Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Geology

Major Professor

Jason Gulley, Ph.D.

Committee Member

P.J. Moore, Ph.D.

Committee Member

Patricia Spellman, Ph.D.

Committee Member

Glenn Thompson, Ph.D.

Committee Member

Stephen McNutt, Ph.D.

Keywords

Banana holes, Flank margin cave, Geomorphology, Karst, Paleosol

Abstract

Carbonate rocks contain about a third of the worlds drinking water and host 60-70% of proven hydrocarbon reserves. Effective development and management of these resources relies heavily on geologic concepts used to predict the distribution, and magnitude of porosity and permeability in the aquifer or reservoir. Most geologic concepts used for flow prediction have been developed in telegenic limestones, where fracture networks, bedding plains, and conduits hosted in effectively impermeable bedrock control the movement of fluids, and evolution of porosity. However, a growing body of work has recognized fluid flow within eogenetic limestones is fundamentally different, and that new concepts for pore system evolution and permeability distributions are needed for effective flow prediction. Our study utilizes San Salvador Island, Bahamas as a natural laboratory for investigating the magnitude, distribution, and connectivity of extreme porosity and permeability end members controlling the flow network. We investigate the formation and hydrologic role of flank margin caves, exposure surfaces and banana holes using drilling, hydrological, geomorphological, geospatial, and petrophysical data. Our findings suggest flank margin caves can represent extreme end members of touching vug networks that form though repeated episodes of dissolution within multiple freshwater lenses during sea-level still stands. In the largest flank margin caves, the initial void space forms in paleo-lenses below modern chambers and subsequent collapse migrates void space vertically to their current elevations. Overprinting of the collapse chamber in post-collapse lenses results enlarges cave chambers and results in their young appearance. Subaerial exposure surfaces represent low permeability end members that can function as aquitards. Because exposure surfaces can focus flow, they can result in aquifer compartmentalization, alter the shape of freshwater lenses, and focus dissolution. As a result, exposure surfaces likely provide the pre-existing architecture controlling diagenetic patterns within simple eogenetic islands. Banana holes are dissolutional voids which form in aquifers perched on exposure surfaces. Evidence for perching is observed in water-filled banana holes, which are disconnected from the underlying tidally controlled aquifer. Processes controlling their formation include the development of surficial calcretes, focusing of runoff to catchment basins, injection of meteoric waters though fast flow routes, and focusing of flow along exposure surfaces in the subsurface. Because their position is decoupled from the freshwater lens, banana holes could have formed during sea level low stands, thereby increasing timeframes and reducing dissolution rates required for their formation. As a result, the generation of macroporosity in eogenetic limestones is not uniquely controlled by the position of paleo-lenses. Our observations shed new light on processes controlling the development of porosity and permeability in eogenetic carbonates. As a result, the concepts presented herein provide powerful inputs to models used to predict fluid flow in freshwater aquifers and hydrocarbon reservoirs alike.

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