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Journal of Petroleum Geology, vol.
DETERMINATION OF FAULT SLIP COMPONENTS USING SUBSURFACE STRUCTURAL CONTOURS: METHODS AND EXAMPLES
S-S. Xu1, L. G. Velasquillo-Martinez1, J. M. Grajales-Nishimura*1, G. Murillo-MuÃ’etÃ›n1, J. GarcÃŒa-Hernandez2 and A. F. Nieto-Samaniego3
1 Instituto Mexicano del Petróleo, Programa YNF, Eje Central Lázaro Cárdenas No.152, Col. San Bartolo Atepehuacán, C.P. 07730, Mexico D.F., Mexico.
2 Petróleos Mexicanos Exploración y Producción Región Marina NE, Activo Cantarell (PEP-RMNE), Cd. del Carmen, Campeche, Mexico.
3 Universidad Nacional Autónoma de México, Centro de Geociencias, Apartado Postal 1-742, Querétaro, Qro., 76001, Mexico.
* Corresponding author, email: [email protected]
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:
(i) If a dip-slip fault offsets a tilted bed, the dislocation (Sc) of contours can be estimated from the vertical component (Sv) of the fault slip and the dip (β) of the bedding according to the following relationship: Sc = Sv/tan β. Since Sc 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 (Scs) of contours is equal to the strike-slip of the fault (Ss), that is, Scs = Ss.
(ii) If a fault offsets a symmetric fold, the strike component (Scs) of fault slip and the dislocation of the contours (Sc) can be calculated, respectively, from the equations Scs = (Smax + Smin) / 2 and Sc = (Smax - Smin) / 2. Smax is the greater total dislocation (Sc + Scs) of a contour line between the two limbs of the fold and Smin is the smaller total dislocation (Scs - Sc) for the same contour line. In this case, Sv can be also calculated using the obtained value of Sc and the equation Sv = Sc tan β.
Similarly, for an asymmetric fold, the dislocation of contours due to the vertical slip component is Scb = (Smax - Smin)/(n + 1), and the strike-slip component is Ss = Scs = (nSmin + Smax)/(n + 1), where n is the ratio between the values of interlines of the two limbs, and Scb 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: (i) 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 65o;
(ii) the bed must not be dip more than 35o; and (iii) 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.
The slip on a fault provides critical information on the kinematics of the blocks involved in the faulting (Bishop, 1960; Ramsay and Huber, 1987; Tearpock and Bishchke, 1991). The fault slip can be used to resolve the tectonic stress tensor (Etchecopar et al., 1981; Angelier, 1989). However, the fault slip can be difficult to estimate if fault-plane striations are not observed, if clear indicators of fault slip are absent, or if the tip zone of the fault plane cannot be detected, as the slip can be below the resolution limit of some observation techniques (Yielding et al., 1996).
Subsurface structure contour mapping is useful in unravelling the geological evolution of petroleum reservoir rocks and groundwater aquifers (Dennison, 1968; Tearpock and Bischke, 1991). For example, the strike, dip and dip direction of a reference surface can be calculated from structural contours and the presence of folds and faults can be revealed. Furthermore, a small contour interval on a map or vertical exaggeration on a section can be used to emphasize subtle structures such as gentle warping, open folds or minor faults.
Faults can be identified in subsurface studies from well logs and seismic interpretations. However, only the apparent separation can be obtained and the actual slip on a fault cannot be observed using these methods. Moreover, accurate dip-slip measurement requires that the interpreted seismic line must be oriented perpendicular to the strike of the fault (Tearpock and Bischke, 1991), and only obvious strike-slip faults can be recognized on subsurface structural contour maps (Moody, 1973; Harding, 1974; Tearpock and Bischke, 1991). Slickensides on fault planes can be directly observed in cores, but less straightforward is the determination of slip on subsurface faults at an oilfield scale. In most cases, core reorientation can be difficult.
Fault geometries can be defined using section-balancing techniques but these can be applied only in the case of plane strain (Hossack, 1979). Various techniques to determine finite fault slips have been proposed (Gratier et al.,1991; Gratier and Guillier,1993; Rouby et al., 1993; 1996, 2000). Rouby et al. (2002) described a technique to estimate the finite fault slips in studies of fault and fold evolution; however, there are limitations to the technique’s widespread use. Since unfolding and unfaulting restoration is required, the faults should have been formed before or during (but not after) folding. In other words, a particular deformation regime must be specified before the restoration can be implemented (Cobbold and Percevault, 1983; Kerr et al., 1993; Kerr and White, 1996; Williams et al., 1997). However, in practice, the mechanism of faulting or folding is not generally known before a detailed study is made and hence the modelled deformation regime may not be consistent with reality.
Methods to estimate the strike slip or vertical slip on a fault using subsurface structural contour maps are discussed in this paper. Our approach differs from restoration techniques (unfolding and unfaulting) and is particularly appropriate for faults which formed after folding or flexure and when no knowledge of the deformation regime that caused the fault and fold is required. We apply the proposed methods to the Cantarell oilfield complex in offshore Campeche (southern Gulf of Mexico), the Jordan-Penwell Ellenburger oilfield (Texas) and the Wilmington oilfield (California).
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