The Preservation of Near-Surface, Meteoric Caves at Depth: Observations From Subsurface Data and Numerical Modeling

Abstract

Carbonate rocks often exhibit multi-scale pore networks due to the interplay of depositional, diagenetic, and structural controls. This interplay often leads to carbonate reservoirs with complex pore networks that affect our ability to accurately characterize reservoir quality and be predictive about hydrocarbon recovery. In particular, the development of karst systems can present unique challenges when characterizing carbonate reservoirs due to the difficulty in determining how these large pore systems are distributed and to what degree they remain open during burial. For example, on isolated carbonate platforms, water table caves commonly form along the distal edge of freshwater lenses parallel to shorelines where hydrogeochemical conditions are optimal for their development. Recognizing the processes by which these caves form 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 remain a debatable question.

In order to address the preservation potential of buried flank margin caves, we utilize forward geomechanical numerical modeling that employs physics-based constitutive laws, governing rock deformation to test how caves of different geometries are modified under multiple stress regimes at various depths. Our results suggest that caves that form at near surface conditions and remain air-filled exhibit total collapse around depths of about 1000 m. Conversely, caves that are water-filled can remain open past burial depths of 10 km, which is a more plausible scenario given the likelihood of a cave being flooded as it enters the phreatic realm. These results suggest that once a cave is filled with water, even if later replaced by oil, there is a large preservation potential for caves that develop at near surface conditions to remain open and intact at depth. Such pore volumes likely have an impact on in-place volumes and connectivity within carbonate reservoirs.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019