Geopressures and Hydrodynamics in the Gulf Basin Tertiary
ABSTRACT Deep drilling in the Gulf Basin has revealed geopressures (overpressures) in Teritiary/Quaternary rocks. These geopressures appear to rise stratigraphically in a basinward direction coincident with the regional basinward sand/shale facies change within regressive depositional cycles so characteristic of Gulf Basin sedimentation. These geopressured rocks appear undercompacted for their depths which implies that Tertiary/Quaternary sedimentation rates in depocenters have exceeded the capacity of the rock system to discharge its connate water under overburden stress. Thus, the connate fluids have temporarily assumed part of the overburden load and geopressured the rocks with maximum pressure gradients approaching 1.0 psi/ft. This pressure system "leaks" via transition zones into overlying sediments which reveal a fluid pressure gradient of 0.465 psi/ft., still in hydrodynamic disequilibrium considering the low water salinities involved. A plot of Gulf Basin pressures reveals no rectilinear increase of "hydrostatic" pressure with depth. Instead, there are zones of unequal flow potential juxaposed across faults and within sand/shale cycles which exert a complex force field on all fluid migration. This suggests that the entire Gulf Basin is currently in a hydrodynamic state. Gulf Basin hydrodynamics have strategic consequences for hydrocarbon migration and trapping because the fluid force field can disrupt or reinforce fault traps and other seals. Field examples from Midland-Escerwood, Acadia Parish, Louisiana and Chocolate Bayou, Brazoria County, Texas illustrate how fluid pressure environments affect hydrocarbon distribution and trapping. Safe drilling practice requires that casing be set in the shale transition zone separating hydrostatic pressures from overpressures. Gulf Basin undercompacted sediments create physical anomalies which are useful for prediction and analysis or pressure distribution. These are classified as follows: Before Drilling: seismic analysis and pressure calculations from available logs. During Drilling: drilling penetration rates, mud flow line temperatures and cuttings density, among many other criteria.