Basin-scale hydrogeologic modeling (original) (raw)

1996, Reviews of Geophysics

Mathematical modeling of coupled groundwater flow, heat transfer, and chemical mass transport at the sedimentary basin scale has been increasingly used by Earth scientists studying a wide range of geologic processes including the formation of excess pore pressures, infiltration-driven metamorphism, heat flow anomalies, nuclear waste isolation, hydrothermal ore genesis, sediment diagenesis, basin tectonics, and petroleum generation and migration. These models have provided important insights into the rates and pathways of groundwater migration through basins, the relative importance of different driving mechanisms for fluid flow, and the nature of coupling between the hydraulic, ther-mal, chemical, and stress regimes. The mathematical descriptions of basin transport processes, the analytical and numerical solution methods employed, and the application of modeling to sedimentary basins around the world are the subject of this review paper. The special considerations made to represent coupled transport processes at the basin scale are emphasized. Future modeling efforts will probably utilize three-dimensional descriptions of transport processes, incorporate greater information regarding natural geological heterogeneity, further explore coupled processes, and involve greater field applications. 1. ment. Uplift of mountains and compressional forces are thought to have driven basinai fluids (petroleum and metal-bearing brines) hundreds of kilometers across the continents, forming some of the world's largest mineral and hydrocarbon deposits [Oliver, 1986; Bethke and Marshak, 1990]. Understanding transport processes within sedimentary basins requires an integrated approach involving geological field studies, laboratory investigations, mathematical modeling, and field measurements of pore fluid pressure and brine geochemistry. Measurements of hydraulic parameters in the laboratory or direct field observations of subsurface fluid pressures, temperatures, and pore fluid chemistry [On-and Kreitler, 1985] provide data regarding active transport processes within flow systems and help to constrain rock parameter information. Field studies help to constrain the timing and migration pathways of ancient hydrological systems as they have evolved through time [Shelton et al., 1992; McManus and Hanor, 1993]. However, the study of sedimentary basin evolution in the laboratory and field is hindered, to some extent, by the slow rates and long distances over which transport processes operate in basins. The formation of energy and mineral deposits within the crust, for example, typically occurs over time periods of millions of years and can involve lateral fluid migrations over hundreds of kilometers [Garven, 1995]. The simplifying assumptions required to reproduce these processes in the laboratory or the data limitations associated with making direct observations in the field pages 61-87 Paper number 95RG03286 62 ß Person et al.' HYDROGEOLOGIC MODELING 34, 1 / REVIEWS OF GEOPHYSICS have left gaps in our understanding of all aspects of basin fluid interactions [Bethke e! al., 1988]. During the last decade, mathematical modeling of subsurface fluid flow and heat and chemical mass transport have been increasingly called upon to study a wide range of transport-limited geologic processes within sedimentary basins, including. the formation of excess fluid pressures, anomalous heat flow, hydrothermal ore genesis, sediment diagenesis (physical and chemical transformation of sediments occurring after deposition), faulting and seismicity, and petroleum generation and migration. While these models have differed greatly in their complexity and the processes represented, they all share a common set of assumptions and basic flow laws. Because mathematical models can represent geologic processes that occur at very slow rates and over continental length scales, they complement field-or laboratory-based investigations. In addition, mathematical modeling represents an important research tool because of the need to consider the behavior of the transport processes occurring in sedimentary basins simultaneously [Tsang, 1987; Bredehoeft and Norton, 1990; Person and Gatyen, 1994]. As Chen et al. [1990, p. 104] notes, "Although we tend to think of a single process, it often happens that a variety of processes are coupled so strongly that qualitatively new effects and system behaviors arise because of this coupling." This paper provides a review of the recent advances that have been made in the mathematical modeling of groundwater flow, rock mechanics, heat transfer, and reactive chemical mass transport processes within sedimentary basins. This work is intended to complement the reviews that have been published recently on modeling subsurface fluid flow [Konikow and Mercer, 1988; Bethke, 1989; BjOrlykke, 1993], heat transfer [Furlong et al., 1991; Lowell, 1991], and petroleum generation and migration [Ungerer e! al., 1990]. 2. DESCRIPTION OF TRANSPORT PROCESSES IN SEDIMENTARY BASINS 2.1. Fluid Flow Subsurface fluid flow plays a critical role in a number of geochemical, geothermal, and tectonic processes within sedimentary basins. Groundwater flow is the most important agent in solute mass transport and is a ratelimiting step in hydrothermal ore genesis [Garven, 1985; Raffensperger and Garven, 1995a, b] and in the formation of diagenetic cements and minerals [Wood and Hewett, 1984]. Basin hydrodynamics also has important implications for long-range (10 to 1000 km) secondary petroleum migration within gently dipping carrier beds [Garyen, 1989; Bethke et al., 1991; Berg et al., 1994]. While it has been known for some time that groundwater flow also has an important, albeit second-order, effect on subsurface heat transfer within basins [Bredehoeft and Papadopulos, 1965], petroleum researchers have only recently shown that thermal anomalies induced by vertical groundwater flow rates of a few millimeters per year can shift the depth to petroleum generation by over 1000 rn within actively subsiding basins [Person e! al., 1995]. Finally, excess fluid pressures play a critical role in fault mechanics [Hubbert and Rubey, 1959; Rubey and Hubbert, 1959] and primary petroleum migration out of low-permeability source rocks [Ungerer et al., 1990]. Subsurface fluid migration within sedimentary basins is driven by a number of mechanisms including sediment and tectonic loading, gradients in (water table) topography, lateral variations in fluid density, seismogenic pumping, and the production of diagenetic fluids. Different fluid flow-driving mechanisms interact within various tectonic environments and during different periods of the plate tectonic cycle (Plate 1). The relative importance of these driving forces on fluid flow varies depending on the tectonic and lithologic conditions (e.g., permeability, porosity, and mineralogy). Some studies have examined the role of specific driving mechanisms on fluid flow within different tectonic environments [e.g., Garven and Freeze, 1984a, b; Bethke, 1985; Ge and Garyen, 1992]. Other studies have examined how several different driving mechanisms interact with each other through geologic time [Garven et al., 1993; Person and Garven, 1994]. Reviews of these different fluid-flowdriving mechanisms are provided by Bethke [1989] and Garven [1995]. An important feature of deep groundwater flow systems within sedimentary basins is that hydrological, mechanical, thermal, and chemical mass transfer processes are all closely coupled. Increases in subsurface fluid pressures induce rock dilation and porosity increase. Mineral precipitation reduces porosity, decreasing groundwater flow rates. Increases in temperature with depth in permeable sediments and sedimentary rocks create density instabilities that induce free convection. These conditions necessitate greater complexity in basin-scale hydrogeological models than in models required to simulate groundwater flow in shallow aquifers over human timescales. Quantifying the complex feedbacks between fluid flow, rock deformation, heat transfer, and reactive mass transport processes will continue to be an area of active research over the next decade [National Research Council, 1990; Steefel and Lasaga, 1994]. 2.2. Heat Transfer Thermal processes within basins have important implications for modifying geochemical reaction rates and fluid properties (viscosity and density) and for inducing fluid flow. Temperature increase during burial, for example, is considered by petroleum researchers to be the primary factor controlling petroleum generation within sedimentary basins [Tissot et al., 1987]. This has made the accurate representation of the thermal history of sedimentary rocks a critical component of basin exploration strategies [Doligez et al., 1986]. Geochemical re-34, 1 / REVIEWS OF GEOPHYSICS Person et al.-HYDROGEOLOGIC MODELING ß 63 64 ß Person et al.-HYDROGEOLOGIC MODELING 34, 1 /REVIEWS OF GEOPHYSICS