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Investigating Pluto-Wheatstone Triassic Stratigraphic Architecture of the Mungaroo Formation, Northern Carnarvon Basin: Towards Reduction of Subsurface Uncertainty

Anthony L. DJAKIC
Bachelor of Science (Petroleum Geology and Geophysics) 2013
Australian School of Petroleum
The University of Adelaide


The Late Triassic Mungaroo Formation in the offshore Northern Carnarvon Basin, North West
Shelf Australia contains significant hydrocarbon reservoirs that are a primary exploration and
production focus. This study aims to assess the stratigraphic architecture of the Upper Mungaroo
Formation in the Pluto-Wheatstone Trend of the Northern Carnarvon Basin within an integrated
multidisciplinary workflow.

The absence of significant age equivalent outcrops limits architectural analysis of the Mungaroo
Formation to subsurface core, seismic, image log, and wireline log data. The Mungaroo Formation
main reservoir units contain low accommodation sections of fluvial channel sandstones of various
types that alternate with high accommodation sections of intercalated floodplain deposits such as
crevasse splays, lakes, soils, and swamps. Distributary channels and restricted environment fills are
interpreted to occur within the Upper Mungaroo Formation as it transitions into the marginal
marine Brigadier Formation due to an overall marine transgression.

Paleocurrent data reflects dominantly downstream accreting (70%) unidirectional bedforms
within low accommodation thick multi-storey channel complexes. Complex lateral and vertical
variations and co-associations of channel styles is indicated by geostatistical sinuosity calculations
from paleocurrent data producing a range of high to low sinuosity estimates (1.2 to 1.9). Study of
seismic time-slices further reveals major changes in channel size and fluvial styles within the
Mungaroo Formation.

Width and thickness estimates are used to develop quantitative estimates of fluvial system sizes
and sand bodies within the Mungaroo Formation. Maximum channel-belt dimensions cluster within
a hierarchy of 2.5-5m, 7-11m, and 15+m thick with a wide maximum width range of 40-10,000+m
produced by published channel dimension methods. When integrated with seismic time-slice data,
estimates of channel belt width are interpreted to commonly range between 200m to 2km+.
Smaller channel bodies may be below seismic resolution.

Reservoir quality in the studied sandstones ranges from poor to excellent. Comparison of
porosity and permeability within main facies associations and grainsizes of the cored intervals
indicates a direct function with reservoir quality. Therefore reservoir parameters may potentially be
predictable from depositional models. Although not forming laterally extensive reservoirs,
favourable reservoir quality in crevasse splay and crevasse splay delta sandstones of the Mungaroo
Formation suggests that they could provide connectivity between the various types of fluvial
channel sandstones.

The Upper Mungaroo Formation of the Pluto-Wheatstone trend is interpreted to contain alluvial
to deltaic deposits within an overall marine transgression with relative cyclic sea level fluctuations.
The fluvial system was interpreted to have a drainage area of greater than 100,000km2
with a dominantly east to west sediment transport direction. Sandstone compositions are predominately
subarkose (Q83F12L5) with minor lithic arkose and rare sublitharenite and feldspathic litharenite.
Cratonic and orogenic terranes of the Australian continent, located on the margin of Gondwana
during the Late Triassic, are likely sediment sources

Small-scale autocyclic or large-scale allocyclic variations in accommodation space, sediment
supply and/or climate resulted in changes in depositional architecture within the Mungaroo
Formation. During periods of low accommodation, thick amalgamated multistorey successions are
likely to reflect the development of trunk channels, entrenched channels, and incised valleys as co-
associations within the Mungaroo System. Respective reservoir geometries have significant influences on reservoir development and connectivity analysis of the Pluto-Wheatstone trend.

Australian School of Petroleum



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