Integrated modeling workflows for characterizing offshore freshened groundwater : a New Jersey shelf case study

Thomas, Ariel Tremayne; Clauser, Christoph (Thesis advisor); Micallef, Aaron (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

The continental shelf is an essential domain for natural resources. It typically harbors reservoirs that have long provided humanity with vital resources such as oil, natural gas and minerals. A comparatively recent discovery is the existence of fresh groundwater that has been recorded on continental shelves globally. The term offshore freshened groundwater has been coined to refer to any pore fluid store in sediments beneath the oceanfloor that has a salinity lower than oceanwater. The New Jersey shelf, on the Atlantic margin of North America, hosts a well-documented occurrence of this phenomenon. The extensive system was sampled by scientific drilling campaigns that revealed the layered reservoir distribution. Research on the origins of this system has produced two competing hypotheses. There is an ongoing scientific discussion whether the system consists of modern groundwater, recharged via an active onshore connection, or paleo-groundwater that was emplaced via meteoric recharge during the Pleistocene sea-level lowstand. Previous attempts to model the New Jersey shelf utilized simple conceptual models. This thesis aims to determine which of the two hypotheses is most consistent with the field observations and characterize the New Jersey shelf freshwater system. An integrated modeling approach incorporated depth-migrated seismic and well data into the most geologically representative shelf model produced to date. Stochastic modeling techniques were used to address the challenge of limited borehole data. Numerical simulations, performed using High-performance computing, revealed the influence of geological heterogeneity at a resolution of ten meters vertically, and one-hundred meters horizontally. The numerical study demonstrates how the conditions on the shelf over the past 70\thinspace000 years could result in a complex zone of fresh to brackish water extending over 100 km from the modern shoreline. Simulated results closely match the observed salinity profiles at four well locations across a 2D shelf transect. Meteoric recharge during sea-level lowstand was found to be the primary mechanism of freshwater emplacement. Seafloor connected pathways and cemented layers, strongly influence the geometry and distribution of the surviving freshwater intervals. While it extends far offshore, the system is laterally compartmentalized by downward fingering saline pore fluid. Furthermore, a three-dimensional parameterization of flow properties on the shelf, constrained by well log and seismic attribute data is presented. The model successfully upscales porosity from the core-sample scale to the shelf scale with dimensions 134 km x 69 km x 1.7 km. The fully open-sourced model is adaptable for numerical studies of fresh-salt water transport or geomorphological processes on the New Jersey shelf. The workflows presented in this thesis quantify facies and porosity compaction trends, which reflect the depositional environment and flow conditions on the shelf. Incorporating these details improves our understanding of the emplacement mechanisms, survival rates and lateral connectivity of the offshore freshened groundwater system. These potential resources are still not very well understood, in many cases scarce data leads to high uncertainty about the volumes of fresh water, and the lateral connectivity of the reservoirs. This thesis presents an interdisciplinary approach that can be further developed into a standardized workflow for the characterization of offshore freshened groundwater systems generally.