SAND FAIRWAY AND PLAY FRAMEWORK ON THE DEEPWATER SLOPE AREA OF NORTH KUTAI PROVINCE

摘要

Neogene shelf, slope, and slope-to-basin-floor transition plays in the North Kutai Province shelf reflects a variety of structural and stratigraphic controls, including linked extensional gravity sliding and toe-thrust compression, shale-tectonics, and sea-level fluctuations. This study characterizes the evolution and significance of play elements for Neogene deepwater slope play-types in the Bontang and SE Sangatta PSC areas, offshore East Kalimantan. Based on regional reconstruction, continuous modification of antecedent drainage systems can be observed by the shift of major deltaic depocenter from Miocene paleo-Mahakam delta into several growth-faulted controlled deltaic systems distributed along a present-day shelf area of the North Kutai Province. Neogene deltaic systems in Kalimantan are the primary sediment source-area for deepwater sand reservoir found in the offshore extension of Lower Kutai Basin. The growth and demise of these deltaic systems are closely related to rapid uplift and erosion of the hinterland area since the Middle Miocene. The offshore North Kutai Province can be classified as a mix between active convergent margin that is continuously driven by far-field stress acting on the eastern margin of Sundaland and near-field stress associated with the gravitational collapse due to hinterland uplift and rapid-depositional loading on the shelf area. Neogene strata in the North Kutai area was deposited in a relatively narrow shelf associated with tectonically active coastal area affected by significant hinterland uplift and erosion. The shelf area is characterized by extensional listric growth- faults that sole-out into basal detachment. Further outboard into the slope, shale diapirs and ramp-thrusts rise from detachment surface and formed large fault-propagation folds that extended the slope area into stepped-gradient profile descending toward the basin-floor area at the depths of Makassar Strait. Abrupt changes on the deltaic input area and shelf- to-slope morphology affect the sand fairway in the deepwater slope area. Productive turbidite sandstones with excellent quality occur on a slope as the confined canyon/channel-fill systems. Sand deposition is strongly controlled by syn-kinematic “lows” whereas intra-basinal “highs” are commonly dominated by more silty and muddy deposits. Shale diapirism and penecontemporaneous structural growth of deepwater folds may significantly modify the basic fold geometry that may lead to deflection and increased ponding of turbidite channel sand fairway across the slope area.