The preservation of water-table caves at depth: Observations from subsurface data and numerical modeling

摘要

Carbonate rocks commonly exhibit multiscale pore networks because of the interplay of depositional, diagenetic, and structural controls. This interplay commonly leads to carbonate reservoirs with complex pore networks that affect our ability to accurately characterize reservoir quality and be predictive about in-place volumes and hydrocarbon recovery. In particular, the preservation of paleocavern systems can present unique challenges when characterizing carbonate reservoirs because of the difficulty in determining how these large pore systems are distributed and to what degree they remain open during burial. Recognizing the processes by which these caves develop provides key insight into predicting their distribution within the subsurface; however, whether or not such caves are capable of remaining open during burial and at depth remains controversial. This study investigates the preservation potential of water-table caves that have developed in near-surface conditions (i.e., eogenetic coastal karst) and have undergone significant burial by constructing geomechanical finite numerical models. Modeling results suggest that when water-table caves are filled with a fluid, such as water, within the shallow phreatic realm, they can remain open past burial depths of 10,000 m. Conversely, if for some reason a cave is unfilled with a fluid and buried, it experiences total collapse at depths of approximately 1000 m. These results suggest that there is a large preservation potential for water-table caves to remain open and intact at depth, resulting in an additional contribution of pore volume for in-place calculations within oil and gas reservoirs that often are overlooked.