Simulation of Petroleum Migration in Fine-Grained Rock by Upscaling Relative Permeability Curves: The Malvinas Basin, Offshore Argentina
André Vayssaire, 2012. "Simulation of Petroleum Migration in Fine-Grained Rock by Upscaling Relative Permeability Curves: The Malvinas Basin, Offshore Argentina", Basin Modeling: New Horizons in Research and Applications, Kenneth E. Peters, David J. Curry, Marek Kacewicz
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Early exploration of the Malvinas Basin (1979–1991) targeted Lower Cretaceous sandstones assuming that hydrocarbons would migrate laterally from the basin depocenter in the south to structures located in shallow water. Hydrocarbons were found, but not in large enough quantities to be commercially viable. Recently, exploration has moved closer to the depocenter and focuses on Eocene to Miocene sandstones a few thousand meters vertically above the mature Lower Cretaceous source rock. As no faults crosscut the entire section between the source rock and reservoirs, they cannot be evoked as conduits for hydrocarbon migration. Therefore, kilometer-scale vertical migration across fine-grained sediments was considered as the main process to transport hydrocarbons from source rock to reservoir.
This migration mechanism is commonly mentioned, but poorly constrained. Darcy flow and invasion percolation calculators were used to simulate hydrocarbon migration. If we consider that hydrocarbons migrate along thin stringers, the relative permeability parameters have to be upscaled to consider that not all of the rock is being saturated by petroleum. Furthermore, fine-grained sediments present a very high specific area, which gives a higher sorption capacity for water, and therefore, less petroleum is needed to reach the saturation threshold for flow. Secondary migration across fine-grained sediments takes time to initiate, but as soon as hydrocarbons invade the pore space, the migration is effective; it occurs with minimal hydrocarbon losses and is essentially controlled by the expulsion rate of petroleum from the source rock and the stratigraphic architecture. From a physics standpoint, the Darcy method looks more appropriate because it incorporates the full physics of the problem. However, under these conditions, viscous forces can be ignored and the invasion percolation method seems appropriate to simulate secondary migration of hydrocarbons across fine-grained sediments.
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Temperature-time–based first-order kinetic models are currently used to predict hydrocarbon generation and maturation in basin modeling. Physical chemical theory, however, indicates that water pressure should exert significant control on the extent of these hydrocarbon generation and maturation reactions. We previously heated type II Kimmeridge Clay source rock in the range of 310 to 350°C at a water pressure of 500 bar to show that pressure retarded hydrocarbon generation. This study extended a previous study on hydrocarbon generation from the Kimmeridge Clay that investigated the effects of temperature in the range of 350 to 420°C at water pressures as much as 500 bar and for periods of 6, 12, and 24 hr. Although hydrocarbon generation reactions at temperatures of 420°C are controlled mostly by the high temperature, pressure is found to have a significant effect on the phase and the amounts of hydrocarbons generated.
In addition to hydrocarbon yields, this study also includes the effect of temperature, time, and pressure on maturation. Water pressure of 390 bar or higher retards the vitrinite reflectance by an average of ca. 0.3% Ro compared with the values obtained under low pressure hydrous conditions across the temperature range investigated. Temperature, pressure, and time all control the vitrinite reflectance. Therefore, models to predict hydrocarbon generation and maturation in geological basins must include pressure in the kinetic models used to predict the extent of these reactions.