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Abstract

Post-Ordovician carbonate buildups and buildup plays from around the world have been evaluated to determine the distinctive aspects of hydrocarbon-productive buildups. Ninety percent of the approximately 40 billion barrels of recoverable oil equivalent found within carbonate buildup reservoirs exists within strata deposited during just 15% of post-Ordovician geologic time. These time windows correspond to periods of extensive source rock deposition and, with the exception of the late Paleozoic and late Miocene, to the rising sea-level portions of long-term, second-order eustatic highstands. Nearly three-fourths of buildup reserves are found in buildups deposited during the early phases of periods of rapidly increasing rates of relative sea-level rise. These buildups are found in the basal, transgressive portions of thick sedimentary wedges and display progressive areal restriction through time until they are eventually unable to keep up with rapid increases in accommodation (total space available for sedimentation). A younger regressive wedge progrades over the buildups during a later period of more limited increases in accommodation and seals the mounded reservoir in fine-grained basinal sediment. Buildups deposited during this regressive phase carry higher exploration risk due to leaky top and lateral seals, and they generally have smaller trap sizes. These cycles of basin fill are clearly recognizable on seismic and log data and have a distinctive character on geohistory plots. Although the origin of these long-term accommodation changes is difficult to assess, we conclude that basin tectonism is responsible for these changes in most of our examples. Long-term eustasy plays a secondary role, according to our analysis. The model illustrated here can be helpful in both the geologic and risk assessment of new buildup plays and prospects.

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