ORIGIN OF GASES IN PERMAFROST ASSOCIATED GAS HYDRATE - EXAMPLES FROM ALASKA AND CANADA

Thomas D. Lorenson¹, Timothy S. Collett², and Michael J. Whiticar^3
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 gas 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 gas hydrate deposits occur within clastic reservoirs where gas can be easily produced using existing technology. Conversely, production of marine gas hydrate deposits will be problematic and expensive. Known marine gas hydrate deposits occur in water depths generally >500 meters and little is known about how the gas hydrate occurs in the sediment matrix; a factor of great importance in production. Clearly gas hydrates from continental deposits represent the more technologically feasible target for potential development. Understanding the origin of the gas in gas hydrate may facilitate successful exploration for this potential resource.

In response to the need to assess the energy resource potential of gas 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 oil fields in northern Alaska. In Canada, during1998 and 2002, and an international consortium of industry, academia, and federal researchers drilled a series of gas hydrate research wells. The JAPEX/JNOC/GSC Mallik 2L, 3L, 4L, and 5L-38 gas 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 gas hydrates and complete the first-ever production tests of a known Arctic gas 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. Gas 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 oil fields.

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

The Eileen gas 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 gas occurs down-dip to the northeast, roughly at 700 m, where the sandstone units cross the Structure I gas 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. Gas hydrate occurs within a zone that contains mixed microbial and thermogenic gas. The thermogenic gas is thought to have migrated updip and along faults from underlying oil and gas reservoirs.

In the Tarn/Cirque oil field, 50 km to the southwest, gas 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 gas 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 gas 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 gas has undergone microbial oxidation of methane and higher hydrocarbons. This gas may have migrated up-dip from a large accumulation of biodegraded oil in the West Sak and Ugnu sandstones. This pool of biodegraded oil underlies the Eileen gas 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 oil methane because it is located directly over the West Sak and Ugnu sands biodegraded oil reservoir both of which are cut by the Eileen fault, which is known to act as a conduit for gas. The Tarn/Cirque gas hydrate accumulation has a greater component of microbial/biodegraded oil methane than does the Eileen gas hydrate accumulation.

Canada

Gas 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 gas hydrates indicate lithologic control of their occurrence. Gas hydrate layers occur in coarse-grained sand dominated facies separated by thin non-hydrate-bearing, fine-grained siltstone and claystone facies.

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

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

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