Diagenetic Methane Hydrate Formation in Permafrost: A New Gas Play?

Abstract High-grade permafrost hydrate concentrations in shallow geological traps constitute primary hydrate natural gas exploration objectives. The hydrate in these deposits can be compared with authigenic mineral deposits because the concentrated gas was largely already in the geological traps before being converted to hydrate. These natural gas hydrate deposits have been identified in Russia, Canada, and Alaska using conventional hydrocarbon exploration techniques. In a very different permafrost hydrate paragenesis, diagenetic hydrate deposits have now been identified in veinworks in the shallow permafrost terrain of the Qinghai-Tibet plateau in western China. These are superficially similar to some vein-type oceanic hydrate deposits. They were formed when natural gas was introduced in fractures within and associated with frozen ground in permafrost terrane and are very different from conventional gas traps. To date, hydrate exploration in permafrost terrane has used a petroleum system model. But new system models, geophysical analysis techniques, and production models are required for diagenetic hydrate deposits in which hydrate concentrates natural gas in secondary porosity. The identification of diagenetic permafrost hydrate in China raises a new economic geology proposition. It can be used to suggest that the Arctic permafrost region in Eurasia and North America may also have similar diagenetic deposits that have gone unrecognized. A new type of permafrost hydrate formation mechanism would have the effect of expanding the likelihood of permafrost methane hydrate economic potential. Introduction Natural gas hydrate is a crystalline material composed of water molecules that form cage structures that are occupied by gas molecules, the whole being stabilized by van der Waals bonding. Methane is the dominant gas found in naturally occurring hydrates on Earth, and most of this methane appears to be characterized by a light isotopic fraction consistent with biogenetic origin. Heavier density hydrocarbon gases found commonly where thermogenic gas sources exist are the next most common gases in naturally occurring hydrate. Other gases, such as H2S, N2, and CO2, are found as traces in hydrate. Because the cage structures are close together in the crystalline structure, the gas is compressed about 164 times by volume, at STP. Concentrated deposits of natural gas hydrate constitute a potential unconventional source of natural gas. Natural gas hydrate occurs on earth in two different environments (Fig. 1), within permafrost terrains and in oceanic marine sediments on deep continental shelves and margins (Max et al., 2003). Although both permafrost and oceanic natural gas hydrates were initially scientific curiosities, the volume of gas contained in hydrate was quickly recognized as being of gigantic proportions. Hydrate is now the subject of exploration and assessment as a potential economic natural gas resource. Hydrate deposits identified to date in Arctic permafrost, initially in Siberian gas fields in thick permafrost terrain (Makogan, 1972) and then in Canada and Alaska, appear to have been formed from preexisting conventional gas deposits that were already concentrated in geological traps. The majority of this hydrate is generally regarded as having been formed from the in-place gas as permafrost deepened during intensification of the most recent glacial episode (Max et al., 2006) that reached its maximum a little over 20,000 years ago. These are essentially authigenic hydrate mineral deposits in that the gas was already large present in its geological trap before being converted to hydrate. Recognition of these permafrost hydrate deposits has closely followed a conventional petroleum system model. How much of the hydrate may actually have formed during previous glacial maxima (Winograd et al., 1997) is uncertain but the persistence of permafrost hydrate deposits to the present, well into our interglacial period suggests that some of the permafrost hydrate may have survived from one glacial maximum to the next through the Pleistocene, along with some ice in the permafrost.

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