1985, Choquette and James convened and organized a seminar entitled "Characteristics and Significance of Paleokarst System and Unconformity" in Colorado College, USA. The 1987 Atlas of Paleokarst Research compiled after the meeting reflects the achievements and present situation of paleokarst research by experts in different fields in the world in recent years from different carbonate strata in different times. At the same time, under the auspices of Institute of Karst Geology of Chinese Academy of Geological Sciences and Geological Society of China Karst Geology Professional Committee, the first and second national academic seminars on karst deposits were held. 1In April, 1992, the Geological Committee of China Natural Gas Society held the "Symposium on Carbonate Karst Reservoir Research and Modern Marine Deposition" in Wuxi. During this period, China not only introduced, absorbed and applied a large number of advanced technologies, methods and theories of international related research, but also made in-depth exploration based on the ancient karst phenomena actually exposed in Sichuan, Ordos and Tarim since the 1980s, combined with the characteristics of China's geological evolution and drawing lessons from the research theories and methods of related disciplines. And a lot of research work has been carried out on the development characteristics, formation mechanism and controlling factors of ancient karst, and gratifying results have been achieved (Mei Bowen,1992; Tang,1995; Wang Baoqing et al.,1996; Huang Shangyu,1997; Zheng,1997; Deng,1998; Lan Jianghua,1999; Chen, 2000; Zheng Congbin, 20065438+0; An Runlian et al., 2002; Wang et al., 2002; Wu Changwu et al., 2002; Xu, 2004; Jia Zhenyuan et al., 2004), in some respects, has also formed its own characteristics.

Chen et al. (2004) pointed out the vertical zonation, shape and filling type of paleokarst reservoirs in China oil and gas fields by studying the relationship between the development characteristics of karst reservoirs and oil and gas. Xiao Yuru et al (2003) divided it into surface karst residual zone, vertical seepage karst zone and horizontal undercurrent karst zone; Jia Zhenyuan (2004) and others believe that the typical carbonate paleoweathering crust reservoir (body) can be divided into four zones: fracture zone, seepage zone, mixed zone and subsurface flow zone; In addition, the longitudinal section of ancient karst can be divided into surface karst residual zone, vertical seepage karst zone, horizontal undercurrent karst zone and deep slow flow zone.

Xu et al. (2004) think that structure and paleouplift, paleoclimate, rock properties and structure are the controlling factors for the development of karst reservoirs with weathering crust. Liu et al. (2005) divided the controlling factors of paleokarst reservoir development into 10 aspects: climate, groundwater chemical action ability, mineralogy, outcrop time, pre-existing pore network, sedimentary facies and stratigraphic characteristics, hydrological system, outcrop area scale and topographic characteristics, datum level change and structural subsidence characteristics. Xiao Yuru et al. (2003) believed that the main factors affecting the plane heterogeneity of carbonate reservoirs in ancient caves are ancient structures, ancient faults, ancient hydrological systems and ancient karst landforms. Among them, it is most closely related to ancient karst landform, and the development of multi-layer caves is mainly controlled by structural uplift and sea level fluctuation.

Huang Jixin et al. (2006) identified three stages of karstification in the study of karst model of carbonate buried hill of Lower Paleozoic in Chezhen area, and preliminarily discussed the karstification mechanism of the three stages, and put forward a karstification model under the influence of mixed water of atmospheric water and seawater, surface water, groundwater and underground hydrothermal solution. Chen Qinghua et al. (2002) found that ancient karst experienced synsedimentary period, weathering crust period, burial period and epigenetic change period. And the dissolution in the development of ancient karst can be divided into five categories: early exposed atmospheric water leaching, supergene karst, pressure-releasing water dissolution, magmatic activity and other hydrothermal dissolution, and organic acids generated by thermal evolution of organic matter, and ancient karst can be divided into sedimentary karst, weathered crust karst and fracture. Li et al. (2004) divided the Lower Ordovician palaeokarst in Ordos Basin into supergene karst and buried karst according to the formation conditions and controlling factors: supergene karst is divided into syngenetic interlayer karst and bare weathering crust karst, and buried karst is divided into pressure-releasing karst and hot water karst; Li Dinglong et al. (1998) think that ancient karst can be divided into five periods: sedimentary karst period (sedimentary hydrogeological period, with sedimentary karst or interlayer karst), weathering crust karst period (leaching hydrogeological period), buried karst period (buried closed hydrogeological period) and semi-buried tectonic karst period (secondary leaching hydrogeological period, accompanied by strong structure-magma).

Foreign scholars have also noticed the dissolution of organic acids in buried diagenetic stage (David,1989; Stephen, 1989a, b; Wagner 1990), which improves the porosity and permeability of the reservoir and plays a constructive role in the transformation of atmospheric fresh water or contemporaneous seawater karst in the early surface environment or shallow buried environment. In the study of Jurassic to Cretaceous carbonate reservoirs in the Middle East, it is found that (David,1989; Stephen, 1989a), from the atmospheric fresh water environment in the early diagenetic stage to the seawater environment in the shallow diagenetic stage, the recrystallization of high magnesium calcite and aragonite reorganized the micropore network and became an important source of modern reservoir pores. Esteban and taberner (2003) measured the fluid inclusion temperature and stable isotopes (δ 13C, δ 18O) of carbonate cement, and found that secondary pores would be generated during the mixed cooling process of brine in the later stage of carbonate reservoir burial. The accumulation of high concentration H2S in oil and gas fields is usually related to abiotic oxidation caused by carbonate reservoirs and thermochemical degradation of hydrocarbons, and fluids containing high concentration H2S play a key role in the dissolution and hydrocarbon generation of carbonate rocks. Chinese scholar Ma Yongsheng et al. (1999) also confirmed the dissolution and transformation of hydrogen sulfide on carbonate reservoirs of Feixianguan Formation in eastern Sichuan through simulation experiments. Morrow( 1990) proposed in the study of Oligocene strata in Taranaki basin, New Zealand that pore fluid with high salinity played an important role in dolomitization of carbonate reservoirs in burial stage, and dolomite was easy to form a good fracture storage network during fracturing. In addition, Li Dinglong (1999) put forward the concept and research method of "paleokarst geochemistry" considering the differences between paleokarst system and modern karst.

In the study of fluid-rock interaction, simulation experiments are generally used. Rauch et al. (1977), Weng Jintao (1987), Song Huanrong et al. (1990) concluded that the dissolution of carbonate rocks is controlled by lithology, and the dissolution rate of carbonate rocks in acidic media increases with calcite at normal temperature and pressure. Weng Jintao (1987) also came to the conclusion that dissolution is related to the structure of carbonate rocks. Han Baoping (199 1) concluded that dolomite dissolved faster than dolomite when buried at 90℃ and 20MPa. Yang Junjie et al. (1995) simulated the relative stability of calcite and dolomite under two different conditions of near-surface supergene open system and buried environment, and obtained that the dissolution rate of carbonate rocks under near-surface conditions is related to its mineral composition, and the higher the calcite content, the faster the dissolution rate; The dissolution rate of dolomite under burial conditions is higher than calcite, and it is considered that most buried oil and gas reservoirs are dolomite. There are many similar experiments at home and abroad, from which we can see that, on the one hand, it is of great significance to study fluid-rock interaction by means of dissolution experiment; On the other hand, dissolution is quite complicated, and the influencing factors are various, such as lithology, temperature, pressure, organic acid, partial pressure of CO2, etc. (Milliman,1974; Longman,1980; James et al.,1984; Moori et al., 198 1,1984; Friedman,1987; Charles,1988; Quinn,1991; Carter et al.,1994; Salle et al.,1994; Beach,1995; Sun,1995; Leslie,1996; Zheng et al., 1997a, b).

Based on the literature at home and abroad, the progress of paleokarst research includes: ① macro and micro identification marks of paleokarst are put forward from the aspects of stratum, landform, rock minerals and geochemistry; (2) Using modern karst theory for reference, studying the mechanism of paleokarst and internal and external geological factors; ③ Some karst examples since Paleozoic are studied, the development characteristics of surface and underground karst are summarized, and the zoning characteristics of karst in vertical section and the filling situation of karst caves are discussed in detail; ④ Based on basic theories and experimental analysis methods such as carbonate sedimentology, reservoir diagenesis and geochemistry. The physical and chemical fillings of caves are studied, the geochemical, cementation stratigraphy and hydrodynamic models of diagenetic system of atmospheric precipitation are put forward, and an example of comprehensive model of ancient karst development is established. ⑤ Recognizing the controlling role and geological significance of ancient karst in determining the formation and distribution of large-scale storage and permeability space and carbonate reservoirs, and recognizing that the formation of many oil and gas fields with large reserves and high production in the world is mostly related to ancient karst in geological history, the characteristics of karst reservoirs are symbolically described from the aspects of geology, drilling, logging, logging and development performance (Buchbinder et al.,1984; Kovluk,1984; Bhatia,1985; Flexer et al.,1985; Daniels et al.,1986; Li Yongan et al.,1991; Muti et al.,1991; Fritz,1993; Lei et al.,1994; Jia Zhenyuan et al.,1995; Zhang Meiliang et al.,1998; Li Dinglong et al.,1999; Ma et al., 2000; Sui Shaoqiang, 20065438+0; Yan et al., 2005).