Karstification is the joint of effects between the surface water flowing through permeable channels and the surrounding rocks such as erosion, dissolution, deposition, and cementation, when carbonate rocks are exposed in meteoric water diagenetic environment and affected comprehensively by physical factors (surface water flow velocity, temperature, pressure, lithologic characteristic, tectonic movement, base level, hydraulic gradient, and climate, etc.), chemical factors (composition, acidity, CO₂ content, and oxidization of karst water), and biological factors (root ecosystem, microorganism, and cavity dwellers)^[1,2,3,4,5]. Karstification produces 4 action zones in turn below the unconformity, namely, epikarst zone, vertical percolation zone, horizontal underflow zone, and deep slow flow zone. The water table is the boundary between the vertical percolation zone and the horizontal underflow zone. The enhanced water pressure in the horizontal underflow zone promotes erosion and dissolution, so the karst fracture-cave system formed has better connectivity and effectiveness and can become karst reservoir^[2,6-10]. If the base level (generally sea level) rises in a periodic manner, the water table and the horizontal underflow zone below it would develop in multiple stages, giving rise to multiple sets of karst reservoirs stacking over each other vertically, with greater thickness and storage capacity^[2,9-11]. Therefore, if the position of the water table is accurately identified, the development characteristics of the horizontal underflow zone below the water table can be figured out, which is helpful for predicting the distribution of reservoirs, optimizing well location, selecting target layer for horizontal well, and designing well trajectory, etc. However, continuous full coring cannot be conducted for the target layer in actual well drilling, so it is hard to locate the paleo-water table for a well with the petrological and mineralogical characteristics, etc. like that for the cross-section of an outcrop. Enlightened by the wide application of imaging logging in study of depositional reservoirs^{[12,13,14,15]}, taking the Ordovician Yingshan Formation in the north slope of Tazhong low bulge, Tarim Basin as an example, we analyzed the karstification in different karst geomorphic units in different stages of karstification, especially the characteristics and differences of the horizontal underflow zone, with the data from imaging logging, conventional logging, and core, etc., established a method for identifying the water table to sort out the stages, position and distribution of the water table, and to analyze the control of karstification on the reservoir and establish corresponding models in this study.

Tazhong area is located in the central part of the central bulge in Tarim Basin, and is the main part of Tazhong low bulge (Fig. 1a), where Yingshan Formation, with original sedimentary thickness of 900-1 000 m, is divided into 4 members from the top down. The Member 2 and Member 1 of Yingshan Formation in the upper part are dominated by micrite, grain limestone and a little clotted limestone, while the bottom part of Member 2 of Yingshan Formation to Member 4 of Yingshan Formation are characterized by interbeds of limestone and dolomite^[16,17,18]. In the Middle-Late Ordovician, the regional compressional tectonism gave rise to the Tazhong low bulge and caused erosion of the upper part of Yingshan Formation in the study area to various extents. The erosion hiatus lasted nearly 15 Ma^[7,16], during which the karst paleogeomorphy gradually descending from southwest to northeast was formed^[14,19-22] (Fig. 1b). From profile of wells, we can see there develop dissolved fractures and dissolved caverns filled by mud, sand, and breccia, etc. to various extents, in the epikarst zone; high-angle dissolved fractures and a few dissolved caverns in the vertical percolation zone formed by downward percolation of karst water; cystic dissolved vugs, and near horizontal dissolved fractures and dissolved caverns, filled by mud, silt, sand, and breccia, etc. to various extents in the horizontal underflow zone; few vugs and fractures in the deep slow flow zone, as the water becomes stagnant there^{[22,23,24,25]} (Fig. 1c). However, overlay of karst fabrics of the horizontal underflow zone and vertical percolation karstic zone is commonly seen on profile of wells relatively high in topography (Fig. 1d). This is mainly because in subsequent periodic rise of sea level, the higher the karst paleogeomorphic site, the longer the exposure time of the site, and the greater the intensity of karstification would be. The development of multiple stages of horizontal underflow zones caused the karst reservoirs to be much more developed^[19,26-27]. For example, wellblock Zhonggu 8 in karst slope and well block Zhonggu 43 in karst highland both have high-quality karst reservoirs, with proven reserves of the magnitude of one hundred billion cubic meters.

Yingshan Formation in Tazhong area is divided into 4 karst zones: (1) In the epikarst zone, there develop dissolved fractures and dissolved caverns, etc. of relatively small scale, which are likely filled by materials of mechanical origin, such as mud, sand, and breccia. The top boundary of this zone is the uncomformity. The zone features conspicuous high values on gamma log; dissolved cavern responses such as dark brown-black, and yellowish white patches, etc, and dissolved fractures of dark-color in shapes of “near vertical”, “low-amplitude sine curve”, and relatively large included angle “V”, etc. on electric imaging log (Fig. 2a and 2b). (2) In the vertical percolation zone are largely high-angle dissolved fractures and a few dissolved caverns, which take on dark-color or bright-color stripes of “nearly vertical”, “high-amplitude sine curve”, and small included angle “V” shape, etc on the electric imaging logging. This zone exhibits box shape on natural gamma and resistivity curves. (3) The horizontal underflow zone contains karst fabrics like cystic dissolved vugs, near horizontal dissolved fractures and caverns, filled by mud, silt, sand, and breccia, etc. to various extents (Fig. 2d, 2e, 2g and 2i), which are shown as cystic dark spots, mixed dark-color and bright-color patches, or near horizontal dark-color irregular strips on image logging, and features high gamma and low resistivity (Fig. 2c, 2f and 2h). (4) In the deep slow flow zone, the water becomes stagnant, so it is hard to form karst fabrics like fractures and caverns, and this zone has no obvious responses on electric imaging and conventional logs.

It is found from the interpretation of single well karst profile and comparison of wells that influenced by periodic rise of sea level, the stages of karst water table and the characteristics of stacking development of karst zones differ in different karst geomorphic units. There are first-stage karst water table, first-stage and second-stage karst water tables, and first-stage, second-stage and third-stage karst water tables, developed in the karst depression, karst slope, and karst highland, respectively, with 1 stage, 2 stages, and 3 stages of water tables in total, respectively (Fig. 3). From the karst depression to the karst highland, the denudation degree of Yingshan Formation increases, and Members 5 and 4 of Lianglitage Formation in the lower part of the overlying Ordovician Lianglitage Formation are evidently lost accordingly^[19,28]. Geochemical analysis^[26] shows that the sea level rose in a periodic manner after the karstification period. The karst depression was covered by sea water the earliest, receiving the deposits of Lianglitage Formation, with complete deposition of all members of Lianglitage Formation. Therefore, the karstification time of Yingshan Formation was the shortest there, generally with only first-stage karst water table developed. The typical characteristic is that the 4 karstification zones have the characteristic of the above karst fabric, with no obvious stacking development feature. After the karst depression was covered by sea water, the Yingshan Formation in karst highland and slope was still exposed to atmospheric environment, so the second-stage water table and karst zone developed. The second-stage water table and karst zone occurred in position higher, stacking above and reforming the first-stage karst zone, with caverns developed (Fig. 3). For example, 5 338-5 341 m of Well G46, 5 925.2-5 928.5 m of Well G451, 5 820.4-5 821.6 m and 5 828.4-5 829.5 m of Well G7, and 6 217.7-6 218.1 m, 6 218.5-6 218.8 m and 6 219.0-6 219.4 m of Well G9, have evident characteristics of overlay development of different karst fabrics, and karst intensity and fabric scale greater than in the first-stage karstification. With continuous rise of sea level, the karst slope was submerged, and Lianglitage Formation started to deposit, whereas the karst highland was still exposed to the air, and the third-stage water table developed there. The karst zone of this stage further reformed the earlier fractures and caverns in the first-stage and second-stage karst zone, resulting in universal development of multiple meter-level caverns below the water table. For example, there are 4 caverns of 1.4 m, 0.8 m, 1.4 m, and 0.4 m high, respectively, developed below the third-stage water table in Well G46 (Fig. 3). The karst fabrics of this stage had the largest dimensions and the most conspicuous development of overlay combination. Moreover, for the same stage of water table, the water table is larger in depth, and karst fabrics are more richer in type and larger in dimension and thickness in higher paleogeomorphic units (Fig. 3 and Table 1).

The first-stage horizontal underflow zone developed in all of the karst paleogeomorphic units, in which the main karst fabrics are cystic dissolved vug layers and near horizontal dissolved fractures partially filled by mud, sand or burial-stage cements. From the karst highland, slope to the depression, the depth of the horizontal underflow zone of this stage decreases gradually, with mean distances to the unconformity of 154 m, 123 m, and 42 m, mean thicknesses of 45 m, 41 m, and 32 m, respectively, and the mean reservoir thickness of 40 m, 29 m, and 13 m, respectively, all exhibiting a trend of thinning towards lower geomorphic units (Table 1). The reservoirs are mainly grade I and II high-quality ones. Furthermore, the horizontal underflow zones in the karst highland and slope have more cystic dissolved vugs (Fig. 3), which account for 57%, and 63%, respectively, of the total thickness of the karst fabrics thereof. Whereas the karst depression has more near horizontal dissolved fractures, which account for about 58%.

The second-stage horizontal underflow zones occurring in the karst highland and slope are 82 m and 64 m respectively from the unconformity, much nearer than the first stage. They have a mean thickness of 38 m, and 24 m, respectively, in which the part with karst fabrics accounting for 56%, and 46%, respectively. The karst fabrics not only include cystic dissolved vugs and near horizontal dissolved fractures, but also caverns and different combinations of karst fabrics (Fig. 3). The mean diameters of the caverns in the karst highland and slope are 1.9 m, and 0.9 m, respectively, and the percentages of the thickness of overlay combination of different karst fabrics, e.g., near horizontal dissolved fractures cutting the early-stage high-angle dissolved fractures, are 25%, and 11%, respectively. The second-stage karstification in the karst highland is higher in intensity than that in the slope, and the mean thicknesses of reservoirs are 21 m, and 15 m. But the reservoirs in the karst slope are slightly higher in quality than in the karst highland and are mainly Class I-II reservoirs, but the reservoirs in karst highland are largely Class II-III reservoirs (Table 1).

Third-stage horizontal underflow zone developed only in the karst highland, with typical characteristics of multiple caverns and more conspicuous overlay combination of karst fabrics. The overlay combination of karst fabrics takes 36% of the total thickness on average, and the mean diameters of all karst fabrics are the largest of the 3 stages. The horizontal underflow zone of this stage is only 17 m from the unconformity and about 15 m thick on average, in which the reservoirs are 13 m thick on average and mainly Class II-III reservoirs.

The karst fabrics in the vertical percolation zone above the water table have large differences from those in the horizontal underflow zone below the water table, so the characteristics of such karst fabrics as caverns, cystic dissolved vugs, and dissolved fractures with different occurrences can be analyzed through the electric imaging logging calibrated by core, and then the karst zones can be delimited and the position of water table identified based on the data of natural gamma and resistivity.

Karst fabrics developed earliest in the horizontal underflow below first-stage water table are simple in types and mainly cystic vugs along bedding or near horizontal dissolved fractures (Figs. 4a, 4b and 5), filled by mud, and silt-sand debris, etc. to different extents, and there are almost no caverns developed. In the vertical percolation zone above the water table, there are mainly high-angle dissolved fractures. The electric imaging logging of the horizontal underflow zone shows layered cystic dark-color patches in concentration and near horizontal irregular dark-color strips (Fig. 5), whereas the karst fabrics in vertical percolation zone are shown as dark-color strips in “high-amplitude sine curve” and small included angle “V” shape etc. Furthermore, according to the conventional logging, the water table is located at the position where the natural gamma value spikes and the resistivity decreases. For example, in Well G451 in the karst highland, cystic dissolved vug layer and near horizontal dissolved fractures are below the well depth of 5 957.6 m, and the vertical percolation zone feature, high-angle dissolved fractures, developed above this depth. Combining with the spike positions of natural gamma and resistivity, it is confirmed that the first-stage water table is located at well depth of 5 957.6 m (Fig. 4b).

With the rise in sea level, the second-stage water table was formed in the karst highland and slope, with depth moving upwards to the vertical percolation zone of the first stage, and the second-stage horizontal underflow zone reforming the early-stage vertical percolation zone. The water table of this stage is mostly located at the position where such typical characteristics as caverns, near horizontal dissolved fractures cutting pre-existing high-angle dissolved fractures, and cystic dissolved vugs with relatively large diameter occur, and the karst fabrics below the water table increase significantly in dimension, thickness, and scale, etc., and show obvious mutual cutting, combination and overlay (Fig. 4a and 4c). The corresponding electric imaging logging image shows near horizontal dark-color irregular strips cutting high-amplitude sine strips or small included angle “V” dark-color strips, mixed cystic dark-color patches and bright-dark-color patch masses, and alternating occurrence of these characteristics (Fig. 6). Below the second-stage water table, the natural gamma shows apparent high value section, with a few small spikes, and the resistivity decreases considerably. For example, in 5 884.7-5 885.2 m of Well G451, there are caverns associated with high-angle dissolved fractures, which are cut by near horizontal dissolved fractures at the bottom (Fig. 4c). The part below 5 884.7-5 885.2 m is dominated by cystic vugs and near horizontal dissolved fractures, or high-angle dissolved fractures cutting the first-stage vertical percolation zone, and two visible mud filled caverns. The part 50 m above 5 884.7-5 885.2 m is still dominated by high-angle dissolved fractures. With the characteristic of natural gamma spike and evident small resistivity below 5 884.7 m, it is confirmed that this depth is the position of the second-stage water table.

The latest-stage water table, i.e., the third-stage water table, and its karstification zones moved upwards and further overlay reformed the karst fabrics of the previous two stages in the karst highland, and the vugs and fractures formed are the largest in dimension and scale out of the 3 stages (Figs. 4d and 7). Below the water table of this stage, there are usually several caverns or aggregation of caverns and dissolved fractures partially filled by mud, and breccia, etc, with diameter reaching meter level, nearly horizontal dissolved fractures cutting high-angle dissolved fractures (both large in width), and cystic vugs in large diameter (Fig. 4a and 4d). The corresponding electric imaging features multiple dark-color patch masses in meter order height, relatively wide sine, “Λ” or “V” dark- color strips, and relatively large dark-color patches (Fig. 7). The water table is located on the top of the high value section of natural gamma composed of multiple spikes, and the resistivity below the water table decreases. The vertical percolation zone above the water table of this stage has mainly wide and large high-angle and oblique-crossing dissolved fractures, and there are caverns of epikarst zone developed to the unconformity at the top of Yingshan Formation (Fig. 4a). For example, below 5 819.5 m in Well G451, there are 4 caverns, with diameters of 0.7 m, 0.9 m, 1.0 m, and 1.1 m, respectively, and overlay combinations of near horizontal dissolved fractures, high-angle dissolved fracture and large-diameter cystic vugs as well (Fig. 4d). In addition, the depth section of 5 819.5-5 826.0 m corresponds to high natural gamma section, with resistivity decreasing evidently, so it is confirmed that 5 819.5 m is the development depth of the third-stage water table.

Karst profiles of wells in the study area were delimited and compared according to the above identification marks of various stages of karst water tables. Combining with the distribution of karst paleogeomorphic units in the study area, water table of the first stage was distributed in the whole area, and it is found that in the karst depression - the lowest paleogeomorphic unit, from the north of the Well G104-Well G101-Well G102 district and the Well G5-Well G9-Well G701 district (the green line shown in Fig. 8) to Tazhong No. 1 Fault, there is only the first-stage water table developed and the development range of reservoirs is small. From the south of the above districts to the boundary of three-dimensional seismic data of the study area, are the karst slope and karst highland with relatively long exposure time, where the 2 stages of water table developed, the reservoirs are larger in development range and mainly Class Ⅰ-Ⅱ , and the well block G8 with proven reserves obtained is in this region. From the south of the Well G111-Well G8-Well G47-Well G7-Well T161 district (the blue line shown in Fig. 8) to the boundary of three-dimensional seismic data of the study area, except the local relatively-low karst platform zone on the south of Well G432 (blue line closed region shown in Fig. 8), are residual mound peaks in karst highlands and relatively-high karst slope, highest in position and longest in exposure time, this area has 3 stages of water table developed (Fig. 8); the reservoirs are large in development range and mainly Class Ⅰ-Ⅱ reservoirs, and the G43 well block with proven reserves obtained is in this region.

There are multiple sets of karst reservoirs developed in Members 1 and 2 of Yingshan Formation in the study area, which have distribution characteristic of “regional crossing bed, local along-bedding” on the profile perpendicular to the stratum strike^[9,26]. Horizontally, reservoirs occur in large ranges, and are mainly high-quality Class Ⅰ-Ⅱ reservoirs in the karst highland and slope, but come in relatively small range in the karst depression (Fig. 8). This is primarily controlled by the karstification under periodic rise in sea level, that is, by multiple sets of reservoirs formed by the karst fractures and caverns in the 3 stages of horizontal underflow zone. In the Middle-Late Ordovician, tectonic movements tended to be active^[16,27], the Yingshan Formation deposited on the wide and gentle platform was still in the original near horizontal state, that is, was lifted and exposed, and underwent karstification, with extremely small stratum dip angle. The karst paleogeomorphy lowered from southwest to northeast, with the relative elevation difference of less than 300 m in most of the area and a width span of 27 500 m^[21]. Accordingly, the slope angle from the karst paleogeomorphic high point to sea level in early stage of karstification could be calculated, that is, the maximum included angle between the unconformity and the horizontal plane was less than or equal to 0.63°. Even if in the well block T161 with the local maximum elevation difference of 600 m, the included angle mentioned above was less than or equal to 2.8°, thus the included angle between each stage of water table below the unconformity and the horizontal plane was smaller. Therefore, on the southwest-northeast profile perpendicular to the stratum strike, multiple sets of reservoirs in various stages of horizontal underflow zone exhibit small angle intersection with various sub-members of strata, and are distributed in a “crossing bed” manner on the whole (Fig. 9). Nevertheless, in local blocks with consistent residual characteristics of strata or northwest-southeast profile parallel to the stratum strike, various stages of reservoirs show quasi-layered distribution in various sub-members and have a characteristic of “along- bedding” development. From low karst paleogeomorphic unit to high ones, the number of stages of the karst water tables and horizontal underflow zones in Yingshan Formation increases, and the thickness of the horizontal underflow zone of the same stage increases (Fig. 9). Therefore, not only there develop second-stage water table and third-stage water table in the karst highland and slope with relatively high topography in the study area, but also the thickness of each stage of horizontal underflow zone is larger than that in the karst depression, so high-quality Class Ⅰ-Ⅱ reservoirs occur in a large range (Fig. 8). Therefore, the karst highland and slope are relatively favorable exploration regions. Once the three-dimensional distribution of each stage of water table can be delineated using methods and techniques such as single well identification, combination of logging and seismic data, and geological modeling, this would provide important guidance to geological modeling of reservoirs, optimization of well location, and even to the selection of horizontal well target layer and trajectory design, etc.

Based on the karst characteristics of the horizontal underflow zone below the karst water table, the identification method and marks for different stages of water table have been worked out. The typical identification marks for the horizontal underflow zones corresponding to the first-stage, second-stage and third-stage water tables in Yingshan Formation, Tazhong area are as follows respectively: simple karst fabric, e.g., cystic dissolved vug layer, or near horizontal dissolved fracture; development of small caverns or overlay combination of different karst fabrics, e.g., near horizontal dissolved fractures cutting early-stage high-angle dissolved fractures; and development of multi-layer caverns and overlay combination of karst fabrics with larger diameters. On one profile, the types of karst fabrics in the middle-stage and late-stage water tables are richer than in the early-stage water table, and the diameters and overlay combination thickness percentages of karst fabrics are larger than in the early-stage water table.

Periodic rise in sea level caused the development of the first-stage water table, the first-stage and second-stage water tables and the first-stage, second-stage and third-stage water tables in the karst depression, slope, and highland, respectively, in the Middle-Lower Ordovician Yingshan Formation. The depth of the same stage of water table to the unconformity and the thickness of horizontal underflow zone tend to increase with the heightening of karst landform.

Controlled by the near horizontal mode of occurrence of Yingshan Formation in the karstification period, the third- stage water table formed by the periodic rise in sea level and the karst fractures and caverns in the horizontal underflow zone below the water table have extremely small included angle with the stratum, and the thickness of each stage of horizontal underflow zone tends to increase gradually from the low to the high landform. This controlled the distribution characteristic of reservoirs, that is, “regional crossing bed, local along- bedding”, making the karst highland and slope have more stages of water table and thicker horizontal underflow zone, more sets and larger thickness of reservoirs, and more high-quality Class I and II reservoirs than in the karst depression, so the karst highland and slope are favorable exploration regions.