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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Abstract
Fault
-seal Prediction in the Gulf of Mexico:
Empirical Data
By
Originally presented at the 1998 Hedberg (AAPG) Research Conference at Galveston, TX
Book/CD-ROM Title:
Edited by
Publication Subject: (
Fault
-seal prediction is a
long-standing risk of both exploration and production. Faults of course may
trap
hydrocarbons, subdivide fields into separate pressure compartments and well drainage
areas, or appear completely transparent to hydrocarbons. In the GOM, Shell Offshore Inc.'s
extensive and longstanding leasehold and production databases, coupled with streamlined
fault
-seal analysis software, have made it possible to compile empirical characteristics
of well-constrained sealing and non-sealing faults. The results show that many of the
common assumptions regarding
fault
seals are, in fact, not well justified. Accurate
assessment and prediction of
fault
seals cannot be done using these assumptions, but can
be done from empirical databases.
The main cause of sealing faults and sealing portions of
faults is generally assumed to be cross-fault
juxtapositions of shales with reservoir
sands, which are characterized in terms of Allan maps and juxtaposition diagrams (Allan,
1989; Knipe, 1997). The sealing capacity of those presumably few sand-on-sand
fault
contacts which do seal is thought to be due to the seal capacity of the gouge, which can
be described by either of two types of equations. One type of equation, typified by
Shell' Clay Smear Potential, describes the gouge' seal capacity in terms of a
physically continuous, wedge-shaped smear of clay or shale. The smear' seal capacity
is related to the length and continuity of the smear, which in turn are directly
proportional to the overburden pressure and thickness and viscosity of shale, and
inversely proportional to the magnitude and rate of displacement (e.g., Bouvier et al.,
1989; Fulljames et al., 1996).
The second type of equation, typified by Linsay's Shale Smear Factor (Lindsay et al., 1993) or Badley's Shale Gouge Ratio (Yielding et al., 1997), describes the gouge's seal capacity in terms of a homogeneous mix of the sands and shales. In this type of equation, the seal capacities thought to be inversely proportional to the sand/shale ratio of the faulted rocks. Faults cutting high net/gross sections are though to have proportionally sand-rich gouges, and hence have little pressure-sealing capacity.
Even though the two types of equations represent
fundamentally different physical processes, recent but limited comparisons of the two have
suggested that they yield equivalent results (e.g., Yielding et al., 1997; Handschy &
Alexander, 1998). Both types of equations, however, require empirical calibration in order
to relate actual fault
-seal capacity (in terms of cross-
fault
differential pressures or
hydrocarbon column heights) to either clay smear potential numbers, shale smear factors,
or shale gouge ratios. Until recently, the efforts required to compile such calibration
sets were simply too overwhelming to justify their collection.
Shell Oil Co. has developed proprietary software which
greatly expedites these efforts. It directly incorporates all available well and 3D
seismic data, enabling construction of highly constrained fault
-plane maps. The results
show that it is essential to collect data only from
fault
sets where the structure and
stratigraphy are highly constrained by both well and 3D seismic control. Moreover, where
fluid pressure gradients exceed hydrostatic, calculations of
fault
-sealed pressures must
be constrained by measured formation pressures, or at least mud weights, on both sides of
the faults.
The actual results of such data-intensive studies show that
pressure-sealing faults in the Gulf of Mexico commonly include significant areas of
sand-on-sand contact. Sealing sand-on-sand fault
contacts occur in both hydropressured and
geopressured reservoirs, and in at least one case, along an apparently long-lived and
currently active
fault
. Conversely, traps formed by san-on-shale
fault
contacts are
commonly underfilled with respect to the structurally highest possible sand-on-sand
cross-
fault
leak point. Thus juxtaposition diagrams alone do not constrain
fault
-seal
risks.
Comparisons of the two types of gouge equations with the in situ cross-fault
pressure differences, show that
one equation correlates well with the observed pressures in most cases, while the other
does not.