Abstract

Sequence stratigraphic techniques are widely applied in many parts of the world, their use having expanded from original development sites in Offshore Morocco, North Sea, Atlantic Shelf, and other areas. One of the more challenging places to employ such technology is in high-sediment supply deltas where is it sometimes assumed that autocyclic variations like lobe-switching tend to swamp the allocyclic eustatic signal. Recent work in several locations demonstrates that the sequence stratigraphic approach is viable even given complexities such as compensatory deposition, interlobe carbonate reef build-ups, and growth-faulting.

The subsurface sections of the Mahakam delta of Indonesia and Rajang-Lupar delta of Sarawak (Malaysia) provide examples of the utility of this approach in stratigraphic correlation, explanation of existing production, and pre-drill reservoir prediction. In Sarawak, established stratigraphic schemes emphasize palynological-defined flooding surfaces, similar to the genetic stratigraphic system advocated by some workers. Comparison of well correlations within this system with that of the sequence stratigraphic methodology, which emphasizes unconformities, results in significant differences in pre-drill predictions of reservoir and source rock distribution. Recent drilling has validated the sequence stratigraphic framework.

In the offshore Kutei basin of Indonesia, sequence stratigraphic techniques are used to subdivide the Upper Miocene producing succession into four major depositional sequences, each bounded by unconformities. Sequences are characterized by distinctive map geometries, lithologic variations, and vertical stacking patterns within systems tracts. Major highstand depocenters often occur at or updip of the shelf margin for each succession, but generally offset in a compensatory depositional pattern. Carbonate deposition is restricted to interdeltaic bights.

Present Mahakam delta production is largely restricted to the sand-prone portion of the highstand systems tract for each sequence, although some recent discoveries exploit sandstones of the transgressive systems tract. The sand-prone nature of the updip highstand and transgressive systems tracts logically points to an equally sandy lowstand systems tract, although little drilling has targeted this fairway. Both here and in Sarawak, sequence stratigraphic techniques are applicable, in spite of complexities imposed by tectonics and autocyclicity.