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Structure and Filling Characteristics of Paleokarst Reservoirs in the Northern Tarim Basin, Revealed by Outcrop, Core and Borehole Images

Fei Tian
  • Corresponding author
  • Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
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/ Xinbian Lu / Songqing Zheng / Hongfang Zhang / Yuanshuai Rong / Debin Yang / Naigui Liu
  • Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
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Published Online: 2017-06-26 | DOI: https://doi.org/10.1515/geo-2017-0022

Abstract

The Ordovician paleokarst reservoirs in the Tahe oilfield, with burial depths of over 5300 m, experienced multiple phases of geologic processes and exhibit strong heterogeneity. Core testing can be used to analyse the characteristics of typical points at the centimetre scale, and seismic datasets can reveal the macroscopic outlines of reservoirs at the >10-m scale. However, neither method can identify caves, cave fills and fractures at the meter scale. Guided by outcrop investigations and calibrations based on core sample observations, this paper describes the interpretation of high longitudinal resolution borehole images, the identification of the characteristics of caves, cave fills (sedimentary, breccia and chemical fills) and fractures in single wells, and the identification of structures and fill characteristics at the meter scale in the strongly heterogeneous paleokarst reservoirs. The paleogeomorphology, a major controlling factor in the distribution of paleokarst reservoirs, was also analysed. The results show that one well can penetrate multiple cave layers of various sizes and that the caves are filled with multiple types of fill. The paleogeomorphology can be divided into highlands, slopes and depressions, which controlled the structure and fill characteristics of the paleokarst reservoirs. The results of this study can provide fundamental meter-scale datasets for interpreting detailed geologic features of deeply buried paleocaves, can be used to connect core- and seismic-scale interpretations, and can provide support for the recognition and development of these strongly heterogeneous reservoirs.

Keywords: Paleokarst Reservoirs; Structure; Filling; Borehole Images; Tarim Basin

1 Introduction

Deeply buried carbonate paleokarst reservoirs are some of the most important hydrocarbon targets in many basins worldwide [17]. Since the 1980s, the Ordovician strata and the associated reservoirs in the Tarim Basin have become important exploration targets in China [8]. Works have identified huge oilfields containing reserves of more than 1 × 108 tons of oil, including the Tahe, Halahatang, YingMaiLi, and Tazhong oilfields [913]. The Tahe oilfield, which is located in the northern uplift of the Tarim Basin, is considered to be the largest giant Paleozoic marine oilfield in China (Figure 1) [14]. Paleokarst reservoirs, which develop near unconformities, are the main reservoir type in the Tahe oilfield. In this type of reservoir, caves and their surrounding fractures are the main storage spaces for hydrocarbons.

Location of the Tarim Basin and the study area 
Tectonic components of the Tarim Basin, including three uplifts and four depressions with east-west orientations (modified from [11]). The Tahe oilfield is located in the Northern Uplift of the Tarim Basin. The paleokarst outcrops in the Liuhuang Valley are located in the Bachu Arch, which is approximately 50 km north of Bachu City.The study area is in the centre of the Tahe oilfield, which is located in the southwestern part of the Akekule Arch in the Northern Uplift of the Tarim Basin (modified from [14]).The time-domain depth map of Ordovician strata in blocks 4 and 7 in the main area of the Tahe oilfield. The wells with FMI data are marked in red.
Figure 1

Location of the Tarim Basin and the study area

  1. Tectonic components of the Tarim Basin, including three uplifts and four depressions with east-west orientations (modified from [11]). The Tahe oilfield is located in the Northern Uplift of the Tarim Basin. The paleokarst outcrops in the Liuhuang Valley are located in the Bachu Arch, which is approximately 50 km north of Bachu City.

  2. The study area is in the centre of the Tahe oilfield, which is located in the southwestern part of the Akekule Arch in the Northern Uplift of the Tarim Basin (modified from [14]).

  3. The time-domain depth map of Ordovician strata in blocks 4 and 7 in the main area of the Tahe oilfield. The wells with FMI data are marked in red.

Previous publications on collapsed paleocave structures based on outcrops, petrographic analysis of core samples, and paleokarst reservoir prediction using seismic data have revealed some of the characteristics of paleokarst reservoirs. Loucks et al. [4, 1518] investigated the paleokarst outcrops of the Lower Ordovician Ellenburger Formation in Texas. These authors constructed a three-dimensional (3D) seismic-scale model of the paleokarst system, established a structural model of the collapse of paleocave passages, and proposed a classification of six basic paleocave facies. Li et al. and Jiang et al. analysed in detail the thin sections and C/O isotope ratios of core samples [19, 20]. These authors found that the paleokarst systems in Tahe mainly formed via atmospheric fresh water dissolution and that the breccias formed primarily through the collapse of local caves. Additionally, Sun et al. predicted regional sweet spots in paleokarst reservoirs at a seismic scale [21]. Zeng et al., Dou et al. Ahlborn et al. and Tian et al. described the spatial structures of these reservoirs based on seismic datasets using 3D delineation methods [7, 2224].

However, because of the low core recovery rate from sections with caves, the core samples only reflect some characteristics of the paleocaves at the centimetre scale. Additionally, due to the limitations of seismic data resolution (>10 m), the seismic-based results only show the macroscopic outlines of paleokarst reservoirs. The Ordovician paleokarst reservoirs in Tahe oilfield, with deep burial depths exceeding 5300 m, experienced multiple geologic processes, including karstification, filling and deep burial collapse, resulting in strongly heterogeneous characteristics [24]. To date, some questions remain unaddressed. What are the structures of the deeply buried paleocaves, e.g., the numbers of caves penetrated by single well or the heights of the caves? What are the fill characteristics of the caves, e.g., the distributions of the fills, including sedimentary, breccias, and chemical fills? How have fractures developed around the caves, and what is the thickness of the fractures? The meter-scale structure and fill characteristics of paleokarst reservoirs remain unclear, thereby significantly limiting any improvements in reservoir exploration and development.

Electric imaging logging has high longitudinal resolution (~5 mm) and makes continuous measurements. This method can be used to clearly identify the sedimentary fill, collapse breccia, chemical fill and fractures and is a reliable tool for studying the characteristics of paleokarst reservoirs at the meter scale. The paleokarst reservoirs in the Tahe oilfield were studied in detail by analysing core samples and borehole images from 14 wells and studying outcrops of coeval rocks. The structure and fill characteristics of the deeply buried paleocaves were also analysed. Then, the effects of paleogeomorphology on the structure and fill characteristics were also examined. The results of this study provide a fundamental dataset for the detailed interpretation of the geologic features of deeply buried paleocaves based on the core and borehole images from the Tahe oilfield and can be applied to similar paleokarst oilfields.

2 Geologic Setting

The Tahe oilfield, which has an area of 2400 km2, is located on the southwestern slope of the south-central Akekule Arch (Figure 1b) [25]. The Akekule Arch is located in the North Uplift (Shaya Uplift) of the Tarim Basin and lies to the east of the Hanikatam Sag, to the south of the Yakela Faulted Arch, to the west of the Caohu Sag, and to the north of the Shuntuoguole Lower Uplift and the Manjiaer Depression (Figure 1b). The rocks of the Akekule Arch experienced multiple stages of tectonic activity, including the Caledonian, Hercynian, Indo-Yanshanian and Himalayan events. The area finally became a large nose-like arch with southward-dipping Paleozoic strata during the Himalayan period [9, 2628].

From bottom to top, the Ordovician strata are divided into the Penglaiba, Yingshan, Yijianfang, Querbake, Lianglitage and Sangtamu formations [26]. Two primary stages of karstification affected the Ordovician strata: a middle Caledonian karstification event and an early Hercynian karstification event. In the middle Caledonian, the arch developed into a monocline dipping gently to the southeast. As the arch was uplifted several times, three episodes of karstification, designated episodes I, II and III, occurred during this period [29]. Because most of the Sangtamu Formation is resistant to dissolution, only minor caves formed in the Lianglitage Formation along paleo-faults that penetrated the Sangtamu Formation. In summary, following the middle Caledonian karstification event, only a few small parallel layers of caves and dissolution fractures developed in the Ordovician strata. At the end of the Devonian Period, the early Hercynian tectonic event caused considerable uplift of the arch, which structurally changed from a monocline in the middle Caledonian to a large nose-like structure [3, 26]. The strata experienced significant weathering and erosion. The Devonian, Silurian and some of the Upper Ordovician strata were eroded away in the north but were preserved in a southward-dipping sequence in the south. A mature karst system developed on the exposed Yijianfang and Yingshan formations where the Sangtamu Formation was completely eroded away. Due to its long duration, widespread distribution, and great intensity, the early Hercynian karstification event was the most significant karstification phase in the formation of the paleokarst reservoirs in the Tahe oilfield [30].

3 Data and Method

Actively explored Ordovician strata are exposed in the subsurface on the edge of the Tarim Basin along the Kalpintag Mountains [7]. Related rocks crop out approximately 600 km west of the actual subsurface region in the Bachu area. The Liuhuang Valley and Yijianfang profile sections have four typical outcrop profiles [31, 32]. In the outcrop area, the Ordovician strata feature multiple levels of paleokarst systems. Outcrop data and analyses, including of cave height, fractures around the caves, sedimentary fill, breccia and chemical cave fill, are used to improve the understanding of the paleokarst subsurface features.

The outcrops provide a preliminary geological model for this study. Approximately 600 m of core samples and borehole images (Formation MicroScanner image, FMI) from 14 wells in the northern Tarim Basin have been analysed in this study. The borehole images are calibrated with the core samples to analyse various geologic features in the reservoirs, including cave height, fracture features, sedimentary fill, breccia and chemical fill. The structure and fill characteristics of the paleokarst reservoirs in these 14 wells can be described in detail using these data. In combination with the restored paleogeomorphology, which is interpreted from 3D seismic data, the distributions of paleokarst reservoir structures and fills in ancient highland, slope, and depression areas are discussed.

4 Results

4.1 Characteristics of Paleocaves in Outcrops

Ordovician strata are exposed in the Bachu Arch on the northern edge of the Tarim Basin [7, 31]. Representative paleokarst outcrops are located in the Liuhuang Valley and in Yijianfang profile sections, which are located in the northern Bachu Arch area (Figure 1a). Tian et al. performed a detailed examination of these outcrops and concluded that they experienced the same episodes of karstification as the reservoirs in the Tahe oilfield [24]. Thus, the outcrops are physical equivalents that are useful in characterizing the structures and fills of the Ordovician paleokarst reservoirs.

A three-layered paleokarst system with 8 paleocaves is located in the No. 4 Liuhuang Valley (Figure 2a). The caves developed parallel to the formation, with three high-level caves, four mid-level caves, and one low-level cave (Figure 2b). The heights of these caves ranged from 1 to 2 m. Approximately 1-m-high fractures developed on the top of these caves. A paleocave dominated by breccia fill and with a height of 5.8 m formed in the No. 3 Liuhuang Valley (Figure 2c). From bottom to the top, the breccia size decreases. Additionally, a sedimentary fill occurs between the breccia (Figure 2d). No. 3 Liuhuang Valley also features a paleocave dominated by sedimentary fill with a height of 6.1 m (Figure 2e). The lower part of this cave consists of well-sorted sand, the middle part consists of breccia, and the upper part consists of sand. Due to the complete filling of the cave, the cave fill affected the formation pressure, and a 1-m-thick fracture zone developed in the roof and sides of the channel (Figure 2f). A completely filled paleocave, which is 3.6 m wide and 1.2 m high, is located in the Yijianfang outcrop (Figure 2g). This cave is completely filled with pure calcite.

Paleocave outcrops in the Liuhuang Valley and Yijianfang profiles. Locations are shown in Figure 1a. (a) and (b) Typical multi-layered paleokarst system in the No. 4 Liuhuang Valley. (c) and (d) Typical cave dominated by breccia fill and an intensely deformed roof in the No. 3 Liuhuang Valley. (e) and (f) Typical completely filled sedimentary cave with a slightly deformed roof in the No. 3 Liuhuang Valley. (e) and (f) Typical fully filled calcite cave in the Yijianfang Profile, with few fractures around the cave.
Figure 2

Paleocave outcrops in the Liuhuang Valley and Yijianfang profiles. Locations are shown in Figure 1a. (a) and (b) Typical multi-layered paleokarst system in the No. 4 Liuhuang Valley. (c) and (d) Typical cave dominated by breccia fill and an intensely deformed roof in the No. 3 Liuhuang Valley. (e) and (f) Typical completely filled sedimentary cave with a slightly deformed roof in the No. 3 Liuhuang Valley. (e) and (f) Typical fully filled calcite cave in the Yijianfang Profile, with few fractures around the cave.

4.2 Paleokarst features in Borehole Images and Cores

Borehole images are modern logs that are frequently used to detect complex geologic features in carbonate and clastic reservoirs [33]. The FMI logs used in this study display variations in microresistivity using 192 electrodes located on 8 pads. The directional information and position of the tool in space are calculated using inclinometer measurements (accelerometers and magnetometers). The borehole coverage is a function of the borehole diameter and is equal to approximately 80% in an 8.5-inch borehole. High-resolution (~5 mm) and nearly complete borehole coverage can greatly increase the detail and precision of geological interpretations. In the displayed FMI borehole images, resistive rocks are bright, whereas conductive material is dark.

In the borehole images, the fractures and sedimentary, breccia and chemical fills appear as conductive or resistive anomalies with well-defined shapes (Figure 3). The fractures are shown as sinusoidal lines in the FMI, providing a resistivity contrast between fractures and the existing formation. There is a fracture with a dissolution vug in the core of well S67 at a depth of 5666.93-5666.97 m. In the static FMI, the sinusoidal line and black dissolution patches are very clear (Figure 3a). The paleocaves are usually filled with breccia, sedimentary and/or chemical fills. Each type of fill has distinct image patterns that can be used for identification. Breccias, which experienced water transport, can be identified in the static FMI because the clasts are smaller at the bottom and larger at the top. A small cave developed in well T615 at a depth of 5557.78-5558.65 m. A core sample of this section shows that it is a breccia-dominated cave, which can also be inferred from the FMI (Figure 3b). The chemical fills are mainly calcite in the Ordovician Tahe reservoirs. Similar to the outcrops in the Yijianfang profile (Figure 2g), the calcite is mainly pure (Figure 3c). Thus, in the static FMI, the pure calcite of well S75 exhibits a uniform resistivity (Figure 3c). Sedimentary fills can be subdivided into sandstone and clay. Well T615 features well-sorted sandstone and stratified clay. Because the sedimentary fills have low resistivity values, they are dark stratified areas in the static FMI (Figure 3d).

Paleokarst features in cores and static FMI.
shows the core and images of a typical fracture with a vug in well S67.shows the core and images of a typical breccia fill dominated cave in well T615.shows the core and images of chemical fill in well S75.shows the core and images of typical sedimentary fill in well T615.
Figure 3

Paleokarst features in cores and static FMI.

  1. shows the core and images of a typical fracture with a vug in well S67.

  2. shows the core and images of a typical breccia fill dominated cave in well T615.

  3. shows the core and images of chemical fill in well S75.

  4. shows the core and images of typical sedimentary fill in well T615.

4.3 Structure and Fill Characteristics of a Single-Layered Paleocave

Typical near-surface karst terrain was summarized in a block diagram by Loucks [15]. The schematic diagram shows the geometry of a single cave passage with fractures around the caves. As the burial depth of the cave passage increases, the roof and walls collapse, forming more breccia [7]. This phenomenon can also be observed in the outcrops in the northern Tarim Basin (Figure 2). Through detailed analyses of cores and borehole images, we found this characteristic in the structure of the paleokarst reservoirs. Well S75 in the Yingshan Formation contained host rock, cave sediments, collapse breccia and fractures at depths from 5521 to 5539 m. A typical fracture-paleocave complex in this interval consists of a 10.58-m-tall cave located at depths of 5525.31 to 5535.89 m, with 0.63-m-tall fractures in its roof at depths from 5524.68 to 5525.31 m and 0.93-m-tall fractures in its floor at depths from 5535.89 to 5536.82 m. Additionally, there are two fracture-developing zones that are 1.67 m tall and 0.29 m tall at depths from 5520.26 to 5521.93 m and from 5523.08 to 5523.37 m.

Based on a detailed core description, conventional well logging data, and interpretations of borehole images, a detailed core log was drawn (Figure 4). Nearly all collapsed paleocave facies [4, 18] were present in the interval. Cave collapse breccia represents a major proportion of the cave fills and can be divided into water-transported and local-collapse breccia. The clasts in the water-transported breccia were typically small in size, transported over long distances, and well sorted (Figure 5a). At the top of the water-transported breccia section, clay was found (Figure 5c). The local-collapse breccia formed via local failures in the cave roof, and the clasts are large in diameter. Some fractures developed in the large breccia (Figure 5b, Figure 5d, e.g., 5528.95-5529.91 m and 5533.02-5533.66 m in well S75). Some sedimentary fills with low resistivity values and high gamma ray activities are also present in the top section of the cave and are indicated by dark colours in the borehole images (Figure 4, e.g., 5525.31-5526.19 m in well S75).

Borehole images and well logs of the paleokarst interval in well S75
Figure 4

Borehole images and well logs of the paleokarst interval in well S75

The core photos show the cave fills in well S75. See locations in Figure 4
Figure 5

The core photos show the cave fills in well S75. See locations in Figure 4

4.4 Paleokarst Zonations and their Structure and Fill Characteristics

Ford and Williams subdivided the unsaturated and saturated zones of an unconfined aquifer [34]. The Tahe paleokarst system was modelled on a mature authigenic karst [24]. In consideration of the practical needs of hydrocarbon exploration, the Tahe paleokarst profile was divided into the following four zones: the epikarst zone, the vadose zone, the run-off zone, and the deep phreatic zone.

The epikarst zone occupies the uppermost portion of the paleokarst system and is generally a zone of heavily weathered limestone that directly underlies the soil. In well T615, the epikarst zone is located at depths of 5520.18-5522.21 m (Figure 6 and Figure 7). There are some residual sediments and fractures at the top, exhibiting light-dark sinusoidal line contrasts compared with the host rock (Figure 6). The vadose zone is the zone of less weathered bedrock extending down to the high water table. Atmospheric precipitation seeps down to the water table through the vadose zone. In well T615, the vadose zone is at a depth of 5522.21–5527.90 m. There are 1.86-m-tall vertical fractures with dissolution features, which are shown in the borehole images (Figure 6). The run-off zone is characterized by predominantly horizontal passages and conduits with erosion features whose orientations are locally controlled by bedding planes. The extent of the run-off zone is primarily controlled by the water table, and rises and falls in the regional base level and the associated water table significantly alter the run-off zone. Multi-layered caves with fractures and various fills commonly develop in the run-off zone. The typical single-layered paleocave in well S75 is shown in Figure 4. However, more caves can be identified in wells penetrated by paleokarst systems. In the run-off zone of well T615, there are 6 paleocaves with different fills. The largest paleocave, with a height of 18.70 m, is present at depths of 5534.41-5553.11 m. The cave fills include well-sorted fine-grained sandstone, clay, breccia, and chemical fills. Sedimentary fills represent a major proportion of the fills. The heights of the other 5 caves range from 0.7 to 1.3 m. The roof and floor of these 6 caves feature many fractures. The caves and fractures provide enormous storage spaces for hydrocarbons. The deep phreatic zone exhibits incipient corrosion and/or cementation grading into the fresh (unweathered) formation. Because this zone is deeper than the present reach of hydrocarbon exploration, the available data is insufficient to characterize this zone.

Paleokarst features of the Tahe Ordovician reservoirs characterized by borehole images from well T615
Figure 6

Paleokarst features of the Tahe Ordovician reservoirs characterized by borehole images from well T615

Geological interpretations of the paleokarst structures and fill characteristics in well T615
Figure 7

Geological interpretations of the paleokarst structures and fill characteristics in well T615

4.5 Paleogeomorphologic Control on the Structure and Fill Characteristics

The development of paleokarst reservoirs is controlled by rock properties, karstification, tectonic activities and geomorphic conditions, resulting in very strong heterogeneity. The paleogeomorphology has a major impact on the development and types of paleokarst reservoirs. Fractures and caves in the weathering crust primarily developed in the highlands and slopes [7, 35]. The Ordovician strata in the study area are directly covered with Carboniferous strata (without Silurian strata), and the main karstification occurred before the deposition of the Carboniferous strata in the early Hercynian. Because multiple tectonic events occurred after the early Hercynian in the Tarim Basin, the present structures do not represent the paleogeography at the time of the early Hercynian karstification.

To reconstruct the early Hercynian paleogeography, we flattened the seismic volume on the lower Carboniferous Bachu Formation reflection (T56). The Bachu Formation contains primarily mudstone (Figure 7), which has a filling effect on early Hercynian differences in geomorphology [36]. The flattened seismic section readjusts the top Ordovician unconformity and shows the macroscale morphology of the Ordovician karstification [37]. The seismic section AA′ is oriented in a north-south direction and has a length of approximately 14 km (Figure 8). In the seismic image, “bright spots” appear below the Ordovician unconformity and are typical indications of paleocaves. A 3D paleogeomorphological map can also be created based on the seismic dataset (Figure 9). Typical karst features, such as rivers, karst towers, canyons and sinkholes, are obvious in the seismic section and the 3D map. We divided the paleogeomorphology into highlands, slopes and depressions. The karst system is mature and is similar to the modern karst system in the Chinese Guilin area.

Seismic section AA′ flattened on the lower Carboniferous Bachu Formation reflection (T56) to reveal the original paleogeomorphology of the top of the Ordovician unconformity. See location in Figure 9.
Figure 8

Seismic section AA′ flattened on the lower Carboniferous Bachu Formation reflection (T56) to reveal the original paleogeomorphology of the top of the Ordovician unconformity. See location in Figure 9.

A time-structure map on top of the karstified Ordovician Yingshan Formation (T74), picked in a seismic volume, flattened on the lower Carboniferous Bachu Formation reflection (T56) (e.g., AA′ in Figure 8)
Figure 9

A time-structure map on top of the karstified Ordovician Yingshan Formation (T74), picked in a seismic volume, flattened on the lower Carboniferous Bachu Formation reflection (T56) (e.g., AA′ in Figure 8)

Karst highlands are located in the high regions of the karst area. Atmospheric precipitation infiltrated or flowed through the karst highlands via karst slopes to karst depressions. Thus, karst highlands feature poorly developed surface water systems and are primarily dominated by isolated karst towers. The epikarst zone sediments have been carried away by water, forming erosional grooves. The large cave-like features are mainly composed of sinkholes, which also had relatively isolated distributions. Karst highlands are mainly distributed in the central part of blocks 4, 6 and 7 and are represented by red and yellow colours in the paleogeomorphological map (Figure 9). Five vertical and three horizontal wells were drilled to collect borehole image logging data. The structure and fill characteristics of these wells indicate that thick fractures developed in the highland and that the caves in the karst highlands were small (mostly vertically less than 20 m). Based on the cave fill characteristics, the paleocaves in the karst highlands are dominated by collapse breccia and, in some locations, chemical fills (e.g., Well S67).

Karst slopes were the main area of flow in the paleokarst water system and developed numerous caves, forming complex underground drainage systems. In contrast, valleys on the surface and surface water system were not well developed. Karst slopes are mainly distributed on the edge of blocks 4, 6 and 7, marked with green and light blue colours in the paleogeomorphological map (Figure 9). Four vertical wells with borehole image logging data were drilled. The structure and fill features of these wells indicate that the caves on the slopes were large (exceeding 10 m), with large heights and good geometric connectivity. Underground rivers transported sedimentary fill from the karst highlands and peripheral areas. However, because of variations in precipitation and underground river diversions, some caves filled with sediments, whereas others developed collapse breccias. Because the caves were filled with sediment and breccias, thereby influencing the effects of the overlying formation pressure, fewer peripheral fractures developed around these caves compared to the caves in the highlands (Table 1).

Table 1

Statistics of the multiple-layer paleokarst reservoirs in the area of well T615

Karst depressions, located in low-lying areas of the karst systems, functioned as water collection areas for the surrounding regions. These regions featured surface rivers, which became filled at a later time. Karst depressions are mainly distributed in the external areas of the southern block 7 and the western block 6, marked as blue and purple in the paleogeomorphological map (Figure 9). Well TK429 in a paleo-channel of a karst depression contains weathering residues in the epikarst zone and penetrated only a 2-m-tall cave fully filled with sediments (Table 1). Well TK635 is a horizontal well that drilled into less than 30 m of the caves, demonstrating that caves were not well developed in the studied karst depressions.

In summary, the paleogeomorphology controlled the development and fill characteristics of the paleokarst reservoirs. The karst highlands developed relatively isolated sinkholes and small local caves, which were filled with collapse breccia and developed fractures. The karst slopes developed numerous tall caves as part of the underground drainage systems. Multi-layered cave systems developed in the karst slopes due to the changing paleowater table. The caves were largely filled with sediments (Figure 10), and the fills bore part of the overlying strata pressure; hence, fewer fractures formed around the caves. The karst depressions developed fewer fractures and caves, which were mainly filled with sediments (Figure 10). Based on the borehole image interpretations, we found that the thickness of the paleokarst reservoirs in the horizontal wells is significantly greater than that in vertical wells. Thus, according to the structure and fill characteristics, we suggest that horizontal wells should be preferentially drilled in this type of paleokarst reservoir.

Cave fill classification and the fill characteristics of drilled wells in different paleogeomorphologic settings. The caves in the highlands are mostly filled with breccia, and those on the slopes and depressions are primarily filled with sediments
Figure 10

Cave fill classification and the fill characteristics of drilled wells in different paleogeomorphologic settings. The caves in the highlands are mostly filled with breccia, and those on the slopes and depressions are primarily filled with sediments

5 Discussion

Based on outcrop investigations, core sample observations, and interpretations of borehole images, this paper identified the characteristics of caves, cave fills (sedimentary, breccias and chemical fills) and fractures above and below caves penetrated by individual wells in strongly heterogeneous paleokarst reservoirs at the meter scale. The paleogeomorphology of study area was reconstructed using a 3D seismic dataset. The karst highlands, slopes and depressions were interpreted and their effects on paleokarst reservoir development were summarized.

Previous studies used seismic datasets to predict the sweet spots in paleokarst reservoirs and delineate their 3D outlines. This type of work can effectively guide drilling during the early exploration period. The thin section observations, inclusion homogenization temperature measurements and isotope analysis of representative core samples can reveal the main development periods and geologic processes experienced by the paleokarst reservoirs. However, seismic datasets can only reveal the macroscopic responses of the reservoirs at the >10-m scale and are unable to accurately characterize the structure and fill inside the caves. Core analysis focuses only on selected representative points, and the scale is on the order of centimetre. Therefore, this method cannot effectively evaluate the petrophysical properties of strongly heterogeneous reservoirs. This paper characterized the paleocaves, their fills and the related fractures at the meter scale. The results effectively characterize the heterogeneity within and around the paleocaves and effectively connect core analysis and seismic interpretations. Therefore, these results can be used to evaluate the petrophysical properties of a reservoir, guide fluid flow simulations, and improve paleokarst reservoir evaluations during the development period.

The Ordovician paleokarst reservoirs in the Tahe oilfield, with burial depths of over 5300 m, experienced multiple phases of geologic processes and exhibit strong heterogeneity. In future studies, outcrop and subsurface data should be used to character the reservoirs. Core samples and thin sections should be used to study the characteristics at the microscopic to centimetre scale, borehole images and conventional well logging information should be used to study the structures, fill and petrophysical characteristics of the reservoirs drilled by single wells at the meter scale, and seismic datasets and production data should be used to study the 3D distributions and macroscopic properties of the reservoirs at the 10-100-m scale. These multiscale, multi-disciplinary comprehensive characterizations can provide strong support for recognizing and efficiently developing these strongly heterogeneous paleokarst reservoirs.

6 Conclusions

The structure and fill characteristics of the deeply buried Ordovician paleokarst reservoirs in the Tahe oilfield of the Tarim Basin were characterized via a comprehensive utilization of outcrops, core samples, and borehole image logging data. Field outcrops and core samples, which provide the most direct evidence of collapsed caverns, show that the caves contain sediments, collapse breccias, and chemical fills. The fill characteristics of different caves vary. Fractures developed around the caves. The extent of the peripheral fractures was smaller for completely filled caves.

Using core-calibrated borehole images, the structure and fill characteristics of paleokarst reservoirs were studied at the meter scale. Through the analysis of a single-layered paleocave, we found that the cave was filled with multiple types of fill and that fractures were present in the cave roof and floor. An analysis of the paleokarst features in each vertical zone of the paleokarst reservoir showed that the epikarst zone was primarily characterized by weathering residues and dissolution fractures, the vadose zone was mainly characterized by high-angle fractures, and the run-off zone was characterized by multi-layered paleocaves filled by sediments and collapse breccias.

The structure and fill characteristics of the paleokarst reservoirs were significantly controlled by the paleogeomorphology. Using a 3D seismic volume that was flattened on the Carboniferous Yingshan Formation mudstone, the early Hercynian karstic paleogeomorphology was restored and divided into highlands, slopes and depressions. Paleokarst structures preferentially formed in the highlands and slopes, and relatively fewer fractures and caves form in the depressions in the catchment areas. The caves in the highlands were mainly filled of breccias, the large multi-layered caves in the slopes were associated with drainage systems and were mainly filled with sediments, and the few small caves in the karst depressions were filled with sediments.

Acknowledgement

This study was supported by the Chinese National Major Fundamental Research Developing Project (Grant No. 2017ZX05008-004), the Chinese National Natural Science Foundation (Grant No. 41502149, No. 41372151 and No. 41102078), and the China Postdoctoral Foundation Funded Project (Grant No. 2015M570148). We are deeply grateful to the Tahe Oilfield Branch Company SINOPEC for supplying the data and allowing this paper to be published. We thank three anonymous reviewers for their thorough and critical reviews and suggestions, which have improved the manuscript.

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About the article

Received: 2016-05-11

Accepted: 2017-03-22

Published Online: 2017-06-26


Citation Information: Open Geosciences, Volume 9, Issue 1, Pages 266–280, ISSN (Online) 2391-5447, DOI: https://doi.org/10.1515/geo-2017-0022.

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© 2017 F. Tian et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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