The formation and burial evolution of multi-level karst cave system

The vast majority of karst caves are epigenetic caves, developed in limestone-dominated carbonate formations, and distributed in form of interconnected or related cave systems, moreover, their three-dimensional geometry is complex and cross-sectional scales vary greatly. Multi-level cave systems are common in areas where epigenetic karst develops on a large scale, but they are less studied as a whole as a cave system. In China, the paleo-karst multi-level caves buried in deep have been the subject of extensive research along with the exploration of hydrocarbon resources in marine carbonate reservoirs, for example, the Tahe Oilfield in the Tarim Basin, mainly focusing on their three-dimensional distribution characteristics, filling and connectivity, and subsequent evolution after burial. However, further research is needed to understand the process of its burial. Here we provide a detailed mapping of the Yanzi(Swallow)Cave multi-level cave system in Guiding County located in the center of southwestern karst region in China, using three-dimensional laser mapping techniques. Then we compare the data with those obtained through seismic, core drilling, and logging data from the Tahe Oilfield, to analyze their morphology and distribution patterns, aiming to infer the formation and burial processes of the epigenetic multi-level caves as well as the factors influencing them. The results show that both exhibit a multi-level structure connected by vertical shafts, with a narrow, elongated strip-like distribution controlled by fractures in plane, hence a significant similarity in their structural causes. We present models of the complete evolutionary process of a multi-level cave system, from its formation period dominated by dissolution-erosion in epigenetic stage to the burial stage which had undergone marine inundation filling, mixed dissolution, and compaction modification; And the mechanisms by which various factors also analyzed, such as rock type, tectonics, hydrology, climate, and time, jointly control the development and preservation of these caves. By drawing on modern dating data of karst cave fillings, we analyze the issue of the burial time scale of cave in paleo-karst reservoir studies, namely, that suitable multi-level cave evolution timescale(in millions of years)is essential for the development and preservation of such a system. Were the time span of the marine inundation process too long, the cave systems formed during the epigenetic stage would be completely eroded away. The findings deepen understandings of the evolutionary patterns of multi-level cave systems and could provide reference for the exploration of fractured-vuggy carbonate reservoirs.