Abstract

Faulting either aids or impairs hydrocarbon entrapment depending on the associated circumstances; it is fundamentally important to know whether faulting has joined rocks of differing capillary properties and differing fluid pressures (Downey, 1984). The primary focus of attention should be on the structural attitude, fluid pressures, and rock properties of the strata that are joined at the fault trace.

A fault plane itself generally has little effect on migrating hydrocarbons. In areas such as the Teritary of the Gulf of Mexico and the Niger Delta, the sedimentary section is composed of high proportions of soft clay. These clay layers can be dragged into a fault plane, provide local shale smears, cause local reservoir disruptions, and even traps (Smith, 1966). Even in such circumstances, the alterations localized along the fault plane are more likely to affect fluid transmissibility, rather than to act as absolute barriers over geologic time.

The fault plane itself can behave as a transmissive open fracture under the following conditions: 1) in tensional settings at very shallow depths, and 2) in tensional settings in strong geopressures. Prospects in these settings carry higher risks of leakage.

In analyzing the likelihood that a fault will offset a porous bed laterally against a sealing bed, the lateral variation in throw must be considered. It is best understood through actual mapping of the structural attitude of the co-joined lithologies at the fault plane (Allan, 1989).

In a typical sedimentary section composed of alternating layers of reservoirs and seals, the probability that any reservoir is trapped across a fault by a sealing layer is dependent on the relative proportions of reservoir to seal thickness within the fault-affected section. If two or more sub-parallel faults affect the rock section, the probability of a reservoir eventually being laterally sealed is greatly increased (Fig. 1).

Careful mapping may reveal the presence of multiple sub-parallel faults that will provide enhanced probabilities for hydrocarbon traps. Examples from the major offshore fields of Brunei demonstrate that an important element of entrapment is the presence of numerous sub-parallel faults (James, 1984).