Abstract: Deltas represent the major sediment source for tectonically confined, tide-dominated seaways or straits. Modern examples show how along-shore tidal currents are able to modify the impinging delta shape, generating asymmetrical coastal plains, deflected delta fronts and elongate sandbanks. Seaway or strait deltas can become become tide-influenced or tide-dominated, assuming physical attributes that may depart from classical models. Ancient deltas in seaways and straits can also reveal unexpected facies stacking and stratigraphies, which can be misinterpreted or attributed to different depositional settings. Two ancient analogues of deltas that prograded into elongate basins dominated by amplified tidal currents are presented here. A common element in these deltas is the progressive-upwards change in the dominant process of sediment dispersion recorded in the delta facies. Early stages of progradation are dominated by river- and wave-influenced lithofacies, whereas late deltaic advancements occur with a dominance of tidal current circulation on the delta fronts and the consequent morphologies are deflected/elongated in the direction of tidal flow. This study provides the basis for a preliminary stratigraphic framework for the depositional style of these types of delta. The studied deposits also suggest analogies with the spatial distribution of many hydrocarbon reservoirs investigated along the margins of confined, narrow, linear basins, the interpretation of which is still debated.

Abstract: The Balingian Province of NW Borneo is a major hydrocarbon-bearing region with production dominantly from Early Miocene (cycle II) coastal to lower coastal plain deposits. A regional sedimentological synthesis is outlined from the main productive interval of three offshore oilfields, which are representative of the depositional environments and stratigraphic architecture in cycle II. Four types of vertical facies successions are recognized: (1) fluvio-tidal channel; (2) tide-dominated delta front; (3) wave-dominated shoreface; and (4) barrier lagoon. The facies successions demonstrate that cycle II consists of a tide-dominated delta system that is partly analogous to the modern Rajang Delta and Lupar Embayment of southern Sarawak. Fluvio-tidal channel and tide-dominated delta successions represent periods of river-fed progradation. Wave-dominated shoreface and barrier lagoon facies successions represent depositional systems that developed during periods of transgression and/or delta lobe abandonment. Cycle II consists of several high-order sequences stacked together into two lower-order, c . 100–300 m thick fining-upwards, unconformity-bounded megasequences. This stratigraphic architecture reflects a combination of hinterland denudation and long-term eustatic sea-level rise. These trends were interrupted by frequent, higher-order relative sea-level falls, which were a result of movement along the West Balingian Line fault zone. Variations in the thickness of megasequences between the three oilfields reflect fluctuations in tectonically driven subsidence rates. This exerted a fundamental control on the reservoir architecture, zonation and correlation and provides a basis for comparing field behaviour.