オーストラリア北西大陸棚Carnarvon堆積盆におけるMutineer油田およびExeter油田の発見

抄録

Nippon Oil Exploration Ltd. farmed into the WA-191-P permit located in the Dampier Sub-basin, North West Shelf, off Australia in March, 1997. Exploration Well, Pitcairn-1 was drilled immediately after the farm-in, and oil was found at the top of Upper Jurassic Angel Formation sandstone. After the drilling of Pitcairn-1, it turned out that there is a marked difference in a time-depth relation between Pitcairn-1 and a nearby well previously drilled, and also that there is a lateral change on the stacking velocity map generated from 2D seismic data. The velocity trends were carefully reviewed with the existing check shot data and seismic stacking velocities, with the result that lateral change in p-wave velocity is conspicuous especially in the Tertiary Mandu Formation which mainly consists of carbonates, and this velocity change corresponds to the interval indicating a distinct clinoform, suggesting that the lithology laterally changes. Depth conversion using a newly constructed velocity model demonstrated the presence of another anticlinal closure with a low relief to the north of Pitcairn-1. For the better definition of depth mapping with dense velocity data, a new 3D seismic acquisition was conducted in the western part of WA-191-P including Pitcairn and other leads. After constructing a new depth map using a detailed velocity model utilizing this 3D seismic data, Mutineer, Norfolk and Exeter prospects were identified to the north, east and west of Pitcairn prospect respectively. Later, these prospects were drilled and oil was found in the Angel Formation, proving to have commercial size of oil accumulation. Currently, detailed studies are being conducted toward the development of the Mutineer/Pitcairn/Norfolk complex and Exeter oil fields by the Joint Venture Partners.