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The AAPG/Datapages Combined Publications Database
Journal of Petroleum Geology
Abstract
Journal of Petroleum Geology, vol.
2004
DETERMINATION OF
FAULT
SLIP COMPONENTS USING SUBSURFACE STRUCTURAL CONTOURS: METHODS AND EXAMPLES
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S-S. Xu1, L. G. Velasquillo-Martinez1, J. M. Grajales-Nishimura*1, G. Murillo-MuÒetÛn1, J. GarcÌa-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.
INTRODUCTION
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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