Abstract

The distribution of trapped hydrocarbons sometimes show strange patterns that are not easily explained by the existing knowledge of source and migration. These patterns may include gas trapped below under-saturated oil, with a thin seal in-between, and dry reservoir units in-between hydrocarbon filled units. For the first case it is often tempting to propose e.g. a new (deeper) source for the deeper gas simply because it is the most natural explanation. Such a solution to the hydrocarbon trapping distribution may, however, lead to new exploration plays in mature areas, and significant amounts of capital may be spent investigating the “new play” with sourcing from an e.g. older, more mature, source rock.

Before a decision to interpret the complex vertical trapping in such a manner is made, an alternative is to investigate whether the trapping pattern may be explained by a complex migration route through e.g. a series of fault compartments. Here we discuss how such an analysis sometimes can explain migration from a source rock above the carrier and reservoir rock, through a series of fault blocks, into older flow units. Gas will ususally be generated further away from the trap (deeper in the basin) than the oil, and this may result in cross-fault migration into older flow units for the gas. Using numerical experiments, we can study the criteria for fault throw distribution, carrier and seal thickness etc. needed for migrating gas below oil into traps.

The problem described here is a 3 dimensional hydrocarbon flow problem, and a solution requires a complex geological description. We use a multi-layer ray-tracing technique to trace the hydrocarbons from source to trap through complex migration pathways. The secondary migration process is suitable for parallel processing, with each flow unit represented by one process in the computer(s). Thus we can quickly assess whether a geological scenario is likely to or not likely to cause trapping of e.g. gas below oil.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah