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