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Abstract: Faulting: Leak or Seal for Petroleum Accumulations?

DOWNEY, MARLAN W.

Faulting juxtaposes different rocks on opposite sides of a Previous HitfaultNext Hit plane. Understanding whether faults assist trapping or cause leakage depends on the properties of the Previous HitfaultNext Hit plane, and on knowing the properties of the rocks joined by faulting.

Previous HitFaultNext Hit Previous HitPlanesNext Hit as Seals--Previous HitFaultNext Hit Previous HitplanesNext Hit are readily observed and mapped. Because the plane of the Previous HitfaultNext Hit is represented by bold black lines in maps and sections, Previous HitfaultNext Hit Previous HitplanesNext Hit are often accorded more importance than deserved. In many cases, the Previous HitfaultNext Hit plane itself has little to do with entrapment (or leakage). The Previous HitfaultNext Hit plane itself is almost never a magic impermeable membrane prohibiting cross-Previous HitfaultNext Hit flow. However, where faulting dynamically offsets a series of interbedded reservoirs and plastic clays, smearing of the clay layers often creates local Previous HitfaultNext Hit plane seals. Strong granulation and re-cementation along the Previous HitfaultNext Hit 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.

Previous HitFaultNext Hit Previous HitPlanesNext Hit 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 Previous HitfaultNext Hit plane may behave as a transmissive open fracture in three general cases: (1) a tensional Previous HitfaultNext Hit plane will transmit fluids during Previous HitfaultNext Hit movements; (2) a tensional Previous HitfaultNext Hit plane will generally behave as a transmissive open fracture at shallow depths; (3) a Previous HitfaultNext Hit plane will often transmit fluids in tensional settings in geopressures. In each of these cases, the Previous HitfaultNext Hit is likely to behave as an open fracture and to provide passage of hydrocarbons along the plane of the Previous HitfaultNext Hit.

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 Previous HitfaultNext Hit will be sealed by rocks on the other side of a Previous HitfaultNext Hit, one needs to know the buoyancy pressure of the hydrocarbon phase and the effective capillary entry pressure (the resistance) of the receiving rocks. All Previous HitfaultNext Hit 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 Previous HitfaultNext Hit plane. Previous HitFaultTop plane maps are powerful tools to begin such analyses.

AAPG Search and Discovery Article #90943©1996-1997 AAPG International Distinguished Lecturers