Print this pageTropical Delta/Carbonate Stratigraphy on the Mahakam Shelf, Kalimantan: Detailed, Time Calibrated Model of a Depositional Cycle
SYDOW, J., H. H. ROBERTS
Mahakam shelf stratigraphy reflects the interaction of eustacy, active tectonics, fluvio tidal delta deposition, and prolific carbonate accretion. These variables have impacted Neogene stratigraphy, to produce a thick succession of stacked delta depocenters and carbonates which contain over 5 billion barrels of recoverable hydrocarbons. Over 3000 line km of high-resolution seismic, ground truthed with cores, were analyzed to produce a detailed, regional sequence stratigraphic analog model of one depositional cycle. Falling-stage, lowstand turnaround, and transgressive rise phases of relative sea level are clearly identified in the stratigraphy. Deltaic architecture reveals that these phases are represented by a progradational continuum from the inner shelf to the steep shelf edge.
Change in shelf accommodation from falling-stage to lowstand turnaround significantly impacts deltaic architecture, with implications for thicknesses and distributions of reservoir and source rocks. Accommodation created behind the prograding delta front during lowstand turnaround is filled by aggrading organic-rich, horizontal delta platform strata. This thick delta plain succession encases aggraded distributary and alluvial-valley-fill trends. The delta platform occupies the outer shelf during more than 30 m of relative sea level rise (<10 ky). Ultimate transgression is accompanied by tidal and longshore scour of the shelf in a setting of widespread rapid carbonate mound accretion (5 to 10 m/ky). High-order (<100ky) eustatic cyclicity, modified by structural deformation, controls stratigraphic architecture, and ultimately, reservoir and source rock distribution. Stratigraphic resolution achieved in this study is not attainable in multi-channel seismic. Results have thereby improved our understanding of the detailed stratigraphy, and will impact future exploration and production efforts in this setting.