Karstification is the comprehensive effect of corrosion, erosion, transportation and deposition of water on soluble rocks. Its essence is that carbonic acid formed by solution of carbon dioxide in water reacts with calcium carbonate in carbonate rocks^[1,2]. Water is the power in the whole process, and its flow mode and distribution determine the structure and distribution of karst fractures and caves^[2,3]. As an important product of karstification, karst landform plays an important role in controlling the distribution and occurrence of karst drainage^[4]. Because of extensive karst drainage and strong flowing dynamics, karst slope zone is characterized by the strongest karstification and the most developed karst fractures and caves^[3,4]. The karst reservoir in the Tahe oilfield is located on the Ordovician karst slope. It has karst caves and fractures as main oil and gas storage space, and the large underground river caverns of up to 80 m in diameter, making it a unique fracture-cave reservoir different from the traditional fracture pore carbonate reservoir^[5,6,7]. With deep burial depth and old age, the karst reservoir in the Tahe oilfield has experienced complex structural deformation and multi-stage karstification, and thus has very strong heterogeneity^[8,9,10]. Based on the National Major Oil and Gas Projects and National Key Basic Research and Development Program of China, major progress has been made in the study on structural evolution^[11], karst landform restoration^[12], karst drainage division^[13], karst facies distribution^[14], fracture-cave structure and filling characteristics^[15], and oil and gas distribution^[16,17], etc. It has important guiding significance for the exploration and development of Tahe oilfield^[6]. By the end of 2019, the Tahe oilfield had the cumulative oil production of over 1×10⁸ t, becoming the Paleozoic marine carbonate oilfield in China with the largest oil reserves and highest oil production^[7].

The exploration and development practice of the Tahe oilfield reveals that the wells in local uplift (hoodoo-upland) area of the Ordovician Karst slope have high oil production and long water-free production period, nearly 60% of the wells have a cumulative oil production of more than 20×10⁴ t per well. But the wells in depression areas (karst depression) between the uplifts, have stopped production before reaching cumulative oil production of 10×10⁴ t. In addition, drilling data reveals that the reservoir in the hoodoo-upland in the Tahe area has a large number of karst vugs, fractures and small and medium- sized fracture-cave groups (with a thickness of more than 100 meters) developed and has rare large underground river caves. Accordingly, it shows chaotic weak reflection, but no "bead string" type strong reflection on seismic sections. Currently, the research on this type of small and medium-sized fracture-cave body in hoodoo-upland area is still in its infancy, and focuses on the identification and three-dimensional delineation of the fracture cavity group by geophysical methods^[18,19]. The genetic type, spatial structure, hydrocarbon migration and accumulation model of this kind of fracture-cave reservoir need to be further studied urgently. Based on the abundant geological and geophysical data of the Tahe area, the genetic mechanism and evolution process of the hoodoo-upland on the karst slope are studied from the perspectives of distribution of faults and thickness of residual strata. Following the idea of discussing the Paleokarst based on current karst, the genetic types and spatial structure of fractures and caves in the hoodoo-upland are identified by core observation and description, well logging and seismic interpretation, and then in combination with the oilfield production practice, the hydrocarbon migration and accumulation model in the hoodoo-upland is established.

From Cambrian to Early Ordovician, carbonate rocks of platform facies were deposited in the Tarim Basin^[4]. In the Middle and Late Ordovician (middle Caledonian movement), the tectonic plates of Paleo-Tethys Ocean began to subduct toward the Tarim plate, and then the Tarim Basin gradually changed from extensional system to compressional system. Under the effect of North-South compression, the Tazhong and Tabei uplifts were formed successively, and multiple strike-slip faults in NE and NWW trend were formed at the meantime^[20] (Fig. 1a). The NE sinistral strike-slip fault system was formed in east Tahe area (east of Well S99), and the "X" conjugate strike slip fault system was formed in west (west of Well S99) Tahe area (Fig. 1b). During this period, the Ordovician carbonate rocks exposed to the surface several times, and experienced short-term karstification (Caledonian karst), hence, multiple unconformities in the Ordovician were formed, including the top of the Yijianfang Formation (T₇⁴), Lianglitage Formation (T₇²) and Sangtamu Formation (T₇⁰)^[11].

At the end of Silurian, the Southern Tianshan Ocean Crust began to subduct toward the Tarim plate, and the early strike-slip faults moved again under the action of NW trending stress^[20,21]. At the same time, the Tahe area experienced strong uplift again and suffered denudation, with denudation strength increasing from south to north. In Tahe 3, 4, 5, 6 and 7 well blocks with the strongest erosion, the T₇⁴ unconformity directly overlaps with T₇⁰ (Fig. 1c). Meanwhile, the limestone of middle and lower Ordovician (Yijianfang Formation and Yingshan Formation) exposed to the surface and suffered strong karstification (Early Hercynian karstification), forming a nose like karst landform in northeast trend and southwest dip. From northeast to southwest, the karst highland, karst slope and karst basin came up in turn^[4].

From the end of Late Devonian to the beginning of Early Carboniferous, sea water gradually intruded from the southwest, forming delta and tidal flat deposits^[22]. The mudstone of the Bachu Formation directly overlapped the Ordovician karst landform, isolating the contact between surface water and Ordovician limestone, ending the Early Hercynian karstification in the Tahe area. At the end of Early Permian (the Late Hercynian), the early faults in the Tahe area had sinistral strike-slip successively, which resulted in tension-torsion deformation of the overlying Carboniferous and formation of right-step en echelon normal faults^[23]. From Late Mesozoic to Paleogene, under the regional tension stress, the early NE deep faults moved again^[20,23], inducing a series of en echelon right-step NE normal faults in the Mesozoic and Cenozoic (Fig. 1b).

By using the impression method and trend surface technology^[12,13], the Pre-carboniferous geomorphology (karst paleogeomorphology) and karst drainage distribution in the study area are restored (Fig. 2). The results show that there are many karst peaks, stone forests, canyons and depressions in the Ordovician Paleokarst landform, with an elevation difference between north and south of over 300 m. The landform mainly includes two hoodoo-uplands in the east and west and a depression between them, as well as a karst lagoon in the south. The highlands are mainly composed of high peaks and narrow canyons. On the west side is S74 hoodoo-upland in NNE strike, high in the north and low in the south, steep in the east and gentle in the west, and gradually inclining to the south. It covers an area of 32.8 km², has an elevation difference of 122 m with the adjacent karst depression. The peak area (peak base area) and height (elevation difference with adjacent depression) gradually decrease from north to south in the S74 hoodoo-upland. For example, the S74 and TK604 have an area of 2.13 km² and 1.68 km², and height of 101.8 m and 82.7 m, respectively. The S71 in the south has an area of 1.02 km² and height of only 48.5 m. In the east, the karst canyons are distributed in the NE direction, while in the west, they are distributed in the NW direction.

On the east side is T402 hoodoo-upland in nearly NS strike, which is also higher in the north and lower in the south, steep in the east and gentle in the west. Compared with S74 hoodoo-upland, the T402 hoodoo-upland has a smaller area of 25.6 km², a bigger height, and an elevation difference of 146 m with the adjacent karst depression. In the north, the TK439 and T402 hoodoo-uplands have a peak area of 2.88 km² and 2.06 km², and height of 128.5 m and 106.6 m, respectively. In the south, the S48 and T403 hoodoo-uplands have a peak area of 1.56 km² and 1.21 km², and height of 76.4 m and 56.2 m respectively. In the west side of the T403 hoodoo-upland, there are wide snakelike valleys in southeast strike formed by river erosion. In the east of T403 hoodoo-upland, there is only one snakelike river with a depth of 100 meters. Its bed depth is consistent with the depth of the S75-TK673 river in the middle of the study area, showing that this depth represents the karst erosion base level in the karst period^[3,17], and is the dividing line between surface water erosion and corrosion capacity, which affects the structure of karst fracture-caves. It is worth noting that the eroded valleys of rivers in the karst period all end near the pinchout line of Ordovician Qarbake Formation(O₃q), indicating that there was water in the south of that line, which is defined as karst lagoon.

Combined with the regional tectonic background, the faults on the East-West seismic section across the hoodoo-upland are interpreted. The results show that the high angle strike slip faults are very well developed in the study area, mainly distributed in the lower Paleozoic (especially the Ordovician), and decrease in number significantly in the Carboniferous and Triassic. In S74 and T402 hoodoo-uplands, faults constitute into positive flower structure, end at T₇⁴ (top of Ordovician) upward and cut down into Cambrian Sinian system. In contrast, the karst depressions between the hoodoo-uplands have fewer faults and the faults mostly occur as single ones (Fig. 3a). The bedding slice of ant tracking volume clearly shows that the study area has mainly NNE and NNW faults in Ordovician (Fig. 3b). According to the fault cutting depth and combination patterns on plane and section, the faults can be divided into main faults and secondary faults. Among them, the NNE main faults have large cutting depths and can break into Cambrian system, such as T606-S74 fault zone and TK439-T402 fault zone. The NNW main faults have different cutting depths, and some of them can break through Ordovician. The NNE and NNW secondary faults are mainly distributed inside the Ordovician (Fig. 3c). It is worth noting that the NNE main fault zone gradually bends eastward in the north of Well S66 and TK411, meanwhile, there are many NNW and NNE trending secondary faults developing around them. As a result, the fault development degree is higher in the north and lower in the south, higher on both wings and lower in the middle. The areas with dense faults are consistent with the distribution range of hoodoo-uplands.

According to the characteristics of regional fault activity, the study area can be divided into three structural layers from bottom to top^[4,11]. Below T₇⁴ (Lower Paleozoic) is the Caledonian-Early Hercynian structural layer, where a sinistral compressive and torsional stress field formed at the right-hand bending part in the north of the NNE main fault under the background of sinistral strike-slip (Fig. 1a)^[24], which induced multiple secondary faults; these faults combine into a positive flower structure (Fig. 4), giving rise to a local raised zone (rudiment of hoodoo-upland) on the karst slope. Formations between T₇⁴ and T₅⁰ (Upper Paleozoic) belong to the Late Hercynian structural layer, where the early faults continued the sinistral strike-slip (especially the NNE main fault), as the regional stress state transited from compressional torsion to tensional torsion^[20], resulting in the development of NNW trending en echelon normal faults in the Carboniferous System; while the Lower Paleozoic still maintained the positive flower shape. Formations above T₅⁰ (Meso Cenozoic) belong to the Yanshanian-Himalayan structural layer, where the early NNE trending main faults moved again and reversed to dextral strike- slip under the background of regional extension^{[20, 23]}, forming a NE trending dextral right-hand normal fault (Fig. 4b) that breaks through the whole Mesozoic, up to the Paleogene, down to the Carboniferous; but the lower Paleozoic still maintained a normal flower shape. It can be seen that the positive flower structure was formed in the Early Hercynian period (karst period), and it has remained the positive flower shape later (Fig. 4e). It is worth noting that the strata from the interface of T80 to T50 on the seismic profiles of Figs. 3a and 4a show positive flower shape, but are different in genesis. From bottom to top, the structural amplitudes of T₈⁰, T₇⁶ and T₇⁴ interfaces increase in turn, indicating typical tectonic stress origin^{[20, 25]}, that is the formation mechanism of the flower structure; while from T₇⁴ to T₅⁰ interface, the structural amplitude gradually decrease, and is nearly flat to T₅⁰ interface, suggesting draping structure formed by compaction difference^[25].

In the Early Hercynian period during the formation of the positive flower structure in Lower Paleozoic, Ordovician in the Tahe area experienced strong karstification. The positive flower structure formed the rudiment of the Hoodoo-uplands on the karst slope and controlled the differential distribution of karst drainage. The hoodoo-uplands were areas lack of water relatively, while the residual karst depressions were areas with ample water. Under the karstification conditions of the same tectonic background and lithology (sand-clastic limestone of Yingshan Formation), the distribution of karst drainage controlled the intensity of karstification. The karst depressions had perennial rivers, with abundant water, so water-rock reaction was longer in duration and wider in range there, and long river erosion and corrosion made the thickness of Yingshan Formation gradually decrease and the landform lower (Fig. 5). The hoodoo-uplands had only seasonal (intermittent) streams with limited water, so the water-rock reaction there was shorter in time and smaller in scope. Occasionally in rainy season, intermittent streams could erode and corrode limestone in the hoodoo-uplands in short periods, forming karst gullies and canyons. Therefore, compared with the karst depression areas, the hoodoo-uplands had weaker karstification, smaller denudation amount and larger residual thickness of Yingshan Formation. With the karstification going on, the difference became more and more evident, the karst depression had more and more serious erosion, and the hoodoo-uplands relatively elevated. Meanwhile the karst canyons constantly cut and destroyed the hoodoo-upland into many high peaks and narrow canyons.

To sum up, the formation and evolution of hoodoo-upland are controlled by paleotectonic compression and corrosion of paleokarst drainage. In the Early Hercynian karstification period, the positive flower structure formed the rudiment of hoodoo-upland on the karst slope, and then controlled the direction and distribution of karst drainage, resulting in the difference of karstification. The depression suffering strong karstification had a large corrosion rate, and the landform gradually lowered, while the hoodoo-upland suffered weak karstification had a smaller corrosion rate, and the landform was relatively uplifted; together with the erosion of streams, the hoodoo-upland with peaks and canyons came up finally (Fig. 5). After the mudstone of Bachu Formation deposited, the hoodoo-upland was buried, and the differential karstification was over. Meanwhile, the later tectonic movement had little influence on the hoodoo-upland, the hoodoo-upland has always maintained the positive flower shape, with the characteristic of inherited development.

As flow channels, faults control the progress of karstification and the distribution of fractures and caves^[20,23]. Due to the multi-phase activities of the faults in karstification stage, the faults and fractures in the hoodoo-upland interweaved with each other, forming a complicated channel network. Whenever the rainy season came, the meteoric water scattered on the hoodoo-upland with cracks, part of the water seeped along the complex network channels inside the hoodoo-upland, and caused diffusive karstification to the Ordovician limestone^[13], forming a large number of densely distributed vugs, fracture-cavity complexes and isolated small caves^[9], which are collectively referred to as small and medium-sized fracture-cavity group. The other part of water flew down the slope along the fractures or faults, and strongly eroded and dissolved the Ordovician limestone, forming a series of river valleys and hills, which then gradually evolved into canyons and karst stone forests. This phenomenon has been confirmed by karst outcrops in northern Tarim and Hunan. For example, the Ordovician karst outcrops in Dawangou, Keping area, where positive flower-like structures developed under strike slip compression, the density of strike slip faults gradually increases from the wing to the axis of the anticline, and the limestone of Yijianfang Formation at the axis has dense small and medium-sized fracture-cavity groups (Fig. 6a). Another example is the exposed stone tooth karst landform formed by corrosion and erosion in the Ordovician geological outcrops of Wulong Mountain in Hunan Province (Fig. 6b). The phenomenon of corrosion expansion along fractures can also be seen in the Ordovician carbonate rocks at 5400 m depth in the Tahe area. For example, Well TK604 located on the S74 hoodoo-upland is at the intersection of the NNE and NNW faults. Imaging logs show that there are several nearly vertical fracture zones (dark bands) around the well, which are obviously open due to corrosion expansion (Fig. 6c).

The exploration practice of Tahe Oilfield reveals that the hoodoo-uplands of S74 and T402 have very abundant small and medium-sized fracture-cavity groups. For example, during the drilling of Well T402, the drilling speed suddenly accelerated when reaching the Ordovician at 5538.5 m, and abnormal oil and gas shows were detected, and then several overflows and well leakage occurred. In the open hole section of 5358.09-5412.84 m in the Lower Ordovician was tested with 9.53 mm nozzle, and the daily oil output was 350.8 m³ and natural gas output was 9860 m³. After the test, the well was drilled further to the depth of 5602 m, in the course, drilling fluid lost continuously to a total of 1908.49 m³. The seventh, eighth and ninth coring were done at 5360.95-5377.75 m in this well. The cores are seriously broken, with a recovery rate of 19%, 55% and 63%, respectively. The cores have many high angle opening fractures and corrosion fractures stained grayish-brown by crude oil (Fig. 7a). Thin sections and physical properties of small and full-diameter cores were analyzed. It was found that the limestone samples have a large number of vugs and fracture-cavity complexes (Fig. 7b, 7c). The full-diameter cores have a porosity of 7.0% (which may be even higher underground). It can be seen that the paleokarst phenomenon in this section is very similar to that in Fig. 6a (modern karst), both of which are sections with dense small and medium-sized fracture-cavity groups. The logging results show that the tri-porosity logging of 5361.5-5464.1 m shows peak cluster high values, which correspond to the characteristics of finger-like low resistivity. Meanwhile, the bilateral lateral resistivity shows obvious positive amplitude difference (Fig. 7d), which also indicates that this section has extremely developed corrosion fractures and vugs. The natural gamma ray curve is in a low and flat state in this section (although the natural gamma curve at 5523 m has a high peak value, the uranium-free gamma log still remains low and flat state), indicating that the fractures and caves are not filled with argillaceous and other fine-grained sediments or have a lower filling rate, which is consistent with the core description results.

During investigation of the modern karst in Luotaxigou Stone Forest area, Longshan city, Hunan Province, we found that some near-surface "caves" formed by chaotic pile-up of collapsed breccias were not caused by corrosion (Fig. 8a, 8b). According to field survey statistics, 12 such "caves" (possibly more) were found in an area of nearly 1km². These caves are various in shape and scale, 0.85 to 12.29 m high (on average 6.68 m), and are mainly filled with limestone breccias, with a filling rate from 10% to 68%, on average 48.7%. The "caves" are all distributed in the valleys or canyons between the karst peaks. Under the guidance of this model, the causes of near-surface "caves" distributed in the valleys in the Tahe area are reasonably explained. For example, Well S66 located in the valley of S74 hoodoo-upland, the well reaches Ordovician at 5495.2 m. During the 11th coring (well depth 5495.97-5501.18 m), oil and gas anomalies were detected at 16:45, brown crude oil was seen in the drilling mud tank, drilling fluid density decreased from 1.12 g/cm³ to 1.08 g/cm³. At 17:51, drilling fluid flow disrupted for about a minute, losing about 2 m³ of drilling fluid. At 17:53, 3-4 intermittent kicks took place, with a kick volume of 0.16 m³. The well kicked again after 1 min of flow interruption, indicating that the caves are limited in scale and connected with other fracture-cave bodies. The cores and imaging log of this section also clearly show that the 5495.8-5498.1 m has a 2.3 m high "cave", which isn’t filled in the upper 1.3 m and filled with limestone breccia in the lower 1.0 m. Both the tri-porosity and dual-lateral logs show cave-like characteristics, while the natural gamma log show hardly any change, which also indicates that the cave is hardly filled with argillaceous and other fine grains (Fig. 8c). Therefore, it is the existence of the "cave" that led to a short flow interruption during the drilling process and the well kick again, resulting in only 14.4% recovery of the 11th coring (5.21 m coring footage, 0.75 m core length, and serious core breakage).

In addition, it is worth noting that, in the core of the 10th coring there is also lime-conglomerate in the valley; the gravels in it are of medium roundness and poor sorting, 5-60 mm in grian size, and are Ordovician limestone. This section has obviously different logging characteristics from the upper and lower strata, and is in gradual contact with the mudstone of Bachu Formation (Fig. 8c), which is obviously different from abrupt contact of Ordovician limestone with the mudstone of Bachu Formation at the karst peak (Fig. 7d). In this paper, it is called karst stage deposits.

The results of drilling calibrated seismic sections show that the small and medium-sized fracture-cavity groups occurring densely in the hoodoo-upland exhibit disorderly weak seismic reflection characteristic on the conventional seismic section^[19], with the fracture-cave structure difficult to identify. In this study, frequency extension processing technology^[26] was used to expand the seismic frequency of the target in the study area from 10-50 Hz to 10-90 Hz, as the high frequency weak signals greatly increased, the characteristics of fracture-cavity structure and faults or fractures can be shown clearly (Fig. 9a). The root mean square seismic attribute (RMS) after iterative processing was used to delineate the small and medium-sized fracture-cave groups and near-surface "caves" in the hoodoo-upland (Fig. 9b). The RMS values of the cave section, small and medium-sized fracture-cave group section, and fracture section are 3200-4000, 1600-2400, and 800-1200 respectively. For example, both Well TK408 and TK411, located in the karst valley, encountered "caves" with height of 5.8 m and 10.9 m, which can also be clearly identified on the RMS profile (Fig. 9b). According to this method, 28 near-surface "caves" are identified in the study area, of which 6 are located in the abandoned river valley of karst depression and completely filled with limestone breccia and sand or mudstone; the remaining 21 caves are evenly distributed in the karst valley, from 1.2 m to 13.5 m high, on average 6.12 m, and filled with limestone breccia, with a filling degree from 15% to 72%, on average 51.6%. This is similar to the statistical results of Xigou Stone Forest in Hunan Province (48.7%), indicating that this kind of cave is low in filling degree and can be good storage space for oil and gas, so they should be paid more attention to.

It can be seen from the RMS seismic attribute section that the fractures and caves in the hoodoo-upland mostly are small-medium sized fracture-cave groups, and the closer to the Ordovician unconformity, the more developed the small and medium-sized corrosion fractures and caves are. The fractures and caves interweave with each other in a network, forming a nearly homogeneous peak fracture-cave reservoir (Fig. 9). For example, Well S48, the highest production well in Tahe oilfield, the seismic section doesn’t show "strong beading reflection" indicating karst cave at the location of this well; but when drilling into the Ordovician system at 5 363.5 m on October 17, 1997, the well had drilling fluid loss, and no cuttings return from the wellhead, and the drilling fluid loss reached 2318.8 m³ when it was drilled to 5370 m, so it was impossible to continue drilling, and the well was completed directly by tubing. The well has been producing at high rate in natural flow, with a cumulative oil production of 74×10⁴t. Another example is Well T402 located in T402 upland with an area of 2.06 km², the well revealed small and medium-sized fracture-cave groups of 102.6 m thick, which is equivalent to the peak height (106.6 m), indicating that the development of the small and medium-sized fracture-cave groups is controlled by local erosion base level. However, in the karst valley with faults, intermittent streams were likely to seep along the faults, resulting in infiltration and corrosion and formation of vertical continuous fracture-cave belts. The lower limit depth of fracture and cave development of this kind of fracture-cave body is far beyond the boundary of the erosion base level. For example, Well TK408 and TK411 have a 118.6 m and 132.5 m thick fractured-cave belt below the near-surface caves respectively. Therefore, faults and geomorphology jointly control the structure and distribution of fractures and caves in Hoodoo uplands. What needs to be emphasized here is that, as the hoodoo-upland was in the area lack of water, so it suffered weak karstification, and had mainly small and medium-sized fracture-cave groups around the peaks, and no large karst caves, which was different from the fault reservoir in Shunbei area with large karst caves developing along the fault zone^[27].

According to the RMS data after frequency extension processing, the distribution of abnormal fracture-cavity bodies 50 ms (about 150 m) below T74 was calculated (Fig. 10a). The results show that hoodoo-uplands of S74 and T402 have mainly small and medium-sized fracture-cavity groups. Wells T402, S66, TK408 and TK411 mentioned above are all distributed in small and medium- sized fracture-cavity areas. In contrast, large underground river karst caves take majority in the karst depression, for example, underground rivers in the well block TH12112 and TH10115 in the northwest of the study area, and underground rivers in direction of T601-S67- T615 and S65-TK451-T444 in the karst depression between hoodoo uplands.

The Ordovician paleo-structures (paleomorphology and faults) and cumulative oil production of wells (reflecting oil and gas enrichment degree) were analyzed together. It is found that high-yield wells in the study area are mainly distributed in the areas of S74 and T402 hoodoo-uplands, and the larger the scale (area and height) of the karst peak, the higher the oil production is (Fig. 10b). The wells in T402 hoodoo-upland have higher oil production than those in S74 in general, while the wells in karst depression area between the hoodoo-uplands have lower cumulative oil production generally. 60% of wells in the hoodoo-uplands have cumulative oil production of more than 20×10⁴ t each. As mentioned above, Well S48 and TK408 have a cumulative oil production of 74.1×10⁴ t and 39.3×10⁴ t respectively. 80% of the oil wells in the karst depression stop production before reaching the cumulative oil production of 10×10⁴ t, for example, Well T601, T415 and T615 have a cumulative oil production of 1.8×10⁴ t, 5.1×10⁴ t and 2.1×10⁴ t respectively. It is worth noting that wells in the NE main fault zone have oil production significantly higher than those in the area with no faults, for example, wells in the fault zones of S74-S67 and T402-T452 direction (Fig. 10b). Therefore, the deep and large faults have important influence on oil and gas enrichment.

The southeast of the Tahe area is adjacent to the huge hydrocarbon source (Manjiaer sag). From the middle and late Caledonian to the Himalayan period, the Cambrian- Ordovician source rocks in the Manjiaer sag entered hydrocarbon generation and expulsion periods several times^[16,28]. A large amount of oil and gas migrated from the karst basin to the karst upland along channels such as unconformities, fracture-cave zones and faults of the Ordovician (especially the deep and large faults cutting into the Cambrian). The karst slope was on the route of hydrocarbon migration, and the local uplifts on the slope were favorable areas for oil and gas accumulation^[28]. The hoodoo-uplands at Ordovician karst slope in the Tahe area are favorable places for hydrocarbon accumulation with conditions for large scale hydrocarbon enrichment. First of all, the karst peaks were formed from positive flower structure in the Early Hercynian under the background of left compression and torsion, with abundant faults. Especially in the late Hercynian and Himalayan periods, the multiple active periods of the deep and large NNE trending faults coincided with the hydrocarbon generation and expulsion periods of the Manjiaer hydrocarbon source kitchen, so oil and gas charged into the positive flower structure along faults in multiple periods. Secondly, the formation period of the hoodoo-upland was consistent with the karstification stage, and a large number of small and medium-sized fracture-cave groups were formed during the long-term karstification, which provided ample effective reservoir space for oil. Moreover, the hoodoo-uplands had the characteristic of inherited evolution, and they had been in positive structures since formation, and had always been on the direction of hydrocarbon migration laterally or vertically. In addition, with the effective sealing of mudstone of Bachu Formation and tight limestone around the fracture-cavities, multi-stage oil continuously migrated horizontally and vertically to the reservoir space in the hoodoo-uplands to accumulate, forming a peak-hill fracture-cave reservoir with vertical quasi-continuous distribution and horizontal continuous enrichment (Fig. 11).

In the early Hercynian period, the positive flower structures developed under the background of left-lateral compression and torsion formed the rudiments of hoodoo-upland on the Ordovician karst slopes in the Tahe area, which controlled the differential distribution of karst drainage and led to the differential karstification and formation of the hoodoo-upland composed of towering hills and narrow valleys. The dense small and medium-sized fracture-cave groups around the hill peaks and caves formed by collapsed breccias in the valleys are the main oil and gas storage spaces. The hoodoo uplands developing successively have been always on the oil-gas migration direction. Under the effective sealing of the lateral tight limestone and overlying mudstone, large amounts of oil and gas charged and accumulated into the medium and small size fracture-cavity bodies during the multi-phase activities of the NE-trending source-connecting faults, forming fracture-cavity reservoirs in quasi- continuous distribution vertically and laterally in the karst peaks. 60% of oil wells there have a cumulative oil production of over 20×10⁴ t. Local positive landforms on the karst slope are mostly related to deep and large faults under the background of strike-slip compression. The effective space configuration between them not only provides fluid migration channels, but also a large amount of effective reservoir space, also they are the dominant area for hydrocarbon migration and accumulation.