Abstract An active exploration campaign in the Exmouth Plateau has yielded gas discoveries in a coeval wave-influenced delta and deep-water, sand-rich fan succession. Depositional elements were organized into clinoform seismic stratigraphic units that blanketed irregular topography created by extensional tectonics. Clinoform geometries revealed steady, rising and falling shelf-slope break trajectories. Slope successions associated with rising trajectories were devoid of deep-water feeder systems. In contrast, during steady to falling trajectories, the slope was characterized by numerous gullies. These gullies served as the main delivery system for sediment gravity flows into the basin. In some instances, an individual gully dominated and captured the flows of subordinate gullies and developed into a larger feeder system. The feeder systems were self-sourced and cannibalized the deltaic and slope successions through knickpoint retreat. Arcuate strandplains organized into wave-influenced cuspate lobes characterized the deltaic succession. Littoral drift was locally to the east. Delta front well information indicated excellent reservoir quality. Sedimentological analysis of core data indicated different depositional processes as a function of the clinoform geometries. High quality delta front sands were fed into the slope and basin floor as sediment gravity flows and deposited as coalescing sand-rich fans. The fan cores were composed of high-density turbidites that graded into debrites and linked debrites along the margins. The deep-water fans were of favorable to excellent reservoir quality.

Abstract The late Triassic Mungaroo Formation is a prolific gas-condensate reservoir along the landward margins of the Exmouth Plateau ( i.e. , Gorgon and Rankin trends). Recent exploration drilling has stepped outboard of these trends into the Exmouth Plateau adding calibration to an area of sparse well control. Reservoir units were primarily channelized complexes of variable widths and orientations within a large fluvial-deltaic complex that formed a thick (>2.5 kilometer) succession. Channel complexes were attractive exploration targets and have been the reservoirs of recent discoveries. Mungaroo channelized complexes were thick >10 meters, multistorey, and greater than 1 kilometer in width. Their depositional origins were considered either distributaries or channels within an incised valley. Reservoir characterization studies has provided ranges in the properties of the channels, internal reservoir architecture and expected facies distribution that allowed for gas initially in place (GIIP) and estimated ultimate recovery (EUR) calculations.

In order to better define the late Eocene clinopyroxene-bearing (cpx) spherule layer and to determine how the ejecta vary with distance from the presumed source crater (Popigai), we searched for the layer at 23 additional sites. We identified the layer at six (maybe seven) of these sites: Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Holes 592, 699A, 703A, 709C, 786A, 1090B, and probably 738B. The cpx spherule layer occurs in magnetochron 16n.1n, which indicates an age of ca. 35.4 ± 0.1 Ma for the layer. We found the highest abundance of cpx spherules and associated microtektites in Hole 709C in the northwest Indian Ocean, and we found coesite and shocked quartz in the cpx spherule layer at this site. We also found coesite in the cpx spherule layer at Site 216 in the northeast Indian Ocean. This is the first time that coesite has been found in the cpx spherule layer, and it provides additional support for the impact origin of this layer. In addition, the discovery of coesite and shocked quartz grains (with planar deformation features [PDFs]) supports the conclusion that the pancake-shaped clay spherules associated with quartz grains exhibiting PDFs are diagenetically altered cpx spherules. An Ir anomaly was found associated with the cpx spherule layer at all four of the new sites (699A, 709C, 738B, 1090B) for which we obtained Ir data. The geometric mean of the Ir fluence for the 12 sites with Ir data is 5.7 ng/cm 2 , which is ~10% of the fluence estimated for the Cretaceous-Tertiary boundary. Based on the geographic distribution of the 23 sites now known to contain the cpx spherule layer, and 12 sites where we have good chronostratigraphy but the cpx spherule layer is apparently absent, we propose that the cpx spherule strewn field may have a ray-like distribution pattern. Within one of the rays, the abundance of spherules decreases and the percent microtektites increases with distance from Popigai. Shocked quartz and coesite have been found only in this ray at the two sites that are closest to Popigai. At several sites in the Southern Ocean, an increase in δ 18 O in the bulk carbonate occurs immediately above the cpx spherule layer. This increase may indicate a drop in temperature coincident with the impact that produced the cpx spherule layer.