PETROLEUM AND MINERAL RESOURCES: THE ROSETTA STONES OF BASIN FLUID FLOW

Integrating observations using increasingly sophisticated quantitative models of large spatial scale has proven effective in the quest to find and understand economic resources. Sedimentation at >2 km/Myr in the Gulf of Mexico basin drives salt and mud diapirs, and petroleum generation, migration, and venting. Because this and most other basins are giant flow-through reactors that vent almost all the petroleum that escapes their thin source strata, future petroleum supplies will be either those retained in the source (oil shales) or frozen in the process of venting (hydrates and tar sands). In general, central portions of basins can be filled entirely with gas. In active basins of low intrinsic permeability, permeability adjusts dynamically to allow the escape of fluids driven by compaction, thermal expansion, and petroleum generation. If petroleum can migrate, by buoyancy and super-hydrostatic pressure gradients faster than it is generated, the petroleum fingers through the brine, as in the Gulf of Mexico. If not, brine displacement occurs, and this is promoted by richer, thicker organic source strata, and by rapid sedimentation under hydrostatic conditions. Both fingering and displacement can be expected, and both can occur in basins that are over-pressured or hydrostatic. The Appalachian, Arkoma, and Western Canada basins are under-pressured gas-filled basins, and are also genetically associated with the extensive North American Mississippi Valley-type (MVT) Pb-Zn mineralization. The gas in these basins complicates the two currently favored hypotheses for MVT mineralization. Cross-basin gravity-driven flow is blocked by the gas, and the overpressures that could rapidly expel brine could not have existed during or after the basin filled with gas because erosion leads to overpressuring. Slow brine expulsion could produce the deposits, but their thermally anomalous nature at mineralization time and their low thermal maturity require short pulses of rapid brine expulsion. We do not understand how such pulses can be produced. The intertwining of organic and inorganic materials and fluids in forming resources means that resources reflect changes that impact living organisms. Economic geology will continue to be a scientific window to the most interesting and important aspects of the Earth's past and future.