DETERMINATION OF FAULT SLIP COMPONENTS USING SUBSURFACE STRUCTURAL CONTOURS: METHODS AND EXAMPLES

Abstract

Problems with measuring fault slip in the subsurface can sometimes be overcome by using subsurface structural contour maps constructed from well logs and seismic information. These maps are useful for estimating fault slip since fault motion commonly causes the dislocation of structural contours. The dislocation of a contour is defined here as the distance in the direction of fault strike between two contours which have the same value on both sides of a fault. This dislocation can be estimated for tilted beds and folded beds as follows: If a dip‐slip fault offsets a tilted bed, the dislocation (S c ) of contours can be estimated from the vertical component (S v ) of the fault slip and the dip (β) of the bedding according to the following relationship: S c = S v /tan β. Since S c and β can be measured from a contour map, the vertical component of fault slip can be obtained from this equation. If a strike‐slip fault offsets a tilted bed, the dislocation (S cs ) of contours is equal to the strike‐slip of the fault (S c ), that is, S cs = S s . If a fault offsets a symmetric fold, the strike component (S cs ) of fault slip and the dislocation of the contours (S c ) can be calculated, respectively, from the equations S cs = (S max + S min ) / 2 and S c = (S max ‐ S min ) / 2. S max is the greater total dislocation (S c + S cs ) of a contour line between the two limbs of the fold and S min is the smaller total dislocation (S c ‐ S cs ) for the same contour line. In this case, S v can be also calculated using the obtained value of S c and the equation S v = S c tan β. Similarly, for an asymmetric fold, the dislocation of contours due to the vertical slip component is S cb = (S max ‐ S min )/(n + 1), and the strike‐slip component is S s = S cs = (nS min + S max /(n + 1), where n is the ratio between the values of interlines of the two limbs, and S cb is the dislocation of contours due to the vertical slip component for either of the two limbs (here it is for limb b). In all cases, three conditions are required for the calculation of contour dislocation: the contour lines must be approximately perpendicular to the fault strike; the intersection angle between the fault strike and the strike of bedding should be greater than 65°; the bed must not be dip more than 35°; and folding or flexure of the stratigraphic horizons must have occurred before faulting. These methods for determining fault slip from the dislocation of structural contours are discussed using case studies from the Cantarell oilfield complex, Campeche Sound (southern Gulf of Mexico), the Jordan‐Penwell Ellenburger oilfield in Texas, and the Wilmington oilfield in California.