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Abstract: Ichnology and Sedimentology of the “Main” Interval Upper Cibulakan Formation, Offshore Northwest Java: Implication for Depositional Interpretation

REKSALEGORA, SENA W., Atlantic Richfield Indonesia, Inc., ADE DOROJATUN, S. GEORGE PEMBERTON, University of Alberta, Canada, PHIL LOWRY and YOHAN KUSUMANEGARA


The Middle Miocene “Main” interval of the Upper Cibulakan Formations is the primary hydrocarbon producing interval in the ARCO Indonesia Offshore Northwest Java contract area. Over 600 MMBO have been produced from this interval since 1971. The lithofacies and associated trace fossils identified from the available conventional cores and an outcrop analog of this interval are useful in environmental interpretation and shed more light on the controls acting on the depositional system.

The Middle Miocene “Main” interval consists of approximately 750 m thick of middle shoreface to shelf marine succession of interbedded sandstones, shales and limestones. The primary reservoir comprises of sharp based, bioturbated, glauconite rich sandstone bodies. Netsand maps and 3D seismic amplitude maps within this interval indicate elongate features (sandbodies) that are 1-2 km wide and 5-8 km long. Abundant ichnofossils are observed in both conventional cores and the outcrop analog. The interval consists of stacked paresequences in an overall transgressive package. The lower portion is characterized by the dwelling burrows of suspension feeding organisms which gradually give way to ichnofossils characteristic of the Cruziana ichnofacies in which feeding structures of deposit feeding organisms dominate in the upper part. Recurring thin layers of shale contain profuse numbers of Chondrites and may be suggestive of low oxygen, offshore environments. Relative sea level variations are well documented in this overall transgressive package and a number of well developed Glossifungites surfaces delineate possible transgressive surfaces of erosion. The integration of the sedimentology and the ichnology indicates that the “Main” interval was deposited in a tide dominated offshore setting and the elongated sand bodies are interpreted as tidal sand ridges.

The integrated sedimentological-ichnological model helps explain the factors controlling the deposition of the “Main” interval, thereby leading to a more effective exploration strategy for the contained sandstone bodies. It also facilitates the prediction of the extent of the reservoir away from areas of well control on both reservoir and regional scales.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah