The Tazhong region is a key area for oil and gas exploration and development within the Tarim basin. In this region, development of Ordovician carbonate paleokarsts has been found to be extremely heterogeneous. To investigate the developmental stages of these Ordovician carbonate karsts and their corresponding paleoenvironmental conditions, oxygen and carbon isotopic characteristics of calcite paleokarst fissure fillings were examined in conjunction with rock-structure analyses. Results show that δ¹⁸O (PDB) values tend towards negative values in general, ranging from −3.97% to −12.7% (average value −7.64%), which is indicative of the presence of paleokarstificationviadissolution by atmospheric freshwater. Values of δ¹³C (PDB) span a relatively large range, from 2.48% to −2.13% (average value of 0.23%). This shows that the paleokarst in this area has gone through two stages of supergene and burial diagenesis, and the process of paleokarst is complex. The paleotemperature at which the karst fissure-filling deposits were formed ranged from 6.5 to 47.1°C (average value of 21.8°C), and the salinity of the medium has Z values from 117.48 to 130.24 (average value of 123.94). Four different paleokarsification settings were thus revealed: a marine depositional environment, an atmospheric freshwater karst-filling environment, a shallow-burial paleokarsification environment and a deep-burial high-temperature environment. It has thus been shown that the karst pores, fissures and caves formed by paleokarstification over multiple stages are the main reservoir spaces in this region. This study will serves as a basis for karst reservoir predictions, and exploration and development in this region.

The karst fracture-cave oil and gas reservoirs of the Yingshan Formation in the northern slope of the Tazhong Uplift are well developed and have achieved good exploration results. However, the karst fracture-cave near the top of the Yingshan Formation is basically filled with mud fillings, which seriously affect the reservoir property, and the source and filling environment of the mud fillings have been unclear. Through the petrological and geochemical analysis of the fracture-cave fillings system in the typical wells of the Yingshan Formation, it has been found that (1) the fracture-cave fillings are mainly composed of a mixture of the bedrock dissolution dissociation particles, clay minerals, and calcite cements of the Yingshan Formation, and the content of each component in the different wells or in the cave interval is quite different. (2) Rare earth element analysis shows that the rare earth distribution pattern of the fracture-cave fillings is similar to the bottom marlstone of the Lianglitage Formation, indicating that the fracture-cave fillings should be mainly derived from the early seawater of the deposition during the Lianglitage Formation. (3) Cathodoluminescence, trace element analysis, and previous studies have shown that the formation and fillings of the fractures and caves mainly occurred in the hypergene period, which had the characteristics of an oxidized environment, and that there are two filling effects. First, the limestone of the Yingshan Formation experienced the formation of karst caves due to meteoric freshwater dissolution during the exposure period, and the limestone of the Yingshan Formation was dissolved, resulting in some insoluble clay and residual limestone gravel particles brought into the cave by the meteoric freshwater for filling. Second, the seawater transgression also played an important role during the deposition of the Lianglitage Formation. The clay content in the seawater was high during the early deposition of the Lianglitage Formation, which led to the clay being brought into the caves by the seawater during the deposition of the Lianglitage Formation for further filling; at the same time, calcite deposited into the caves with the clay. The above research promotes the study of the formation mechanism of the karst cave reservoir in the Yingshan Formation and has important theoretical significance for the guiding of the next oil and gas exploration in this area.

Hypogene karst is a special manifestation of karst development in spatial scale. Intensive study of its development mechanism has significant meaning for engineering construction, shale gas and geothermal exploitation. To reveal the developing pattern of hypogene karst in Huanjiang syncline, karst groundwater at different depths in wells HD1-2 and HD1-4 and karst springs was selected as the research object. Through the analysis of geochemistry and stable isotopes of karst groundwater, it was revealed that the circulation pattern of deep karst water came from the common recharge of meteoric water and fossil water hosted in karst caves, runoff of deep faulting belts and discharge of large karst springs, over Huanjiang syncline, which provides good hydrodynamic conditions for hypogene karst development. Meanwhile, the widely developed faulting belts and structural fissures provide primitive dissolution space. Through the above analysis, the paper constructs a hypogene karst development pattern controlled by the deep cycle of groundwater in Huanjiang syncline.

In recent years, it has become more and more common to drill deep karst caves as a part of deep shale gas resource exploration and engineering construction in South China. However, the amount of research on the genesis and development mechanism of deep karst caves is relatively low. Based on drilling core karst morphology analysis and two-dimensional (2D) seismic and wide-field geophysical exploration methods, it is revealed that the deep karst in the Huanjiang area is mainly composed of net cracks, holes expanding along cracks and dolomite honeycomb pores, and that large karst caves are also developed, which are related to NW-trending faults. The deep karst is developed in the hanging wall of the fault, with a width of 500 m and a height of 1500 m, and a linear distribution along NW faults in the region. Based on Th-U dating, inclusion testing and rare earth elements from cave fillings, it is revealed that the development of deep karst space is related to two deep karst genetic types: The first is the hypogene hydrothermal karst, which developed in the Yanshanian period and is related to regional magmatism. The second is the groundwater deep circulation karst, mainly developed after the Himalayan period, which is related to the deep circulation of meteoric water. The genesis of deep karst space is the result of multi-stage karst superposition and is mainly controlled by faults. It is difficult to determine the specific time points of these two types of deep karst transformation, but according to regional tectonic evolution, we speculate that the Yunnan-Guizhou Plateau has been uplifted since the Himalayan period (>3.54 Ma), and the Carboniferous carbonate rocks and early faults in the Huanjiang area have been exposed, leading to the change and evolution of deep karst. Through comprehensive analysis, a fault-controlled hypogene hydrothermal karst pattern and a meteoric water deep karst pattern are established. The genetic pattern of deep karst provides theoretical support for predicting this kind of karst in southern China and for avoiding drilling deep karst caves as a part of resource exploitation.

The dolomite of the fourth member of Dengying Formation in Gaoshiti-Moxi area of central Sichuan Basin is rich in hydrocarbon resources. It has experienced superimposition-reformation of multistage karstification, and is the key target for studying deep ancient carbonate reservoirs. Exploration and development practices show that there are great differences in the development of karst reservoirs of the fourth member of Dengying Formation between the platform margin and intraplatform in Gaoshiti-Moxi area. However, the differences in the genetic mechanism of karst reservoirs between these two zones are unclear. Therefore, based on an integrated analysis of core, thin section, drilling, logging, and geochemical test data, this study clarifies the differences in karstification between the platform margin and intraplatform and conducts a comparative analysis of the controlling factors for the differences in karst reservoirs. Results show that the fourth member of Dengying Formation experienced superimposition-reformation of four types of paleokarstification, including eogenetic meteoric water karst, supergene karst, coastal mixed water karst, and burial karst. Large-scale dissolved fractures and caves are mainly controlled by meteoric water karstification, primarily developing three types of reservoir space: vug type, fracture-vug type, and cave type. Dolomite and quartz fillings are mainly formed in the medium-deep burial period. Four types of paleokarstification are developed in the platform margin, while the coastal mixed water karst is not developed in the intraplatform. Eogenetic meteoric water karst and supergene karst in the platform margin are stronger than those in the intraplatform, while burial karst shows no notable difference between the two zones. The thickness of soluble rock (mound-shoal complex), karst paleogeomorphology, and different types of paleokarstification are the main controlling factors for the difference in karst reservoirs between the platform margin and the intraplatform.