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ORIGIN OF GASES IN PERMAFROST ASSOCIATED Previous HitGASNext Hit HYDRATE - EXAMPLES FROM ALASKA AND CANADA

Thomas D. Lorenson1, Timothy S. Collett2, and Michael J. Whiticar3
 1 U.S. Geological Survey, Menlo Park, CA, USA, 94025
2 U.S. Geological Survey, Denver, CO, USA
3 University of Victoria, Victoria, BC, Canada

Notable permafrost-associated continental Previous HitgasNext Hit hydrate deposits are located in areas that already have petroleum exploration and development infrastructure, including the North Slope of Alaska, the Mackenzie Delta of Canada, and the West Siberian Basin of Russia. These continental Previous HitgasNext Hit hydrate deposits occur within clastic reservoirs where Previous HitgasNext Hit can be easily produced using existing technology. Conversely, production of marine Previous HitgasNext Hit hydrate deposits will be problematic and expensive. Known marine Previous HitgasNext Hit hydrate deposits occur in water depths generally >500 meters and little is known about how the Previous HitgasNext Hit hydrate occurs in the sediment matrix; a factor of great importance in production. Clearly Previous HitgasNext Hit hydrates from continental deposits represent the more technologically feasible target for potential development. Understanding the origin of the Previous HitgasNext Hit in Previous HitgasNext Hit hydrate may facilitate successful exploration for this potential resource.

In response to the need to assess the energy resource potential of Previous HitgasNext Hit hydrate in northern Alaska, the USGS, the Bureau of Land Management (BLM), the U.S. Department of Energy (USDOE) and industry, has been collecting geochemical samples from wells since 1984 for an ongoing research program in the Prudhoe Bay, Kaparuk River, Milne Point, and Alpine/Tarn Previous HitoilNext Hit fields in northern Alaska. In Canada, during1998 and 2002, and an international consortium of industry, academia, and federal researchers drilled a series of Previous HitgasNext Hit hydrate research wells. The JAPEX/JNOC/GSC Mallik 2L, 3L, 4L, and 5L-38 Previous HitgasNext Hit hydrate research wells were drilled to a depth of 1165 m on top of a broad antiform located in the Mackenzie Delta. The objectives of the research program were to study the geology, geochemistry, geophysics, and engineering properties of Previous HitgasNext Hit hydrates and complete the first-ever production tests of a known Arctic Previous HitgasNext Hit hydrate accumulation.

Alaska

Four stratigraphic sequences, the Franklinian, Ellesmerian, Beaufortian, and the Brookian occur in the Prudhoe Bay-Kuparuk River area of the North Slope, Alaska. The Brookian foreland basin and passive margin rocks are Cretaceous to Holocene in age and originated during the uplift of Brooks Range to the south. Previous HitGasNext Hit hydrate deposits are restricted to the up-dip portion of a series of nearshore deltaic sandstone reservoirs in the lower Tertiary Mikkelsen Tongue of the Canning Formation that overlie the more deeply buried Prudhoe Bay and Kuparuk River Previous HitoilNext Hit fields.

The Eileen and Tarn Previous HitgasNext Hit hydrates are thought to contain a mixture of deep-source thermogenic Previous HitgasNext Hit, and shallow, microbial Previous HitgasNext Hit that was either directly converted to Previous HitgasNext Hit hydrate or was first concentrated in existing conventional traps and later converted to Previous HitgasNext Hit hydrate in response to climate cooling or changes in surface conditions. The microbial gases likely have a biodegraded Previous HitoilNext Hit Previous HitgasNext Hit source contribution. The distribution of the Eileen and Tarn Previous HitgasNext Hit hydrate accumulations appear to be controlled in part by the presence of large scale regional faults that may have acted as vertical Previous HitgasNext Hit migration conduits.

The Eileen Previous HitgasNext Hit hydrates occur in six (Units A-F) laterally continuous Tertiary sandstone units, with individual occurrences in the range of 3 - 31 m thick. A significant accumulation of free hydrocarbon Previous HitgasNext Hit occurs down-dip to the northeast, roughly at 700 m, where the sandstone units cross the Structure I Previous HitgasNext Hit hydrate stability zone. Molecular and isotopic compositions of gases from wells can distinguish an upper microbial-sourced methane zone within and below the permafrost, down to maximum depths of about 150 - 750 m. Mixed microbial and thermogenic gases occur below this horizon. The maximum depth of the microbially-sourced methane becomes deeper down-dip. Geologic control of the deepest limit of significant microbial methane concentration coincides with the Eocene unconformity between the Mikkelsen Tongue and the overlying non-marine Sagavanirktok Formation. Previous HitGasNext Hit hydrate occurs within a zone that contains mixed microbial and thermogenic Previous HitgasNext Hit. The thermogenic Previous HitgasNext Hit is thought to have migrated updip and along faults from underlying Previous HitoilNext Hit and Previous HitgasNext Hit reservoirs.

In the Tarn/Cirque Previous HitoilNext Hit field, 50 km to the southwest, Previous HitgasNext Hit hydrate occurs as a 60-70 m thick zone within the Tertiary Ugnu and West Sak sandstones at about 230 to 300 m depth. The base of methane hydrate stability is about 530 m. Two wells in this area include the Atlas 1 and the Tarn 2N-305. At the Tarn 2N-305 well, a microbial methane zone extends to just above the top of the Previous HitgasNext Hit hydrate at about 270 m deep. Below this depth, the carbon isotopic composition of the methane remains between -50 and -46‰, indicating a mixed microbial and thermogenic methane source. A different pattern is seen at the Altas 1 well, about 10 km to the southwest. Although microbial methane is present, it is likely mixed with thermal sources everywhere below 50 m to about 740 m, where sampling stopped. The carbon isotopic composition of the methane within the Previous HitgasNext Hit hydrate zone ranges from -54 to -50‰ indicating mixed microbial and thermogenic methane.

Carbon isotopic measurements of mixed microbial and thermal ethane (-51 to -35‰) and carbon dioxide (-29 to -12&permil), indicate that some Previous HitgasNext Hit has undergone microbial oxidation of methane and higher hydrocarbons. This Previous HitgasNext Hit may have migrated up-dip from a large accumulation of biodegraded Previous HitoilNext Hit in the West Sak and Ugnu sandstones. This pool of biodegraded Previous HitoilNext Hit underlies the Eileen Previous HitgasNext Hit hydrate deposits and is about 50 to 100 km the northeast of the sampled Tarn wells. The Eileen accumulation may have a component of the same biodegraded Previous HitoilNext Hit methane because it is located directly over the West Sak and Ugnu sands biodegraded Previous HitoilNext Hit reservoir both of which are cut by the Eileen fault, which is known to act as a conduit for Previous HitgasNext Hit. The Tarn/Cirque Previous HitgasNext Hit hydrate accumulation has a greater component of microbial/biodegraded Previous HitoilNext Hit methane than does the Eileen Previous HitgasNext Hit hydrate accumulation.

Canada

Previous HitGasNext Hit hydrates in the Mackenzie Delta region occur exclusively within the Kugmallit, Mackenzie Bay, and Iperk sequences. The Kugmallit Sequence is a delta plain deposit composed mainly of unconsolidated to weakly cemented sand, minor clay, and rare lignite. The Mackenzie Bay Sequence is present mainly offshore and is composed of weakly cemented mudstone and siltstone. The Iperk Sequence is composed mainly of unconsolidated coarse-grained (sand-dominated) clastic sediments of varied depositional origin. At Mallik, the layered or interbedded character of the Previous HitgasNext Hit hydrates indicate lithologic control of their occurrence. Previous HitGasNext Hit hydrate layers occur in coarse-grained sand dominated facies separated by thin non-hydrate-bearing, fine-grained siltstone and claystone facies.

Thermogenic Previous HitgasNext Hit likely migrates up along listric-normal growth faults from depths of at least 5000 m. The Previous HitgasNext Hit is subsequently trapped along the crest of rollover anticlines and tilted fault blocks, and when within the Previous HitgasNext Hit hydrate stability field, forms Previous HitgasNext Hit hydrate. Further Previous HitgasNext Hit migration is impeded by the formation of Previous HitgasNext Hit hydrate and the presence of ice-bonded permafrost.

Two general types of Previous HitgasNext Hit observed in the Mallik wells. Microbial Previous HitgasNext Hit is characterized by high C1/(C2+C3) ratios (>1000) and methane carbon isotopic ratios between -70 to -93‰. Thermogenic Previous HitgasNext Hit is wetter and has carbon isotopic ratios for methane of around -35 to -45‰. The carbon isotopic ratios for ethane and propane of this thermogenic Previous HitgasNext Hit are -31‰ and -26‰ respectively. Methane isotopic compositions of 12 Previous HitgasNext Hit hydrate samples averaged -42.7‰ and clearly indicate a thermogenic source. The Mallik Previous HitgasNext Hit compositional trends resemble Previous HitgasNext Hit produced from the nearby Taglu Previous HitgasNext Hit field where there are multiple sandstone reservoirs below about 2,700 m. The Previous HitgasNext Hit within the Taglu field is associated with nonbiodegraded Previous HitoilNext Hit.

Previous HitGasNext Hit hydrate occurrences in Arctic Alaska and Canada share a significant thermogenic input of methane. In both cases thermal gases come from either existing Previous HitoilNext Hit and Previous HitgasNext Hit accumulations or from source rock within the Previous HitoilNext Hit and Previous HitgasNext Hit generating window that have migrated up-dip and or up-fault and formed Previous HitgasNext Hit hydrate. The Alaska Previous HitgasNext Hit hydrate accumulations have a greater microbial methane input that likely, in part, originates from gases associated with microbial hydrocarbon oxidation of Previous HitoilNext Hit.