PROSPECTING FOR GAS HYDRATE ACCUMULATIONS USING 2D AND 3D SEISMIC DATA, MILNE POINT, NORTH SLOPE ALASKA

Tanya L. Inks¹, Timothy S. Collett², David J. Taylor², Warren F. Agena², Myung W. Lee^2
1 IS Interpretation Services, Inc., Denver, Colorado
² U.S.G.S, Denver, Colorado

In September 2003 a study was initiated, using 3-D seismic in the Milne Point area of northern Alaska, in support of a proposed pilot drilling program that will help answer questions about gas-hydrate reservoir properties, possible production methods, and economics. Historical log correlation work and analysis of gas hydrates in the Milne Point area (Collett, et al., 1993, 2001) was used as a starting point for a seismic driven analysis of the Milne Point 3-D survey area. Modern 3-D and 2-D seismic data are being used to gain a better understanding of the geologic controls related to gas hydrate petroleum systems in the Milne Point area. The Landmark software suite was used to integrate and analyze detailed log correlations, specially processed log data, gas-hydrate composition information and specialized 3-D seismic volumes. The seismic data was also used to analyze reservoir fluid properties, in both stack and offset domains, in comparison to theoretical modeling results by Lee (2004). The primary result of the study has been the development of viable intra-hydrate stability zone prospects and sub-hydrate free gas prospects. For the purposes of this paper, “intra-hydrate stability zone” prospects are defined as those prospects that are below the base of ice bearing permafrost (IBPF) and above the base of the gas hydrate stability zone. “Sub-hydrate free gas” prospects are defined as free gas reservoirs trapped at the base of the gas hydrate stability zone by the gas hydrate framework.

The study focused on the Milne Point 3-D seismic survey, provided to the USGS by BP Exploration Alaska, Inc. Regional 2-D seismic data, licensed by the USGS, supplemented the 3-D seismic data and was used along with well data to constrain and improve the quality of critical maps, such as time structure maps, fault maps and base hydrate stability zone maps, in the Milne Point area. These data were also used to improve the quality of the velocity model used to depth convert the Milne Point 3-D seismic data.

The initial interpretation of the structural framework in the Milne Point 3-D seismic survey area shows that faulting may play a significant role in the migration and trapping of the gas associated with the gas-hydrate accumulations. North Slope gas-hydrates are known to be composed of mostly methane gas from deeply buried thermogenic sources; thus, a detailed fault interpretation is critical to understanding how faults act as gas conduits for shallow gas-hydrate accumulations. The age relationship between various fault sets may play a significant role in determining migration pathways and the compartmentalization of these gas-hydrate reservoirs. Fault analyses on a 3-D seismic volume enhanced by ESP (coherency) processing show that the fault orientation above and below the Canning Formation is distinctly different, and, as such, the secondary and tertiary migration from Ellesmarian reservoirs are more complex than originally thought. Clearly, some faults are not connected through the Canning Formation to deeper faulting.

Theoretical seismic modeling of boundaries between ice-bearing permafrost to gas hydrate reservoirs, shale to gas hydrate reservoirs, and shale to free gas reservoirs, as well as transitional gas hydrate to free gas reservoirs at the base of the gas hydrate stability zone have been used to understand the acoustic properties of these complex systems in the pre and post stack domain. The similarity in acoustic properties between ice and gas-hydrate makes it difficult to differentiate between ice- and gas-hydrate-bearing sediments. Therefore, prospective gas hydrate reservoirs adjacent to permafrost are difficult to quantify. In the Milne Point 3-D area, some assumptions can be made to constrain modeled results describing the relationship of these boundaries in the stack and offset domains. First, we can assume that thermogenic gasses migrated into what are now gas hydrate reservoirs, allowing us to assume a gas hydrate concentration in sandstone reservoir rock of about 80-85% – similar to conventional gas reservoirs. Second, unconsolidated sandstone reservoirs in the Sagavanirktok Formation that contain the majority of Milne Point gas hydrates typically have 30-40% porosity. This leaves reservoir thickness as the main variable used in modeling acoustic attributes and in calculating volumetrics.

The focus of the project centered on the interval below the base of ice bearing permafrost (IBPF) to just below the base of the hydrate stability zone. The base of the gas hydrate stability zone was computed using well log derived IBPF depths and high-resolution borehole temperature surveys. A pair of horizons representing the upper and lower limits of the base gas hydrate stability zone were mapped and displayed on the seismic data. (Error was considered to be plus or minus 75 ft., or plus or minus 15 ms.)

Intra-hydrate stability zone reservoirs have acoustic properties allowing them to be identified by several simple seismic attributes. Additionally, these gas hydrate reservoirs can theoretically be differentiated in the offset domain from free gas in the same reservoir. Free gas trapped in a sub-gas hydrate stability zone prospect can be easily identified by seismic attributes in this geologic setting. It can be shown that the seismic amplitude anomalies are associated with free gas at the base of the gas hydrate stability field, and are connected to up-dip gas hydrate reservoirs. In some cases, no distinct amplitude anomalies attributed to gas hydrates above the free-gas to gas-hydrate boundary have been identified, even though convention would indicate that gas-hydrates must be present to form the trap. One hypothesis would be that there were changes in migration pathways and the rate of migration during the formation of the gas hydrate stability zone. This may have allowed separation between sub-hydrate gas and areas of concentrated intra-hydrate stability zone gas. The recent movement along younger faults in the post-Canning interval likely influenced migration pathways, and may effect the location of sub-hydrate free gas accumulations. These free gas accumulations are also prospective as exploration targets.

From the analysis of the seismic data, several intra-gas-hydrate stability zone prospects have been identified in the Milne Point 3-D survey area. Intra-gas-hydrate prospects are typically fault bounded and are identified primarily by their acoustic properties. As a rule, areas that are currently structurally high within prospective fault blocks can be shown to have acoustic properties that correspond to high concentrations of gas-hydrate. This structural relationship is similar to conventional gas prospects, pointing back to the free-gas origin of these gas hydrates. It is also clear that some of these fault blocks do not appear to be “fully charged,” as there are down-dip limits to the mapped acoustic anomalies. Several of these intra-hydrate prospects appear to be candidates for gas-hydrate production testing, due to their proximity to existing roads and infrastructure.

Free-gas prospects that are associated with the base of the gas-hydrate stability zone represent the second type of target for our prospecting effort. Our work shows that there is a predictable relationship between the base of the gas hydrate stability zone and amplitude anomalies thought to represent free gas. Initially, the BP Cascade-1 well in the far southeast portion of the 3-D survey was thought to show an example of such a sub-hydrate trapped free-gas column. The well has a 300-ft-thick free-gas accumulation in an excellent reservoir interval just above the zone mapped as the Staines Tongue of the Sagavanirktok Formation. However, due to structural complexity, the gas column in this well is as likely trapped by one of the numerous mapped faults as by gas hydrates at the base of the gas hydrate stability zone. (The trapping mechanism is ambiguous.) However, northwest of the Cascade block the same reservoir interval displays high amplitudes typical of a gas charged reservoir directly below the interface between the gas hydrate stability zone. Along this fairway, free gas appears to be trapped at the base of the hydrate stability zone in some cases, and by faulting elsewhere. In the main Milne Point Field area, the seismic amplitudes lessen dramatically above the base of the gas-hydrate stability zone. This sedimentary section could contain numerous prospective blocks made of up thick reservoirs that potentially host both gas hydrate and conventional free-gas accumulations.

The Milne Point area study has been successful in that we have been able to find both intra-gas hydrate and sub-gas hydrate free-gas prospects that are appropriate for the proposed production test well drilling. The historical log analysis work conducted by the USGS in this area, combined with knowledge gained from 3-D seismic attribute analysis, has helped us to understand the geologic setting for these unconventional reservoirs. Future work should verify the assumptions used in the theoretical versus real-world modeling that was necessary to make an evaluation of the proposed prospects.

References:

Collett T.S., 1993, Natural gas hydrates of the Prudhoe Bay and Kuparuk River area, North Slope, Alaska: AAPG Bulletin, v77, p. 793-812

Collett, T.S., 2001, Natural-gas hydrates: Resource of the Twenty-First Century?, in M.W. Downey, J.C. Threet and W.A. Morgan, eds., Petroleum provinces of the twenty-first century: AAPG Memoir 74, p. 85-108.

Collett, T.S., Subsurface temperatures and geothermal gradients on the North Slope of Alaska, in Cold Regions Science and Technologies, 21 (1993) Elsevier Science Publishers B.V., Amsterdam, p. 275-293.

Lee, M.W., in press, Well Log Analysis to Assist the Interpretation of 3-D Seismic Data at the Milne Point, North Slope of Alaska, U.S.G.S. bulletin.

Taylor, D.J., et al., 2003, Imaging Gas-Hydrate Bearing Zones Using 3-D Seismic Data- Milne Point, North Slope, Alaska: abstract 2003 3-D Symposium, Denver, Colorado.