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Sampling Procedure Analytical Method? Analytical Result^ Summary Acknowledgements References Appendices Explanation Page 1 1 5 9 11 12 13 13 15 TABLES Table 1. Production wells sampled for this study. ILLUSTRATIONS Figure 1. Graph of the depth-temperature zone in which gas hydrates are stable in a permafrost region. 2. 3. 4. Map of the distribution of gas hydrate & free gas units in the Prudhoe Bay-Kuparuk River area Map of the production wells sampled for this study Drilling rig mud return and separation system diagram 4 7 8 APPENDICES I II m IV V VI vn vm IX X KRU2B10 KRU2D15 KRU3A9 KRU3H9 KRU3K9 MPUE4 PBUR1 PBUS26 PBUZ7 PBUZ8 17 23 32 41 43 52 56 74 78 82 ABSTRACT Gas hydrates, which are crystalline substances of water molecules that encase gas molecules, have the potential for being a significant source of natural gas. World-wide estimates for the amount of gas contained in hydrates range from 1.1 x 10^ to 2.7 x 10° trillion cubic feet. Gas hydrates exist in many Arctic regions, including the North Slope of Alaska. The two primary objectives of the U.S. Geological Survey Gas Hydrate Research Project are (1) to map the distribution of in-situ gas hydrates on the North Slope of Alaska, and (2) to evaluate the geologic parameters that control the distribution of these gas hydrates. To aid in this study, British Petroleum Exploration, ARCO Alaska, Exxon Company USA, and the Continental Oil Company allowed the U.S. Geological Survey to collect geochemical samples from drilling North Slope production wells. Molecular analysis of gaseous drill cutting and free-flowing gas samples from 10 production wells drilled in the Prudhoe Bay, Kuparuk River, and Milne Point oil fields indicates that methane is the primary hydrocarbon gas in the gas hydrate-bearing stratigraphic units. Isotopic data for several of these rock units indicate that the methane within the inferred gas hydrate occurences originated from both microbial and thermogenic processes.Gas hydrates, which are crystalline substances of water molecules that encase gas molecules, have the potential for being a significant source of natural gas. World-wide estimates for the amount of gas contained in hydrates range from 1.1 x 10^ to 2.7 x 10° trillion cubic feet. Gas hydrates exist in many Arctic regions, including the North Slope of Alaska. The two primary objectives of the U.S. Geological Survey Gas Hydrate Research Project are (1) to map the distribution of in-situ gas hydrates on the North Slope of Alaska, and (2) to evaluate the geologic parameters that control the distribution of these gas hydrates. To aid in this study, British Petroleum Exploration, ARCO Alaska, Exxon Company USA, and the Continental Oil Company allowed the U.S. Geological Survey to collect geochemical samples from drilling North Slope production wells. Molecular analysis of gaseous drill cutting and free-flowing gas samples from 10 production wells drilled in the Prudhoe Bay, Kuparuk River, and Milne Point oil fields indicates that methane is the primary hydrocarbon gas in the gas hydrate-bearing stratigraphic units. Isotopic data for several of these rock units indicate that the methane within the inferred gas hydrate occurences originated from both microbial and thermogenic processes. INTRODUCTION Gas hydrates are naturally occurring solids composed of rigid cages of water molecules that entrap gas molecules. While methane, propane, and other gases can be included in the clathrate structure, methane hydrates appear to be the most common in nature (Kvenvolden, 1988). At standard conditions (STP), one volume of saturated methane hydrate will contain as much as 164 volumes of methane gas (Davidson and others, 1978). Because of this large gas storage capacity, gas hydrates may represent an important commercial source of natural gas. Significant quantities of naturally occurring gas hydrates have been detected in many regions of the Arctic including Siberia (Makogon and others, 1972), the Mackenzie River Delta (Bily and Dick, 1974), and the North Slope of Alaska (Collett, 1983; Collett and others, 1988). Estimates of the amount of gas within the hydrates of the world are highly speculative and range over about three orders of magnitude, from 1.1 x 10^ to 2.7 x 1()8 trillion cubic feet (adapted from the Potential Gas Committee, 1981). The amount of gas in the hydrate reservoirs of the world greatly exceeds the volume of known conventional gas reserves. The production history of the Russian Messoyakha gas hydrate field demonstrates that gas hydrates are an immediate source of natural gas that can be produced by conventional methods (Makogon, 1981). Gas hydrates also represent a significant drilling and production hazard. Soviet, Canadian, and American researchers have described numerous problems associated with gas hydrates, including blowouts and casing failures (Bily and Dick, 1974; Franklin, 1980; Makogon, 1981). Recent studies have indicated that atmospheric methane, a potential greenhouse gas, is increasing at such a rate that the current concentrations («1.7 ppm) could double in the next 50 years (Rasmussen and Khalil, 1981). It has been suggested that destabilized gas hydrates may be contributing to this build-up in atmospheric methane. Because gas hydrates occur close to the earth's surface, they are easily affected by near-surface changes in pressure and temperature; thus, destabilized gas hydrates may be an important source of atmospheric methane (Kvenvolden, 1988). Gas hydrates exist under a limited range of temperature and pressure conditions. The depth and thickness of the zone of potential gas hydrate stability in permafrost regions can be calculated, if the geothermal gradient and gas chemistry are known (Bily and Dick, 1974). In figure 1, the methane hydrate stability curve and the depth to the base of the icebearing permafrost were used to determine the depth and thickness of the potential methane hydrate zone at the Northwest Eileen State-2 well on the North Slope of Alaska. In this example, the depth to the base of the ice-bearing permafrost, as determined from well logs, is «530 m (Collett and others, 1988). Temperature surveys from surrounding wells (Lachenbruch and others, 1982) suggest that the mean-annual surface temperature at the Eileen well is -11°C, and that the base of ice-bearing permafrost is at -1.0°C. These temperature data have been used to project a geothermal gradient that intersects the methane hydrate stability curve at 210 m and 950 m, delineating a 740-m zone in which methane hydrate would be stable. Results reported in Collett and others (1988) indicate that methane hydrates would be stable beneath most of the coastal plain province on northern Alaska, with thicknesses being greater than 1,000 m in the Prudhoe Bay area. Thermal conditions, however, preclude the occurrence of gas hydrates in the north-central part of the National Petroleum Reserve in Alaska, located =300 km west of Prudhoe Bay, and in the foothills east of the Umiat oil field, which is located «150 km southwest of Prudhoe Bay. Within the methane hydrate stability field on the North Slope, evidence for gas hydrates has been found in 50 wells by means of well log responses calibrated to the logs within the confirmed gas hydrate occurrences in the Northwest Eileen State-2 well (Collett, 1983; Collett and others, 1988). Most of the gas hydrates identified by well logs are

u.s. geological survey open file report/open-file report

Molecular and isotopic analyses of the hydrocarbon gases within gas hydrate-bearing rock units of the Prudhoe Bay-Kuparuk River area in northern Alaska