Abstract: Faulting: Leak or Seal for Petroleum Accumulations?
DOWNEY, MARLAN W.
Faulting juxtaposes different
rocks on opposite sides of a fault
plane. Understanding whether faults
assist trapping or cause leakage depends on the properties of the
fault
plane, and on knowing the properties of the rocks joined by faulting.
Fault
Planes as Seals--
Fault
planes are readily observed and mapped. Because the plane of the
fault
is represented by bold black lines in maps and sections,
fault
planes are
often accorded more importance than deserved. In many cases, the
fault
plane itself has little to do with entrapment (or leakage). The
fault
plane
itself is almost never a magic impermeable membrane prohibiting cross-
fault
flow. However, where faulting dynamically offsets a series of interbedded
reservoirs and plastic clays, smearing of the clay layers often creates
local
fault
plane seals. Strong granulation and re-cementation along the
fault
plane can also locally alter lateral transmissibility in co-joined
reservoirs. Such phenomena are real and create important impediments to
fluid movement that profoundly affect reservoir performance, but they rarely
create absolute seals to hydrocarbon movement over geologic time spans.
Fault
Planes as Leaks--Faults
record dynamic movements of rock strata over time. Analysis of the influence
of faults on migration and entrapment of hydrocarbons presumes that one
knows when hydrocarbons were generated and migrated. A
fault
plane may
behave as a transmissive open fracture in three general cases: (1) a tensional
fault
plane will transmit fluids during
fault
movements; (2) a tensional
fault
plane will generally behave as a transmissive open fracture at shallow
depths; (3) a
fault
plane will often transmit fluids in tensional settings
in geopressures. In each of these cases, the
fault
is likely to behave
as an open fracture and to provide passage of hydrocarbons along the plane
of the
fault
.
Importance of Knowing the Properties
of Co-Joined Rock Layers--Seal rocks have pore throats that are too small
and poorly connected to allow passage of adjoining hydrocarbons. To understand
whether hydrocarbons on one side of a fault
will be sealed by rocks on
the other side of a
fault
, one needs to know the buoyancy pressure of the
hydrocarbon phase and the effective capillary entry pressure (the resistance)
of the receiving rocks. All
fault
traps will leak. The question is, what
is the vertical extent of hydrocarbons that can be trapped before a structural
leak point is reached, or the effective capillary entry pressure is somewhere
overcome? This question cannot be answered without analyzing structural
attitude, fluid pressures, and capillary entry pressures of the rocks joined
at the
fault
plane.
Fault
plane maps are powerful tools to begin such analyses.
AAPG Search and Discovery Article #90943©1996-1997 AAPG International Distinguished Lecturers