The costly well designs and delays caused by the presence of overpressured shallow sand zones or "flowing water sands" in the deep water Gulf of Mexico has focused attention on the need for better detection and mapping of these deep water hazards. Neither exploration 3-D nor conventional high resolution 2-D were proving adequate for the complex areas. Short Offset High Resolution 3-D was introduced to combine the spatial benefits of exploration 3-D with the resolution of a conventional 2-D high resolution survey. This paper discusses the use of short offset high-resolution 3-D data for the mapping of deep water hazards with a focus on flowing water sands.
Typical hazards encountered the deep water Gulf of Mexico include:
Faults and Buried Channels
Pressurized water sands
Of these, Geopressured shallow water flows have been the most difficult to map. They have been primarily encountered on the continental slope at 500 to 2000 feet below the mudline in water depths ranging from 1,000 to 7,000 feet.
Understanding the creation process of h s hazard provides the key for mapping the potential for flowing sands One source of this over pressure is under compaction due to rapid burial. This increases the pore pressure of underlying porous sediments. If these porous sediments are contained within a seal (typically shale or clay with very low permeability), the overburden will transmit pressure to this sand faster than the seal allows fluids to escape. In a second mechanism, the pressures can be transmitted laterally through silty shale/mudstone into a high permeable area with flow potential. Thus the area of rapid buildup of overburden may be offset from the flowing sand.
The goal is to identify those areas with seals and with rapid deposition above as having a high potential as a flowing sand. Exploration 3-D conventional data, reprocessed 3-D data, 2-D high resolution data and 3-D conventional used in conjunction with 2-D high resolution data have all been used to map areas with flow potential. 3-D conventional data with its maximum frequency of 60 Hz is limited in resolution and in its ability to represent areas of steep dip due to the large trace spacing. Reprocessed 3-D may increase the frequency content up to 100 Hz and thus improves the vertical resolution. 2-D high resolution data provides resolution at frequencies above 300 Hz and with its tighter trace spacing provides better imaging of steep dips. However it is subject to out of plane artifacts and provides no information between lines which are spaced 50 to 300 apart.
If control information is available from nearby wells or bore holes, the seismic data can be used to map flowing sands and seals from these offset wells back to the target area.
In complex areas, two solutions have been used. The first is to supplement Exploration 3-D (including reprocessed) with high resolution 2-D. The second is to collect high resolution 3-D directly. Reprocessed exploration 3-D is usually processed short offset