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Abstract

The Central Sumatra basin contains the Pematang-Sihapas(!) petroleum system, the most prolific petroleum system in southeastern Asia. A chronostratigraphic framework based on well logs and cores provides insights concerning the occurrence of seals, reservoirs, and the distribution of hydrocarbons. Oil sourced from lacustrine lithofacies of the Pematang Group (i.e., the Brown Shale) in the underlying rift sequence migrated vertically until reaching a thick paleosol horizon (representing the 25.5 Ma sequence boundary). Thereafter, oil migrated toward the eastern margin of the basin charging the giant Minas and Duri fields. Erosional truncation (incised valley development) of paleosols and faults provided “windows” for migration of oil into overlying Miocene (Sihapas Group) marine, sandstone reservoirs. Well log correlations and core data reveal the common presence of incision along the 25.5, 22, 21, and 17.5 Ma sequence boundaries. Oil accumulated preferentially in basal transgressive sandstones. Approximately 80 percent of the recoverable oil resides in the lower part of the 21 Ma depositional sequence. These well sorted, medium-grained sandstones (Bekasap Formation) record deposition in estuarine (presumably macro-tidal) settings. Marine sandstones within the overlying 16.5 and 15.5 Ma depositional sequences are oil-saturated; however, they are very fine-grained and have inherently low permeability. The regional top seal for Sihapas reservoirs is formed by calcite-cemented, glauconitic shales and siltstones (Telisa Group) that record the maximum Miocene transgression. Relatively small oil accumulations in the underlying alluvial-fluvial and lacustrine sandstones of the Pematang Group are sealed by paleosols. The permeability of the fluvial reservoirs is degraded by poor sorting and pervasive authigenic kaolinite. In distinct contrast, Sihapas sandstones have undergone minimal diagenesis. This newly developed sequence stratigraphic framework has dramatically improved the understanding of the correlation and distribution of Miocene hydrocarbon reservoirs and seals in Central Sumatran oil fields.

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