Redefining a Sequence Stratigraphic Framework for the Miocene to Present in Brunei Darussalam: Roles of Local Rectonics, Eustacy and Sediment Supply

摘要

A Play Based Exploration (PBE) approach has been applied to the Brunei onshore, principle, second and third offshore agreement areas (OAA) of the Neogene (Serravalian to recent) deltaic sequences of the Baram – Balbac Basin operated by Brunei Shell Petroleum (BSP). Previous studies, utilizing a global sequence framework, are now considered to have underestimated the influence of local autocyclic forcing, manifested through high sedimentation rates, extensional delta-top tectonics and mutli-phase crustal shortening. Integration of shelf margin trajectories, biostratigraphy and structural kinematics has enabled a revised tectono-stratigraphic framework consisting of 10 major 3rd order sequences to be established in the Late Miocene to Present section. Regional compressional pulses are at the origin of some of these sequences, with syn- tectonic sequences showing onlap terminations on the flanks and/or thinning on the top of inverted structures and significant dislocations in the shelf edge trajectory. A rigorous reassessment of the biostratigraphic framework has necessitated a revision of the sequence definitions and the synethsis of these observations is detailed. Two sequence boundaries are now calibrated to two major global eustatic falls (11.7 and 5.73 Ma). The 11.7 Ma event, a period of tectonic quiescence, is linked to a ~15 km basinward shift of the shelf edge trajectory without any clear sub-aerial erosion on the emerged shelf. The 5.73 Ma event, also a period of tectonic quiescence, is linked to a more modest basinward shift of the shelf edge, but accompanied by widespread canyon incision along most of the margin, and a major incision of the fluvial system feeding the Champion Delta. Establishing a chronostratigraphic framework in deltaic sequences deposited on active margins requires a combination of pragmatism and caution. The same sequence may show very different stratal patterns and inferred systems tract along the same margin.