Abstract: Miocene Carbonate Buildups in Southeast Asia: Relationship of Basin Type to Reef Type

Mark W. Longman

Miocene carbonate buildups forming hydrocarbon reservoirs in southeast Asia range from typical "walled-reef" coral-rich and red algal-rich complexes with up to hundreds of meters of vertical relief to low-relief carbonate mudbanks with only a few meters of relief during deposition. Recognizing the geometry, facies distribution, and tectonic setting of these different types of buildups will influence exploration and development programs involving carbonate reservoirs.

Whereas a classification system of reefs based on overall morphology (e.g., barrier reef, pinnacle reef, etc.) is useful, insight into reservoir types can be improved by using a facies approach. Many modern coral reefs are "walled-reef complexes" with a marginal rigid framework, back-reef skeletal sands, and steep fore-reef slopes. Lateral correlation of facies in these reefs can be difficult because the different facies accumulate nearly vertically through time. Miocene walled-reef complexes formed commonly in the rift margin basins surrounding the South China Sea.

Another common type of carbonate buildup, particularly in tectonically stable back-arc basins, is the low-relief carbonate mudbank. The sheltered depositional setting in these basins, in combination with deposition during times of gradually rising sea level (mainly in the early Miocene), favored formation of these mudbanks over walled-reef complexes. Reservoir units in these buildups formed mainly in response to sea level fluctuations. Thus, they tend to be laterally correlative across the field. Such mudbank reservoirs are common in the South Sumatra, Sunda, and Northwest Java basins.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994