At the beginning of the 20th century, Grund first proposed the concept of saturation zone, and believed that groundwater has zonation^[1,2]. Later, Cvijić combined hydrology with geomorphology and put forward the viewpoint of karst-hydrological zoning^[3], and then Swinnerton proposed a two-part scheme of vadose and phreatic karst zone based on karst outcrop^[4]. On this basis, the vadose karst zone was subdivided by Esteban into upper infiltration zone and lower percolation zone. the phreatic karst zone was subdivided into active variation zone and deep phreatic zone, meanwhile, established a typical karst hydrodynamic profile^[5,6]. Loucks identified the genetic types and structure of fracture-caves in different karst zones through outcrop observations, and established the development model of subaerial exposure karst, which clearly shows the spatial structure of karst cave system^[7]. At the early 21st century, the view of karst-hydrological zones was gradually introduced into the paleokarst studies in the Tarim, Ordos, Bohai Bay and Si-chuan basins in China^[8,9,10], especially in the last 10 years, breakthrough progress has been made in the study of Ordovician karst zones in Tahe Oilfield, Tarim Basin, NW China^{[11,12,13,14]}. Jin and Li et al., combined with the karst geological conditions of Tahe area, proposed a four-division scheme of karst zones, from top to bottom: epikarst zone, vadose karst zone, runoff karst zone and phreatic karst zone^[12,13]. On this basis, Zou et al. established the identification methods for different karst zones by integrating drilling and logging data, fracture-cavity types, filling characteristics and karst morphology^[14].

However, the exploration and development of Tahe Oilfield in recent years has revealed that the combination structures of karst zones are different in different karst geomorphic units. No underground river caves have been found in the local highlands (watershed) on the Ordovician karst slope^[15,16,17], and no karst zoning has been found in the Upper Ordovician covered area with fault reservoirs^[18]. How such differences are formed and distributed is directly related to the deployment of Tahe Oilfield exploration and development schemes. In this study, based on abundant geological and geophysical data in Tahe Oilfield, the differential combination structures of karst zones in different geomorphic units are studied by analyzing the karst drainage flowing channels and fracture-cave characteristics of different karst zones, meanwhille, combination with the oilfield production practice, the hydrocarbon accumulation is studied in order to provide guidance for the exploration and development of fracture-cave reservoirs in Tahe Oilfield and other areas.

The Tahe Oilfield is located in the north of the Tarim Basin (Tabei area), and the main reservoirs are the Yingshan Formation and Yijianfang Formation of Ordovician with karst fractures and caves developed^{[19, 20]}. During the Early and Middle Ordovician, Tabei area was in a carbonate platform environment, and a set of thick carbonate rocks were deposited^[21]. Among them, bioclastic limestone of Yijianfang Formation (O₂yj) and arenaceous limestone of the upper member of Yingshan Formation (O_1-2y^I) are the most soluble rocks, followed by dolomitic limestone of the lower member of Yingshan Formation (lower O_1-2y^II) and dolomite Penglaiba Formation^{[11, 21]}. In the Late Ordovician, influenced by multi-stage lifting of Caledonian movement^[22], marl and nodular limestone of mixed shelf facies as well as mudstone and siltstone were deposited in Tabei area^[21], with poor solubility (it can be used as water barrier layer in karst area)^[23]. From bottom to top, there are Qarbake Formation, Lianglitag Formation and Sangtamu Formation (Fig. 1).

At the end of the Silurian, the northern part of Tarim was strongly uplifted, and many NNE and NNW strike- slip faults were developed at the same time^[24,25], which led to the long-term exposure of the Ordovician carbonate rocks, and intense karstification occurred (the Early Hercynian karstification). The meteoric water erosion and corrosion occurred strongly along the faults and fractures, forming numerous branching karst drainage. Between each karst drainage is the banded watershed, and between the watersheds is the karst valley formed by long-term erosion and corrosion of rivers. The two watersheds and one karst valley formed an independent karst basin or unit^{[16-17, 26]}. As shown in Fig. 1, there are four karst drainages with nearly north-south distribution in Tahe area, corresponding to four banded watersheds and karst valleys with different scale and shape. Among them, a dustpan-shaped karst valley was developed between the watershed S71-S74 and watershed S48-T402, which constituted a relatively independent karst unit, where many surface rivers (the main stream and distributary rivers) were developed. It is worth noting that the rivers all disappeared near the pinch-out line of the Qarbake Formation. It is speculated that there may be water body (lagoon) south of the pinch-out line. It is called a karst polje, and the pinch-out line of Qarbake Formation is regarded as the dividing line between karst valley and polje. The watershed and karst valley developed weathering crust karst due to the direct exposure of the middle and lower Ordovician soluble limestone. In karst polje area, fault-controlled burial karst was developed because the insoluble strata of the Upper Ordovician series overlay the Middle-Lower Ordovician^[18].

At the end of Devonian to early Carboniferous, Tahe area was in a delta-tidal flat environment^[27], and the Bachu Formation overlay gradually the Ordovician unconformity from southwest to northeast, and the Early Hercynian karst was over^{[21, 28]}. During the later tectonic evolution process, the Ordovician system in Tahe area has been buried deeply underground, with the characteristics of inheritance evolution^[22].

During the Early Hercynian karst process (Karst stage), the epikarst zone was developed close to the surface of weathering crust, in which the structural fractures and weathering fissures were extremely well developed, with two flowing pattern, namely surface runoff (slope flowing) and infiltration near the surface^[12,17]. When atmospheric rainfall scattered in limestone of weathering crust surface, in the steep slope area, the flow was mainly slope flowing along the slope, resulting in formation of erosion karren (surface river), the karst hill and the stone forest. Among them, the surface river flow along faults and fissures,at the intersection of faults and fractures, with the progress of karst was developed doline (the bottom of river valleys or doline is usually the bottom boundary of the epikarst zone)^[12]. In the gentle slope area, due to the small gradient of water flow, the water can enter the interior of weathering crust along the intricate crack channels, resulting in infiltration corrosion, forming combination groups of karst vugs and fracture-caves (small and medium-sized fracture-cavity) with quasi-contiguous distribution. This type of fracture-cave has the characteristics of low filling, and small amount of non-soluble fine grain material filling was seen locally. Expanding (borehole diameter expansion) and leakage (drilling fluid was lost) occurred frequently during drilling (Fig. 2). For example, in Well T403, the Ordovician Yingshan Formation was drilled at a depth of 5408.2 m, with oil and gas shows immediately, and then a densely developed small and medium-sized fracture-caves were drilled (Fig. 2a). At the depth with the fracture-caves, the natural gamma ray curve is low (close to the limestone background value of 7-9 API), the compensated acoustic time difference value is 240-277 μs/m, and the compensated neutrons value is 0-2%. The Dual lateral resistivity curves showed a bulge type with medium-high resistivity (400-500 Ω·m). The core showed that crude oil was filled in the karst fracture-caves, and the slice also showed that the karst vugs (blue in slice) were densely developed in the limestone, with pore diameter sizes ranging from 0.1 mm to 0.7 mm (Fig. 2d).

During karst stage, the vadose karst zone was located under the epikarst zone. It was far away from the surface of weathering crust, the fractures and weathering fissures in it were reduced, and the relatively isolated faults and high-angle fractures were the main flow channels. Water flow percolated downward through the epikarst zone, forming karst fracture-cave complex, or directly percolated downward through the doline, forming a karst shaft, or expanded corrosion at the intersection of faults and fractures, forming a water-storage cave^[14].

In karst period after the burial process, this type of fracture-cave has the characteristics of low filling (A small amount of collapse breccia and calcium clay fillings), expanding and leakage. But, it is mainly in the shape of relatively isolated vertical belt, which are different from the densely developed epikarst zone in lateral quasi-continuous. So, the boundary between these two zones can be divided (the top boundary of the vadose karst zone or the bottom boundary of epikarst zone). For example, in Well T403, at a depth of 5414.6 m, the fracture density began to reduced significantly, and the values of R_LLD and R_LLS were reduced suddenly (40-60 Ω·m), and there is a clear positive amplitude difference (R_LLD is greater than R_LLS), formed a discontinuous beaded fracture-cavity complexes by corrosion and expansion of vertical fractures (Fig. 2e). Among them, a 7.3 m water storage cave was developed at the depth of 5438.5-5445.8 m, where the GR value was 10-15 API, Δt value was 240-277 μs/m, R_LLS value was 100-220 Ω·m. It can be seen that it is completely different from the densely developed karst small and medium-sized fracture-cavity in the epikarst zone (Fig. 2d, 2e). Therefore, the depth of 5414.6 m is the boundary between the vadose karst zone and the epikarst zone.

During the karst period, the runoff karst zone was located near the underground water table, and water flow mainly along this level as near-horizontal runoff. Faults, fractures and unconformities were the main flow channels. After the meteoric water reached the underground water table through the fault, dolines, shaft, or other channels of fracture-cave, due to the water lifting (buoyancy) effect, not downward percolation occurred, but radial flowing, which was the biggest difference between the runoff karst zone and the vadose karst zone. At the beginning, the water flow percolated radially along the fractures near the underground water table. As the karstification continued, the fractures continued to expand, forming corridors or karst caves, and then the water flow changed into a radial pipe flow. Due to the intense erosion and corrosion of pipe flow, a complex karst caves of underground rivers were formed gradually, which were often filled with sedimentary sand and mudstone, and the expanding and leakage occurred easily. As shown in Fig. 2, in Well T403, underground river caves of 66.7 m were drilled at the depth of 5487.6-5554.3 m, which was filled with a large amount of sedimentary conglomerate, the gravels are composed of Ordovician limestone clastics, with a moderate degree of rounding, with characteristics of water flow transportion, meanwhile, a large number of exogenous other-shaped quartz minerals are filled between the gravels (Fig. 2f). At this depth with cave development, the GR shows a box-shaped or peak-cluster middle-high value (10-80 API), and the maximum values of Δt and ϕ_CNL exceed 333 μs/m and 30%, respectively. The R_LLD and R_LLS are also box-shaped, with extremely low values (3-30 Ω·m). In addition, this type of cave can be clearly identified on the seismic attribute of wave im-pedance (IMP), which distributed in a continuous curved band on the plane and in a continuous pipe shape on the profile at 3.48-3.51 s. It is completely different from the quasi-continuous and isolated fracture-cave in the epikarst zone and vadose karst zone (Fig. 2b, 2c). Based on this, it can be determined that the top boundary of the runoff karst zone is at 5487.6 m in Well T403. It should be emphasized that the underground water table is not horizontal, it rises slowly with the elevation of the terrain, and its position is not stable, but fluctuation up and down with seasonal change, resulting in the development of a complex underground river karst cave system in runoff karst zone.

During the karst period, the Phreatic karst zone was located below the underground water table, and the water source was mainly formation water (CO₂ capacity balance, poor corrosion) mixed with a small amount of meteoric water. The water flow mainly flowed slowly along the bedding or fissures, and weak karstification, forming a small amount of corrosion vugs or fissures along the bedding. Because of its slow water flow and a weak enclosed environment, the corrosion fissures were mostly filled with gray-green mud. Compared with the runoff karst zone, the phreatic karst zone has greatly weakened in fracture-cavity development, and it is easy to distinguish. For example, the 9th core from Well T403 showed that the Ordovician limestone (arenaceous limestone) was drilled at the depth of 5562.1 m, and the logging response characteristics also returned to the background value. The 10th core showed that only some structural fractures are developed in the arenaceous limestone with almost no signs of corrosion expansion, but bedding corrosion fissures filled with gray-green muddy are found (Fig. 2g). In addition, it can also be seen from the IMP attribute profile that below 3.51 s, the karst fracture-caves response bodies are basically not developed. Based on this, it can be determined that the top boundary of the phreatic karst zone is at 5562.1 m (the bottom boundary of the runoff karst zone). The bottom boundary of the phreatic karst zone is usually insoluble and non-permeable rock formations^{[2, 5, 14]}. However, in Tahe Oilfield, for the platform facies thick massive limestone strata (Middle-Lower Ordovician) with developed faults, the bottom boundary of the phreatic karst zone may be very deep.

To sum up, in the epikarst zone, quasi-continuous small and medium-sized fractured-vugs are well developed. The vadose karst zone is characterized by the development of isolated vertical fracture-caves belts, and the runoff karst zone is characterized by the development of a large underground river caves. The phreatic karst zone is characterized by the development of only a few bedding corrosion fissures. These karst fracture-cave combinations, fillings, and geological-geophysical response characteristics can be used as important indicators to identify and classify different karst zones in karst periods (Table 1).

The watershed is a local highland on the karst slope, with densely developed faults and fractures, which intersect with weathered fissures, forming an extremely complex water flow network channel. However, the watershed is a poor water area, lacking the supply of surface river water, and atmospheric rainfall is its main source of water. In addition, due to the large topographical drop of the watershed, water flowed easily and quickly into the adjacent karst valley along the slope, the water-rock contact time was short, and the karstification was weak. In the rainy season, meteoric water infiltrated along the complex network channels in the watershed, and diffuse infiltration and corrosion occurred, resulting in the formation of the densely developed small and medium-sized fracture-cavity bodies, that is epikarst zone. After that, if there was sufficient water, it could penetrate the vadose karst zone and reached near the groundwater table, and runoff erosion occurred. However, for the watershed with poor water, due to its high landform and deep the table, the water was difficult to reach the groundwater table. Even if a small amount of water arrived, due to the limited amount of water, it was difficult to form an underground river channel, but to form a small amount of bedding corrosion fissures similar to the phreatic karst zone.

As shown in Fig. 3, the landform of the watershed in Well Block T402 has a height difference of 300 m, and faults and fractures are densely developed (Fig. 3a, 3b). Well T402 drilled into the Ordovician at a depth of 5358.5 m, and then the epikarst zone (thickness of 76.3 m) and the vadose karst zone (thickness of 103.6 m, Fig. 3c) were drilled. Among them, the epikarst zone is densely developed with small and medium-sized fracture-vugs, and its well logging is characterized by flat-low GR, clustered medium-high Δt and low R_LLD. At the depth of fracture and cavity development, the coring recovery rate was less than 20%, and the core was severely broken, being angular rock blocks (fault breccia), with good oil content (Fig. 3d). The vadose karst zone is dominated by relatively isolated fracture-caves in vertical, and the well logging shows the characteristics of flat-low Δt and medium R_LLD (Fig. 3e). The amplitude seismic attributes of iterative root mean square (RMS) show that the interconnected karst fracture-vugs in the epikarst zone are distributed in sheets around the top of the watershed, and the isolated fracture-caves of the vadose karst zone are distributed in a vertical belts. Meanwhile, the development degree of fracture-caves decreases suddenly below 3.48 s, and there is almost no fracture-cave response. That is to say, the karst cave of underground river in the runoff zone is not developed. In addition, the results of multiple production tests show that the oil-water interface has risen gradually (as shown in Fig. 3e, the oil-water interface was not seen in the years of 1998, and it was speculated that the oil-water interface was below 5468 m. By 2002, the oil-water interface had risen to 5430 m. By 2004, the oil-water interface had risen to 5380 m). The results show that the densely developed karst fracture-caves in the watershed are interconnected, forming a quasi-continuous peak-hill reservoir.

The karst valley is a negative topography and belongs to a catchment area with abundant water and widespread karst drainage, where topography is flat, water flow is slow, water-rock contact time is long, karstification is strong, and the amount of karst is large (thickness of the residual stratum of the Yingshan Formation in the karst valley is greater than the watershed), and the epikarst zone and seepage karst zone are thinned due to strong karst. At the same time, due to the low topography and shallow depth of the groundwater table, sufficient water flowed continuously to reach the groundwater table through faults, cracks or sinkholes, causing radial corrosion, gradually forming underground river caves and corridors (drainage channels), and then the water flowed out of the surface or merged into a karst polje (drainage basin) to form a complete karst water drainage system. However, due to the complex structure of underground river (sometimes wide and sometimes narrow), a large amount of clastic materials carried by surface rivers were easily filled in the caves, resulting in serious filling of caves in underground rivers. At the same time, affected by the fluctuation of the groundwater table, multiple layers of complex underground river channels were developed.

As shown in Fig. 4, the TK730 karst valley is higher in the north and lower in the south, with a landform height difference of less than 40 m. Drilling and seismic data reveal that there are epikarst zones (average thickness of 15.6 m), vadose karst zones (thickness of 22.8 m), runoff karst zones (thickness of 126.5 m), and phreatic karst zones (not drilled through) from top to bottom. Among them, in the runoff karst zone, the upper and lower double-layer "overpass" network-shaped underground rivers are developed, which are filled by sedimentary sand, mudstone and collapsed breccia. Three filling sequences are developed, and the filling rate ranges from 65% to 100%, with an average of 86.8%. The planar distribution of the underground river mainly spreads along the faults and fractures in the north-south direction (Fig. 4c), and the section is continuously distributed from high to low near the groundwater table (affected by faults, fractures, geomorphology, and groundwater table). Underground river flows from the northern TK644 Well Block through the TK632-TK730 Well Block, and returns to the TK 734 Well Block and merges into the drainage basin, forming a huge lenticular runoff karst zone.

The karst polje area had been submerged by water for a long time. Since CO₂ was concentrated in the water, which greatly reduced the corrosion capability. In addition, the Upper Ordovician insoluble rock layer was overlaid on the Middle-Lower Ordovician soluble limestone, the water and rock cannot be contacted, and karstification cannot occurred. Therefore, the karst zone is not developed in the polje. However, due to fault activity in some areas, the water was turbulent (the CO₂ concentration changed and was corrosive), and poured into the ground along the faults to contact the soluble rock, resulting in karstification and forming fault-controlled karst fracture-caves distributed only along the fault. That is, fault reservoir^{[18, 29]}.

As shown in Fig. 5, in Well T807, which is located on the NW faults in the polje, a marl (insoluble rock layer) of the Qarbake Formation was drilled at the depth of 5532.6 m, with no oil or gas shows. When the limestone of Yijianfang Formation was drilled at the depth of 5552.2 m, oil and gas shows were found ( indicating the existence of karst fracture-caves), and then a caves as high as 52.7 m of karst caves were drilled at the depth of 5696.5-5748.2 m. This type of cave is mainly filled by loose fault breccia, while clay and other fine-grained materials are filled less (the GR value is close to the limestone base value 7-9 API, the Δt value is as high as 500 μs/m, and the R_LLD/R_LLS values are as low as 10 Ω·m with obvious positive difference), indicating that a large amount of reservoir space is still reserved. The fractures and cavities are mainly distributed around the fault belt within the range of 400-500 m, and the vertical depth can reach 600 m (the karst fracture-caves are suddenly reduced below 200 ms), showing a quasi-continuous "waterfall" type spatial structure. It is completely different from the underground river caves in the runoff karst zone (GR value is 60-80 API, horizontally continuous belt-like distribution), and also deeper than the small and medium-sized fracture-vugs in the epikarst zone and vadose karst zone.

With the 3D carving technology for fracture-cave, the karst fracture-cavity (wave impedance seismic attribute response scale of the fracture-caves) between T₇⁴ and T₇⁶ interfaces in the Tahe area during the Early Hercycyan period were carved^[30]. It can be seen from the superposition map of fracture-cave anomaly body and karst landform of this period that the fracture-cave structure combination of different geomorphic units is different. The distribution of fracture-cavity in the watershed is mainly isolated quasi-continuous sheets, continuous belts (underground rivers) in the karst valley area, and linear discontinuous in the karst polje. Among them, underground rivers are connected with surface rivers, which are distributed in the karst valley around the watershed and end in the karst polje, forming a complete karst water discharge system from surface river to underground river and then to drainage basin (Fig. 6).

The interpretation results of karst zone structure in the north-south cross-well section showed that in Wells S88, TK688 and TK653 in the watershed, densely developed small and medium-sized fracture-caves, namely the epikarst karst zone, at 30.4-83.5 m below the weathering crust, with a thickness of 28.2-105.6 m and an average thickness of 57.8 m (Fig. 7). In Well TK610, drilled in isolated karst caves at 126.4 m below the weathering crust (at 3.48 s), and these karst caves were all distributed in the time range of 3.48 s to 3.50 s. That is to say, the vadose karst zone is developed, and the thickness ranges from 54.3 m to 132.1 m, with an average value of 115.2 m. Below 3.52 s, the fracture-cavity decreases suddenly and almost disappears. That is to say, there is no underground river cave system in runoff karst zone.

However, in the karst valley, in Wells T615, TK734 and TK647, underground river karst caves were drilled at 10.5-22.3 m below the weathering crust. In Wells TK643 and TK650, underground river karst caves were drilled at 53.2 m and 68.8 m below the weathering crust respectively. That is to say, the runoff karst zone is developed, and the thickness ranges from 61.5 to 188.2 m, with an average value of 132.6 m (Fig. 7). Due to the strong erosion of water flow in karst valley area, the epikarst zone and vadose karst zone are relatively thin, 26.4 m and 14.6 m respectively. At the same time, below 3.54 s, the development of fracture-cavity decreases sharply. In Well TK734, the corrosion cracks filled with grey-green clay were drilled, which is known as the phreatic karst zone.

In Wells T703 and T807 in the karst polje, there is no karst zone. In Well T703, no fracture-caves were drilled, while in Well T807 in the fault area, a cave with height of 51.7 m was drilled in 144.3 m below the weathering crust. The wave impedance attribute also showed that the vertical continuous fracture-cavity was developed at the faults belt around the well T807 (Fig. 7). This result is consistent with the 3D carving results in Fig. 5.

To sum up, during karst period, under the condition of the same karst basin and the same lithology, in different karst geomorphic units, due to the difference in drainage distribution, flow channels and modes, different structural combinations of karst zones were developed. In the watershed, the epikarst zone and vadose karst zone are mainly developed, in which the small and medium-sized fracture-vugs and isolated caves are developed, and the underground river are not developed. In the karst valley, there are epikarst, vadose, runoff and phreatic karst zones, in which the thickness of runoff karst zone is the largest, and a large underground river cave karst system is developed. There is no karst zone in karst polje area, only fault reservoir are developed in local active fault belt (Fig. 8).

The Ordovician karst geomorphology, faults, and underground rivers in the Tahe area were superimposed with the current cumulative oil production of a single well (reflecting degree of oil and gas enrichment). The results show that in 76.5% wells in the watershed, the accumulative oil production per well is more than 5×10⁴ t, and it is distributed in sheets, such as in Well Block S66-S74 and Well Block S48-T402. In karst valley area, in 32.8% wells, the accumulative oil production per well is more than 5×10⁴ t, and it is mainly distributed along faults, such as in Well Block T607 and Well Block T436. However, in the underground river development area, the wells are generally low productive, in 84.9% wells, the accumulative oil production per well is less than 2×10⁴ t, such as in the Well Block of T615 and Well Block TH10125. In karst polje area, in 83.6% wells, the accumulative oil production per well is less than 5×10⁴ t, but the oil wells on the fault belt, the accumulative oil production per well can exceed 5×10⁴ t, such as in Well Block of T702b and Well Block T781, while the oil wells outside the faults belt, the accumulative oil production per well is generally less than 1×10⁴ t, such as in Well Block T707. In addition, in 56% oil wells in the NNE faults (F1, F2) in the study area, the accumulative oil production per well is more than 3×10⁴ t, and 18% of wells had cumulative production per well of more than 30×10⁴ t (Fig. 9).

Overall, oil and gas is best enriched in the watershed, followed by karst valleys, and karst basins are worse. However, due to the local multi-stage activities (both active in karst periods and hydrocarbon accumulation stage), source faults (faults communicating with hydrocarbon source kitchens) are not only karst water channels, but also spaces for hydrocarbon migration and accumulation, resulting in the same degree of hydrocarbon enrichment in different geomorphic units.

In the watershed area, densely faults and fractures are well developed, which provided complex network of water channels, and guaranteed the development of densely distributed small and medium-sized fractured-vuggy reservoirs under the condition of limited water. In addition, with the characteristics of inheritance evolution, it has always been in the structural high position, and has always been the direction of hydrocarbon migration and accumulation, oil and gas migrated and accumulated continuously along faults, unconformities and connected fractures and vugs, forming hill-peak reservoirs around the watershed in quasi-continuous distribution, with large hydrocarbon abundance and high oil production (Fig. 10), such as in Well Blocks S48, S74, T402 and other well areas.

In the karst valley, faults are also important paths for karst water and hydrocarbon migration. Adequate water flowed along faults and fractures into the underground and radial corrosion near the groundwater table, forming complex underground river caves. Due to the strong transported capacity of the underground rivers, the caves were filled seriously, and a large amount of oil and gas storage space was lost. In addition, the karst valley is adjacent to the watershed, and is in negative structure, which belongs to the disadvantage area of oil and gas migration and accumulation, and the degree of oil and gas enrichment is poor, and the oil wells generally have low production. However, in the area without underground river, the multi-stage active source faults not only provided a huge fault reservoir space, but also communicated the deep oil source, making up for the shortcomings of small space and insufficient oil source, forming a high-yield oil and gas enrichment area (Fig. 10), such as in Well Block T607 and Well Block T436.

Although there is no karst zone in the karst polje, the karst fracture-cave reservoirs are developed around the faults belt, and the high-yield oil and gas enrichment area has been developed by effective matching with the source faults (Fig. 10), such as in Well Block T702B.

In Tahe area, Tarim Basin during the Early Hercynian Period, the Ordovician karst landform consisted of three geomorphic units, namely watershed, karst valley and polje. In different geomorphic units, the distribution of karst drainage and water flowing pattern were different, resulting in different structure of karst zones. In the watershed, there are epikarst and vadose karst zones, but no cave of underground river in runoff karst zone. In the watershed, there were densely developed faults, fractures, fracture-vugs complex and water storage caves, which were connected with each other through fractures or fissures, and forming the quasi-continuous hilly-peak type reservoir with high hydrocarbon abundance and high production. In the karst valley, there are epikarst zone, vadose karst zone, runoff karst zone and phreatic karst zone, among which runoff karst zones account for the largest proportion, where large underground river caves were developed and had been mostly filled by sedimentary sand and mud, resulting in serious loss of reservoir space, poor hydrocarbon enrichment and low production. In the karst polje, there is no karst zone, because it had been submerged by water for a long time, only fault reservoirs are developed in the part of local active fault zone. Faults are not only the channels of karst drainage, but also the migration path and accumulation space of oil and gas. The fault reservoir is an important target area for exploration and development of karst carbonate reservoirs.

GR—natural gamma, API;

R_LLD, R_LLS—deep and shallow lateral resistivity, Ω•m;

Δt—compensated acoustic time difference, μs/m; φ

ϕ_CNL—compensated neutron, %.