Structural and Tectonic Processes
Construction of three-dimensional (3-D) basin models in areas of detached salt tectonics poses difficult challenges but is necessary to simulate the 3-D thermal effects of salt and correctly model subsalt burial histories. Over much of the offshore northern Gulf of Mexico Basin, a regional salt canopy detaches shallow structures, formed by growth and subsequent collapse of allochthonous salt sheets, from subsalt structures formed mostly in response to movement of deep (autochthonous) salt. Dynamic simulation modeling of this system requires (1) understanding the evolution of salt distribution and thickness through time, (2) a methodology to incorporate thickness changes within the simulation model, and (3) geometric solutions to account for the fact that allochthonous salt occurs at various stratigraphic levels across the basin.
Twenty regionally mapped horizons, including top and base of allochthonous salt and a composite weld representing areas of collapsed salt canopy, were used to build a regional Gulf of Mexico numerical simulation model. Salt isopach maps for sequential stages of the basin evolution were derived by vertical backstripping using “regionals” constructed to approximate the predeformation geometry of selected horizons. For a given horizon, the salt thickness changes since horizon deposition is represented by the difference between a mapped horizon and its regional. Simple rules were applied to partition the derived salt thicknesses between the allochthonous and autochthonous salt levels. To model the climb of the salt canopy across stratigraphy, the basin model was divided into subsalt and suprasalt parts containing horizons of equivalent age separated by an intervening allochthonous salt layer. The thickness of both the allochthonous and autochthonous salt layers were altered through time using the salt isopachs.
The resulting simulation model reasonably represents the large-scale structural evolution of the northern Gulf of Mexico Basin, including (1) progressive southward displacement and evacuation of salt along the Louann level, (2) basinward stratigraphic climb and progressive welding of salt within the canopy, (3) seaward progradation of depositional systems throughout the Mesozoic and Cenozoic, and (4) Miocene uplift and erosion of the onshore part of the basin. The methodology outlined can be adapted to assist in building basin models in other structurally complex basins.
Figures & Tables
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.