Theory and Approach for Mapping Hydrodynamic Traps
Authors: Yang Yunlai , Mahmoud Khalid A. 
Published: May 2016

Abstract
Flowing water beneath an oil accumulation causes the oil-water contact (OWC) tilted. Obviously, for a given trapping structure, the boundary and volume of oil accumulation for a tilted OWC are different from those of a horizontal one. Previous work has defined the tilting angle of an OWC, but the equation defining a tilted OWC surface is lacking. From fundamental hydraulic principle, we derived an equation defining the surface of a tilted OWC:
Where, Z, Z0 - depth of the OWC at any an given location and a reference (well) location, respectively, L; rw, ro - the density of formation water and hydrocarbon, respectively, FL-3; hw, hw_0 - hydraulic head at the given location and the reference (well) location, respectively, L. The derived equation has two important applications. First it can be used directly to delineate OWC for a given hydrocarbon accumulation. Second, it can be used to map hydrodynamic traps for a given region. For the second application, we developed an easy to understand alternative approach to map hydrodynamic structural traps by employing some simple map operations. We also developed a simple approach to map a special type of hydrodynamic stratigraphic traps - "Open Hydrodynamic Stratigraphic Traps". Unlike normal hydrodynamic stratigraphic traps, oil accumulation in an open hydrodynamic stratigraphic trap is not confined by a closure formed by a curved-tilted OWC, rather its boundary is not fixed. Along a mountain foot of a prospective basin is the favorite place for forming such traps. A field example is presented to show the results of mapping hydrodynamic traps using the developed procedure and to demonstrate the importance of the hydrodynamic effect on the traps' boundary.

Introduction
Provided the hydraulic heads in the groundwater surrounding a hydrocarbon accumulation are the same, i.e., there is no water flow beneath the hydrocarbon accumulation, the only force acting on the hydrocarbon body is buoyancy, which is vertical. This vertical force results in a horizontal contact between hydrocarbon and water. If hydraulic heads are different, i.e., if the water beneath the hydrocarbon accumulation is flowing, the hydrocarbon body is also subjected to a lateral hydraulic force. The direction of the resultant force of buoyancy and hydraulic force is not vertical. As the result, the hydrocarbon water contact is tilted. This effect has been observed in reservoirs worldwide (Hubbert, 1953; Pelissier et al., 1980; Berg et al., 1994; Thomasen and Jacobsen, 1994; Underschultz, 2005; Tozer and Borthwick, 2010). Hubbert (1953) provided an equation defining the angle of the tilted OWC.
A trap whose hydrocarbon water contact is tilted by hydrodynamic force is termed as hydrodynamic trap. Obviously, for a given trapping structure, the boundary of hydrocarbon accumulation of a tilted hydrocarbon water contact is different from that of a horizontal one. Hubbert (1953) developed an approach for mapping hydrodynamic traps using a concept of "Minimum Energy Level".
Although an equation describing the tilting angle of a hydrodynamic hydrocarbon water contact has been developed (Hubbert 1953), the equation defining a tilted hydrocarbon water contact surface is lacking. Based on explicit hydraulic principles we derived an equation defining the surface of hydrocarbon water contact. We then developed a clear and easy to understand alternative approach to map hydrodynamic traps using our constructed equation. Further, another approach is proposed to locate a very special type of traps, "Open Hydrodynamic Stratigraphic Traps". For simplicity, oil is used for hydrocarbon hereinafter. Hydrocarbon water contact is then simply called oil-water contact (OWC).