U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY MOLECULAR AND ISOTOPIC ANALYSES OF THE HYDROCARBON GASES WITHIN GAS HYDRATE-BEARING ROCK UNITS OF THE PRUDHOE BAY-KUPARUK RIVER AREA IN NORTHERN ALASKA Open-File Report 92-299 by Zenon C. Valin and Timothy S. Collett This work was funded by the U.S. Geological Survey Global Change and Climate History Program and the U.S. Department of Energy (Agreement No. DE-AI21-83MC20422). This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Zenon C. Valin U.S. Geological Survey 345 Middlefield Road Menlo Park, California Timothy S. Collett U.S. Geological Survey Federal Center, Box 25046 Denver, Colorado 80225 CONTENTS Page Abstract 1 Introduction 1 Sampling Procedure 5 Analytical Methods 9 Analytical Results 11 Summary 12 Acknowledgements 13 References 13 Appendices Explanation 15 TABLES Table 1. Production wells sampled for this study 6 ILLUSTRATIONS Figure 1. Graph of the depth-temperature zone in which gas hydrates are stable in a permafrost region 3 2. Map of the distribution of gas hydrate & free gas units in the Prudhoe Bay-Kuparuk River area 4 3. Map of the production wells sampled for this study 7 4. Drilling rig mud return and separation system diagram 8 APPENDICES I KRU2B10 17 II KRU2D15 23 III KRU3A9 32 IV KRU3H9 41 V KRU3K9 43 VI MPUE4 52 VII PBUR VIII PBUS26 74 IX PBUZ7 78 X PBUZ8 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 105 to 2.7 x 108 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 105 to 2.7 x 108 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 (about 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 ice- bearing 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 Å530 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 degrees C, and that the base of ice-bearing permafrost is at -1.0 degrees 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 Å300 km west of Prudhoe Bay, and in the foothills east of the Umiat oil field, which is located about 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 geographically restricted to the eastern part of the Kuparuk River Production Unit and the western part of the Prudhoe Bay Production Unit (figure 2). Stratigraphically below the mapped gas hydrate occurrences are several super-giant oil fields that contain enormous quantities of gas in solution or as a gas cap. These accumulations include the Prudhoe Bay and Kuparuk River oil fields, and the heavy-oil and tar deposits within the West Sak and Ugnu sands (Werner, 1987). West Sak and Ugnu are informal terms used by ARCO to identify a series of shallow oil-bearing sandstone reservoirs in the Kuparuk River area. Oil-source-rock correlations indicate that the oil in the entire Prudhoe Bay area came from a common source (Seifert and others, 1979); however, little is known about the history of oil or gas migration. The objective of this study was to document the molecular and isotopic composition of the gas trapped within the gas hydrate-bearing stratigraphic intervals overlying the Prudhoe Bay and Kuparuk River oil fields. To reach this objective, we have analyzed cuttings gas and free gas samples collected from 10 drilling-production wells in the Prudhoe Bay and Kuparuk River fields. A description of the sampling procedures and analytical results follow. SAMPLING PROCEDURE For this study, drill cuttings and free gas samples were obtained from 10 wells drilled in the Prudhoe Bay, Kuparuk River, and Milne Point oil fields (table 1, figure 3). The drill cuttings were collected from the shaker table (figure 4) and placed in either quart or pint size metal cans. Water was added to submerge the cuttings, leaving about 1.5 cm air-space or headspace at the top of the can. In most cases a bactericide (Zephiran Chloride or Sodium Azide) was added to the water to prevent biological activity in the sample. The can was sealed with a metal lid. The free gas samples were collected from the mud logger's gas trap, which is positioned within the mud pit (figure 4) at the back of the shaker table. The gas trap consists of a pipe (15 cm in diameter) which is sealed at one end and inverted in the opossum belly. At the top of the trap is an agitator which separates the formation gas from the drilling muds. Both the shaker table and gas trap are open to the atmosphere. Most of the free gas samples were collected in 400 ml glass bottles by a simple displacement method, which consisted of using a small vacuum pump to withdraw the gas sample from the mud logger's gas trap and inject the sample into a glass bottle that was filled with water and inverted in a water bath. ANALYTICAL METHODS Two U.S. Geological Survey laboratories (Branch of Petroleum Geology laboratories, Denver, Colorado and Branch of Pacific Marine Geology laboratories, Palo Alto, California) and two contract laboratories (Geochem Research Incorporated, Houston, Texas and Global Geochemistry Corporation, Canoga Park, California) have been used to analyze the samples collected from the 10 wells in this study. Cooperative research efforts also permitted the West German Federal Institute for Geosciences and Natural Resources (BGR) to analyze samples from the Kuparuk River Unit 2B-10 and Kuparuk River Unit 2D-15 wells. A description of the analytical methods used in each of these laboratories follows. USGS Branch of Petroleum Geology The canned drill cuttings were permitted to outgas for several weeks prior to analyzing the headspace gas. The cans were punctured, internal pressures were measured, and a sample of gas was acquired. The headspace gas sample was injected into a gas chromatograph and the components air, carbon dioxide (CO2), methane (C1), ethane (C2), and propane (C3) through normal-pentane (NC5) were separated and identified with a thermal conductivity detector. During the chromatographic separation, the methane peak was diverted into a syringe for injection and subsequent oxidation to CO2 in a Leco induction furnace. The oxidized methane was dehydrated and the stable C isotopic composition of methane was measured on a Nier- McKinney mass spectrometer, and reported in the delta notation relative to the Peedee belemnite (PDB) marine carbonate standard (written communication, 1987, C. N. Threlkeld, U.S. Geological Survey, Denver, Colorado). The bottled free gas samples were analyzed in the same manner as the canned headspace samples. USGS Branch of Pacific Marine Geology The canned drill cuttings analyzed in this laboratory were equipped with septa covered ports. Prior to analysis, the can was shaken for ten minutes. A 8-ml aliquot of helium was injected into the can and an equal amount of headspace gas was withdrawn and analyzed. The gas analysis was performed on a Karl model 311 gas chromatograph equipped with flame ionization and thermal conductivity detectors. The gas chromatograph was calibrated with standard mixtures of hydrocarbon gases, CO2, and air. Calculations of gas concentrations were made by integrating the areas of the chromatograph peaks and comparing the values with the standards. The bottled free gas samples and a limited number of vacuum tube-stored samples were analyzed in a manner similar to the analyses of the canned headspace samples. Geochem Research Incorporated At the laboratory, a silicone rubber septum was attached to the lid of the canned sample in preparation for the C1-C7 headspace gas analysis. Prior to analysis, the can was shaken by hand for 1 minute. A small hole was pierced through the septum, and a sample of gas was withdrawn with a 2-ml syringe after a positive pressure (one atmosphere) was created in the can by the injection of 2 ml of degassed water. The 2-ml headspace gas sample was injected into a standard 1-ml gas sample loop attached to a Varian Aerograph 1400 isothermal gas chromatograph, equipped with a 3.18 mm by 2. 44 m alumina-packed column and a flame ionization detector. This column resolves C1, C2, C3, iso- and normal butane, and if present, the C2, C3, and C4 olefinic hydrocarbons. After the normal butane peak eluted, the flow of carrier gas through the system was reversed with a back-flush valve, and the C5-C7 hydrocarbons were eluted as a single composite chromatographic peak. The concentration of each hydrocarbon was computed from the peak area by means of an electronic integrator with baseline correction. Before a suite of samples were analyzed, a light gas standard containing 100 ppm each of methane, ethane, propane, iso-butane, and normal butane were analyzed in triplicate. Analytical reproducibility is consistently within 2-3% of the observed value. After the can was opened, an aliquot of 10-ml of wet cuttings was placed in a specially designed, sealed blender for the C1-C7 cuttings gas analysis. The sample was disaggregated for 2-3 minutes. A 2-ml sample of degassed water was injected into the 10-ml air space at the top of the blender and an equal amount of headspace gas was withdrawn. This 2-ml gas sample was analyzed in the same manner as the canned headspace gas. Global Geochemistry Corporation The C1-C5 hydrocarbon composition of the canned headspace gases and free gas samples were measured on a Hach 400 gas chromatograph equipped with a sample/back-flush valve, two 3.18 mm by 2.44 m stainless steel columns in a series/bypass configuration, and a flame ionizer detector. Analyses were completed in 15 minutes employing an He carrier gas (30 ml/min) and an oven temperature of 60¡C. As the individual hydrocarbon components eluted they were channeled to a vacuum line for isotopic analyses. The hydrocarbons were combusted to CO2 and H2O in a cupric furnace (held at 800 degrees C). The CO2 was collected at the sample tube, and the H2O was collected in a separate sample tube containing precombusted zinc reagent. The zinc tube was heated at 500¡C for one hour to convert the H2O to H2 gas. Carbon (d13C) and deuterium (dD) isotopic measurements were made on a Nuclide (7.62 cm, 60¡C) dual-collecting stable isotopic ratio mass spectrometer and were reported relative to the Peedee belemnite (PDB) marine carbonate standard and the Standard Mean Oceanic Water (SMOW) international standard, respectively. Analytical reproducibility is typically +-0.2 ppt for carbon and +-3 ppt for hydrogen. The gas chromatograph was calibrated with in-house gas standards. West German Federal Institute for Geosciences and Natural Resources (BGR) See Faber and Stahl (1983) for an analytical description of the laboratory methods methods employed. ANALYTICAL RESULTS To characterize the gas within the interval of gas hydrate stability on the North Slope, 10 drilling-production wells have been sampled and analyzed (table 1, figure 3). Results of these analyses have been included in the appendices of this report. An important assumption in our work is that the gas compositions of drill cuttings and free gas samples reflect the in-situ gas composition of the stratigraphic interval that was sampled. These analyses suggest that methane is the principal hydrocarbon gas in the near-surface (0-1500 m) strata of the North Slope. Stable methane carbon isotopic analyses of gaseous drill cuttings from several gas hydrate-bearing rock units yielded carbon isotopic values averaging approximately -49 ppt, thus suggesting that the methane within the inferred gas hydrate occurrences is from mixed sources, microbial and thermogenic (Collett and others, 1990). Gas with stable carbon isotopic compositions of -50 ppt and heavier is considered to be thermally generated; conversely, an isotopic composition of -60 ppt or lighter indicates that the gas was formed by microbial processes (Schoell, 1983). However, due to the arbitrary nature of these analytical boundaries, we have defined an isotopic transitional or mixing zone (values ranging from -65 to -45 ppt) between the thermogenic and microbial end members. Vitrinite reflectance (Ro) measurements of about 0.4 percent show that the gas hydrate-bearing rocks have never been subjected to temperatures within the thermogenic window (Collett and others, 1990). Thus, the thermogenic gas must have migrated from greater depths. The gas cap of the Prudhoe Bay field is composed primarily of methane (83 to 88 %) along with small quantities of ethane (5 to 7 %) and propane (1 to 2 %) (written communication, 1989, M.C. Davidson, BP Exploration, Anchorage, Alaska). If the gas within the near-surface sediments migrated from deeper structures, these shallow gases should have geochemical constituents similar to those of the deeper gases. However, no significant amounts of ethane or propane were detected within the interval of gas hydrate stability. The depletion of heavier hydrocarbons such as ethane and propane from gas mixtures by stripping during migration has been suggested by Schoell (1983) and Jenden and Kaplan (1986) to explain natural gases containing thermogenic methane but only minor amounts of heavier hydrocarbons. The thermogenic component of the gas within the interval of gas hydrate stability on the North Slope may have been stripped of most of its heavier hydrocarbons. Such a process could account for the molecular and isotopic compositions observed. By comparing the methane carbon isotopic composition of this apparent gas mixture to the isotopic composition of the Prudhoe Bay gas cap it is possible to calculate the relative volume of thermogenically versus microbially sourced gas within the hydrate stability field. The methane carbon isotopic analyses of the Prudhoe Bay gas cap yield an average value of approximately -39 ppt (written communication, M.C. Davidson, BP Exploration, Anchorage, Alaska). The microbially-sourced methane component likely had an original methane isotopic composition ranging from -60 to -70 ppt (Collett and others, 1990). Because the mixing of two gases results in a linear and proportional change in isotopic composition (Schoell, 1983), it is estimated that about 50 to 70 percent of the methane within the hydrate stability field is thermogenic and has migrated from the Prudhoe Bay gas cap. SUMMARY Methane is the most abundant hydrocarbon gas within the gas hydrate- bearing rock units of the Prudhoe Bay-Kuparuk River area in the North Slope of Alaska. Isotopic analysis indicates that both microbial and thermogenic processes have contributed to the formation of this methane. The thermogenic component probably migrated into the rock units from greater depths, since vitrinite reflectance measurements show that the units never endured temperatures within the thermogenic range. Approximately 50 to 70 percent of the methane within the gas hydrate units is thermogenic in origin. ACKNOWLEDGMENTS Special thanks are extended to P. Barker, M. Werner, and D. Suchomel of ARCO Alaska, and M. Davidson and C. West of British Petroleum Exploration for providing critical data and much appreciated technical guidance. We also thank British Petroleum Exploration, ARCO Alaska, EXXON Company USA, and Continental Oil Company for permission to sample development wells in the Prudhoe Bay, Kuparuk River, and Milne Point oil fields. REFERENCES CITED Bily, C., and Dick, J.W.L., 1974, Natural occurring gas hydrates in the Mackenzie Delta, Northwest Territories: Bulletin of Canadian Petroleum Geology, v. 22, no. 3, p. 340-352. Collett, T.S., 1983, Detection and evaluation of natural gas hydrates from well logs, Prudhoe Bay, Alaska, Proceedings of the Fourth International Conference on Permafrost, Fairbanks, Alaska: National Academy of Sciences, Washington D.C., p. 169-174. Collett, T.S., Bird, K.J., Kvenvolden, K.A., and Magoon, L.B., 1988, Geologic interrelations relative to gas hydrates within the North Slope of Alaska: U.S. Geological Survey Open-File Report 88-389, 150 p. Collett, T.S., Kvenvolden, K.A., and Magoon, L.B., 1990, Characterization of hydrocarbon gas within the stratigraphic interval of gas hydrate stability on the North Slope of Alaska: Applied Geochemistry, v. 5,. p. 279-287. Davidson, D.W., El-Defrawy, M.K., Fuglem, M.O., and Judge, A.S., 1978, Natural gas hydrates in northern Canada: Proceedings of the 3rd International Conference on Permafrost, National Research Council of Canada, v. 1, p. 938-943. Faber, E., and Stahl, W., 1983, Analytic procedure and results of an isotopic geochemical surface survey in an area of the British North Sea, in Brooks, James, ed., Petroleum Geochemistry and Exploration of Europe: Blackwell Publishing, London, p. 51-63. Franklin, L.J., 1980, In-situ hydrates - a potential gas source: Petroleum Engineer International, November, p.112-122. Jenden, P.D., and Kaplan, I.R., 1986, Comparison of microbial gases from the Middle American Trench and Scripps Submarine Canyon--Implications for the origin of natural gas: Applied Geochemistry, v. 1, p. 631-646. Kvenvolden, K.A., 1988, Methane hydrate--a major reservoir of carbon in the shallow geosphere? in Schoell, Martin, ed., Origins of Methane in the Earth: Chemical Geology, v. 71, p. 41-51. Lachenbruch, A.H., Sass, J.H., Marshall, B.V., and Moses, T.H., Jr., 1982, Permafrost, heat flow, and the geothermal regime at Prudhoe Bay, Alaska: Journal of Geophysical Research, v. 87, no. B11, p. 9301-9316. Makogon, Y.F., 1981, Hydrates of natural gas: Tulsa, Penn Well Publishing Company, 237 p. Makogon, Y.F., Trebin, F.A., Trofimuk, A.A., Tsarev, V.P., and Cherskiy, N. V., 1972, Detection of a pool of natural gas in a solid (hydrate gas) state: Doklady Academy of Sciences U.S.S.R., Earth Science Section, v. 196, p. 197-200. Rasmussen, R.A., and Khalil, M.A.K., 1981, Atmospheric methane (CH4): Trends and seasonal cycles: Journal of Geophysical Research, v. 86, no. C10, p. 9826-9832. Schoell, Martin, 1983, Genetic characterization of natural gases: American Association of Petroleum Geologists Bulletin, v. 67, no. 12, p. 2225-2238. Seifert, W.K., Moldowan, J.M., and Jones, J.W., 1979, Application of biological marker chemistry to petroleum exploration: 10th World Petroleum Congress, p. 425-440. Werner, M.R., 1987, Tertiary and Upper Cretaceous heavy oil sands, Kuparuk River area, Alaskan North Slope, in Tailleur, I.L., and Weimer, Paul, eds., Alaskan North Slope geology: Bakersfield, California, Pacific Section, Society of Economic Paleontologists and Mineralogists and the Alaska Geological Society, Book 50, v. 1, 109-118. APPENDICES EXPLANATION The following appendices give the results of the headspace, free gas, and blended headspace analyses performed on the drill cutting and free gas samples from the wells listed in table 1. Each appendix includes an identification table which gives information for each sample analyzed. The numbers in the first column specify a sample number for which information about that sample is indicated in the boxes across from the number; the corresponding number can be found on the headspace, headspace/ free gas, and blended headspace tables. A blank box indicates that the test was not performed; a dashed box indicates that the element was not detected. All data are expressed in parts per million. Free gas samples for the KRU3A9 and PBUR1 wells were not analyzed. Blended headspace analyses were not performed for the KRU3H9, MPUE4, PBUS26, and PBUZ7 wells. The following abbreviations are used for the sample ID table: Sample Type Container Bactericide CT - Cuttings CP - Pint Can A - Sodium Azide GS - Free Gas CQ - Quart Can Z - Zephiran Chloride GB - Glass Bottle N - None VT - Vacuum Tube Laboratory BG - Federal Institute for Geosciences and Natural Resources GC - Geochem Research Incorporated GG - Global Geochemistry Corporation GM - USGS Branch of Pacific Marine Geology GP - USGS Branch of Petroleum Geology The following abbreviations are used for the headspace, headspace/free gas, and blended headspace analyses tables: N2 - Nitrogen CO2 - Carbon Dioxide C1 - Methane C2 - Ethane C2:1 - Ethene C3 - Propane C3:1 - Propene IC4 - Isobutane NC4 - n-Butane IC5 - Isopentane NC5 - n-Pentane C5-C7 - sum of the C5(Pentane), C6(Hexane), and C7(Heptane) fractions d13C1 - stable carbon isotope ratio (C13/C12) of the methane fraction dDC1 - deuterium isotope ratio (H2/H1) of the methane fraction d13C2 - stable carbon isotope ratio (C13/C12) of the ethane fraction d13CO2 - stable carbon isotope ratio (C13/C12) of the carbon dioxide fraction (end)