Distribution of Paleokarst Water Table and Fault-Related Control on Reservoirs: Yingshan Formation, Tahe Oilfield, Tarim Basin Open Access

The paleokarst presents distinct zonal characteristics, forming four sequential zones beneath the unconformity surface: epikarst zone, vertical vadose zone, horizontal runoff zone, and subsurface flow zone [1-3]. Below the vertical vadose zone, lithostatic pressure and surface water influx can induce multistage fluctuating dissolution of the water table and develop a well-formed karstic fracture-cave system within the horizontal runoff zone [4-6]. The water table representing the onshore extension of the ancient sea level has its location and thickness controlled by the tectonic amplitude of the paleogeomorphy [7-9]. During periods of sea level change, water table fluctuations within horizontal runoff zones generate multiple sets of bedding-parallel or cross-strata karstic fracture-cave reservoirs [10, 11]. The rise of the datum plane has caused vertical karstic modifications over different periods, resulting in complex fracture-cave systems in the Tahe area, where the fracture-cave reservoirs in horizontal runoff zones are believed to have been transformed from earlier vertical vadose zones [12-14]. In numerous karst regions, dissolution pores, fractures, and caves serve as vital records of ancient water table evolution, offering key insights into their formation and development [3, 9].

Faults play a critical role in influencing ancient karst fracture-cave systems, which are widely developed in carbonate rock formations. They significantly affect the dissolution of surface or hydrothermal fluids and are essential for the migration, accumulation, and preservation of oil and gas [15-17]. Field surveys in the Tarim Basin have revealed that faults possess complex internal structures consisting of fault cores and damage zones, with variations in lithology, layer thickness, fault morphology, and displacement contributing to the asymmetric development of these zones [5, 18-20]. The influence of faults on karst processes depends on their timing: early-stage faults primarily cause rock collapse, whereas later-stage faults enlarge preexisting fissures or damage zones, facilitating fluid activity [19, 21, 22]. The phase of fault activity and the development of secondary faults and fractures are key factors in the reconstruction of carbonate reservoirs. Strike-slip faults are prevalent in the Tahe area and serve as important conduits for atmospheric precipitation, hydrothermal dissolution, and TSR processes [23, 24]. Reservoir development is influenced by the hierarchy and positioning of faults [25-27], with major faults connected to the Cambrian system being particularly favorable for the formation of karstic fracture-cave bodies [14, 23, 28]. Fault intersection points exhibit the most developed reservoirs, followed by flower-like structures, whereas linear structural belts have a comparatively limited effect on reservoir reconstruction [29, 30].

The development of fracture-cave systems during groundwater dissolution is influenced by factors such as surface water infiltration, temperature-pressure conditions, lithology, tectonic activity, and fault scale. Karst processes are the primary drivers of heterogeneity in carbonate reservoirs, with differential dissolution causing lateral and vertical variations in reservoir properties [22, 31]. Field investigations of paleokarst caves in Kentucky have indicated that the water table, faults, and lithology (dense layers) are key to the formation of sinkholes and karst channels [32, 33]. Similarly, seismic interpretations of paleokarst caves in the Benedune and Ellenburger Formations of West Texas suggest a coeval relationship between cave formation and faulting [34, 35]. Therefore, identifying the locations and fluctuation ranges of the water table as well as the scale and phases of faults is critical for exploring the distribution of karst reservoirs in horizontal runoff zones [36, 37]. Coring wells typically do not continuously core into target layers, making it challenging to analyze water table fluctuations [9]. The low resolution of seismic data for deep reservoirs and interpretational ambiguity in conventional well logging further complicate research on the water table in the Tahe area. This study integrated imaging, conventional well logging, and core data to identify multistage water table control features and predominant reservoir types within various horizontal runoff zones using paleogeomorphy reconstruction and hierarchical fault delineation. Additionally, a karst-controlled reservoir model was implemented by incorporating the structural evolution features. This study aimed to explore the characteristics of reservoirs within multistage horizontal runoff zones under denudation conditions, providing valuable insights for applying paleokarst water table-controlled reservoir studies in similar structural settings globally.

The Tarim Basin, located in the northwest of the Xinjiang Uygur Autonomous Region, China, spans approximately 560,000 km². It is the largest petroleum- and gas-bearing sedimentary basin in the country (Figure 1(a)). The basin is underlain by a metamorphic basement of the Sinian system and overlain by Paleozoic carbonate rocks and Meso-Cenozoic continental deposits, which collectively hold significant oil and gas potential. Tectonically, it is bordered by the Tianshan, Western Kunlun, and Altun mountain ranges, while its internal framework consists of a complex arrangement of uplifts and depressions, forming a tectonic pattern characterized by “five uplifts and four depressions” [38-40]. The Tahe Oilfield is located in the southern part of the Akekule Uplift within the Shaya Uplift of the Tarim Basin, bordered by the Caohu Depression to the east, the Halahatang Depression Trough to the west, the Manggar Depression to the south, and the Shuntuoguole Uplift (Figures 1(a) and 1(b)). The primary oil-bearing formations in the Tahe Oilfield are Ordovician fracture-cavity carbonate reservoirs, with paleokarst reservoirs contributing 73% of the production [41, 42]. The Ordovician strata are divided into three sections: Lower Ordovician (Penglaiba (O_1p) and Yingshan (O_1y)), Middle Ordovician (Yingshan (O_1-2y) and Yijianfang (O_2yj)), and Upper Ordovician (Qiaerbake (O_3q), Lianglitag (O_3l), and Sangtamu (O_3s)). The Middle to Lower Ordovician formations consist of open and restricted platform facies including crystalline grain limestone, mudstone, and sparry intraclastic grainstone [43]. The Yingshan Formation, distributed throughout the Middle to Lower Ordovician, has a sedimentary thickness of 800–1000 m and is subdivided into four members, including Ying I, Ying II, Ying III, and Ying IV. This study focused on the uppermost Ying IV member characterized by yellow-gray packstone, intraclastic grainstone, sparry intraclastic grainstone, and wackestone (Figure 1(c)).

The Akekule Uplift is a NE-SW trending, nose-shaped structure underlain by Precambrian metamorphic basement rocks. It experienced multiple phases of complex tectonic movement, including those from the Caledonian, Hercynian, Indosinian, Yanshan, and Himalayan periods [44, 45]. The Caledonian period is divided into three stages: early, middle, and late Caledonian, corresponding to the late Cambrian to early Ordovician (488, 460 Ma), the middle Ordovician to early Silurian (460, 430 Ma), and the middle to late Silurian (430, 418 Ma), respectively. The Hercynian period is further subdivided into early and late stages. The early Hercynian lacks a well-defined time boundary but generally corresponds to the early Devonian, and the late Hercynian extends from the late Carboniferous to the Permian [6, 40]. Erosion during the mid-Caledonian and early Hercynian periods played a key role in the formation of karst features in the region. During the mid-Caledonian period, regional stress shifted from extensional to compressional, leading to differential erosion in the Akekule area [25, 42]. This period saw three episodic uplifts that formed unconformities at T₇⁴ (top of the Yijianfang Formation), T₇² (top of the Lianglitag Formation), and T₇⁰ (top of the Sangtamu Formation) (Figure 1(c)).

By the end of the early Ordovician (early Hercynian), intense regional compression caused prolonged weathering and erosion of the Upper Ordovician carbonate rocks, leading to the formation of a widespread and continuous T₇⁴ unconformity marked by strong seismic reflection across the Tahe area. During the late Silurian to Carboniferous, collisional uplift from the compression between the eastern Tarim Basin and Tianshan island arc completely eroded the Silurian, Devonian, and Upper Ordovician strata in the Tahe region, exposing the middle Ordovician Yingshan Formation and Carboniferous Bachu Formation. The bimodal limestones at the top of the Bachu Formation served as key marker horizons, corresponding to the T₅⁶ reflection interface in the seismic profiles (Figure 1(c)). During this period, fluctuations in the water table and weathering erosion within the Ordovician strata created a substantial network of internal fractures [43, 46, 47], whereas unevenly sized karst landscapes formed along the T₇⁴ unconformity surface [48].

Paleogeomorphology is a critical reference for studying paleokarst water table fluctuating dissolution, as it controls the locations where these water tables develop. Therefore, restoring paleogeomorphological features is essential before investigating the dissolution processes of paleokarst water tables. In this study, based on the extensive Carboniferous Bachu Formation mudstone overlying the Ordovician in the Tahe region [45], the paleogeomorphology was classified into four sary units using the “impression method” described by [48, 49]: karst highlands, gentle karst slopes, steep karst slopes, and karst depressions. Each secondary unit was further refined into tertiary geomorphic units based on variations in paleogeomorphological relief (Figure 2 and Table 1). Paleogeomorphology plays a crucial role in controlling the elevation and developmental stages of paleowater tables. In the karst highlands, the water table is generally the deepest and exhibits the most stratified stages from top to bottom. On karst gentle slopes, the water flow velocity decreases, facilitating frequent infiltration of atmospheric precipitation and groundwater convergence. Consequently, multiple stable water tables may form within a single stable geological period. In contrast, on karst steep slopes, the steep terrain causes water table fluctuations that are primarily governed by precipitation intensity and frequency. In low-lying areas of both gentle and steep slopes, some paleowater tables may emerge at the surface.

During the Caledonian-Hercynian period, multiple phases of tectonic activity in the Tahe region led to the formation of a complex network of multiscale fractures of varying sizes [46]. The faults in this area were influenced by the direction and intensity of tectonic stress fields, exhibiting significant variations in scale. Different fault levels play distinct roles in the formation of fracture-cave reservoirs [25, 50, 51]. Based on the vertical discontinuities and fault classification standards of the Northwest Petroleum Bureau, faults in the Tahe region were categorized into three levels: levels I, II, and III. Coherent attributes were extracted within 0–20 ms below the T₈⁰, T₇⁸, and T₇⁶ unconformity surfaces to map the planar distributions of these faults. The first-level faults, serving as the main trunk faults, are characterized by prolonged activities and primarily exhibit conjugate features in the northeast and northwest directions. These faults intersect the T₈⁰ unconformity surface and control a series of secondary strike-slip faults in the Tahe area. The second-level faults developed within the interior of the level I fault systems as subsidiary faults, primarily distributed between the T₇⁸ and T₈⁰ surfaces. The third-level faults were located on both sides of the level I and II faults, comprising both the main and secondary fault systems and were distributed between the T₇⁶ and T₇⁸ surfaces (Figure 3).

The data collected for this study included 12 full-bore imaging logs, conventional logs, seven coring wells, seismic data from the Tahe main area, and five seismic reflection interfaces (T₅⁶, T₇⁴, T₇⁶, T₇⁸, and T₈⁰). The secondary paleogeomorphic units of the Tahe region, including karst highlands, gentle karst slopes, steep karst slopes, and karst depressions, were reconstructed using the “impression method,” which involved subtracting the T₅⁶ unconformity surface from the T₇⁴ surface in two-way travel times. The internal paleogeomorphic units were delineated based on height variations relative to the T₅⁶ unconformity surface. Coherent processing of seismic data identified the plane distribution of coherent attributes from 0 to 20 ms below the T₇⁶, T₇⁸, and T₈⁰ unconformity surfaces, mapped the fault locations, and classified the fault levels. Through detailed reservoir characterization of 12 full-bore imaging logging wells across different geomorphological units, key karst facies such as planar-aligned pores, near-horizontal fractures, and caves were identified, helping to delineate the fluctuating range of paleowater tables and the locations of horizontal flow zones. Based on these findings, the stages of paleowater surfaces in karst highlands, gentle slopes, and steep slopes were determined. Additionally, the distribution of horizontal runoff zones across different stages was examined, and a statistical analysis was conducted on reservoir types, reservoir thickness, and the proportion of dominant reservoir bodies under both single paleowater table controls and paleowater table-fault composite controls. This analysis enabled the identification of dominant reservoir types in different paleogeomorphological settings. Next, the reservoir characteristics and thickness variations of horizontal flow zones under different geomorphological conditions were analyzed. To assess productivity, the cumulative production of individual wells and reservoir types within the same geomorphological unit was compared to verify the effectiveness of wells located in dominant reservoir areas. Furthermore, the formation mechanisms of favorable reservoirs were explored by integrating geomorphological features and structural evolution characteristics. Finally, a paleokarst reservoir model for the Tahe region was established, considering the coupled influence of paleowater levels and faults.

In the horizontal runoff zones, varying paleogeomorphological features exhibited distinct karstic facies formed by fluctuating erosional processes of the paleokarst water tables [9, 37]. The karst highlands primarily exhibited small-scale discrete spot-like planar-aligned pores and nearly horizontal dissolution fractures, as observed in wells S66, S74, and TK408 (Figures 4(a), 4(b), and 4(d)). On gentle karst slopes, intense water-rock interactions could produce small caves over 1 m thick, characterized by yellow specks and patches intermixed with brown-black banding, as observed in well T7-615 (Figure 4(c)). Similarly, steep karst slopes demonstrated horizontal dissolution pores and fractures (Figure 4(f)), where fracture zones could lead to the development of larger caves exceeding 3 m in thickness (Figure 4(e)). The presence of planar-aligned pores, near-horizontal fractures, and caves of various sizes served as key indicators for identifying the position of paleokarst water tables [52].

This study analyzed typical full-bore image logging in karst highlands and delineated multiple stages of horizontal runoff zones based on the distribution of planar-aligned pores, fractures, and caves. In well S74, three stages of horizontal runoff zones were vertically identified. The high-angle fractures formed by vertical vadose zones were above 5651 m and below 5672.5 m. The water table modifications reshaped the earlier vertical vadose zone reservoirs, resulting in overlapping planar-aligned pores, vertical fractures, and oblique intersecting fractures between 5651 and 5672 m, creating a complex pore-fracture combination in the first-stage horizontal runoff zone. Additionally, multiple sets of planar-aligned pores and near-horizontal fractures were observed between 5592–5604 and 5505–5510 m, corresponding to the second- and third-stage horizontal runoff zones (Figure 5).

Three stages of paleokarst water erosion generated horizontal runoff zones on gentle karst slopes, with high-resistivity dense limestone layers distributed between each stage. In well S75, water table erosion was evident in the segments at 5716.8–5730, 5635.4–5666.7, and 5525–5561.7 m. Indicators such as planar-aligned pores, near-horizontal fractures, and small caves filled with collapsed breccia cemented by fine sandstone were characteristic features of these horizontal runoff zones. Moreover, the slight variations in natural gamma readings and minor peaks or fluctuations in the resistivity curve further confirmed the presence of planar-aligned pores and small caves, substantiating the distribution of the three-stage horizontal runoff zones (Figure 6).

Karstification processes on steep karst slopes were identified over shorter durations than those on the karst highlands and gentle karst slopes. The limited infiltration of surface water resulted in highly developed surface water systems, where fluctuations in high-altitude groundwater can often be discharged near the surface [53, 54]. Owing to the absence of vertical high-angle fractures, planar-aligned pores and nearly horizontal dissolution fractures were widely developed in runoff zones. Research analyzing the full-bore imaging logs of wells S94 and S93 at a single paleokarst water table and paleokarst water table-fault compound locations identified the distribution of two stages of horizontal flow zones on steep slopes (Figures 7 and 8). In areas influenced by a single water table action, inclined fractures and pore-fracture complexes were confined within 30 m of the surface, transitioning into near-horizontal fractures and bedding-parallel dissolution voids with depth. Based on natural gamma and resistivity curves, horizontal runoff zones were inferred at depths of 5980–6012 and 6030 m in well S94 (Figure 7). In well S93 located in the zones influenced by both the water table and fault activity, pore-fracture complexes, inclined fractures, and mud-filled caves were present with planar-aligned pores and vertical fractures ceasing below 5798 m. The distribution of caves, planar-aligned pores, and near-horizontal fractures suggested that 5765–5798 and 5845–5900 m represented two stages of horizontal runoff zones (Figure 8).

The thickness of horizontal runoff zones varied significantly across the karst highlands, gentle karst slopes, and steep karst slopes owing to overlapping karst facies characteristics. The karst highlands and gentle karst slopes developed water tables in stages 1, 2, and 3, whereas steep karst slopes predominantly exhibited stages 1 and 2 water tables. In the first-stage horizontal runoff zone (outlined by the green line in Figure 9), the karst highland feature reservoirs are composed of horizontally vertically combined fractures, such as those in well TK408 (5557.5, 5588.4 m) and well S74 (5652.7, 5680 m), as well as single dissolution pores and fractures, as seen in well S66 (5678.8, 5700 m, bottom not observed). On the gentle karst slope (well S75), the reservoirs transitioned into collapse-angle gravel-filled dissolution cavities, and on steep karst slopes (wells S93 and S94), they developed into multiple sets of overlapping near-horizontal fractures. The second-stage horizontal runoff zone exhibited minimal thickness variation (bounded by the blue line in Figure 9) and mainly consisted of composite overlays of planar-aligned pores and fractures. In some areas within karst highlands, composite features, such as planar-aligned pores and oblique cross-joints, were also observed, as seen in wells TK408 and S74. The sites influenced by the interaction of level II faults and the water table, such as well S93, developed mudstone-filled and collapsed breccia-filled karst caves (ranging from 2.5 to 7.5 m in thickness) and composite types of horizontal fractures. The third-stage horizontal runoff zone had a small fluctuation range, with a distance of less than 50 m from the T₇⁴ unconformity surface (outlined by the red line in Figure 9). In this phase, the reservoir types included planar-aligned dissolution pores and oblique intersecting fractures in karst highlands. As the zone transitioned toward a gentle karst slope, it shifted to composite overlays of planar-aligned pores, oblique intersecting fractures, and caves filled with dissolution breccia.

Following the karstification period and influenced by sea level rise, various scales of subaqueous dissolution fracture-cavity bodies developed vertically in the Tahe region [55]. The first-stage horizontal runoff zones were formed through superimposed transformation of the water table on preexisting vertical vadose zones [9, 56, 57]. In the areas affected only by paleokarst water table processes, the dark bands of karstic cavities (thickness < 1 m) and complex high-angle “∧” shaped fracture-cavity assemblages are evident in imaging logs of the karst highlands (Figure 10). The reservoir types in these zones included planar-aligned pores, fractures, inclined intersecting fractures, reticular fractures, and caves, with individual reservoir thicknesses ranging from 0.1 to 4 m. The high-angle fracture-cavity assemblages and reticular fractures with the thicknesses of 9.5 and 3.8 m, respectively, dominated the karst highlands, constituting 80% of the total reservoir thickness (Figure 11(a)). In contrast, the karst slopes exhibited a significant increase in reservoir thickness (Figure 9), characterized by imaging logs of angular conglomerate pebbles and alternating bright and dark patches above the bottom bands. The dark-colored mottles featuring the high-angle “V” and “∧” shaped bands were observable cutting through larger dark mottles (Figures 10(c) and 10(d)). The dominant reservoir types in well S75 included pore–fracture complexes and large cavities (thickness > 4 m), comprising 66% of the reservoir thickness. In well T7-615, high-angle fractures accounted for 48% of the reservoir thickness. Secondary reservoir types included planar-aligned pores and near-horizontal fractures (Figures 11(c) and 11(d)). On steep karst slopes, the reservoir vertical continuity was relatively poor, marked by superimposed dark planar-aligned pores and ribbon-like near-horizontal fractures (Figure 10(f)). The dominant reservoir types in these areas were planar-aligned pores and pore-fracture composites, with thicknesses of 6.57 and 8.78 m, representing 52% and 25% of the total thicknesses, respectively. The single-layer thickness was less than 1 m (Figure 11(f)).

In areas where water surfaces interact with fractures, reservoir thickness and type are determined by the fault levels [58]. Large caves with a total thickness exceeding 18 m, comprising 85% of the reservoir, developed at the intersections of karst highlands and level I faults. The individual cave heights ranged from 2 to 4 m, and the imaging logs demonstrated vertically stacked fractured zones with alternating bright and dark bands (Figures 10(b) and 11(b)). At the intersections of steep karst slopes with level II faults, dark-colored planar-aligned pores and ribbon-like near-horizontal fractures were predominant. These two reservoir types together accounted for 89% of the total thickness, with the individual reservoir thicknesses typically ranging from 0.1 to 0.25 m (Figures 10(e) and 11(e)).

Following the formation of the first-stage water table, subsequent sea-level rise modified the vertical vadose zone, leading to the development of fracture-cave systems within the second-stage horizontal runoff zone [21, 55]. Under the impact of a single water table, the reservoirs in the karst highland, gentle karst slope, and steep karst slope regions (corresponding to wells TK408, S75, and S94, respectively) were predominantly characterized by planar-aligned pores and dissolution fractures. It shows dark-spotted dissolution pores and ribbon-like near-horizontal dissolution fractures on imaging logs (Figures 12(a), 12(c), and 12(e)).

In karst highlands, reservoirs displayed diverse types, including planar-aligned pores, oblique fractures, network fractures, and small caves (Figure 13(a)). The gentle karst slopes with intense water-rock interactions were prone to unfilled large caves, such as the 7.8-meter-thick cave in well T7-615, which accounted for 35% of the reservoir thickness (Figure 13(d)). Imaging logs in this region presented alternating light and dark stripes between 5620.84–5623 m and 5623.77–5625.5 m, indicating dissolution caves (Figure 12(d)). Caves were also identified at the intersection of the water table and level I faults, as evidenced by the bright and dark mixed patches on imaging logs in wells TK604 (5595.6, 5601.2 m) and S93 (5775, 5778 m) (Figures 12(b) and 12(e)). In well TK604, caves accounted for 97% of the reservoir, with a thickness of 34.95 m (Figure 13(b)). In the areas where the paleowater table interacted with level II faults, the reservoirs developed both caves and network fractures. In well S93, these reservoirs had thicknesses of 10.8 and 14.87 m, accounting for 36% and 49% of the total reservoir thickness, respectively (Figure 13(e)).

The third-stage water table occurred most recently, and with the continued rise in sea levels, steep karst slopes were submerged, confining the third-stage horizontal runoff zones to karst highlands and gentle karst slopes [9]. The karst features formed during this stage overprinted the karst facies of the previous stages, including epikarst and vadose zones. In the areas controlled by a single paleokarst water table, such as the karst highland peak clusters (e.g. wells S74 and S67), the planar-aligned dark-colored spot pores and bands developed, with the vertical “V” shaped dark stripes intersecting horizontal stripes (Figures 14(b) and 14(c)). The primary reservoir types in these areas included planar-aligned pores, near-horizontal dissolution fractures, high-angle fractures, and pore-fracture composites, with fractured or pore-fracture composites being the dominant types. In well S74, the pore-fracture composites constituted 72% of the reservoir, whereas fractured reservoirs accounted for 56% in well S67. The individual reservoir unit thickness ranges from 0.1 to 1 m (Figures 15(b) and 15(c)).

Toward the edge of the highland, the reservoir types transitioned to planar-aligned pores and near-horizontal dissolution fractures, as seen in well TK429, with individual thicknesses less than 0.5 m (Figure 14(a)). The imaging logs showed dark speckled pores and bands (Figure 15(a)). In the gentle karst slopes, in addition to the pores and various fracture types (near-horizontal dissolution fractures, oblique fractures, high-angle fractures, and network fractures), meter-scale caves were present, such as the cave in the well T7-615 from 5557.23 to 5558.74 m (Figures 14(e) and 14(f)). The caves dominated the primary reservoir type, with greater thickness than the dissolution pores and fracture reservoirs. For instance, well S75 and well T7-615 had cave thicknesses of 9.2 and 6.22 m, respectively, accounting for 45% and 36% of the total reservoir thickness (Figures 15(e) and 15(f)). In areas influenced by both the paleowater table and faults, single-type cave reservoirs developed. Well TK604 featured a 32.9 m vertical stack of caves, representing 100% of the reservoir thickness (Figure 15(d)), which appeared as alternating bright and dark thick-layer speckled formations in imaging logs (Figure 14(d)).

The Yingshan Formation exhibits three distinct stages of karst fracture cavities shaped by multiple phases of tectonic uplift and episodic marine transgression. These stages, controlled by paleowater tables, water tables, and fault systems, correspond to the three stages of horizontal runoff zones [37, 53]. Statistical analysis of single-well imaging logs under different geomorphologies revealed significant variations in karst reservoir thickness and type across various landforms. The average thicknesses of the three stages of horizontal runoff zones in karst highlands were 26.9, 23.8, and 16.1 m, decreasing progressively from bottom to top (Table 2). During the Silurian-Carboniferous period, multiphase tectonic uplift and intense weathering led to the formation of a well-developed fracture network in the Tahe area [30, 46, 50]. The surface and atmospheric precipitation filtered through this network into the subsurface, creating large-scale fracture-cave systems within 0–300 m below the T₇⁴ interface [21, 45]. The first-stage horizontal runoff zone was modified by dissolution, resulting in a composite of vertical fractures, networked fractures, and small caves with thicknesses less than 1 m (Figures 14(b) and 14(c)). The reservoir type at the intersection of the fault and the paleowater table is controlled by the fault scale and atmospheric freshwater dissolution process. Faults act as open dissolution spaces, where the fragmentation and alteration of the original strata create an unstable formation, providing favorable conditions for subsequent dissolution driven by atmospheric precipitation. Surface water and atmospheric freshwater further enhanced this process, transforming horizontal dissolution fractures or small caves into a vertically stacked cave system (Figure 14(d)). Because of the depth from the weathered crust, portions of mudstone submerged in seawater did not experience thorough infiltration beyond 150 m, allowing the preservation of cave formations [3, 25]. As sea level rose, weathering on karst highlands was concentrated near the top of the weathered crust (within 100 m of the T₇⁴ interface), leading to the development of planar-aligned pores and horizontal fractures within the second-stage horizontal runoff zone (Figure 9). The faults provided key pathways for sediment infiltration, filling large caves (thickness > 3 m) with dissolution breccia formed by multiple phases of weathering and cementation during marine transgressions (Figures 12(b) and 13(b)) [27, 29]. The third-stage karst cavities formed through multiple phases of tectonic activity and dissolution transformed the surface drainage systems into subterranean systems as groundwater descended through fractures at the anticlinal wings. Reservoirs modified by weathering in the first and second stages and runoff effects in the third stage featured discontinuous dissolution pores, caves, horizontal fractures, and vertical fracture complexes in areas influenced by a single paleokarst water table. The high-angle dissolution and networked fractures were the dominant reservoirs in the karst highlands (Figure 11(a)), whereas the karst caves, either unfilled or filled with collapse breccia, were later infilled with mudstone or mudstone-cemented collapse breccia due to mudstone percolation in the paleokarst water table and fault complex zones (Figures 10(b) and 11(b)).

Compared with karst highlands, karst slopes experience reduced weathering and leaching effects, with vertical fractures more developed near the surface [53, 59]. The surface and atmospheric water infiltrated through fractures or sinkholes, and the combination of sufficient karst water and a terrain gradient (2%–3%) promoted frequent water-rock interactions in gentle karst slopes [21, 48, 60]. The thickness of the horizontal runoff zones in these slopes varied within 27–34.7, 6.8–30.8, and 23.5–37.3 m from bottom to top, with the first and third stages having the greatest reservoir thickness and most diverse reservoir types (Table 2). In the deeper subsurface, at the first-stage water table, regional uplift-induced weathered fractures intersect with lateral dissolution, creating a network of cross-cutting fractures, networked fractures, and fracture-cave complexes [25, 46, 50]. In less-weathered areas, strong runoff dissolution led to the formation of planar-aligned pores, dissolution fractures, and large caves exceeding 4 m in thickness (Figure 11(c) and Table 2). Toward the basin, the reduced influence of weathering and runoff dissolution shifted the dominant reservoir types from composite pore-fracture systems and caves to singular vertical fractures, planar-aligned pores, and dissolution fractures (Figures 11(d) and 11(f)) [57]. The second-stage water table fracture-cave bodies formed during episodic sea-level rise, with the increased sea levels reducing the weathering intensity below the T₇⁴ interface (100 m), concentrating the runoff dissolution into the planar-aligned pores, dissolution fractures, and caves exceeding 2 m in thickness (Figure 12(d)) [6]. The third-stage fracture-cave bodies shaped by multistage surface weathering and runoff dissolution formed reservoirs of planar-aligned pores, vertical fractures, network fractures, and fracture-cave complexes [27, 52]. The near-horizontal fractures modified by the paleokarst water surface also formed small caves greater than 1.5 m thick (Figures 14(f) and 15(f)).

The steep karst slopes, as discharge zones for karst water, are characterized by abundant surface water and a high gradient (3%–5%) [48]. The third-stage paleokarst water table distributed in karst highlands and gentle karst slopes underwent dissolution in underground runoff and was discharged into steep karst slopes [38]. Multiple phases of tectonic uplift weaken the weathering and erosion of the basin, reducing the depth of atmospheric precipitation-induced diagenesis [53, 61]. In steep karst slope areas, the underlying carbonate rocks retain their original sedimentary characteristics, making them highly susceptible to dissolution. Only the near-surface environment within 60 m of the T₇⁴ unconformity was significantly affected by weathering and leaching [3, 6]. The interlayer sliding within limestone during deposition facilitated vertical micro-fracture formation, whereas the runoff action formed planar-aligned pores overlapping the vertical fractures to create small-scale pore-fracture complexes. The variations in the gradient and high solubility along steep slopes resulted in multiple sets of thick, stacked planar-aligned pores and horizontal dissolution fractures [62]. Consequently, the first-stage horizontal runoff zone developed three primary reservoir types, including planar-aligned pores, horizontal dissolution fractures, and pore-fracture complexes, accounting for 89% of the total (Figure 11(f)). As the groundwater level continued to rise, the second-stage horizontal runoff dissolution weakened, whereas planar-aligned pores and near-horizontal fractures remained the main reservoir types [9, 53]. At locations influenced by both faults and the water table, such as well S93, the collapse breccias within the fault cores and networked fractures in the fractured zones were filled with marine-invaded mudstones, forming mudstone-filled fractures and mudstone-cemented collapse breccias (Figure 9 and Table 2).

To analyze the karst reservoir characteristics across different hydrological periods, the cumulative oil and water production of wells in various geomorphological settings was statistically evaluated (Figure 2). Scatter plots illustrating the cumulative oil-water relationship under the single and combined control of the water table and faults (Figure 16) revealed significant differences in well productivity based on controlling factors. The highest cumulative oil production was identified in the karst highlands and water table level I fault areas, followed by the gentle karst slopes and water table level II fault areas, with the lowest productivity in the steep karst slopes and water table level III fault areas (Figures 16(C1), 16(C2), and 16(C3)). In the areas influenced by a single water table, the cumulative oil production ranged from 13.7 to 787×10³ tons in the karst highlands, 2.2 to 48.5×10³ tons in the gentle karst slopes, and 0.1 to 41.2×10³ tons in the steep karst slopes (Figure 16(a)). The high-yield wells, typically exceeding 100×10³ tons, were predominantly located in the karst highlands, such as the north-south oriented S48 highland, with decreasing production toward the basin, as exemplified by the T7-615 gentle karst slope and TH12126 steep karst slope (Figure 2). In the Tahe region, oil production was governed by the geomorphological elevation and reservoir type. Ancient uplifts with broad, gentle topography created favorable conditions for oil and gas accumulation, where multiple phases of weathering led to the development of an extensive subsurface fracture network within the weathering crust. Vertically, this network can extend up to 200 m below the T74 unconformity surface, whereas laterally, it exhibits strong continuity, facilitating both vertical and horizontal hydrocarbon migration [29, 63, 64]. The infiltration of atmospheric precipitation has contributed to the formation of various karst reservoirs at both the surface and deeper underground, including surface weathering fractures, pore-fracture complexes, and deep caves. These fracture-cavity systems are vertically stacked and play a crucial role in the high oil-water production observed in karst highlands [25, 53, 65] (Table 2). As the geomorphology changed and surface weathering intensity decreased at lower elevations, the oil and gas migrated vertically because of buoyancy; some moved to the adjacent residual hills and formed oil-gas enrichment in the gentle karst slopes, while others accumulated in the karst highlands [53].

The second- and third-stage karst reservoirs on gentle karst slopes primarily consisted of planar-aligned pores and fractures, with poor vertical connectivity contributing to lower oil yields than the karst highlands. The oil production on steep karst slopes demonstrated variability, with certain wells producing over 2.2×10³ tons, overlapping with yields from gentle slopes (Figure 16(a)). The water system structures on steep karst slopes were similar to those on gentle slopes, as the multistage uplift continually adjusted the karst discharge base level, shifting the erosion of surface waters from lateral to vertical. This infiltration of surface water created a dual water system, both surface and subsurface, in certain areas with steep slopes [53, 54]. Some productive layers on the steep karst slopes developed multiple sets of fracture-cave reservoirs, where the synchronous formation of weathering crust and fracture-cave bodies could lead to oil yields comparable to those on gentle slopes, particularly in wells located above the drainage base level. However, wells situated below the discharge base level (cumulative oil yield less than 1×10³ tons) were more affected by lateral surface river erosion. The limited interaction between surface water and groundwater in these areas restricted the development of karst pore-fracture complexes, thereby reducing the vertical connectivity in planar-aligned pores and near-horizontal fractures.

The interaction between the water table and faults caused significant variations in oil and water productivity, with oil production increasing as the fault scale increased (Figures 2 and 16(B)). The cumulative oil production associated with the water table and faults of different orders (levels I, II, and III) ranged from 6.21 to 479×10³ tons, 5.4 to 198×10³ tons, and 1.1 to 64.8×10³ tons, respectively (Figure 12(b)). The wells located at level I and II faults demonstrated particularly high oil production rates, exceeding 100×10³ and 31×10³ tons, respectively. The fault cores serving as primary stress release zones could extend deep underground (level I fault may reach the Cambrian). During the karstification period, the vertical infiltration of atmospheric precipitation increased the water-rock contact, continuously enlarging the fault cores and surrounding fractured zones and creating significant storage spaces for large solution cavities [66, 67]. The faults affected the groundwater flow by shifting it from horizontal runoff to vertical percolation, leading to the formation of fracture-cavity bodies at different stages along vertical profiles [45, 53]. Furthermore, the volcanic thermal fluids and thermodynamic action of hydrocarbons and sulfates during the burial period could further alter the carbonate rocks through faults, enhancing the connectivity of fracture-cavity systems [68]. The scale and vertical connectivity of these fracture-cavity systems are critical factors in the variability of cumulative oil production at locations affected by the combined effects of the paleokarst water table and faults.

In the Tahe area, NE- and NW-trending shear fractures were predominantly distributed in the upper blocks of fault zones, with the fracture development increasing closer to the fault cores. During fault activity, calcite cements formed by sedimentation and episodic marine incursions preferentially fill open spaces in the fault core and adjacent fractured zones [69]. However, some effective infiltration spaces were preserved within the main faults and fractured zones, which played a significant role in the subsequent hydrocarbon charging and accumulation processes. Subsequent tectonic uplift and multiple stress field effects led to stress re-release at the fault core and in strongly fractured upper blocks, causing renewed rock fragmentation and the formation of large isolated caves through atmospheric precipitation and hydrothermal dissolution [18, 27, 53]. Varying degrees of dissolution caused reservoir fragmentation, with the extension and compression-shear segments demonstrating wider fractured zones and more intense dissolution. In contrast, the high-angle vertical faults in the straight segments result in narrower reservoir ranges, leading to variations in the production capacity at different locations along the same fault [3, 25, 70].

Two main reservoir development models exist for the Tahe area: multistage water table dissolution-controlled reservoirs and water table-fault composite-controlled reservoirs. Phreatic dissolution significantly influenced the vertical and localized lateral characteristics of the reservoirs. Karst water infiltrated along the faults and fracture zones in structurally deformed ancient residual hills, forming different generations of karst water erosion primarily during the first and third stages. In karst highlands and gentle karst slopes, a network of fractures and pore-fracture complexes with cross-stratum characteristics was developed (Table 2). In the basin, the control of ancient karst residual hills on reservoirs weakened, with lower weathering and weak water-rock interactions resulting in the formation of locally developed dissolution pores and near-horizontal fractures on steep karst slopes, particularly in the first- and second-stage horizontal runoff zones [3, 9, 66]. During the early Hercynian, the large-scale regional uplift caused intense weathering of the Silurian, Devonian, and Upper Ordovician strata, forming extensive fracture-cave bodies 150 m below the T₇⁴ unconformity surface [2, 71]. Subsequent marine transgressions led to the gradual formation of second- and third-stage phreatic fracture-cave bodies. Prolonged weathering enhances the vertical connectivity of first-stage fracture-cave reservoirs in karst highlands and gentle karst slopes [14, 38, 53]. As the karst landforms decreased, insufficient weathering on steep slopes failed to alter the deep strata or were covered by seawater, leading to the formation of horizontal cavities and near-horizontal fractures through phreatic flows and transitioning reservoir types from cross-strata to along-strata formations (Figures 9 and 17).

Faulted karst reservoirs in the Tahe Oilfield can be strongly influenced by both paleokarst water tables and faults. Drilling practices have indicated that tool sticking and mud loss commonly occur beneath weathered zones within fractured karst cores [63]. Major strike-slip faults in the area, particularly level I and II faults, significantly controlled the Ordovician carbonate reservoirs by penetrating the top of the Ordovician (T₇⁴ unconformity) and creating open passages on the surface [20, 29]. Mudstones from the Bashu Group preferentially fill these fault zones, with collapse breccias cemented by mudstone forming approximately 100 m beneath the weathered zone [3, 6, 15, 20]. In the fault zones extending into karst depressions, ample water supply and prolonged water-rock interaction resulted in larger, better-connected karst cavities. Mudstones can infiltrate and fill these cavities through fault cores beneath 200 m of the weathered zone or cement collapse breccias formed during fault activity (Figure 17) [18, 19, 54]. Karst cavities predominantly formed where faulting intersected the water table, causing mechanical fracturing of carbonate rocks between water tables. Vertically, these cavities shaped by the overlapping effects of multiple water tables and faults were connected through fracture networks. Level I and II faults in residual hills (karst highlands and gentle karst slopes) diverted most surface water underground, forming extensive karst channel systems aligned with fault directions below 150 m of the weathered zone [20, 69]. Under strong hydrodynamic conditions, the supply areas of these channels frequently accumulate sediments, such as collapse breccias, sandstones, and sandstone-cemented breccias. As the depth increased, the filtration effect of the mudstone gradually weakened. Meanwhile, atmospheric precipitation alters the surface weathered crust, leading to the precipitation of calcite within fractures during episodic marine transgressions. This calcite deposition reduces the infiltration of surface mudstone into deeper layers, thereby preserving the deep fracture-karst system (Figure 17).

Detailed geological analyses of water table fluctuating dissolution and reservoir characteristics offer a valuable approach for exploring fracture-cavity reservoirs in denudation settings, enhancing our understanding of the structural influences on the development of multistage water surface- and fault-controlled reservoirs. This study provides an interpretation of superior reservoir formation by integrating paleogeomorphology, karst water movement patterns, and faults in the Tahe oilfield. Several key conclusions were drawn from this study.

1. The Yingshan Formation in the Tahe area demonstrated three stages of horizontal runoff zones from bottom to top. In the karst highlands and gentle slopes, all three stages of horizontal runoff zones were present, whereas on steep karst slopes, only the first and second stages were developed.

2. The three stages of fracture-cave systems exhibited distribution characteristics of both cross-strata and parallel to the strata. Under the control of a single water table, the reservoirs in karst highlands and gentle karst slopes, associated with first- and third-stage phreatic surfaces, were characterized by the superposition of the current horizontal runoff zone and the vertical vadose zone from the previous stage. Interlayered networks and pore-fracture composites were recognized as prime areas for hydrocarbon exploration.

3. The first-stage horizontal runoff zone on steep karst slopes and the second-stage horizontal runoff zone across all landforms (highlands, gentle slopes, and steep slopes) were characterized by dissolution pores and near-horizontal fractures along the strata. The interaction between level I and II faults and the paleokarst water table led to the formation of large dissolution caves superimposed across multiple stages. The caves formed under the influence of faults and the first-stage water table were predominantly filled with sandstone, sandstone-cemented collapse breccia, and poorly cemented collapse breccia, whereas those associated with the second- and third-stage water tables were filled with mudstone and mudstone-cemented collapse breccia.

All relevant data are within the paper.

The authors declare that they have no financial interests and personal relationships with other researchers or organizations that may have potential conflicts.

This work was supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. 2016ZX05014-001, 2016ZX05053-001, Sponsor: Hui Lv).

We would like to thank the SINOPEC Research Institute of Exploration and Development, Northwest Oilfield Branch Company for the detailed data and support provided. We also thank the Editor and anonymous reviewers for their constructive suggestions and critical comments, which have greatly improved this manuscript.