Abstract: Seal
Capacity in Dynamic Petroleum Systems: Example from
Pagerungan Gas Field, East Java Sea, Indonesia
KALDI, JOHN G., - ARCO Indonesia (presently National Centre Petroleum Geology & Geophysics); DUNCAN MACGREGOR - BP Indonesia; GREG P. O'DONNELL - ARCO Indonesia (Presently ARCO Exploration & Production Technology)
Seal
capacity is the calculated amount of hydrocarbon column
height a particular lithology can support. Most analyses of
seal
capacity are made with the assumption that the petroleum system has
reached equilibrium; i.e. all generation, migration, structural
adjustments etc. have ceased, and that either the structural or
capillary spill point has been reached. In such static systems,
determination of
seal
capacity is simply a calculation of the
relationship between the buoyancy pressure of the hydrocarbon
column and the capillary properties of the up dip, lateral or
bottom
seal
. However, in many young or rejuvenated petroleum
systems, active generation and migration may still be occurring. In
these dynamic systems the present hydrocarbon column height is a
function of not only the
seal
capacity, but also of the difference
between the rate of influx of hydrocarbon through a feeder or
carrier bed, and the rate of outflow (leakage) of hydrocarbon
through the
seal
. This rate difference, or “lag” is
controlled by the relative permeability contrasts between the
carrier bed and the
seal
. Such mechanisms may be responsible for
the common observation of stacked hydrocarbon pools within
Indonesian fields.
An example of such a dynamic systems is the Pagerungan gas field
of the East Java Sea, Indonesia. The reservoir at Pagerungan is the
Mid to Late Eocene Ngimbang Clastics Formation. Prior to this study
there was considerable uncertainty as to which formation was the
seal
. Therefore,
seal
capacities were determined for various
formations using mercury injection capillary pressure (MICP)
analyses. These analyses indicate that the Late Eocene Ngimbang
Shale is the best top
seal
over the Pagerungan field, with
seal
capacity of approximately 900 ft. of reservoir gas. However, the
field contains an observed 1100 ft. gas accumulation. Gas chimneys
are seen on seismic sections through the field, and shallower
reservoirs on the same structure contain gas of the same
composition as is produced in the main reservoir zone. In addition,
rocks which contain up to 10% residual (trapped) gas extend several
hundred feet below the interpreted present field Free Water Level
(FWL).
Assuming that the analyzed samples are truly representative of
the best seals in this field, it is concluded that gas is actively
migrating, at geological rates, through the Pagerungan system. An
earlier FWL existed below the one found today, and gas is presently
leaking through the Ngimbang Shale. This migration has resulted in
the charging of shallower reservoirs up-structure in Pagerungan,
and the creation of “waste zones” in the poorer reservoir
quality rocks between the Ngimbang Clastics Fm. and the Ngimbang
Shale Fm. Presumably, this migration process will continue until
the column height equilibrates to capillary top seal
constraints,
or until the column is drawn down by production of
hydrocarbons.
The application of seal
capacity measurements in prospect and
reserves assessments needs to take account of the possible dynamic
nature of the petroleum system involved. The Pagerungan field
provides an analogue for situations where measured
seal
capacities
may be exceeded and where charging of younger reservoirs through
capillary leakage may have occurred.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah