Element Complex Distribution in The McArthur River Delta, Gulf of Carpentaria, Austrlia
Robert K. Kibiwot, 2013
Master of Science (Petroleum Geoscience)
Australian School of Petroleum
The University of Adelaide
The broader objective of this project and indeed other efforts related to mapping and characterisation
of a mixed-influence deltaic or coastal depositional environment where fluvial, tidal and wave processes
interact, is to provide an understanding of the linkage between the evolution of the system and the
distribution of emplaced sedimentary facies as a result of the depositional processes. In ancient deltaic
or coastal settings, this can be related directly to potential hydrocarbon elements such as source rock
and reservoir. It can further provide the basis for in-depth analyses on heterogeneities in the subsurface
whose intention is to enhance the predictability of reservoir connectivity or compartmentalization,
which directly impacts on oil exploration and production.
The formation of the Holocene McArthur River delta situated on the south western shoreline of the Gulf
of Carpentaria, northern Australia from about 5 ka BP was as a result of sea level fall of up to 2 m
attributed to hydro-isostatic deformation. During this period of forced regression, the delta has
prograded approximately 18 km resulting in the emplacement of various architectural units related to
tidal, fluvial and wave processes. These units have been grouped into Elements, Element Sets, Element
Complexes, Element Complex Sets and Element Complex Assemblages based on various criteria
including the depositing process, increasing scale of observation, genetic relations and environment of
emplacement. Where applicable, the architectural units have further been categorised into shorelineto-
offshore, channelized and onshore zones based on location of emplacement.
The McArthur River delta is predominantly influenced by monsoonal fluvial flows and diurnal tides.
Morphologically, it is divided into two main sections; the upper and lower deltaic plains. The upper
deltaic plain is characterised by significant lateral migration of fluvial dominated channels resulting in
deposition of extensive nested sequences of scroll bars. The lower deltaic plain is dominated by
intertidal and supratidal saline mudflats underlain by sand and shelly sands which contain in situ shell
beds interpreted as delta front deposits. There are two active fluvial distributary channels approximately
dip-aligned and are characterised by widths that reduce gradually upstream. In contrast, the near
abandoned west-east flowing tide dominated channels, confined to the lower deltaic plain are
characterised by rapidly tapering widths and have been interpreted as former fluvial distributaries that
underwent sequential avulsions.
On the sub aerial delta, tide related Elements occupy more than 56%, followed by composite of fluvial
and tide related Elements at approximately 42%. Wave related Elements are the least in the delta and
occupy only approximately 2%. The surface Element Complexes have been categorised into shorelineto-
offshore, channelized and onshore. These occupy approximately 49%, 10% and 41% respectively.
Three Element Complex Assemblages (ECA's) have been delineated in the delta. ECA A which is the
largest and most landward in the delta is categorised as tide dominated, wave influenced and fluvially affected (Twf); ECA B is classified as tide dominated, fluvially influenced and wave affected (Tfw) while ECA C is classified as fluvially dominated, tide influenced and wave affected (Ftw). Although ECA's B and C show that the modern shoreline stabilized from around 2 ka BP, ECA C shows continued accretion of the delta interpreted from the islands forming to the north of the delta and also the relatively recent fossil shell bed dated about 0.6 ka BP found within the forming island.
In terms of reservoir potential, the predicted subsurface Mouthbar Element Complexes portend great potential as they occupy an area in excess of 184.14 km2 representing approximately 51% of the area occupied by Element Complexes in the shoreline-to-offshore zone. The depth of these units however needs to be established for purposes of determining possible reservoir volumes. The saline mudflats characterising most of the delta surface are relatively thin and are underlain by sand and shelly sands that gradually thicken from the upper deltaic plain through the lower deltaic plain to the modern shoreline. This demonstrates the fact that whereas the McArthur River delta is a tidally dominated system, the underlying extensive deposits of sand and reservoir analogue deposits indicate great reservoir potential that needs to be investigated further.