Abstract: Reducing Reservoir and Source Rock Risk in Deepwater Plays: Examples from Southeast Asia
Snedden, John W. and J. F. (Rick) Sarg - Mobil E&P
Deepwater exploration efforts require means of reducing pre-drill reservoir risk. In Southeast Asia, additional risk pertains to increased distance from Type III coaly source rocks formed in updip coastal-plain paleoenvironments. Lessons drawn from exploration in the Sarawak and Kutei (Mahakam delta) Basin deepwater regimes provide an understanding of how to reduce these two key risk elements of the petroleum system.
In Sarawak (northern Borneo), successful pre-drill prediction of reservoir type and content in deepwater blocks was based upon regional sequence stratigraphic correlation and seismic facies calibration. Previous stratigraphic schemes had emphasized palynologically-defined flooding surfaces, similar to the genetic stratigraphic system advocated by some workers. Comparison of well correlations within this system with that of the sequence stratigraphic methodology, which emphasizes unconformities, resulted in significant differences in pre-drill predictions of reservoir type and distribution. For example, the genetic stratigraphic approach predicted a NW-SE trending strandplain system. By contrast, a NE-SW trending fluvial-dominated delta model resulted from the sequence stratigraphic analysis.
Calibration of seismic response within the sequence
stratigraphic framework allowed lowstand reservoirs
to be identified with
some certainty. Abundant, thick
deltaic
siliciclastic
reservoirs
were penetrated
in the Mulu-1 and Bako-1 wells drilled in the deepwater blocks. This confirmed
predictions made with the sequence stratigraphic analysis. However, less
success was experienced with predicting hydrocarbon charge which post-drill
studies indicate are related to maturity and the distance (>10 km) from
coeval coastal-plain coals.
In the Mahakam delta (southeast Borneo), regional
mapping of Miocene and Pliocene shelf sequences proved critical to identifying
sand-prone fairways for deepwater lowstand reservoirs
(Fig. 1). Prior drilling,
made without the benefit of regional sequence stratigraphic studies, had
found limited reservoir volume. New maps constructed from seismic, cores,
and logs pointed to key canyon system entry points and adjacent sand-prone
highstand deltas (Fig. 2). Defining the location of lowstand depocenters
also reduced risk regarding source rock, as these provide the major kitchens
for hydrocarbon migration.
Recent drilling has validated the reduction in
reservoir and source rock risk, with discoveries flowing gas, condensate,
and oil in thick Upper Miocene and Pliocene deepwater sandstone reservoirs
.
Channel-levee systems on the upper slope are the active targets but downdip
drilling will eventually focus upon probable sheet-type turbidite
reservoirs
.
Geochemical analysis of discovered oils also confirms the pre drill lowstand
source kitchen model.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil