The region immediately south of West Timor, offshore Indonesia, has been largely underexplored, with only one well drilled onshore in the West Timor Block operated by eni, and no wells drilled offshore. The area is located along the Outer Banda Arc, a geologically complex, non-volcanic semi-circular belt where the Australian and Asian Plates obliquely collide. The main reservoir target is the clastic Plover formation. Imaging and resolution of the Top Jurassic horizon and the overlying accretionary section is the primary geophysical objective for prospect generation and poses a formidable challenge to marine seismic acquisition and processing.
The seismic exploration history dates back to a legacy 2D survey in 1991. In 2009, a regional 2D survey using towed streamer dual-sensor broadband technology yielded significantly improved continuity of events beneath the accretionary section. Encouraged by these results, in 2010 eni acquired a pilot study of 2D lines and subsequently a 3D survey using the same broadband acquisition technology.
Broadband marine seismic via dual-sensor streamer resulted in improved resolution of the overburden and greater penetration at the target level. These benefits are a direct consequence of eliminating the receiver ghost. Ghost-free data is rich in both low and high frequencies, has improved signal to noise ratio, and is easier to interpret.
A second important contribution comes from utilizing a unique implementation of Beam Depth Migration to correctly image the complex overburden and underlying target structure. Unique aspects of this implementation include near-vertical steep dip imaging, residual multiple attenuation in the depth domain, and the ability to detect and correctly position weak signal. These features play an important role in imaging both the accretionary prism and the target structure.
The combination of the broadband dual-sensor acquisition and the Beam migration imaging provided significant uplift in the understanding and interpretability of the seismic data promoting the development of a new exploration play in the region.
In this case study we present the results from time-lapse analysis on a Wide Azimuth Towed Streamer (WATS) data set from 2010 that was compared to a pre-production Narrow Azimuth Towed Streamer (NATS) data set from 2002 at King / Horn Mountain (KHM) Fields in the Gulf of Mexico. The WATS data set was acquired to improve the overall imaging of the fields and was not acquired for the goal of time-lapse. Nevertheless, we were able to extract a clear time-lapse signal from the WATS survey. The cost of extracting this time-lapse signal was highly reduced compared to a conventional dedicated time-lapse streamer survey acquisition as it only required reprocessing.
Combined processing of these datasets was a technological challenge. The two datasets were co-binned onto a common grid and a common narrow azimuth dataset was extracted. This dataset was analyzed and interpreted and a clear time-lapse signal was observed in the extra-salt areas. The observations in the time-lapse signal were similar to the previously acquired dedicated time-lapse survey from 2005. The noise level from the WATS on NATS time-lapse was higher than from the conventional dedicated time-lapse survey, but the signal was strong enough to be observable above background noise. Observations from this time-lapse project allow us to better understand the production history of the field, lower the risk on some of the infill targets and avoid drilling wells into potential gas caps that may have formed in the fields. The results from this study demonstrate that usable time-lapse observations can be extracted from combining WATS and NATS data at a much reduced cost.