LATE JURASSIC-EARLY CRETACEOUS DEPOSITIONAL SYSTEMS OF THE DAMPIER SUB-BASIN-QUO VADIS?

Abstract

A diminishing prospect inventory based on traditional structural targets, has placed increasing emphasis upon finding commercially viable stratigraphic traps. This is especially true in the Dampier Sub-basin where recent drilling has identified at least twelve depositional sequences within the Late Jurassic to Early Cretaceous succession. These sequences document a far more complex Late Jurassic to Early Cretaceous sedimentary history than previously acknowledged using traditional litho-stratigraphic mapping techniques. Use of a chronostratigraphic framework to develop Late Jurassic and Early Cretaceous systems tract models, reveals that several repetitive facies suites may be penetrated by the drill, depending on geographic location and frequency of lowstand events. These models have profound implications for prediction of Late Jurassic to Early Cretaceous submarine fan systems, their potential reservoir distribution, and sealing capacity for hydrocarbon entrapment.

At least eight lowstand events have been recognised in the Oxfordian to Tithonian sequence alone, associated with both synrift crustal extension and/or related fluctuations in global-eustatic sea level. During these lowstand episodes, huge volumes of coarse clastics were transported by mass-flow into the Lewis Trough with sediment transport of some 5–40 km. The magnitude of shelf-margin tectonic instability, sediment supply, and relative sea level change, controlled the geometry of submarine fan complexes that developed within each lowstand depositional cycle. In general, earlier (Oxfordian) basin-floor sand cycles comprise mixed channel-fill and submarinefan lobe moundforms confined to the Lewis Trough. Following collapse of the Madeleine intra-basinal high in early Kimmeridgian times, these earlier channelised lobes were progressively replaced by more widespread, massive, detached, non-channelised basin floor lobes, which became dominant by end-Tithonian time. The Early Cretaceous succession comprises mainly transgressive-highstand shelf to slope sediments, heralding a change from synrift lowstand to post-rift highstand depositional cycles.

Sequence boundaries identified in the Dampier Sub-basin demonstrate a remarkable synchroneity with the worldwide global-eustatic curve, suggesting that a symbiotic relationship may exist between major tectonic events and third-order eustatic cycles, at least within the synrift section of the Dampier Sub-basin.