Fluid Flow and Petroleum and Mineral Resources in the Upper (<20-km) Continental Crust

Fluid flow in the upper continental crust is driven by energy from the sun through precipitation, sediment deposition, compaction, and petroleum generation, evaporative brine formation, as well as by heat from the Earth’s interior. Flow over geologic time and commonly across large distances has produced a diverse suite of resources with physical or chemical characteristics that reflect climate, changes in atmospheric and oceanic chemistry, and tectonics. Review of manifestations of fluid flow and economic resources in North America shows that the juxtaposition of highly oxidized and highly reduced sediments, the generation of nonaqueous fluids, and the presence of nonaqueous fluids in the pore space is important to resource formation. Mississippi Valley-type (MVT) lead-zinc deposits and trace mineralization stretching across ~1,000 km are associated with basins that now are filled with underpressured gas (Arkoma, western Canada, Appalachian basins). Oxidized brines produced sedimentary rock-hosted stratiform deposits when they were driven through organic shales capping the midcontinent rift at several discrete times separated by ~10 m.y. Accumulations of nearly 100 wt percent uranium formed when oxidized brines encountered methane seeps in the Athabasca basin. Hydrocarbons leaking through hundreds of salt-margin faults support gas seeps and methane hydrate accumulations in the offshore Gulf of Mexico. Hydrocarbon dikes in Utah, sand flame structures in Delaware, mineral deposition followed by gangue and ore mineral corrosion in the midcontinent, lead-zinc veins in the anhydrite caps of salt domes in the Gulf of Mexico suggest pulses of fluid flow. To quantitatively assess these phenomena, we review the basics of nonisothermal, multiphase fluid flow at underand overpressured conditions. The maximum intrinsic permeability of clean sand is summarized; ways to estimate, measure, or infer intrinsic permeability are tabulated. It is shown that free convection in clean sands will fill pores with silica most rapidly at a 4-km depth in an overpressured basin and that filling will take ~25 m.y. We calculate the conditions under which generated petroleum will displace rather than percolate through basin pore waters and we show that the permeability of basin sediments is dynamically determined by the escape of fluids while sediments are actively accumulating or hydrocarbons generating. The calculations show that the observed suite of gasand water-filled, overand underpressured basins are the expected consequences of the likely range of basin organic content and sedimentation rates. The quantitative review and parameter constraints provide a basis for assessing an extensive literature on MVT deposits in North America, the Kupferschiefer sedimentary-hosted rock stratiform deposits in Europe, and convectively formed sediment-hosted stratiform deposits. The Kupferschiefer deposits require expulsion of brines from nearly the entire Permian Zechstein basin to ore districts on its southern margin. The brief elevation of temperature during Permian MVT ore deposition in North America requires short pulses of rapid brine expulsion. Currently popular suggestions such as cross-basin gravity-driven flow or episodic expulsion from overpressured basins are not compatible with geologic and physical constraints. Viable mechanisms are difficult to identify. We suggest glacial meltwater loading of the gas-filled portions of basins as one possibility. Where the subsurface plumbing system is favorable, convective sediment-hosted stratiform deposits may accumulate in lithified sediments as the result of brine reduction. More commonly ore metals sulfides accumulate at or very near the sediment-water interface, and fault configuration is an important control. Our review and analysis suggest that identifying the factors critical to or strongly enabling the formation of resources may be a good way to penetrate the combination-lock complexity of their formation. Of the many factors potentially involved in the formation of MVT and sedimentary rock-hosted stratiform deposits, for example, capped regional-scale permeable aquifers that contain oxidized sediments and brine seem most important. Modeling coupled hydrocarbon-mineral resource systems over the tens of millions of years required for their formation, and the thorough testing of these models against field data, are suggested as a strategy for identifying the critical relationships between resources and the operation and evolution of the Earth system. † Corresponding author: e-mail, cathles@geology.geo. cornell.edu ©2005 Society of Economic Geologists, Inc. 100th Anniversary Volume, pp. 000–000