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).