effectively improve the quality and interpretation accuracy of 2D seismic data. The bottleneck problem in predicting source rock by seismic data has been solved by using seismic facies and amplitude attribute extraction tech-nology. (2) In the deployment of exploration wells, taking into account the needs of trap and source rock evaluation, coring work has been strengthened, by systematic study of cores from three wells, Songtan 1, Songtan 2, and Long 1, it is confirmed that Guyang Formation and the Oligocene have saline lacustrine source rocks of high quality. Following the idea of selecting favorable belt according to the sag confirmed based on the “source control theory”^[9], Well Song 5 and Jihua 2X drilled in the faulted horst adjacent to source and faulted anticline around the major sag in 2018 obtained high-yield industrial oil flows of 62.6 m³/d and 10.26 m³/d respectively, marking major breakthroughs in exploration. A series of basic geological questions such as if the crude oil is mainly from the Guyang Formation or from the Linhe Formation, what migration and accumulation characteristics of the oil are, and if there are exploration prospects of natural gas have not been answered. In this study, through sorting the oil and gas geological conditions in the Linhe Sag systematically, the source rock quality, oil and gas sources, migration and accumulation characteristics of hydrocarbon were examined carefully in the hope of solving the abovementioned issues. On the basis of this, the favorable zones for exploration were predicted to provide a basis for making exploration decision.

The Hetao Basin is an important part of the arc-shaped tectonic system around the northwestern corner of the Ordos block. The formation, evolution and development of the basin are closely related to the arch structure^[1,2]. The three major faults controlling the Linhe Depression in the western segment of Hetao Basin are all arc-shaped faults. From north to south, they are the Langshan Fault (F1), Hangwu Fault Zone (F2, F2-1, F2-2, F3), and Huanghe Fault (F4).

The Langshan Fault (F1) is located in front of the Langshan Piedmont in the northern margin of the basin. It is a basin- control fault zone in arc shape on the whole with an extension distance of 360 km (Fig. 3). Trending SE-nearly due south, with a dip angle of 50°-80°, it is a normal fault that breaks through the basement. The fault throw in the basement is generally above 10 000 m, and reaches a maximum of up to 14 000 m in the Taiyangmiao Area. The fault throw reduces at both ends in the south and in the north, and the fault throw in the basement in the west of Well Songtan 1 in the southern end reduces to less than 500 m.

The Hangwu Fault (F2, F2-1, F2-2, and F3) is also a arc-shaped fault zone. The previous study concluded that the Hangwu Fault consists of the eastern and western sections, and the western section is the Hangjinhouqi Fault, which is a NW-trending normal fault with a dip of 50-70°. The eastern section is the Wuyuan Fault (Fig. 3), which is distributed in nearly EW direction. Currently, it is a normal fault trending north now, and was a reverse fault in Cretaceous. It began to reverse in the Eocene and became a normal fault in the Oligocene. Recent studies have shown that the Hangjinhouqi fault terminates at the Toudaoqiao, and the southward extension consists of two roughly parallel branch faults. The western branch dips NW and the eastern branch dips SE. Well Song 5 fault horst zone is held between them. Generally, the Hangwu fault has obvious segmentation characteristics and arc shape combination on the plane. There are several branch faults, mainly tensile faults with both shearing and twisting features that break through the basement.

The Huanghe Fault (F4) extends from the southeast of Jilantai to the northeast, passes through the Dengkou, and extends to Tuoketuo in the east, with a total length of 400-500 km. It is parallel and similar in shape with the arc-shaped fault of the Langshan Mountain. And the Huanghe Fault (F4) is the boundary fault between the southern part of the Hetao Basin and the Ordos Basin.

The study of regional stress field shows that the Hetao area was generally in the compression tectonic stress field in the Early Cretaceous due to the Yanshan Movement^[1,2,3], and the basin had a depression characteristic. Since the Eocene, affected by the Pacific tectonic domain and the Himalayan tectonic domain, the Ordos Massif moved southeastwardly relatively, and the Hetao area was strongly extended and subsided^[1,2,3]. During different tectonic periods of depression and faulting, the major faults such as Langshan Fault and Hangwu Fault always controlled the distribution pattern of basin subsidence and sedimentary environment, which determined that ​​the northwestern Linhe Sag was relatively deeper in water body, and was the main hydrocarbon-generating sag.

In the Early Cretaceous, the regional tectonic stress field was generally in the extrusion environment, and the Wuyuan Fault in the eastern flank showed the nature of thrust. The depocenter and subsiding center were located in the south of the Wuyuan Fault (F3) and north of the Fuxingbei Fault (F9). The sedimentary facies belts distribution was characterized by sandy conglomerate deposits of the alluvial fan facies in the north of the Wuyuan Fault. The southern part of the Wuyuan Fault mainly was shallow lacustrine facies in arid climate, where the lithologies were dominated by brown-red, brown mudstone and argillaceous sandstone. Although there was also squeezing in the west of the Linhe Depression, the thrust mainly developed in the Langshan Fault. The Hangjinhouqi Fault had only weak deflection. Therefore, the main body of the depression was located in the Hangjinhouqi-Taiyangmiao area, in which the depocenter was distributed in the northwest and was semi-deep lacustrine facies belt composed of mainly dark-colored mudstone. In the vast area in the south of Hangjinhouqi-Fuxing Fault, the topography was relatively flat during sedimentation, characterized by shallow lacustrine and fluvial environments (Fig. 4). In the Late Cretaceous, the Hetao Basin was uplifted gradually, and Late Cretaceous and Paleocene sediments are missing in the Linhe Depression.

Since the Cenozoic, the Hetao Basin has entered a new geological historical period. The Ordos Massif has transformed from suffering NW extrusion in Mesozoic to SE tension and the Hetao Basin has faulted under the influence of tension. The Linhe Depression perpendicular to the direction of the maximum tension had the largest tensile tensor. It became a depression with the thickest deposits, largest area, the most tectonic faults in the Hetao Basin^[1,2,3]. Due to the opposite distribution of the Hangwu Fault and the Langshan Arc Fault, the northern deep sag between the two faults has become the largest graben in the Hetao Basin since the faulting activity in the Cenozoic, and has been in the deep waters of low-lying areas during the faulting period. Under arid climatic conditions, the water body of the lacustrine basin was likely to have salinity and density stratification due to salinization, thereby forming a strong reducing environment at the bottom of lacustrine basin in which source rock in saline environment developed (Fig. 5). For the southern fault step zone adjacent to the hydrocarbon-generating sag, the water in the lacustrine basin was shallow, and the source rock only developed in the thin interlayer of mudstone in semi-deep lacustrine facies during the rising period of lacustrine water, with single layer thickness of only 0.8-1.4 m.

The Eocene-Pliocene tectonic activity gradually intensified, and the tension rate reached the peak in the Pliocene. Affected by the structure of basement, the stress field was not balanced among different basement blocks. A series of secondary faults of different scales were derived from the Hangwu Fault Zone, resulting in the skew, tilting and towing between different blocks, and the formation of a series of associated local structures. In the downthrown wall of the intersection representing arc-shaped convex region between the Hangjinhouqi and Wuyuan Faults in the north, a series of reverse drag syndepositional anticlines in strip shape along the major faults were formed, for instance the Merlin Anticline. A reverse fault (F2-2) opposite to the trending of the major Fault (F2-1) was derived in the upthrown wall of Hangwu Fault in the northwest of Dengkou, forming a horst block trap near Well Song 5 held by two branch faults. At the southwestern ends of the deep sag in the north, the Langshan Fault intersected the Jixi bedrock uplift zone, forming a stepped fault block and a syndepositional anticline in its upthrown wall, forming a faulted anticline trap near Well Jihua 2X.

In the Late Pliocene, the periphery of the basin was uplifted for short period. The Quaternary unconformably overlying the early sediments can be observed in outcrops, but the formations in the basin deposited continuously. Since the Quaternary, the activity of deep fault has significantly weakened. Except for the continuous activities of a few primary faults such as the Langshan Fault and the Hangwu Fault, most of the early faults were no longer active. Instead, a large number of fourth- to fifth-order small shallow faults with smaller fault throw and shorter extensions moved frequently. The basin entered the stage of filling and denudation, and the paleotopography height difference gradually disappeared. The lacustrine facies deposited across the whole basin in the Late Pleistocene^[10].

Seven wells penetrated in the Linhe Depression drilled through the Guyang Formation. Among them, six wells revealing source rocks are all distributed in the southern fault steps and the Jilantai shallow sag. The data of these 6 wells shows that the dark-colored mudstone is distributed at the bottom of the upper member of the Guyang Formation, with a TOC value of 0.10%-2.63%, averaging 0.78%; and a chloroform bitumen “A” value of 0.004 1%-1.315 3%, averaging 0.185 8%. Excluding the non-source rock with TOC value less than 0.4%, the source rocks have an average TOC value of 0.89% and high content of soluble organic matter, with soluble extract (bitumen “A” extracted by chloroform) of 0.697 4%. The parent material types are mainly Type II₁ and Type I, with a minor of Type II₂ (Fig. 6).

The geochemical indexes of the source rock show that the saturated hydrocarbon chromatogram of source rock of the Guyang Formation in Well Linshen 3 of the Linhe area has the dominance of pristine (Pr/Ph ranging between 1.53 and 1.64), indicating freshwater sedimentary environment; however source rock in Well Songtan 1 of the semi-deep lacustrine facies shows the dominance of phytane (Pr/Ph only ranging between 0.07 and 0.09), high content of gammacerane higher than C₃₀ hopane content, the ratio of gammacerane/C₃₀ hopane as high as 1.9-4.06, and boron content of (21.9-110.0) × 10^-6, which suggests the saline depositional environment under strong reduction conditions^[11,12,13]. These are very similar to the source rocks in the lagoon basins such as Qaidam Basin and Jianghan Basin^{[14,15,16,17,18]}, but different from the Late Triassic Yanchang freshwater lacustrine source rock in the Ordos Basin^{[19,20,21,22]}. Studies have shown that the productivity of organic matter is not high in saline lacustrine basin, but the stratification of water body resulted in a strong reduction environment at the bottom of the lacustrine basin due to high salinity of lacustrine water, which is conducive to the preservation of organic matter, and thus high-quality source rock can be formed in this kind of environment^[16,17]. The thermal simulation in the closed system gold tube carried out by the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences showed that the saline source rocks in the Linhe Depression had strong hydrocarbon-generating capacity, and the peak oil production rate of the Guyang Formation source rocks was as high as 857 kg/t, and the peak gaseous hydrocarbon yield could reach 460 m³/t.

Since the Cenozoic, the Linhe Depression has subsided constantly, with no magmatic activity and thermal anomalies. The present and past geothermal gradient of the Depression was relatively stable at 2.7-2.9 °C/100 m, so the thermal evolution of the source rocks was mainly controlled by the burial depth, and the hydrocarbon generation threshold depth was 3000-3 400 m. More than 5 000 m in burial depth, the source rock of the Guyang Formation in Well Linshen 3 in the area has a measured R_o value of 1.00% to 1.25%, and has reached the mature stage. At the burial depth of around 2 450 m, the source rocks in Well Songyang 1 and Jishen 1 are only 0.48% to 0.55% in R_o value and in the immature-low maturity stage. In the southwestern section of the northern deep sag and the area adjacent to the Hangwu Fault Zone, the Guyang Formation has a relatively shallow depth and produces oil primarily. The source rock near the Langshan Fault has a depth of more than 9 000 m and produces gas largely.

The dark-colored mudstone of the drilled well was 29.0- 100.8 m thick, in which the source rock is 16.8-64.8 m thick, accounting for 17%-85% of the thickness of dark-colored mudstone. In general, the source rock distribution is controlled by sedimentary facies belt. The source rock of coastal shallow lacustrine facies in Well Song 5 and Linshen 3 have low organic matter abundance and soluble organic matter content; the source rock of semi-deep lacustrine facies in Well Jican 1 and Songtan 1 have high abundance of organic matter, but the rock of lacustrine shore in Well Songtan 2 has no source rock. By calibrating known well, the thickness of source rock in the northern deep sag area was semi-quantitatively estimated by analyzing seismic facies and amplitude attribute data. The area with the largest thickness was predicted to be in the semi-deep lacustrine facies belt of the Aolunbulage-Hangjinhou area, with a thickness of 200-500. The source rock of shallow lacustrine facies in the south of Wuyuan is the second, with a thickness of 200-300 m (Figs. 4 and 7). These areas are the major hydrocarbon-generating centers.

Nine exploration wells in the Linhe Depression drilled through Linhe Formation. Among them, four wells including Songtan 1, Songtan 2, Jican 1, and Linshen didn’t encounter dark-colored mudstone. The rest exploration wells in the offshore lacustrine facies and delta facies such as Song 5 and Linshen 3 all encountered dark-colored mudstone which is mainly distributed in the Ling 2 Member in the middle of the Linghe Formation. The mudstone consists of dark gray, gray-black laminated mudstone, dolomitic mudstone and anhydrite-bearing mudstone. The dark-colored mudstone has a TOC value of 0.07%-5.10%, 0.76% on average, chloroform bitumen “A” of 0.004 5%-1.847 3%, on average 0.227 3%, and a total hydrocarbon content of (8.49-4 079.55) × 10^-6, on average 725.40 × 10^-6. According to the statistics of Well Song 5 and Long 1 with rich core data, excluding the non- source rock with TOC value of less than 0.4%, the mudstone source rock has an average TOC of 1.87%, and high content of soluble organic matter, with average chloroform bitumen “A” of 0.305. 8%. The parent materials are mainly Type II₁ and Type I, mixed with a small amount of Type II₂ (Fig. 6).

Similar to the source rock in the Guyang Formation, the source rock of the Linhe Formation is also characterized by high boron content (121-344) × 10^-6, low Pr/Ph ratio (0.09- 0.20) and high content of gammacerane, indicating that the parent material was in strongly reduced saline depositional environment. The source rock and anhydrite are interbedded or symbiotic. The euryhalinity foraminifera fossils were found in the strata. They are monotonous in genus, small in size, abundant in quantity, and have mutations observed, reflecting the sedimentary environment of arid lagoon basin^[11]. Thermal simulation experiment shows the source rock has strong hydrocarbon-generating capacity, with a peak oil production rate of 730-790 kg/t and a peak gaseous hydrocarbon yield of 600 m³/t.

The source rocks in the Linhe Formation was formed late, and its hydrocarbon generation threshold is slightly higher than that in the Guyang Formation, at about 3 400-3 600 m. In the area south of the Hangwu Fault, the source rock in the Linhe Formation is generally less than 3 500 m in burial depth, and mostly in the immature-low mature stage, for example Well Long 1, Song 5 and Linshen 2. The source rock in Well Lingshen 3 well with a burial depth of more than 4 000 m has a measured R_o value of 0.8%, reaching the mature stage. In the area south of Aolunbulage-Well Song 5 in the northern deep sag, the source rock in the Linhe Formation, relatively shallow in burial depth, is in the low-mature to mature stage, and largely produces oil. In the area with a burial depth of more than 8 000 m close to the flank of Langshan, the source rock is over-mature, and produce gas primarily. The source rock in the rest of northern deep sag is in mature to high-mature stage, and produces oil and wet gas.

On the plane, the source rock of littoral facies is thin, generally less than 200 m thick (Figs. 5 and 8). It is predicted that the source rock is mainly distributed in the semi-deep lacustrine facies near the flank of the Langshan Fault in the northwestern part of the northern deep sag, with a thickness of 200-550 m, making this area a hydrocarbon-generating center.

The Linhe Depression has the characteristics of typical continental lacustrine graben basin, with multiple provenances, short water systems, oscillation of lake level, and frequent intercalations of sand and mud^[23]. Therefore, reservoirs developed at various stages. Coupled with rapid settlement and weak diagenesis, the reservoirs above 5 100 m have better physical properties, with a porosity of greater than 8% in general. In the depth interval of 5 100-5 500 m, reservoirs with porosity of 6% to 8% still can be found (Fig. 9). So far, the oil-bearing strata are mainly found in the Guyang Formation and Linhe Formation, and also in the fractures of metamorphic rock. Therefore, the 3 sets of reservoirs will be discussed in this paper.

The reservoirs of the Guyang Formation are mainly composed of medium-fine feldspar sandstone and lithic feldspathic sandstone. The physical properties of the reservoirs are closely related to their depth and cement components. Generally, the reservoirs less than 5 000 m deep have good physical properties and mainly intergranular pores, with a porosity of greater than 8% in general. According to analysis of 64 samples from Well Songshen 1 and Linshen 4, they have an average porosity of 13.5% and an average permeability of 135.4×10^-3 μm². The physical properties of reservoirs with burial depths exceeding 5 100 m are poor. The sandstone at 5100-5 500 m depth in Well Linshen 3 has mainly secondary dissolution pores, an average porosity of 6.3%, and an average permeability of 1.4×10^-3 μm². The development of reservoirs more than 5 500 m deep is still unclear. However, according to the exploration practice of the Tarim Basin, in some exploration wells, reservoirs at more than 8 000 m deep still have a porosity of 6% to 8%^[24]. In addition, calcareous cementation can undermine reservoir physical properties, for example, the shoal facies reservoir in the Guyang Formation of Well Songtan 2. Although less than 2 000 m in burial depth, the reservoir has an average porosity of only 11.9% and a maximum permeability of only 33×10^-3 μm², on average of 11×10^-3 μm² due to calcareous cementation.

On the plane, underwater fan delta sandy conglomerate reservoir develops in the steep slope zone of the downthrown wall of the Langshan Fault. The reservoir is generally greater than 10 000 m in burial depth, and less than 6 000 m in burial depth only in the south of Aolunbulage and in the east of Well Linshen 6, and it is speculated that the reservoir has good physical properties. In the semi-deep lacustrine facies belt in the northern deep sag, fine reservoirs of delta front and turbidite sandstone developed during the intermittent descending of lacustrine level. With a burial depth of 6 000-10 000 m, the reservoir is inferred as tight reservoirs. The southern coastal shallow lacustrine facies belt is a favorable area for the development of delta front and beach bar sandbodies, and reservoirs with better physical properties should exist in areas with a burial depth of less than 5 500 m.

Compared with the Guyang Formation, the Linhe Formation shallower in depth has better physical properties. The drilled wells are mainly located in the southern fluvial facies and shallow lacustrine facies sedimentary areas, where channel sandbodies, delta front sandbodies and turbidite sandbodies develop, the sandstone is 5-12 m thick in single layer and 9.5 m at maximum. They are largely medium to fine-grained feldspathic sandstone. The target layer is shallower than 4500 m, and very rich in primary intergranular pores. According to the statistical results of 438 samples, they have a porosity of 2.4%-27.1%, on average 15.1% and a permeability of (0.1- 2199.0)×10^-3 μm², on average 162×10^-3 μm², representing medium porosity and medium-high permeability reservoir. The pore throats of the Linhe Formation reservoir are fine and well sorted. The capillary pressure curve shows thick skewness of good sorting and obvious platform segment (Fig. 10). The median value of the pore throat radius is 0.69-5.42 μm, the average throat radius is 3.29-3.30 μm, representing fine pore and fine throat structure. In the southern slope of the northern deep sag, the Linhe Formation is less than 5000 m in burial depth and of shallow lacustrine facies. The formation has lobe-shaped sandbodies of delta front formed by progradation in intermittent descending of lacustrine level and turbidite sandbodies in the downthrown wall of fault, and reservoirs in which have good physical properties.

Well Jihua 2X in front of the Langshan Fault encountered alluvial sandy conglomerate of 300 m thick at the burial depth of only 1746 m, which has a porosity from logging interpretation of 17% to 25%, suggesting good storage capacity. Similar to the situation of the Guyang Formation, in the area north of Aolunbulage, the sandy conglomerate in the steep slope of the Langshan fault has a relatively large thickness, but it is relatively deeply buried and has relatively poor physical properties.

There are also fracture reservoirs in the basement of the Wulashan Group in the western Jixi Uplift of the Linhe Depression, mainly the Archean biotic plagioclase gneiss. These reservoirs have no matrix pores, but some dissolved fractures and vugs along the fracture surface locally. The fractures contain oil and have certain storage capacity. However, drilling in other areas revealed that the metamorphic rocks of the Wulashan Group were generally tight, with few pores and fractures and no storage capacity.

On the premise of a large amount of work in the early stage, especially in recent years, important progress has been made in comprehensive geological research and seismic research. In 2018, PetroChina Changqing Oilfield and Huabei Oilfield drilled six exploration wells in the major hydrocarbon-generating sags, the adjacent Hangwu Fault zone and the Jixi Uplift. Among them, four wells, Song 5, Jihua 2X, Jihua Drill 2 and Jihua Drill 7, obtained industrial oil flow and three types of oil reservoirs were discovered.

5.1.1. Fault-block reservoir

Well Song 5 in the Hang 5 Fault in the Hangwu fault zone on the southeastern part of the northern deep sag discovered a fault-block oil reservoir. The reservoir is the medium feldspar sandstone in delta front facies in the middle of the Linhe Formation. The burial depth of oil layer is 2 918-3 144 m. Comprehensive interpretations of log and well logging showed there were 5 oil layers of 20.4 m combined, and oil-bearing water layer of 5.0 m. The depth interval of 2 983.4-2 995.7 m had an initial oil production of 12.6 m³/d. Due to high viscosity and poor fluidity of the crude oil, the hydrothermal jet pump was subsequently used to obtain a high-yield industrial oil flow of 62.6 m³/d. The reservoir has a temperature of 94.8 °C, a formation pressure of 29.86 MPa, and a pressure coefficient of 1.02, representing normal pressure system. The crude oil contains saturated hydrocarbon of 57.39%, aromatic hydrocarbon of 22.32%, non-hydrocarbon of 13.4%, and asphaltene content of 6.89%. The crude oil has a density of 0.9366 g/cm³ (50 °C), kinematic viscosity of 60.78 mm²/s (50 °C), and viscosity of 15.5 mPa·s under the original temperature of the oil layer, indicating it is heavy oil with high density, high viscosity and high freezing point (heavy oil).

5.1.2. Fault-anticline reservoir

Well Jihua 2X discovered a small fault-anticline reservoir the downthrown wall of the Langshan Fault in the southwestern part of the northern deep sag, with a trap area of ​​19 km². The well is completed at the depth of 1 746 m, and doesn’t drill through the Oligocene Linhe Formation. The well section from 1 397 m to the well bottom all contains oil, oil layers of 195.8 m were interpreted from logs and well logging. The interval of 1 651-1 736 m in the well was perforated, and tested a daily oil production of 10.26 m³ by the screw pump thermal recovery. The crude oil contains saturated hydrocarbon of 38.46%, aromatic hydrocarbon of 32.93%, non-hydrocarbon of 15.07%, and asphaltene content of 13.54%. The crude oil has a density of 0.927 8 g/cm³ (50 °C), a kinematic viscosity of 122.2 mm²/s (50 °C), and a freezing point of 20.5°C , belonging to heavy oil.

5.1.3. Bedrock fracture reservoir

The fracture-type reservoir of the bedrock uplift belt is located in the deep sag near the northern Jixi bulge, and is structurally adjacent to the Jihua 2X Fault anticline. It is a bedrock monoclinic zone surrounded by normal faults in the northwest and northeast, and is uplifted in the northwest and tilted in the southeast. Nine wells were drilled in this belt and oil shows were generally observed in the top weathering fracture belt of the metamorphic rocks of the Wulashan Group. Industrial oil flows were obtained from Well Jihua 2 and Jihua 7. Among them, Well Jihua 7 produced oil of 1.92 m³/d, and Well Jihua 2 produced oil of 2.84 m³/d and water of 10.64 m³/d in drillstem test. The reservoir is shallow in burial depth at only 350-700 m, and strongly controlled by structure. Both wells with industrial oil flow are located at the structural highs on the northwest side. The crude oil from Well Jihua 2 has saturated hydrocarbon of 51.37%, aromatic hydrocarbon of 30.05%, non-hydrocarbon of 5.93%, and asphaltene of 12.66%. Crude oil from Well 230 has saturated hydrocarbon of 26.39%, aromatic hydrocarbon of 28.09%, non-hydrocarbon of 35.50%, and asphaltene of 10.02%. The crude oil from Well Jihua 7 has a density of 0.897 g/cm³ (20 °C), a kinetic viscosity of 21.95 mm²/s (20 °C), and a freezing point of 2 °C, representing medium crude oil.

The comparative analysis of crude oil and source rock biomarker compounds shows that the crude oil from Well Song 5 is sourced from the Oligocene Linhe Formation, and the crude oils from Well Jihua 2X and the fracture reservoir in the bedrock uplift zone are derived from the Lower Cretaceous Guyang Formation. The evidences are the following:

The crude oil from Well Song 5 has low Pr/Ph ratio (0.28), high concentration of gammacerane, and a gammacerane/C₃₀ hopane ratio of 1.43. The terpanes are dominated by pentacyclic terpane and the steranes are dominated by the regular type, with no diasterane. The ratios of C₂₉ sterane 20S/(20S + 20R) and C₂₉ sterane ββ/(αα + ββ) are respectively 0.10 and 0.23, showing low-maturity, similar to sterane composition of dark-colored mudstone in Well Song 5 and Long 1, indicating that the oil is mainly sourced from the source rock in the Linhe Formation (Fig. 11).

The geochemical characteristics of the Linhe Formation crude oil in Well Jihua 2X also has a low Pr/Ph ratio (0.28), a high gammaracene content, and a gammacerane/C₃₀ hopane ratio of 2.02. The terpanes are dominated by pentacyclic terpane and the steranes are dominated by the regular type, with very low content of diasterane. The ratios of C₂₉ sterane 20S/(20S + 20R) and C₂₉ sterane ββ/(αα + ββ) are respectively 0.41 and 0.39, indicating the oil is much higher in maturity than that in Well Song 5, and belonging to mature oil. The geochemical indexes are similar to that of source rock in the Guyang Formation of Well Songtan 1, and quite different from source rock in Well Song 5 and Long 1, indicating that the crude oil is derived from the Guyang Formation (Fig. 11).

The characteristics of biomarker compounds of the crude oil from Well 230 located in the bedrock uplift zone are similar to those of crude oil from Well Jihua 2X and source rocks from Well Song 5. The steranes are dominated by the regular type, with low content of diasterane. The ratios of C₂₉ sterane 20S/(20S + 20R) and C₂₉ sterane ββ/(αα + ββ) are both 0.42, representing mature oil. All these indicate that the oil is sourced from the Guyang Formation (Fig. 11).

The biomarker compounds of crude oil from Linhe Formation in Well Jihua 2X and of crude oil in the metamorphic rock of Well 230 are consistent with that of the Guyang For-mation in Well Songtan 1, indicating that the oil and gas are mainly sourced from the Guyang Formation. But the source rock of the Guyang Formation south of Well Songtan 1 is thin and can’t provide sufficient oil, so the oil is mainly from the Guyang Formation in the northern deep sag area. Additionally, the study area is far from the Oligocene favorable hydrocarbon- generating area, so the Oligocene is not the major oil source.

Well Song 5 is within the effective hydrocarbon supply range of the Linhe Formation and the Guyang Formation, but the Guyang Formation in the adjacent area is deeply buried and has entered a highly mature stage (Fig. 7), so it is impossible to produce low-mature oil. Only the source rock in the Linhe Formation has low maturity (Fig. 8), and thus is the major source of reservoir in Well Song 5.

The Hangwu Fault and the Langshan Fault not only controlled the distribution of hydrocarbon-generating sag, but also acted as primary pathways connecting oil sources. The fault zone is also a development zone of structural traps. Oil and gas migration and accumulation are closely related to these two fault zones. The oil reservoirs discovered so far are all located near the major fault zone. The sags, migrating pathways, and the migration and accumulation are controlled by the fault system obviously.

In the southeast flank of the hydrocarbon-generating center in the northern deep sag, the Hangjinhouqi-Wuyuan Fault had long-term activity and penetrated the basement. It showed strong tensile activity since the Cenozoic, and could effectively link the source rock layers of the Guyang Formation and the Linhe Formation. The hydrocarbons migrated along the up-dipping direction of fault and accumulated in suitable traps. In the northwest of Dengkou-Toudaoqiao Belt, the major fault of Hangjinhouqi (F2-1) tilts northwest, with steep and straight fault plane. There is no syndepositional anticline in the downthrown wall and it is intersected by branch opposite direction faults (F2-2) formed in the Pliocene in the upthrown wall, forming an elongated horst structure between the two oppositely trended faults (Figs. 3 and 12a). The low-mature crude oil formed by the Linhe Formation migrated upward along the major fault (F2-1) and accumulated in the sandstone reservoir in the Linhe Formation. The up-dipping direction is occluded by the reverse fault (F2-2) and the top is sealed by the mudstone in the upper Linhe Formation and regional caprock in the Wuyuan Formation, forming fault-block heavy oil reservoir in Well Song 5 (Fig. 12a).

In the southwest flank of the northern deep sag, the Langshan Fault intersects the Jixi uplift, forming a fault step to the northern deep sag which communicates the Lower Cretaceous oil sources. During the upward migration of along the fault zone, the oil accumulated in the fault anticline where Well Jihua 2X is located, firstly in the alluvial fan reservoirs in the Guyang Formation and Linhe Formation. The oil reservoir is formed under the sealing by overlying mudstone caprock in the Wuyuan Formation (Figs. 3 and 12b). The remaining oil passed through the fault zone and entered the metamorphic rock fractures of the Wulashan Group in the Jixi uplift, and migrated laterally along the unconformity between the Wulashan Group and the Lower Cretaceous and weathering fracture layers; the reservoir with the top sealed by the mudstone layer in the Guyang Formation, and the upward tilting direction occluded by the Gingler Fault come about (Fig. 12b).

Despite the fact that oil and gas reservoirs have not been discovered in the northern deep sag and Hangjinhouqi-Linhe area, based on the comprehensive understanding on hydrocarbon-generating sags, fault systems, traps and reservoirs, the characteristics of oil migration and accumulation are speculated as follows:

The northern Hangjinhouqi-Linhe area is located in the arc top area of ​​the Hetao arch structure, where the major faults of the Hangjinhouqi and Wuyuan Faults intersect. On plane, it protrudes to the northwest. The major faults (F2, F3) are steep in the upper and gentle in the lower parts, showing a shovel- like shape. In the downthrown wall, a series of contemporaneous anticline belts occur. Therefore, the oil and gas would preferentially enrich in the contemporaneous anticline belts (Fig. 12c① and ⑧) in the downthrown wall of the Hangjinhouqi Fault during the migration along the primary fault plane. The remaining oil and gas continued to migrate laterally, and accumulated in possible anticline traps or fault block traps formed in the downthrown wall of the Wuyuan Fault (Figs. 12c③). The Lower Cretaceous is dislocated by the Hangwu Fault and the tight layers in the basement of upthrown wall can form lateral sealing. Therefore, fault-block trap or fault-block-lithological trap are possibly formed (Fig. 12c④). Due to the large fault throw of the Hangjinhouqi and Wuyuan Faults, the resistance of lateral migration of oil and gas through the fault zone is much greater than that of upward migration along the fault zone. Therefore, although the fault-block trap is extremely developed in the fault step zone far from the Hangwu Fault, far from the hydrocarbon-generating center in the northern deep sag, the area has insufficient supply of oil and small potential to form reservoir. The wells in the high blocks including Long 1, Linshen 2 and Lintan 1 only showed weak oil traces in the Linhe Formation and the testing results showed water layer. The Lower Cretaceous, deeply buried, has undergone extrusion in the early stage, and anticline traps might develop locally, forming small fault-block oil reservoirs (Fig. 12c⑥). In the structural highs, fault-block reservoir is probably formed due to the intersecting of fault block (Fig. 12c⑦).

In the northern deep sag, the areas where the Linhe Formation and the Guyuan Formation are more than 8 000 m deep produce gas primarily. With sufficient gas source, large burial depth, small thickness and poor physical properties of reservoir sandbody, the exploration there should focus on natural gas (Fig. 12c② and ⑤).

The Linhe Depression of Hetao Basin has a low degree of exploration and only a small number of wells. Through analysis of petroleum geological characteristics of the Linhe Depression and existing exploration results, the area has favorable reservoir and caprock combinations and good exploration prospects. The Linhe Depression of Hetao Basin has geological oil resources of 6.1×10⁸ t, recoverable oil resources of 1.4×10⁸ t, geological natural gas resources of 3 200×10⁸ m³, and recoverable gas resources of 1 600×10⁸ m³.

By considering the accumulation elements including oil source, migration, and trap, it is concluded that the next exploration area of Linhe Depression should be the fault tectonic belt near the hydrocarbon-generating sag. Evaluation shows that the structural traps of the Hangwu Fault zone in the southern slope of the northern deep sag, the bedrock fracture-structural trap of the Jixi Uplift Belt, the structural-lithologic trap in the northern deep sag are three favorable exploration zones.

The structural trap zone of Hangwu Fault is the most favorable exploration zone. This trap zone is adjacent to the Lower Cretaceous Guyang Formation and Oligocene Linhe Formation source rocks. There are many syndepositional anticlines, nasal arcs, and fault blocks, close to oil source. These structures have reservoirs with good physical properties and are on the direction of oil and gas migration. The overlying Miocene mudstone shows good sealing capacity and the configurations of source rock and reservoir are good. Recently, the exploration breakthrough of the Oligocene Linhe Formation in Well Song 5 further confirms that this is the most promising exploration zone.

The favorable zone of bedrock fracture-structural trap in the Jixi uplift belt: two exploration directions are identified according to the type of storage space: (1) area with favorable bedrock fractures, the reservoir is the top fractured layer in the Proterozoic gneiss and the oil is sourced from the trough northwest of Well Songtan 1. The oil and gas accumulated under the blocking of tight crystalline surrounding rock during the upward migration of oil and gas through the unconformity surfaces and faults. The oil reservoirs discovered by Well 230, Jihua 2, and Jihua 7 are of this type, but the size of reservoir is affected by the development degree of fracture. (2) The target zone of the rolling anticline is represented by the reservoir in Well Jihua 2X, which is adjacent to the northern deep sag, with sufficient oil source. The Lower Cretaceous shallow lacustrine clastic rocks have good physical properties. The oil migrated along the unconformity surfaces and faults, and accumulated in the rolling anticline trap.

The favorable zone of structure-lithologic trap in the northern deep sag: it is the development center of the Lower Cretaceous and Oligocene semi-deep lacustrine source rocks in the Linhe area of ​​the Hetao Basin. The terrain is relatively flat with few local structures. The reservoirs are sandbodies in delta front and underwater fan facies. This area has conditions for forming lithologic traps and tectonic-lithologic traps. Although the exploration target layer in the area is deeply buried currently (the Oligocene is more than 4 500 m deep, and the Lower Cretaceous is more than 5 000 m deep). With the advancement of drilling and oil testing techniques and processes, major breakthroughs in exploration will be achieved.

Through comprehensive study of petroleum geological characteristics and hydrocarbon accumulation characteristics in the Linhe Depression, it is concluded that the Langshan Fault, the Hangwu Fault and the Huanghe Fault control the sedimentary evolution and hydrocarbon-generating center of the depression, and the depression experienced the Early Cretaceous depression and the Cenozoic faulting. There develop two sets of source rocks, the Cretaceous Guyang Formation and the Paleogene Linhe Formation, with high organic matter abundance, Type II₁ and I organic matter dominantly, high hydrocarbon-generating potential. Mature to highly mature, the source rocks produce oil, condensates and wet gas primarily. There develop clastic reservoirs of the Guyang Formation and the Linhe Formation, and the reservoirs with a burial depth of less than 5 000 m are better in physical properties. The bedrock weathering fracture reservoirs are developed in the Xiji Uplift and oil reservoirs are discovered. Oil reservoirs of fault-block, fault-anticline, and bedrock fracture types have been discovered, the oil is low-mature to mature oil from the Guyang Formation and the Linhe Formation. The research shows that the Linhe Depression is rich in resources and has great exploration potential. The Guyang Formation and the Linhe Formation are the main exploration targets. The Hangwu Fault zone, Jixi uplift zone and northern deep sag zone are favorable exploration zones.