Abstract

The exploration methods described in this report are effective, low-cost tools that can increase exploration success in ancient sedimentary basins worldwide. These exploration techniques target shallow hydrocarbon reservoirs by integrating geologic, geophysical, and geochemical data with exploration methods contained in a pending patent. Digital databases can analyze numerous layers that describe progressive changes in subsiding sedimentary basins and generate predictive models that show locations of sites that have the greatest potential to contain shallow hydrocarbon reservoirs.

All ancient sedimentary basins are subjected to similar physical and chemical changes during subsidence. The temperature of sediment and interstitial fluid increases during basin subsidence. Static pressure increases in proportion to burial depth, and dynamic pressures are controlled by basin tectonics during subsidence. In general, the salinity of basinal fluids increases with depth at the same time as H₂S increases and O₂ decreases. Kerogen converts to liquid petroleum, which accompanies biogenic gas, both of which are released into migrating fluids. As burial depth increases, thermogenic gas enters basinal fluids. Basinal fluids are expelled from sediment in three main stages: (1) At shallow burial depths pore-water is expelled around detrital particles; (2) At about 3,000 m burial montmorillonite is converted to illite, which liberates a large volume of non-saline water; and (3) Low-temperature metamorphic water is released at temperatures and burial depths that convert illite to chlorite and water is forced from low-temperature hydrous detrital minerals.

Basin subsidence produces predictable changes in the chemical composition of warm, reduced, acidic brine, which causes reduction of porosity and permeability in transmissive rock units during progressive–burial diagenesis. The timing of water-expulsion events is predictable in subsiding sedimentary basins, as is the timing of hydrocarbon generation; burial-history diagrams provide quantification of those relations. Brine- and hydrocarbon-migration pathways are traceable in three dimensions because chemically reactive basinal fluids are expelled under dynamic pressure into permeable strata that are not in chemical equilibrium with invading fluid. The non-equilibrium conditions in permeable layers produces alteration diagenesis at depth, which involves chemical and oxidation-reduction (redox) reactions between migrating warm acidic, reduced brine and formation fluid and minerals in permeable strata. Alteration diagenesis creates secondary porosity. Labile minerals, rock fragments, and low-temperature reactive minerals such as iron oxide, gypsum, and anhydrite are destroyed by warm, reduced, acidic, hydrocarbon-bearing brine. Brine-migration pathways can be mapped because alteration diagenesis permanently changes the mineralogy and textures of permeable strata through which the fluid migrated. Where high-pressure brine and hydrocarbons break confining seals to forcefully intrude overlying semi-consolidated sediment in seal-bypass systems, dynamic pressure is released and fluid migration stops, which may result in high-level hydrocarbon reservoirs. Eventually these hydrocarbon reservoirs are uplifted, and erosion brings them close to the surface in ancient sedimentary basins.

We selected four ancient sedimentary basins to demonstrate how principles of process-predictive methods can be applied to improve exploration success: (1) Surat Basin, Queensland Australia; (2) Arabian Platform, Saudi Arabia; (3) Paradox Basin, Utah and Colorado, U.S.A.; and (4) Anadarko-Ardmore Basins, Oklahoma, U.S.A. In each sedimentary basin, strata modified by alteration diagenesis can be mapped to different degrees of precision, structural compartments were identified in some basins, and ancient seal-bypass systems have been located. Ancient seal-bypass systems locate the final migration sites of basinal brine and hydrocarbons.

To increase exploration success for shallow hydrocarbon reservoirs, geologic maps of alteration diagenesis should be integrated with burial-history analyses, source-rock identification, permeability studies, seal analysis, pressure surveys, and identification of ancient seal-bypass systems to trace hydrocarbon-migration pathways in four dimensions. These methods can be used to find new, shallow hydrocarbon reservoirs on current land positions, rejuvenate old non-productive or poorly productive oil and gas fields, modify drilling patterns to take advantage of unique geometric distribution of hydrocarbon reservoirs, or initiate a global program to explore for shallow hydrocarbon reservoirs in ancient sedimentary basins.

The implications of locating ancient seal-bypass systems and mapping associated fluid-migration systems extend far beyond this effort to make hydrocarbon exploration less costly and more efficient. These concepts and analytical methods have a global impact on exploration for stratabound mineral deposits of uranium, vanadium, copper, silver, gold, platinum-group metals, cobalt, lead, zinc, and other metals that could be entrained in warm, reduced, acidic, hydrocarbon-bearing brine in ancient sedimentary basins. Natural-gas and oil storage as well as CO₂ sequestration will be affected by the occurrence of transmissive breccia bodies in ancient seal-bypass systems, and these structures may adversely affect the integrity of compartments and reservoirs. Finally, seal-bypass systems may have provided downward migration pathways for fresh water to access deep reservoirs during Pleistocene time.