Abstract

Seismic analysis of four Miocene carbonate buildups (the Porong, KE-11C, KD-11E, and BD buildups) in the Madura Strait region, Indonesia, shows that they are constructed of four seismic units (1-4). All the buildups are located atop an east-west-trending Oligocene fault block, which, through its uplift, favored the deposition and aggradation of shoal-water carbonates. Aggradation, platform narrowing, and backstepping of leeward reefs into the upper parts of individual buildups typify the growth profiles. Adjacent off-buildup sedimentary rocks, deposited at the same time as the buildups, show clear horizontal onlap relationships. In combination, these observations imply that all the platforms were drowned. The uppermost parts (unit 4) of each of four major drowned buildups in the trend have been drilled to test for the presence of hydrocarbons. An analysis of the drilling results, in combination with our seismic analysis, shows that those buildups that have relatively high levels of hydrocarbons have suitable trap configurations but lack seal integrity. For example, compactionally induced strain and gravitational gliding created leaky collapse-graben faults (extensional structures) atop some buildups (Porong, KE-11C), which allowed hydrocarbons to drain. Likewise, the updip inclination of some potential seal beds facilitated leakage (KE-11E).

In terms of potential targets in similar drowned buildups in the region, higher priority should be given to targets that (1) show evidence of ongoing platform backstepping and so-called give-up reef growth profiles associated with incipient drowning, (2) contain all four seismic units (units 1-4) in the buildup, (3) lack an extensional collapse graben over the crest of the drowned structure, and (4) demonstrate bed inclinations in the sealing beds that indicate likely closure over the crest of the buildup.

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