Site Summary – Alaminos Canyon block 21 and East Breaks block 992
The final site tested by the Gulf of Mexico Gas Hydrates JIP during Leg II lies in roughly 4900 ft of water within the East Breaks (EB) 992 and Alaminos Canyon (AC) 21 protraction areas. This area is known as the “Diana” sub-basin, and contains several prolific oil and gas fields. The targets for JIP Leg II are shallow sand packages that occur approximately 600 feet below the seafloor and 800 feet above the inferred base of gas hydrate stability. The sands are distributed widely across AC 21 and adjoining areas, and also occur as more isolated units in blocks EB992 and AC24. A small isolated sand body within the target interval was penetrated by the 1995 ExxonMobil “Rockefeller” well (EB992 #001). Log data from that well indicated a sand ~130 ft thick with elevated, but low (~2 ohm-m) resistivity. If that data were correct (and the validity of shallow log data taken in boreholes designed to reach deeper targets are always somewhat suspect), the unit would be expected to contain a low saturation of gas hydrate (~30% Sgh).
Regional seismic data showed the sand to have a highly irregular top and relatively flat base with small, local incisement into subjacent units. The seismic data also suggests that the unit transmits sound waves at significant greater speed than the surrounding shales, consistent with, but not conclusive of, the presence of gas hydrate. Throughout the area, the geophysical expression of the interval is relatively consistent, with few features that reveal the sources or migration pathways for gas. The sediments are also very young (< 0.2 million years), being late Pleistocene in age.
West-to-east seismic section through the EB992 area. Red line denotes location of an 1995 exploration well that encountered a 130’-thick resistive sand. The top and base of that sand, and the correlation of that sand to the seismic anomalies are noted. Data courtesy WesternGeco.
Two wells, the AC21-A and AC21-B, were drilled into this prospective shallow sand facies on March 3 and 4, 2009. The B well targeted a relatively thick occurrence that was seismically typical of the unit’s appearance throughout the sub-basin. This well logged a single sand body 125 ft thick with resistivity remarkably consistent at 2 ohm-m, slightly more resistive than the bounding shales (1.5 ohm-m). The AC21-A location featured a more anomalous high-amplitude geophysical response. Data from this well showed two sands, separated by a 15 foot thick shale, both with consistent resistivity of 2 ohm-m.
Initial interpretation of the AC21/EB992 drilling results is that the sands appear to exhibit uniformly low gas hydrate saturation over a large area. Several industry wells through the same sand interval in the Diana basin show the resistivity of water-bearing sands to be an order of magnitude less than that found in the JIP wells. Further analysis with the higher resolution resistivity, processed sonic, and other measurements recorded in the LWD tool’s memory, will assist in the further evaluation of this reservoir. Additional information on formation mineralogy, bedding characteristics, and pore water geochemistry will also be needed. It is anticipated that further analysis of these very young (late Pleistocene) East Breaks sands can provide important insights into the early development of gas hydrate systems located far above the base of gas hydrate stability.
The JIP permitted additional sites in EB992 to evaluate this target. However, ongoing industry operations in neighboring AC24 block made operations at the most favorable location not possible at this time. Other sites were made available, and the JIP is truly grateful to ExxonMobil for their willingness to accommodate our effort. However, given the remarkably consistent expression of the unit in both the regional seismic data and in well data, the science team found no rationale to expect any significantly new insights into the accumulation by further drilling. Therefore the drilling phase of JIP Leg II was completed at midnight on May 4, 2009.
This 6.75" PDC (polycrystalline diamond compact) bit drilled all 15,700 feet of section examined during JIP Leg II.