OBC Seismic Learnings in Papua
Supriatna, Yayat; Barley, Brian J.; Supriyono, Supriyono
Resource, BP Berau Ltd., Jakarta, Indonesia.
BP's LNG Plant Tangguh in Papua, Indonesia, produces from reservoirs located mostly beneath Berau Bay. Strong tidal currents (up to 6 knots) make conventional seismic streamers difficult to control and noisy, and the 1997 post-discovery streamer survey (3km was the maximum offset employable) did not fully resolve the complexity of the Jurassic reservoir and overburden.
Ocean Bottom Cables (OBC) are not subject to the control and maximum offset limitations of streamer acquisition, and offer a series of technical advantages as well - if these can be realized. These special advantages of OBC and some of the difficulties of achieving them in practice are the subject of this paper.
The karstified limestone that sits on top of a massive limestone sequence causes severe imaging problems for the underlying Pre-Tertiary clastic reservoir in Bintuni Bay. Modeling studies (Schurter et al 2009) and results from other areas implied that high fold wide azimuth data should be able to provide a much better image in this situation.
In 2005, BP acquired a small area of OBC to establish that high, wide azimuth fold and longer offsets did indeed deliver better seismic quality. The cables were trenched in despite the increased cost and complexity because 2D experiments had shown that the types of OB Cables available at the time drifted on the seabed due to the tidal currents, suffered severe seabed noise and often had poor coupling giving usable geophone data.
The results were encouraging, and, when improved OBC designs that promised not to require trenching became available, BP Indonesia acquired a further 600km2 of higher fold (~500) wide azimuth 3D dataset. The data were acquired using an inline parallel geometry and subsequently processed through a Pre Stack Time imaging sequence. Amongst the practical problems faced was the very poor S/N ratio in the prestack records, but PZ summation was successful post-stack (Septyana et al 2010).
Although the pre-stack field data contain high levels of noise, the final migrated image shows significantly enhanced seismic definition at the reservoir interval which justifies the high effort acquisition design. A further 280km2 OBC acquisition commenced September 2010 and promises to give the best results in Berau Bay to date.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012