SEISMIC ACTIVITY

A combination of factors including the DWRRA, several key deepwater discoveries, the recognition of high deepwater production rates, and the evolution of deepwater development technologies, spurred a variety of deepwater activities. One of the first impacts was a dramatic increase in the acquisition of 3-D seismic data (figure 2). (Note that figures 2 and 3 illustrate areas permitted for seismic acquisition. The actual coverage available may be slightly different than that permitted.) Three-dimensional seismic data are huge volumes of digital energy recordings resulting from the transmission and reflection of sound waves through the earth.

Figure 2. Progressive deepwater 3-D seismic permit coverage.

These large �data cubes� can be interpreted to reveal likely oil and gas accumulations.

The dense volume of recent, high-quality data may reduce the inherent risks of traditional hydrocarbon exploration and allow imaging of previously hidden prospects. Figure 2 illustrates the surge of seismic activity in the deepwater GOM during the last 12 years. Seismic acquisition has stepped into progressively deeper waters since 1992. Figure 3 shows the abundance of 3-D data now available. These data blanket most of the deepwater GOM, even beyond the Sigsbee Escarpment (a geologic and bathymetric feature in ultra-deep water). Note that many active deepwater leases were purchased before these 3-D surveys were completed (only the more sparsely populated 2-D datasets were available).

Figure 3. Deepwater Gulf of Mexico 3-D seismic permit coverage from 1992 to 2003..

The seismic permitting coverage shown in figure 3 does not tell the whole story of geophysical activity in the deepwater GOM. Pre-stack depth migration (PrSDM) of seismic data has greatly enhanced the interpretation capabilities in the deepwater GOM, particularly for areas hidden below salt canopies.

While PrSDM was once used sparingly, the availability of large speculative PrSDM surveys allows the widespread use of this technology in the early phases of exploration. Subsalt discoveries like Mad Dog, Thunder Horse, North Thunder Horse, Atlantis, and Tahiti demonstrate the importance of subsalt exploration.

Figure 4 provides a partial inventory of speculative PrSDM coverage.

Figure 4. Pre-stack depth migration coverage from various industry sources.

This figure was assembled from publicly available sources and provides a good indication of the current widespread coverage of PrSDM processing.

Time-lapse seismic surveys (also known as 4-D) will likely be the next significant seismic technology to be applied routinely in the deepwater GOM. The technique can be applied to characterize reservoir properties, monitor production efficiency, and estimate volumetrics from inception through the life of the field (Shirley, 2001). The high cost of drilling deepwater wells and challenges associated with reentry of deepwater wells may promote the use of 4-D technology in the deepwater GOM.