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Abstract:

Sedimentological and stratigraphic analysis of outcrops and subsurface data in the Mahakam Delta province, combined with concepts of transgressive sedimentary processes derived from a study of the modern Mahakam Delta, indicate that a significant portion of the palaeo-Mahakam Delta succession was deposited during transgressive phases. Some of the transgressive successions resulted from major transgressions and are regionally extensive, but many reflect small, short-lived transgressions within dominantly progradational phases.

Two sandstone facies with significant reservoir potential are common within the transgressive successions. Backfilled distributary sandstones occur in outcrop as 10–20 m thick, fining-upward channel sands that become more marine upward and in the subsurface as elongate, coastline-perpendicular sand bodies with a back-stepping stratigraphic architecture and highly variable thickness. Transgressive shoreline sandstones also fine upward, but they are shoreline-parallel, their lateral extent is controlled by the pre-transgression delta morphology and their thickness depends on the rate of relative sea-level rise.

Both types of transgressive sandstone can be difficult to distinguish from specific progradational sandstones with wireline log data, especially when they occur within predominantly progradational successions. However, their sand body geometries, volumetrics and connectivity are much different from the stratigraphically adjacent sandstones deposited during progradational phases of deltaic deposition.

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