FROM CHAOS TO CAVES - AN EVOLUTION OF SEISMIC KARST INTERPRETATION AT THE VORWATA FIELD

摘要

Vorwata is a giant gas field located in Bintuni Bay, Papua Barat Province. The Jurassic reservoir has been on production since 2009 with 15 wells drilled from 2 offshore platforms. The overburden in this area is complex and challenging to drill through, especially the thick, karstified carbonate of the Kais and Faumai Formations. These formations have resulted in historical drilling Non-Productive Time (NPT) such as lost circulation, tight hole and stuck pipe. A variety of different seismic surveys have been acquired in this area which includes a large 3D narrow-azimuth towed-streamer survey and a wide-azimuth trenched Ocean Bottom Cable (OBC) survey. The acquired datasets have been reprocessed over time to extract additional value as seismic processing technology evolves. This has provided improvements in subsurface image quality down to the complex carbonate interval. In particular, improvements in the imaging of the Faumai Formation allow more detailed interpretation and have led to an evolution in the understanding of the karst features which range from chaotic to well-defined systems. The 2015 towed-streamer anisotropic Pre-SDM is the latest reprocessing and shows a better image of the karst features compared to the previous towed-streamer isotropic Pre-STM dataset. Further value can be extracted by transforming these new seismic reflectivity volumes to bandlimited impedance. This alternative view of karst features shows a string of low-impedance anomalies which are interpreted as structurally-controlled cave systems instead of tower karst features as previously interpreted on less well focused reflectivity data. These results show some correlation with historical drilling NPT events which make it possible to reduce future occurrences of NPT events for the development wells.