Hydrodynamic effects on low-dip stratigraphic traps

Abstract

Existing descriptions and mapping techniques of hydrodynamic effects on subsurface fluid contacts are generally restricted to relatively thick, continuous reservoirs. These concepts do not readily apply to stratigraphic traps in thin reservoirs that pinch out laterally in some directions yet are normally pressured. Spatial variation in reservoir pinch-out trends, geological depth structure, and hydrodynamic head gives rise to many scenarios of hydrodynamically modified stratigraphic traps. Further complexity arises where stratigraphic traps are developed in unstructured or low relief areas, where a slight tilt angle of a fluid contact can translate into a significant deviation from structural conformance in map view. Hydraulic gradient azimuth relative to structural dip azimuth is a key factor. Where these are parallel, hydraulic gradients have little effect on stratigraphic trapping potential. The closer the hydraulic gradient azimuth is to the structural strike direction, the greater the potential impact of fluid contact tilt in that stratigraphic trap. These results are not predicted by the usual method of revealing hydrodynamic traps via hydraulic head transformations of structural maps. Instead, a modified workflow for hydrodynamic stratigraphic traps combines structure, porosity, and hydraulic gradient maps.