Reservoir Heterogeneity of Miocene–Pliocene Deltaic Sandstones at Attaka and Serang Fields, Kutei Basin, Offshore East Kalimantan, Indonesia
Arthur S. Trevena, Yoseph J. Partono, Tom Clark, 2003. "Reservoir Heterogeneity of Miocene–Pliocene Deltaic Sandstones at Attaka and Serang Fields, Kutei Basin, Offshore East Kalimantan, Indonesia", Tropical Deltas of Southeast Asia—Sedimentology, Stratigraphy, and Petroleum Geology, F. Hasan Sidi, Dag Nummedal, Patrice Imbert, Herman Darman, Henry W. Posamentier
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Heterogeneous sandstones in upper to middle Miocene lobes of the Mahakam Delta are prolific hydrocarbon reservoirs in the lower Kutei Basin, offshore East Kalimantan, Indonesia. At Attaka and Serang fields, more than 99 million cubic meters (627 million barrels) of oil and 37 billion cubic meters (1.3 trillion cubic ft) of gas have been produced from sandstones that we interpret as delta-front bars and tidal/fluvial distributary channels. Sand bodies of the modern Mahakam delta are analogs for many of these reservoirs.
Delta-front bars are burrowed to laminated, fine-grained sandstones that form equant to somewhat dip-elongate bodies that range in thickness from < 1 m to 5 m and may exceed several kilometers in width. Cross-stratified, coarse- to fine-grained tidal/fluvial distributary channel sandstones are 3 to 17 m thick and narrow (< 1.5 km wide).
Distributary-channel sandstones are typically highly porous (20–35%) and permeable (100–10,000 md), although tidal distributaries exhibit permeability heterogeneity, due to mud drapes and local burrows. Delta-front sandstones, although areally extensive, have generally poorer reservoir quality than the distributary channel sandstones (k = < 0.1–1000 md; porosity = 10–25%). Also, the delta-front sands exhibit major centimeter- to decimeter-scale variations in permeability, which are related to variations in clay content and intensity of burrowing.
Stacked, distributary-channel reservoirs are especially well developed in the Upper Miocene “Main Deltaics” interval, which we interpret as a succession of lowstand deltaic lobes.
The coarsest-grained and thickest deltaic sandstones typically accumulate during relative lowstands, times when deltas have prograded long distances across marine shelves. Such lowstand sandstones also form the most porous and permeable hydrocarbon reservoirs. Thin-bedded sandstones and burrowed sandstones are common in distal deltaic deposits, which were probably deposited during times of somewhat higher relative sea level. Such sandstones may form low-resistivity reservoirs, which can, if recognized, contribute substantially to hydrocarbon production.
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It is the objective of this volume to bring to the fore a category of deltas with which many sedimentologists and stratigraphers are, at best, vaguely familiar. It is expected that this volume also will stimulate new research on tropical deltas by highlighting how their facies and stratigraphic architectures differ from mid- and high-latitude ones, by emphasizing their significance to the global sediment budget, and by stressing their uniqueness within a petroleum systems framework. This special publication emphasizes the need for models intrinsic to tropical deltas of Southeast Asia to supplement the more conventional general models currently in vogue, based on past studies of large and small mid-latitude deltas. The papers in this book explore how the combination of these complex factors has shaped deltas in this region. Sedimentological surprises such as distributary channels floored by thick accumulations of fluid mud lend a bit of “mystery” to tropical deltas. We hope that, rather than being merely a summary of tropical deltas, this book may open the door to a new and active phase of sedimentological and stratigraphic research in tropical environments across the globe.