Paleosol Development and Floodplain Sedimentology in Relation to Fluvial Reservoir Distribution of the Late Triassic ‘G’ and ‘H5’ Units, Mungaroo Formation, Goodwyn Field, North West Shelf
Glassborow, Brent Anthony
Honours Degree, 2004
University of Adelaide
The giant Goodwyn gas field is located in the Dampier Sub-basin which is a component of the Exmouth-Barrow-Dampier rift system of the Northern Carnarvon Basin, North West Shelf. Initially Hydrocarbon exploration targeted the Late Triassic fluvio-deltaic sediments of the ‘E’ and ‘F’ units in the upper Mungaroo Formation. However, to optimise field development the underlying ‘G’ and ‘H’ units of the Mungaroo Formation are being investigated for their hydrocarbon reservoir potential.
The ‘G’ unit is dominated by amalgamated channel belt systems considered to be deposited in a low accommodation, moderate sediment supply environment. The Goodwyn 5 exploration well retrieved 60m of core from the ‘G’ unit that consisted of a fluvial succession of channel and floodplain deposits. Of this, 36m display some degree of pedogenic alteration and thus, paleosol development.
Core logging and floodplain sedimentology was the basis for interpreting lithofacies associations and their relative position on the alluvial plain. Geochemical, petrological and palynological analysis reinforced lithofacies signatures. Pedofacies, based on soil horizonization, colour and pedogenic structures, were assigned to the floodplain sediments to distinguish paleosol maturity.
Paleosols from the Goodwyn 5 cored interval exhibited very weak to strong development which reflected their distance from the active channel belt. Sediments of the distal floodplain, swamp and swamp margins commonly exhibited a greater degree of pedogenic alteration.
Characteristic features of paleosol development observed in wireline logs and core samples of the ‘G’ unit were applied to the ‘H5’ unit, which consists of isolated fluvial channel sands and extensive floodplain deposition. Pedogenic alteration was readily observed in cuttings of the ‘H5’ unit. They displayed strong redoximorphic colouring and high carbonaceous contents. Recognition of these features combined with seismic and wireline log data enabled relative lateral distances to be calculated between floodplain sediments and the active fluvial channel belt.
Calcrete development in the less shale-prone sediments of the floodplain was observed to be locally extensive, signifying a period of aridity in the commonly sub-tropical climate of the Late Triassic. The preservation of these calcrete profiles may hinder hydrocarbon recovery from lack of pressure drive or create reservoir baffles.
Paleosol development is regulated by the nature and frequency of overbank flooding. It was interpreted that sediments in the Goodwyn 5 well deposited distally from the fluvial channel were less likely to be influenced by such processes and therefore have a greater residence time in the pedogenic profile. Those sediments proximal to the fluvial channel were subjected to greater rates of sedimentation and therefore a less mature paleosol profile developed. The relative abundance of floodplain sediments in the ‘H5’ unit emphasises the need to understand their origin so that potential localised hydrocarbon traps can be optimised.