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Volume 30 Issue 5
Oct.  2019
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Eogenetic Karst in Interbedded Carbonates and Evaporites and Its Impact on Hydrocarbon Reservoir: A New Case from Middle Triassic Leikoupo Formation in Sichuan Basin, Southwest China

  • Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incom-pleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.
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Eogenetic Karst in Interbedded Carbonates and Evaporites and Its Impact on Hydrocarbon Reservoir: A New Case from Middle Triassic Leikoupo Formation in Sichuan Basin, Southwest China

    Corresponding author: Xiucheng Tan, tanxiucheng70@163.com
  • 1. State Key Laboratory of Oil and Gas Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
  • 2. School of Earth Sciences and Technology, Branch of Sedimentology and Hydrocarbon Accumulation, CNPC Key Laboratory of Carbonate Reservoir, Southwest Petroleum University, Chengdu 610500, China
  • 3. Department of Earth Sciences, Nanjing University, Nanjing 210023, China
  • 4. Institute of Petroleum Exploration and Exploitation, PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China

Abstract: Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incom-pleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.

0.   INTRODUCTION
  • Karst is one of the important mechanisms responsible for the development of high-quality carbonate reservoirs and thus has attracted persistent attention, especially in carbonate sequences (Agosta et al., 2010; Guidry et al., 2007; Florea and Vacher, 2006; Eberli et al., 2004; Hammes et al., 1996). Two types of karst can be distinguished by different bedrocks, i.e., the karst within carbonate successions (Wang and Lu, 2006; Meyers, 1988) and the karst within carbonate-evaporite successions (Gutiérrez et al., 2008; Klimchouk and Aksem, 2002). Of the two types, due to the coexistence of various rock types, the research about the carbonate-evaporite successions, e.g., the characteristics and origins, has been challenging for decades.

    The evaporites (e.g., gypsum and salt rocks) can occur not only as nodules in carbonate rocks but also as layers alternated with carbonate rocks to form rhythmic strata (Rahimpour-Bonab et al., 2010; Sztanó et al., 2005; Zhang et al., 2003; Lu et al., 2002a, b; Song and Huang, 1998; Sando, 1988). In the former case, gypsum nodules are commonly dissolved to form moldic or pinhole pores in carbonates, laying a foundation for reservoir development, e.g., in the case of the Middle Ordovician Majiagou Formation of the Ordos Basin, central China (Wang and Zhang, 2006; Ma et al., 1999; Ma, 1994; Zhang et al., 1992). In contrast, the karst behaves differently in the layered carbonates and evaporites. For example, Sando (1985) examined the karst of the Mississippian Madison carbonate rocks associated with layered evaporites in the northern and central Wyoming and found that the dissolution of evaporites resulted in the karst is not conducive for reservoir formation due to the collapse of overlying rocks. The collapse also led to the difficult identification of the karst because the classic karst structure cannot be easily observed. In addition, although some researchers proposed that the impacts of such karst on reservoir quality were negative as outlined above (Sando, 1985), most scientists still believed that the impacts should be heterogeneous in theory (Kalvoda et al., 2015; Yuste et al., 2015; Brenchley et al., 2006; Altiner et al., 1999; Choquette and Pray, 1970). Thus, how such karst affects the reservoir is critical. In summary, the identification of such karst and understanding its impacts on reservoir have both scientific and practical implications but have not been well constrained.

    The Middle Triassic Leikoupo Formation (Anisian) in the Sichuan Basin, Southwest China is an important marine gas exploration target of the basin (Zhu et al., 2016; Wang et al., 2009; Ma, 2007), such as giant Zhongba Gas Field, with proved reserves reaching 86.30×108 m3, where Well Chuan 19 produces gas at a rate of 25.80×104 m3/d early in 1972, the proved reserves of Moxi Gas Field reaching 253.87×108 m3 (Shen et al., 2008). It classically constitutes interbedded carbonate rocks and evaporites (Wu et al., 2011; Liu W H et al., 2006). A paleokarst unconformity is present between its top and the bottom of the overlying Upper Triassic Xujiahe Formation (Song et al., 2012; Zhong et al., 2011; Meng et al., 2005). This provides a good case of studying karst in interbedded rocks.

    However, only a few works have been conducted on the karst, with preliminary focus on the presence of this type of karst, and the impact of thermal sulfate reduction (TSR) has been considered (Hao et al., 2015; Cai et al., 2014). The investigation of specific karst characteristics and process is still lacking. In this paper, we present the results of a relatively comprehensive geological study, aiming to describe karst-related features observed and interpret their origin and a possible impact on reservoir development. The results can be expected to have significance both regionally and internationally.

1.   GEOLOGICAL SETTINGS
  • Marine carbonates are important targets of global hydrocarbon exploration (Yang et al., 2017; Zhao et al., 2017; Wu et al., 2016; Wang et al., 2015; Youssef and El-Sorogy, 2015; Burchette et al., 1990; Surlyk et al., 1986; Halbouty et al., 1970). The Sichuan Basin of SW China is one of the successful cases of such exploration. The basin, covering an area of approximately 1.8×105 km2, is located in the northwest of the Yangtze Platform, and belongs to one of its secondary tectonic units (Zhu et al., 2011). Resulting from the northeastward and northwestward extension across major faults within the Yangtze Platform, the Sichuan Basin is shaped like a diamond in map-view (Liu and Chang, 2003; Fig. 1).

    Figure 1.  Distribution of paleouplifts during the Middle Triassic Leikoupo period (Anisian) in Sichuan Basin.

    During the deposition of the Middle Triassic Leikoupo Formation of this study (Fig. 2), the basin was tectonically influenced by the early Indosinian orogeny, resulting in a stable uplift (Zhai, 1989; Fig. 3). As a consequence, the Leikoupo Formation of the basin experienced limited development of tectonic fractures, while the uplifting results in the uppermost strata of the Leikoupo Formation to long-term exposure and denudation (Zhai, 1989). This provides basic geological conditions for the development of unconformity and associated eogenetic karst.

    Figure 2.  Generalized stratigraphy of the Middle Triassic Leikoupo Formation.

    Figure 3.  Stratigraphic burial history of the study area.

    Under the general impact of uplifting during the early Indosinian orogeny, an immense northeast-trending uplifting with the Huaying Mountains being the center took place, whose southern and northern sections are designated as the Luzhou uplift and Kaijiang uplift, respectively (Fig. 1). The Leikoupo Formation in these two uplifts as well as the other two uplifts that were formed during the Caledonian orogeny (i.e., the northwestern Tianjingshan paleouplift and Leshan-Longlüsi paleouplift, Fig. 1; Zhou et al., 2007; Li et al., 2000) was denudated to certain degree (Xu et al., 2012; Li et al., 2011).

    Under the influence of the early Indosinian orogeny, the sedimentary environment of the Sichuan Basin during the Leikoupo period (Anisian) was characterized by a restricted environment of epicontinental sea and only the southern and northwestern areas were somewhat connected to external open sea (Li et al., 2012). The environment of the sea basin is characterized by a pattern of "deep in the west and shallow in the east", and its eastern Jiangnan oldland provides provenance (Zhai, 1989). Under the control of this paleogeographic pattern, the marine basin during this period was closed to semi-closed with an arid to semi-arid and hot climate in general (Li et al., 2011). The basin is a restricted and evaporitic marine platform, with the northwestern margin of the basin acting as the periphery of the platform (Li et al., 2012). Consequently, the Leikoupo Formation is characterized by an assemblage between marine carbonates and evaporites in lithology, both of which exhibit multicyclic features. Specifically, the structure displays interbedded beds with unequal thickness, mainly including marine limestones and dolomites, and gypsum and salt rocks (Fig. 2). In addition, argillites can also be observed in certain areas.

    As is shown in Fig. 4, the Leikoupo Formation displays complete denudation in the core zone of the Luzhou paleo-uplift of southern Sichuan. In contrast, the characteristics in the Kaijiang paleouplift of eastern Sichuan are different, where the denudation of the Leikoupo Formation is relatively weaker as the lowermost strata (e.g., first member) are still present. To the northern Longgang area, the uppermost strata (e.g., the fourth member) have not been denudated. These imply that the Luzhou-Kaijiang paleouplift belt was a highland area that gradually transitioned into a depression area northwestward.

    Figure 4.  Schematic map showing stratigraphic distribution in the Sichuan Basin before the Late Triassic.

2.   DATABASE AND METHODOLOGY
  • More than 1 000 wells have been drilled in the Leikoupo Formation of the Sichuan Basin, providing a solid foundation for the present study. We selected 26 cored wells and 3 outcrops (Fig. 1) for detailed analyses. A total of 399 samples were selected for thin section preparation and physical property tests. The preparation of thin sections was performed in the State Key Laboratory of Oil and Gas Geology and Exploration, Southwest Petroleum University. Observation of thin section was completed in the School of Earth Sciences and Technology, Branch of Sedimentology and Hydrocarbon Accumulation, CNPC Key Laboratory of Carbonate Reservoir, Southwest Petroleum University. Porosity was measured using a JS100007 Helium Porosimeter and permeability was measured on an A-10133 Gas Permeameter at the Institute of Petroleum Exploration and Development, PetroChina Southwest Oilfield Company, employing national industry standards.

3.   RESULTS
  • The Leikoupo karst in the study area can be identified as eogenetic based on criteria established in previous studies (Grimes, 2006; Vacher and Mylroie, 2002), although other diagenesis such as in mesogenetic stage can superimpose some characteristics on it (Hao et al., 2015; Cai et al., 2014).

  • Overlying Upper Triassic Xujiahe Formation was widely present (Liu and Dreybrodt, 2011; Zhao, 1991) as clearly shown in outcrop sections (Fig. 5a). The uppermost strata of the Leikoupo Formation was subjected to widespread denudation to varying degrees (Fig. 4). This provides fundamental condition for karst development.

    Figure 5.  General features of the eogenetic karst in interbedded carbonate and evaporitic rocks of the Middle Triassic Leikoupo Formation, Sichuan Basin. (a) The unconformable contact between the Leikoupo Formation and the overlying Upper Triassic Xujiahe Formation, Weiyuan Section; (b) solution-collapse brecciated dolomite showing the complete filling of caves, Well Longgang 160, 3 707.54‒3 707.81 m, Lei-4-3 sub-member, core observation; (c) caves filled with seepage silt, showing the geopetal structure, Well Qinglin 1, 3 724.3 m, Lei-3-2 sub-member, microscopic observation, plane polarized light; (d) weathered residual breccia, Well Longgang 21, 3 778.57 m, Lei-4-3 sub-member, microscopic observation, plane polarized light; (e) solution-collapse brecciated dolomite, Beibei Section, Chongqing City, Lei-1-1 submember; (f) solution-collapse brecciated limestone, breccia composed of oolitic limestone, Well Longgang 19, 3 783 m, plane polarized light; (g), (h) bioclastic (crinoidal coquina) calcarenite with sparry calcite cement, 2 859.8 m, Well Longgang 163, (g) plane polarized light, (h) cathode luminescence; (i) pyrite filling between breccia, 4 573.71 m, Well Longgang 168, plane polarized light; (j) solution-collapse breccia filled with gypsum, Well Long 4, 3 975 m, Lei-3-2 sub-member, core observation; (k) solution-collapse brecciated dolomite, silica-replaced dolomite, Well Longgang 160, 3 711.70 m, Lei-4-3 sub-member, microscopic observation, plane polarized light; (l) secondary limestone resulting from intense dedolomitization, Well Longgang 39, 4 479.75 m, Lei-4-3 sub-member, microscopic observation, plane polarized light; (m) gray-brown muddy-silty crystalline dolomite showing the development of grike filling, Well Bao 16, 1 912.33‒1 912.77 m, Lei-1-1 sub-member, core observation; (n) micritic dolomite interbedded with mudstone showing indiscernible eogenetic karst features, Well Mo 004-H8, 2 232.87 m, Lei-4 member, core observation.

  • In the Sichuan Basin, approximately 1 000 wells have been drilled into the Leikoupo Formation. However, only a few display slight drilling breaks, such as Well Mo 76 of the Moxi Structure in central Sichuan, where there was a drilling break of 0.4 m at the top, and Well Wei 58 of southern Sichuan, where there was a drilling break of 0.3 m at the top.

    Further core observation also indicates that the number of drilling breaks is limited. Only small caves can now be locally found but consistently have a severe filling (Fig. 5b). Microscopic observation shows that pores are often filled or at least partially filled by silty, muddy seepage sediments (Fig. 5c), as well as calcites and dolomites (Fig. 5h).

  • Residual weathered and solution-collapse breccia occurring in carbonate and evaporite strata may be indicative of karst (Weidlich, 2010; Wang et al., 2008). In this study, field survey and core observation reveal that such breccia widely occurs in the uppermost Leikoupo strata of the Sichuan Basin (e.g., in the Longgang area; Fig. 5d). The thickness of the breccia layers varies considerably from several to tens of meters. The breccia contains pinholes, have uneven sizes ranging from a few millimeters to tens of centimeters, and are highly angular with poor sorting (Fig. 5e). The components of the breccia comprises a variety of constituents, such as dolomicrite (Fig. 5e), dolarenite, algal clastic dolomite, argillite (Fig. 5d), oolitic limestone (Fig. 5f) and bioclastic limestone (Figs. 5g and 5h). The components filling in the inter-breccia space is also complex, including calcite and dolomite debris (Figs. 5g and 5h), and a few pyrites (Fig. 5i) and gypsums (Fig. 5j). These rocks are the residual products of karst development in relatively low structures and the variation of these rocks is caused by the different topographic positions of the karst. The different levels of karstification produce the uneven sizes and poor sorting of the breccia (Figs. 5b, 5d, 5e).

  • In this study, solution pores were easily observed with typical geopetal structures (Fig. 5c). The pores were often filled or at least partially filled by silty and muddy seepage sediments, and the chemical deposition of carbonates, such as calcites and dolomites (Fig. 5h; Wang et al., 2008; Kosa and Hunt, 2006).

  • In the vicinity of karst weathering crust, carbonates (e.g., calcites and dolomites) and evaporites (e.g., gypsum and anhydrites) are often replaced by silica; this is controlled by the atmospheric and aqueous flows that are distributed widely in near-surface environments (Rahimpour-Bonab et al., 2012; Kuznetsov and Skobeleva, 2005). In addition, dolomites are prone to be dedolomitized in the interbedded carbonate and evaporitic rocks due to the massive dissolution of gypsum (Choi et al., 2012; Kenny and Krinsley, 1998; Wang and Sha, 1991; Cao and Hu, 1988). The dissolution of gypsum will release Ca2+ and SO42-. Then, along with the rising of Ca/Mg ratio and concentration of SO42- in formation water, the dolomite will tend to be dedolomitized as: CaMg(CO3)2 (s)+Ca2+ (aq)+SO42- (aq)=2CaCO3 (s)+Mg2+ (aq)+ SO42- (aq). The silicification and dedolomitization can be clearly observed in the top of the Leikoupo Formation of the Sichuan Basin (e.g., the karst zone in the Lei-4-3 sub-member in the Longgang area; Figs. 5k and 5l). Therefore, it is implied that the silicification and dedolomitization should be caused by karstification (Choi et al., 2012; Rahimpour-Bonab et al., 2012; Kuznetsov and Skobeleva, 2005; Kenny and Krinsley, 1998; Wang and Sha, 1991; Cao and Hu, 1988).

  • Karst is generally characterized by vertical zonation that can be easily identified, from top to bottom mainly including epikarst zone, vertical vadose karst zone, horizontal phreatic karst zone and deep slow-flow karst zone (He et al., 2013; Ren et al., 1983). However, the karst zonation is difficult to be distinguished in the Leikoupo Formation of the Sichuan Basin (Fig. 6). A representative core examination from Well Longgang 19 reveals that the development of karst leads to severe stratigraphic collapse and culminates in the extensive formation of solution-collapse breccia. Relatively clear karst zonation was only identified in a limited number of strata, such as 3 765.10‒ 3 765.17 m, which shows the development of vertical fractures that were filled by gravel and black argillite.

    Figure 6.  Schematic profile showing the vertical development of the eogenetic karst at the top of the Leikoupo Formation, Well Longgang 19.

    In summary, the non-zonation is a common feature of the Leikoupo karst.

  • Controlled by the early Indosinian orogeny, a sedimentary framework of large-scale uplifts and depressions emerged in the Sichuan Basin during the Middle Triassic (Zhai, 1989), causing differential development of the Leikoupo karst in different areas. Based on the different levels of stratigraphic incompleteness and denudation, the Leikoupo karst in the Sichuan Basin can be divided into three major geological units: karst uplift (e.g., the Luzhou and Kaijiang paleo-uplifts), karst slope (e.g., the transitional belt between the central and southern Sichuan) and karst depression (e.g., the northwestern Sichuan) (Fig. 4). It should be noted that specific characteristics and mechanism of the lateral development of the karst cannot be conducted based on the data available, which can be investigated in the future along with data accumulation.

    In the karst uplifts, the karst features are not obvious in general due to the combined influences of erosion/denudation and high argillite contents (Li et al., 2012; Fig. 7a).

    Figure 7.  Schematic sections showing the lateral development of the eogenetic karst in the Leikoupo Formation. (a) Section of wells Zhongshen 1-Guanji-Pengji-Mo 47-An 9-Bao 31-Yun 6; (b) section of wells Long 4-Longgang 8-Longgang 001-7-Longgang 6-Tieshan 8-Qili 17. See Fig. 1 for the location of the sections and wells.

    In the karst slopes, the karst structures are relatively better preserved in comparison with those in the karst uplift areas. For example, in the central-southern Sichuan transitional zone (i.e., the karst slopes), the development of eogenetic karst is revealed by the observation of fully infiltrated vertical fractures within the cores (Fig. 5m). In the Moxi area of the central Sichuan, the karstification is not so obvious due to relatively high argillaceous content present in the multiple cyclic sedimentation (Fig. 5n). Regarding the southeastern area adjacent to the Kaijing paleouplift of the Longgang area, the top Lei-4-3 sub-member underwent karstification, resulting in the development of solution-collapse breccia and fractures, as well as a large extent of filling (Figs. 5b and 5m), great karst thickness (usually within 60 m) and relatively good lateral continuity (Fig. 7b).

    In the karst depressions (i.e., the northwestern basin), the karstification is generally weak, as evidenced by the rare solution-collapse breccia and solution fractures observed in the cores. In addition, in the deep strata below the unconformity, the karst structures are generally thin with a range mainly between 1 and 2 m in thickness and have intensive filling (Fig. 5n).

4.   DISCUSSION
  • Based on the above results, it can be implied that the Leikoupo karst in the study area took place in near-surface open environments (Berra, 2012). This further has two possibilities, i.e., eogenetic karst and telogenetic karst. The eogenetic karst took place in near-surface open environments with the sediments being shallowly buried; in contrast, the telogenetic karst took place when the deep buried sediments were uplifted to near-surface environments. According to the geological setting, in which unconformity present between the Leikoupo Formation and the Upper Triassic and no uplifting has taken place later, the Leikoupo karst should belong to eogenetic rather than telogenetic (Xu et al., 2012; Huang et al., 1980). Before the deposition of the Upper Triassic Xujiahe Fomation, the maximum burial depth of Leikoupo Formation is less than the depth limit of shallow burial (~1 000 m, Fig. 3; Andreassen et al., 2007), indicating that the uplift and karstification of the Leikoupo Formation occurred before moderate to deep burial. Therefore, carbonate within the Leikoupo Formation can be regarded as eogenetic in origin, and the Leikoupo karst is classified as an eogenetic karst. In addition, the Leikoupo karst is featured by limited development of drilling breaks, which is caused by the contemporaneous filling of fracture/pores, and it is an unique feature of the eogenetic karst (Xiao et al., 2016). However, the telogenetic karst generally has a large drilling breaks (Zhao et al., 2013). There are apparent differences.

    In summary, the Leikoupo karst in this study is an eogenetic karst, and the specialities of this eogenetic karst still need to be investigated in the future in detail.

  • The uppermost strata of the Leikoupo Formation suffered weathering and meteoric leaching during the eogenetic karst. The epikarst zone containing breccia was formed, thereby generating vertical solution fissures and fractures. These spaces were then flushed by corrosive flow with continuously expanding dissolution, eventually resulting in the formation of a vertical vadose karst zone. When karstification reached the water-table, the movement of the karst water changes from vertical flow to lateral transport. Consequently, lateral surface flow emerges. Gypsum and salt beds, which are susceptible to dissolution, provide an ideal site to accommodate these lateral subsurface flows. Hence, as the karst water flows through the strata, dissolution rapidly alters the gypsum and salt beds to form hollow cavities. Subsequently, the overlying strata lose support and undergo collapse, thereby destroying the vertical zonation structure of the karst that has developed previously. In addition, some karst water that has approached saturation continues to infiltrate along the solution fissures and fractures, thus forming a predominant deep, slow-flow karst zone with cementation and filling. Because the horizontal phreatic karst zone has emptied the gypsum and salt beds, leading to stratigraphic collapse, the resulting loss of the overlying strata makes it difficult to recognize the vertical zonation features. Therefore, due to the influence of lithological factors (i.e., the development of layered carbonates and evaporites), the classic zonation of eogenetic karst has not been well observed in the Leikoupo Formation of this study in general.

    The paleo-tectonic and sedimentary framework dominates the differential lateral development of the eogenetic karstification in the Leikoupo Formation of the Sichuan Basin. Accordingly, a schematic developmental model of the eogenetic karstification in the Leikoupo Formation of the Sichuan Basin was established (Fig. 8). As is shown in Fig. 8, different areas have various exposed karst features. As a result, the vertical karst zone could not develop within specific strata. For example, the vadose and phreatic zone mainly develops in the Lei 2 and Lei 1 members in the central Sichuan-southern transition zone, and in the Lei 3 and Lei 4 members in the central Sichuan. However, in the northwestern Sichuan, the vadose and phreatic zone is under-developed; the residual Leikoupo Formation is mainly included in the slow-flow zone of the karst depression.

    Figure 8.  Schematic model showing the development of eogenetic karst in the Leikoupo Formation. T1j. Lower member of Jialingjiang Formation; Ⅰ. epikarst zone; Ⅱ. karst vadose zone; Ⅲ. karst phreatic zone; Ⅳ. karst slow-flow zone.

    To be specific, the karst platform and slope are mainly the infiltration area of the surface karst water. Vertical seepage is the main mechanism responsible for subsurface karstification. As a consequence, solution fractures are susceptible to be filled by terrigeneous debris. Due to a relatively high argillaceous content in rock composition (approximately 15%) and the impact of denudation, the features associated with eogenetic karst in this geological unit are not clear in general. Note that this is a generalized and schematic model, and thus specific geological conditions should be combined when it is used for geological prediction.

  • The Leikoupo Formation in the Sichuan Basin is characterized by interbedded carbonate and evaporitic rocks with complex rock compositions mainly including tight carbonate rocks, porous carbonate rocks and evaporitic rocks (Fig. 2). The different rock material compositions and mineral types further influence and control the types and intensity of karst. Note that the present discussion below is generally qualitative to semi-quantitative. Quantitative discussion and mechanism of the impact of karst on reservoir development cannot be conducted due to the limitation of data available, which can be investigated in the future along with data accumulation.

  • For the three main rock types in the Leikoupo Formation as outlined above, the eogenetic karst has variable impacts on the reservoir formation.

    (1) Eogenetic karst in tight carbonate rocks

    The tight carbonate rocks mainly comprise micritic limestone and micritic dolomite. They are distributed widely and are the main lithologic types of the Leikoupo Formation. Normally, under the influence of eogenetic karst, the dissolution of limestone is more severe than that of dolomite (Li et al., 2018; Feng et al., 2009; Hou et al., 2005). Note that one of the prerequisites for this differential dissolution is the physical property of these two types of rock are similar. In this study, the original physical property of the micritic limestone and micritic dolomite is different. The porosity and permeability of the micritic limestone are < 2% and < 0.01×10-3 μm2, respectively. In contrast, the porosity and permeability of the micritic dolomite are 3%–12% and > 0.04×10-3 μm2, respectively. Thus, it is apparent that the dissolution fluid is easier to impact the micritic dolomite than the micritic limestone. The physical property is the ultimate control for the flow of fluid.

    In theory, limestone is hard and brittle and thus is easily to be crushed under mechanical action (Feng et al., 2009; Hou et al., 2005). Therefore, when the eogenetic karst develops in the tight limestone, vadose conditions are generated via the generation of fractures, through which karst water migrates and dissolves the surfaces of the fractures and enlarges them into caves, resulting in a variety of dissolution (Gabrovšek and Dreybrodt, 2001; Hu and Chen, 1994). The Leikoupo Formation is generally characterized by elevation and uplifting during the early Indosinian orogeny as previously discussed. Consequently, fractures are predicted not to be developed in the Sichuan Basin. This limits the karstification in such tight carbonate rocks (Fig. 9a).

    Figure 9.  Photographs showing the basic characteristics of the impacts of eogenetic karst on reservoir development in the Leikoupo Formation. (a) Tight micrite lacking fractures with limited reservoir improvement of eogenetic karst, Weiyuan Section, Lei-1-2 submember; (b) uniform dissolution of dolomicrite under eogenetic meteoric water, Gusong Section, Lei-1-1 submember; (c) dolarenite with development of primary intergranular pore, Well Jie 22, 1 741.89–1 741.99 m, Lei-1-1 submember, microscope thin-section observation, blue casting, plane polarized light; (d) reservoir improvement by eogenetic karst in karst slope areas, Weiyuan Section, Lei-1-2 submember; (e) tubular gypsum precipitated by the infiltration of saturated karst water, Weiyuan Section, Lei-1-2 submember; (f) dolarenite containing gypsum nodules with the development of moldic pores, Well Han 1, 3 587 m, Lei-3-2 sub-member, core observation; (g) interbedding gypsum and dolomite rocks, Well Dacan, 4 149.38–4 149.63 m, Lei-3-2 submember, core observation; (h) salt rocks, Well Guang 100, 3 112.22–3 112.63 m, Lei-1-1 sub-member, core observation.

    In contrast, the fragility of the dolomite is positively correlated with the granularity in general; the dolomite with smaller particles is easier to be broken and crumbled under physical and mechanical action (Latinwo et al., 2010; Zhang and Lu, 2001). Its karstification is characterized by a uniform corrosion intensity, karst development and water content under the complex control of mineral fluid interaction (Liu Z H et al., 2006; Oblik et al., 2004; Liu and Dreybrodt, 2001). This phenomenon is clearly observed in field survey where tight dolomicrite generally displays a uniform karst development and water content under the influence of eogenetic karstification (Fig. 9b).

    (2) Eogenetic karst in porous carbonate rocks

    In the study area, porous carbonate rocks mainly include thin-layered and coarse-textured granular carbonate rocks of shoal facies that display intergranular pores with high primary porosity (Fig. 9c). As a consequence, when eogenetic karst took place, the karst water could diffuse and easily enter intergranular pores, thereby spreading throughout the beds to form a complex karst cave system with favorable reservoir space (Fig. 9d). This often develops in the karst slope and monadnock areas. Such pore space, however, is commonly filled by saturated karst water. This would lead to mineral precipitation, unfavorable for reservoir formation, e.g., tubular gypsum (Fig. 9e), which is commonly indicative of near-surface environments (Amadi et al., 2012; Acosta et al., 2011; Mees, 2003). This can be further supported by their coexistence with the karst breccia. Furthermore, the stratigraphic collapse caused by karstification has a negative effect on the preservation of the reservoirs. Hence, it is difficult for such reservoirs to grow to large scales. In addition, the porous rocks are not relatively widely developed, constrained by the limited development of shoal facies only in uplift areas (Song et al., 2012).

    Note that the quantitative evaluation of the impact from the different rock types to reservoir formation is hard because of the complexity of reservoir diagenesis and pore evolution. Based on the above discussion, we propose that the different rock types are fundamental for reservoir formation. The most direct evidence includes the positive correlation between reservoir physical property, oil and gas show and oil production results and rock facies. However, their specific contribution to reservoir property compared with other factors (e.g., topographic undulation, stratigraphic structures and atmospheric and hydrological conditions) is hard under the present research conditions. What the main control needs further studies.

    (3) Eogenetic karst in evaporitic rocks

    The Leikoupo Formation in the study area contains large quantities of evaporitic rocks, the main components of which include anhydrites, salts and, occasionally, gypsum (Fig. 2). They are usually present in layered and massive forms (Figs. 9g and 9h), with a small number occurring as nodules in carbonate rocks. Evaporites can be readily dissolved in the presence of formation water or brines, and the dissolution degree depends on the compositions of formation water, temperatures, pressures, etc. (Johnson, 2005; Zhang and Lu, 2001). If the formation water contains H2S, CO2 and organic acids, it can dissolve the evaporites easily (Galve et al., 2009).

    Under similar exposure and karst conditions, the dissolution rate of evaporitic rocks is approximately 30‒70 times greater than that of carbonate rocks (Négrel et al., 2007; Liu et al., 2005; Klimchouk et al., 1996). In addition, evaporitic rocks are too weak to resist mechanical stress and thus are more vulnerable to suffer karst dissolution than carbonate rocks (De Meer et al., 2000; Ford, 1989). These multiple factors all indicate that the evaporitic rocks in the Leikoupo Formation can seldomly be preserved due to dissolution.

  • Based on the comparison between the porosity and permeability of the Lei 32 layer among different karst areas, including the central Sichuan-southern transition zone within karst slope adjacent to karst platform, the central Sichuan within karst slope and the northwestern Sichuan within karst depression (Fig. 8), we can found that the mean porosity of karst slope is less than that of karst depression, and the mean permeability of karst slope is higher than that of karst depression (Table 1). In addition, frequency range of porosity value < 3% within karst slope (approximately 90%) is greater than that within karst depression (only 73%), but the frequency range of higher porosity within karst slope (> 9%) is more than that within karst depression (Fig. 10). Thus, it is implied that the impact of eogenetic karst on reservoir quality mainly manifests the increase in permeability, such as the production of fractures, and the enlargement of original porous layers. However, there are also negative effects. The overall decrease of porosity is mainly caused by the dissolution of evaporitic rocks, the collapse of overlying carbonate rocks and massive argillaceous fillings, as discussed above.

    Porosity (%) Sample No. Permeability (mD) Sample No.
    Average Maximum Minimum Average Maximum Minimum
    Central Sichuan-southern transition zone 1.78 22.71 0.1 135 1.86 18.2 0.001 29
    Central Sichuan 1.51 15.25 0.1 67 1.03 22.7 0.001 29
    Northwestern Sichuan 2.20 11.27 0.1 197 1.01 32.9 0.001 105

    Table 1.  Physical property of the Lei 32 sub-member of different areas in the Sichuan Basin (data from core analysis)

    Figure 10.  Frequency range of physical property within the Lei 32 sub-member of different areas in the Sichuan Basin. (a) Central Sichuan-southern transition zone; (b) central Sichuan; (c) northwestern Sichuan.

    Based on the above discussion in terms of lithologies and physical properties, it can be implied that eogenetic karst can optimize the physical property of primary porous reservoirs to certain extent, but has limited impacts on tight rocks. In the Leikoupo Formation of the Sichuan Basin, the porous reservoirs are generally developed in the karst uplift and slope areas, where a vadose zone occurs in the low permeable layers of the top strata and thus surface water can easily and effectively carry karst products away, resulting in a net increase in reservoir space. This can be illustrated by the dissolution and enlargement of the primary grainstone reservoirs (Fig. 9d). However, this type of reservoir is often thin layered and has limited distribution (Heydari, 2000). Thus, the optimization of such reservoir is generally not significant. In contrast, the karst slope-basin area is characterized by the development of tight rocks and a high argillaceous content in lithology (approximately 15%). Combined with the lack of communicating fractures during the Indosinian orogeny (Ding et al., 2015), the karst water is difficult to penetrate into rock masses. Therefore, the eogenetic karst has weak optimization on reservoir quality here (Figs. 5d and 9d).

  • Based on the above discussion, it is implied that the impacts of eogenetic karst on reservoir development is generally negative, contrary to that in the strata which have carbonates in dominance (as well as bearing a few evaporites) (Ferket et al., 2006). Combined with geological frameworks of the study area during the Leikoupo period, a general model of reservoir development under the impacts of eogenetic karst was established (Fig. 11).

    Figure 11.  Conceptual model of the development of the eogenetic karst in the Leikoupo Formation. (a) Normal stratigraphic sedimentation; (b) stratigraphic elevation, development of eogenetic karst, erosive fluid dissolves the evaporitic beds into "hollow cavities"; (c) dissolved evaporites cause the overlying strata to collapse due to a lack of support against gravity.

    In the interbedded carbonate and evaporitic rocks, the evaporitic rocks can be preferentially dissolved, which further accelerates the dissolution of carbonate rocks (Shalev et al., 2006; Song and Huang, 1998). Hence, the dissolution of gypsum and salt rocks expedites the formation of karst and causes collapse of the overlying strata. Solution-collapse breccia consequently appeared (Johnson, 2008, 2002; Cooper, 2002; Trzhtsinsky, 2002; Martinez and White, 1999; Lu and Cooper, 1996; Paukštys and Narbutas, 1996). The stratigraphic collapse caused by the dissolution of evaporites mainly depends on the thickness and strength of the overlying strata and size and distribution of the solution pores resulted from the dissolution of evaporites (Gutiérrez et al., 2012; Wang and Sha, 1991). The study area experienced a relatively long and closed concentration of saline water during the Leikoupo period (Zeng, 2007), resulting in a huge thickness of precipitated evaporites with the cumulative thickness of an individual cycle greater than 200 m. In addition, the evaporites occur widely and stably in lateral distribution. Thus, when the eogenetic karst took place, the high solubility of widely- and stably-distributed evaporites facilitated the development of hollow dissolution cavities and accelerated the dissolution of carbonate rocks. The overlying thin-layered carbonate rocks collapse due to insufficient support against gravity, thereby forming discontinuous rock masses and filling and destroying the newly developed solution space. This is not conducive to the preservation of karst pores. In addition, the stratigraphic collapse also leads to the generation of monadnock and valley landscapes, which deteriorates the lateral stratigraphic continuity and further exacerbates the heterogeneity of the reservoirs.

    Note that thermal sulfate reduction (TSR) apparently has impact on the reservoir formation, but that has not been discussed here because it took place in mesogenetic stage, which should be addressed in other separated articles. In contrast, bacterial sulfate reduction (BSR), as another type of sulfate reduction like TSR, may be another important process in the reservoir formation. It probably occurred in the process of eogenetic karst and would have significant impact on mineral dissolution and precipitation (Xing et al., 2018; Einsiedl and Mayer, 2005; Paskauskas et al., 2005).

5.   CONCLUSIONS
  • (1) Influenced by the early Indosinian orogeny and sustained regression, the Middle Triassic Leikoupo Formation in the Sichuan Basin has the favorable and necessary geological conditions for the development of eogenetic karst. This was affirmed by five folds of evidence, including stratigraphic incompleteness and the occurrence of unconformity, the presence of residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization.

    (2) Eogenetic karst zonation is not readily distinguishable vertically due to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flows. Laterally, the karst slopes of the three karst geological units exhibit the most developed karst structure, followed by karst uplifts and the least development in the karst depressions.

    (3) Eogenetic karst has negative impacts on reservoir formation in general, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation in interbedded carbonate rocks and evaporites.

ACKNOWLEDGMENTS
  • We sincerely thank anonymous reviewers for their constructive comments, which are helpful to improve the article substantially. We thank the editors for technical handling and editing of this manuscript. This work was funded by the "13th Five-Year Plan" National Science and Technology Major Project of China (No. 2016ZX05004002). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0888-7.

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