Abstract: Controls On Cyclic Sequence Architecture In The Middle Miocene Paleo-Mahakam Delta System, Badak and Nilam Fields, Kutai Basin, East Kalimantan, Indonesia

Sidi, F. Hasan & H. C. Baskara ? VICO Indonesia; G.P. Allen & S.C. Lang ? Queensland University of Technology

Badak and Nilam fields are two of the major hydrocarbon fields located in the Middle Miocene paleo-Mahakam fluvio-deltaic system of the Kutai Basin. The productive horizons consist of numerous isolated mouth bar and distributary channel sandstone reservoirs which were deposited in a passive margin-type basin characterised by (1) high subsidence rates (>3,000 feet/Ma), (2) rapid influx of siliciclastic sediments, and (3) cyclic variations of accommodation resulting from differential subsidence along active fold axes in the proximal portion of the basin and eustatic sea level changes on the shelf.

Detailed stratigraphic correlation of almost a hundred wells in both fields have furnished 30-km-long regional strike sections which indicate that sedimentation patterns are characterised by a high degree of cyclicity. This cyclicity is shown as regressive-transgressive episodes, which can be analysed at three distinct scales. The smallest cycles (100-150 feet thick) represent individual delta lobes, interpreted as parasequences produced by autocyclic processes. Intermediate-scale cycles (800-1,200 feet) form regressive-transgressive parasequence sets, whereas the largest-scale cyclicity (6,000-8,000 feet) is associated with major basin-fill patterns due to progradation of the shelf and slope.

By utilizing a smoothing technique on the gamma-ray logs, it was possible to identify and correlate the intermediate-scale sequences on regional strike sections. These sequences are bounded by more marine-dominated shale-prone intervals within which a point of maximum transgression can be identified and correlated at this scale. These larger-scale maximum flooding events cut across regional stratigraphic markers, indicating that although they are physically correlatable, they are in fact diachronous along depositional strike. This suggests that on any depositional surface, vertical stacking patterns varied along depositional strike, from zones of regressive stacking to zones of transgressive stacking. These diachronous patterns appear to be controlled by migration along strike of zones of preferential sediment influx. Analysis of sediment thickness variations on regional seismic strike sections suggests that these variations in time of the location of the predominant sediment source into the delta could result fromsediment flow deflection related to patterns of differential subsidence along active fold belts parallel to sedimentary strike.

Although eustatic sea level cycles also appear to affect stratal patterns, these local tectonic effects tend to blur the eustatic signatures within the basin by adding diachronous components to the system tracts and bounding surfaces. Even if detailed biostratigraphy might be carried out in the largely benthic-dominated fauna of these deltaic sediments, calibration of the intermediate-scale sequences with global eustatic chart will probably not be possible. Such subtle tectonic effects, which control the location of the zone of predominant sediment influx into a deltaic basin, can probably exert a major control on larger-scale sequence patterns within a basin. Therefore, in this type of tectonic setting it is necessary to analyse stratal patterns on depositional strike as well as dip.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil