Oxfordian Rift-associated Unconformity Characteristics in the Northern Carnarvon Basin, North West Shelf, Australia
Richard T. Fernie, 2013
Honours Degree of Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
The Northern Carnarvon Basin is a major hydrocarbon province in the North West Shelf of Australia, in particular the area encompassing the Rankin Trend and its western flank extending onto the Exmouth Plateau. These areas host significant gas and condensate reserves, in which Woodside Energy Ltd and its partners hold a significant interest. The most prolific play historically has been the Late Triassic Mungaroo Formation with hydrocarbons trapped in large horsts and fault blocks. In addition, there have been several major discoveries made in Middle and Upper Jurassic reservoirs contained in more subtle accumulations associated with the regionally extensive Oxfordian riftassociated unconformity (J40.0 SB). The reservoir quality and configuration of sediments overlying the unconformity vary significantly across the Rankin Trend and the Exmouth Plateau. There is a need for an integrated study of depositional characteristics and sequence stratigraphy that describes the nature of the unconformity and the Oxfordian reservoirs and topseals that post-date it. This study addresses those issues with a view to aiding Woodside Energy's future exploration efforts.
A data set incorporating 8 wells that recovered core across the unconformity was selected as the basis for investigation. These wells cover 6 producing fields: the Io/ Jansz field (Jansz-3), the Wheatstone field (Wheatstone-1, Wheatstone-3), the Echo/ Yodel field (Yodel-2), the Keast field (Keast-2), the Goodwyn field (GWA02, Goodwyn-9), and the Persephone field (Persephone-1).
Based on sedimentological analysis of the available core, several sedimentary facies were established, classified and correlated to wireline logs and seismic data. The J40.0 SB unconformity was identified and correlated on seismic data, with adjacent key surfaces and sequence boundaries, between wells. In this way, important structural and stratigraphic features, controls on sedimentation, seismic characteristics of the depositional environment, structural development and the relevance, or otherwise, of nearby faults and other structural elements, were identified.
Palynology was used as a zonation tool to constrain the position of the J40.0 SB and other significant stratigraphic surfaces, and was used to identify and determine the significance of reworked palynomorphs as a basis for recognising reworked Triassic sediments within Jurassic strata.
The structural evolution of the area is complex, with indications of various phases of fault movement involving different fault blocks at different times creating accommodation space for reservoir to be deposited. It was possible to construct a local depositional and structural history relative to sea level changes to explain the depositional pattern of reworked Triassic sediments across the study area.
This work demonstrates that the reworked Triassic in Lower to Middle Oxfordian sandstones have formed hydrocarbon reservoirs in trapping configurations that often depend on a stratigraphic element to form closure, e.g. Jansz, or complement the overall trap by enhancing reservoir volume, e.g. Wheatstone. The methodology can be applied to further explore the area for J40.0 SB related accumulations.