Establishment of a sequence stratigraphic framework, based on an integration of seismic, well-log, core, and biostratigraphic data, indicates that the Tertiary-aged Gunung Putih carbonate complex in the East Java Sea comprises an asymmetric buildup. This asymmetry was inferred to occur in response to paleo-oceanographic circulation patterns that favored aggradation on the north face and progradation on the south face. The aggradational pattern was associated with carbonate reefal buildups, whereas the progradational patterns were associated with grain-rich forestepping deposits.

Widespread carbonate buildups were initiated on a beveled Cretaceous to early-Tertiary platform during the late Eocene. These buildups exhibit pronounced stratigraphic asymmetry; the northern side is inferred to lie on the paleowindward side and is characterized by bulwark-like framestone-rich buildups. Though the stacking pattern is predominantly aggradational, forestepping and backstepping can occur locally. The southern side is inferred to lie on the paleo-leeward side and is characterized by continuous forestepping buildups with clinoform reflection geometries. Mounded, toe-of-slope, lowstand deposits of several sequences form an apron around the entire Miocene complex.

From the late Eocene through the Miocene, differential subsidence progressively changed the style of carbonate buildup from widespread distribution across the entire platform to more restricted distribution associated with smaller structural highs that occurred on the platform. Eventually, as the platform continued to founder, carbonate deposition ended completely during the late Miocene. The carbonate complex subsequently was buried in terrigenous mudstones and siltstones.

Based on the evolution of the Gunung Putih carbonate complex, a general approach for the exploration for hydrocarbons within carbonate buildups has been developed. Initially, a structural high that caused the sea floor to be sufficiently raised so as to establish a carbonate factory must be identified.

Fig. 2. Log correlation for the Northeast Kangean Block illustrating the relationship between seismic sequences, systems tracts, paleontologic and palynologic horizons paleoecozones, formation names, lithology, and well-log curves.

Oversized Image :click to view

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Subsequently, evidence for deposition at the top and margins of the structure should be identified. The occurrence of backstepping (i.e., transgressive) deposits, in particular, is suggestive of an active carbonate factory on the platform. Further evidence for carbonate buildups is the re-establishment on the structural crests of shallow-water conditions through the development of stacked, mounded, and clinoforming seismic reflections. Shallow-water conditions also can be inferred through the identification of erosionally truncated or toplapping seismic reflections.