“Clathrates” generally refer to non-stoichiometric metastable substances in which lattice structures composed of first molecular components (host molecules) trap or encage one or more other molecular components (guest molecules) in what resembles a crystal-like structure. Clathrates are sometimes referred to as inclusion compounds, hydrates, gas hydrates, methane hydrates, natural gas hydrates, CO2 hydrates and the like. Example properties of such clathrates are described, for example, in Sloan, E. D., 2008, Clathrate Hydrates of Natural Gases, 3rd Edition, Taylor & Francis, as well as in Daigle, H., Dugan, B., 2011, Capillary controls on methane hydrate distribution and fracturing in advective systems, Geochem. Geophys. Geosyst., V. 12, No. 1.
In the field of hydrocarbon exploration and development, clathrates are of particular interest. For example, clathrates exist in which water host molecule lattices encage one or more types of hydrocarbon guest molecule(s). Such hydrocarbon-capturing clathrates occur naturally in environments of relatively low temperature and high pressure where water and hydrocarbon molecules are present, such as in deepwater and permafrost sediments. Clathrates at lower temperatures remain stable at lower pressures, and conversely clathrates at higher temperatures require higher pressures to remain stable. Generally, and as noted in Sloan, above, clathrate formation is a complex dynamic process that occurs in specific geologic and pressure/temperature (P/T) conditions over geologic time.
Hydrocarbons and other gases trapped in clathrates are of biogenic and/or thermogenic origin. Generation of thermogenic and biogenic gases is described in numerous articles and textbooks. Examples of such literature include: Rice, D. D., Claypool, G. E., Generation, Accumulation, and Resource Potential of Biogenic Gas, AAPG Bulletin, January 1981, v. 65, p. 5-25; Fjellanger, E., et al., Charging the giant gas fields of the NW Siberia basin, The Geological Society of London, Petroleum Geology Conference series, 2010, v. 7, p659-668; and Hantschel, Th., Kauerauf, A., I., Fundamental of Basin and petroleum Systems Modeling, Springer Verlag Berlin Heidelberg, 2009, p. 151-340. Clathrates that have a biogenic isotopic signature are formed from gases that migrate over relatively short distances into a zone where temperature and pressure conditions support formation of clathrates, referred to as a clathrate stability zone (“CSZ”). Clathrates that have a thermogenic isotopic signature are formed from thermogenically generated gases that typically migrate upwards into the CSZ from mature source rocks over geologic time. Mixed origin clathrates contain isotopic signatures of both biogenic and thermogenic gases.
In addition to hydrocarbon gases, clathrates may encapsulate non-hydrocarbon gases such as CO2 and H2S. CO2, H2S formation in subsurface locations is described in further detail in a number of publications, for example in Fleet, A. J., et al., 1998, Large volumes of CO2 in sedimentary basins, Goldschmidt Conference Toulouse 1998, Mineralogical Magazine, V.62A, p. 460-461.
In general, clathrates are formed under poor to moderate seals in shallow sediments. Furthermore, once formed, clathrates serve as an additional seal that traps free hydrocarbons, thereby preventing additional free hydrocarbons from rising through the formed clathrates. This may either promote further clathrate formation, or may trap free gas at or below the CSZ, or may cause the free hydrocarbon gases to relocate in order to rise through the permeable portion of CSZ. Furthermore, gases trapped in clathrates are freed once the sealing location is buried deeper, thereby leaving the CSZ due to increased temperature and/or pressure. These released gases may again migrate to the surface and either (1) be lost or (2) contribute to new hydrates being formed at shallower locations within the CSZ. As such, the presence and distribution of clathrates, and in particular clathrates that encapsulate hydrocarbons, is dynamic over time, as changes to locations of clathrates, free gas, and CSZs occur.
Typical analysis of clathrates focuses on present day clathrate stability zones (e.g., as described in Sloan, above), which correspond to the current subsurface locations where temperature and pressure conditions would support clathrate formation. However, such analysis has drawbacks. For example, relying on current temperature and pressure conditions ignores the above-described dynamic aspect of hydrocarbon generation and charge, and the formation and destruction of clathrates as a function of changing PVT (pressure/volume/temperature) conditions due to geologic changes, such as burial or uplift. This may lead to mis-estimation of the type, location, and saturation of various hydrocarbon or non-hydrocarbon gases in the CSZ. Inaccurate estimation of the types and locations of hydrocarbons trapped in clathrates can result in an incomplete analysis and failure to identify economically attractive hydrocarbon-rich clathrate deposits. It could also result in attempted harvesting of clathrates from locations that appear to have high hydrocarbon concentrations, but in fact contain clathrates that encapsulate non-hydrocarbon gases, such as CO2 or H2S. This can result in selection of locations for clathrate harvesting that are at best unproductive, and at worst dangerous.
Accordingly, improvements in such existing analyses are desired.