Pre-Rift Reservoir Architecture of a Fluvial-Deltaic Succession, New Insights from the Pluto Gas Field, Late Triassic, Carnarvon Basin, Western Australia

Chris Clayton¹, Georgia Boyd¹, John Holbrook², Simon Lang³, Neil Marshall³, and Jennifer Wadsworth^3
1Pluto Project, Woodside Energy Ltd, Perth, WA, Australia
²University of Texas, Arlington, TX
³Science and Technology, Woodside Energy Ltd, Perth, WA, Australia

The Pluto Gas Field is hosted within a widespread pre-rift Late Triassic fluvio-deltaic succession. New data confirm a regional scale (>5,000km²) reservoir architecture comprising three (50-100 m thick), widespread sand-prone sheets (reservoir) alternating with thick shale-prone intervals. These sand-to-shale intervals represent 2nd or 3rd order sequences that become progressively more estuarine before widespread marine inundation. The sequences can be correlated regionally, within a broad intracratonic sag. The sediment is sourced from adjacent cratons during the Late Triassic Fitzroy Movement, a major tectonic event prior to onset of rifting in the Early Jurassic along the North West Shelf.

At Pluto Field, the lower two sand sheets are constructed from amalgamated fluvial multivalleys filled mainly with low-sinuosity channel-belt sandstones. They have excellent reservoir quality and are highly interconnected over a large area. The interleaved shale-prone intervals comprise highly aggradational, well drained floodplain deposits grading into poorly-drained carbonaceous lacustrine floodbasin deposits, punctuated by sandy avulsion channels and splays of good to excellent reservoir quality.

The uppermost sandsheet is 10-30m thick and shows the first evidence of estuarine influence in the system. It is interpreted as an amalgamated sheet of fluvial-dominated but tidally-influenced distributary channels within incised or compound valley fills that are generally well-connected.

Controls on the stratigraphic architecture include fluctuating base levels within a modest, but accelerating tectonic component to accommodation, coupled with higher frequency eustatic fluctuations controlling incision and flooding events. Sediment supply was probably high, shed from the uplifted interior within a variable discharge, monsoonal climate regime.

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