ABSTRACT: High-frequency sequence architecture in Upper Miocene and Quaternary strata on the Mahakam shelf, East Kalimantan, Indonesia

Cores and well logs from the Upper Miocene productive interval of the Attaka and Serang oil fields have been analyzed in the context of Quaternary analogs on the same shelf. The overall objective of the project is to establish a better basis for intra-field correlation schemes that honor bedding planes and hydraulic flow units.

Total stratigraphic thickness of the productive interval in a typical Attaka well is about 1325 m. The stack can be subdivided into about 35 individual sequences, each on the average 38 m thick. The age of the succession ranges from 10.7 to 7.3 Ma, or 3.4 my, suggesting that the dominant sequences may represent the Milankovitch 100ky climate cycles. Overall, the sequences are stacked in a forestepping pattern, and wells in Attaka sample distal pro-deltas at the base of the productive interval ('deep undifferentiated') changing upward to delta front and delta plain in the 'main deltaics'. In the upper half of the succession, sequence boundaries are erosional and overlain by fluvial, distributary or estuarine(?) sandstones which in turn are overlain by inferred deltaic clinoforms (inferred from log patterns but below seismic resolution). In some sequences, predominantly in the lower part of the sampled interval, the clinoforms downlap onto (inferred TST and HST) carbonates.

One-hundred-thousand year climate cycles also appear to have dominated late Quaternary sedimentation on the Mahakam shelf. The last such cycle deposited a sequence that on the average is 40 m thick. The basal sequence boundary is erosional and deeply incised by paleovalleys.

Reflector geometry and seismic amplitudes indicate that the boundary is overlain by fluvial and estuarine sediments (0 to 20 m thick), followed by carbonate mounds (also from 0 to 20 m thick).

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia