Remaining Exploration in the Southwest Malita Graben, Bonaparte, Australia

Abstract

Abstract The Malita Graben, situated within the Bonaparte Basin, is highly under-explored relative to similar geologic settings along the North-West Shelf of Australia. The Mid-Jurassic Elang/Plover Formations have historically been the highly successful exploration target across this basin. However, with high reservoir temperatures and deep formation burial within the Malita Graben, reservoir degradation due to quartz overgrowths was considered a large risk at the Jurassic level. In 2011, D-1 and L-1 wells had drilled at the junction of the Sahul Syncline and Malita Graben to test the Elang/Plover play in the block WA. As a result, thick successions of high-quality Upper-Jurassic to Lower-Cretaceous 'Sandpiper' reservoir were intersected. Subsequent seismic, well log synthesis and reservoir characterisation studies have upgraded the potential of Sandpiper sandstones within the Malita Graben, which remain largely under-explored. The strata penetrated by D-1 and L-1 wells provided key geological evidence to constrain the depositional model associated with the infilling stages of the Malita Graben. The wells intersected thick, homogeneous Late-Tithonian to Early-Berriasian quartzarenite units, encased in pelagic, deep-marine shales. 54m of conventional core was obtained in D-1 well, straddling the Tithonian-Berriasian boundary. The core is considered key as it refines biostratigraphic age dates, gives evidence for turbidite deposition in a deep-basin setting, and reveals a pronounced vertical heterogeneity in permeability readings. Correlating D-1 to L-1 wells suggests coeval stratigraphy, and thus possible lateral connectivity of a sand-rich submarine fan system in the vicinity of Block WA, and possibly in broader regions throughout the Malita depocenter. The results of D-1 and L-1wells were significant in advancing the understanding of the Sandpiper sand distribution. The occurrence of thick Tithonian and Berriasian sands, which were not expected prior to drilling, opens opportunities for further review of the play in the Malita and Sahul depocenters. However lateral and vertical reservoir heterogeneity of the sands is both complex and challenging to apply to an exploration strategy across the Malita Graben. Modern seismic data may help enhance the understanding of the Sandpiper play in the area, but predicting clay mineralogy and its association with diagenetic processes on individual lobe sequences may remain elusive. Introduction The Malita Graben is a prominent depocentre within the Northern Bonaparte Basin and contains up to 10km of Mesozoic to Cenozoic sediments. Previous exploration in the area has been focused on the Middle Jurassic Elang-Plover play, but drilling has been largely restricted to structural highs adjacent to the main depocentres. Limited drilling has been undertaken within the Malita Graben, primarily because of reservoir quality risk sassociated with deeply buried, high temperature reservoirs.