Introduction

The North Slope of Alaska contains a significant volume of natural gas.  The known amount of gas within the oil and gas fields is large and the estimated amount of undiscovered resource is even larger (Bird, 2002a) .  Increasing demand for gas as a relatively clean fossil fuel is driving plans to develop this resource either through a pipeline to the lower 48 states, LNG shipments, or gas-to-liquids technologies.

            To increase confidence in U. S. Geological Survey (USGS) estimates of undiscovered gas resources there is a need to understand the role of natural gas in the petroleum systems of the North Slope.  Of particular interest to resource estimates in the National Petroleum Reserve in Alaska (NPRA) is whether the petroleum systems of that part of the North Slope are gas-rich.  Current exploration targets are oil reservoirs, but the historic drilling experience indicates that gas is abundant in the western part of the North Slope within NPRA.

            Initial drilling by the U.S. Navy in 1944 to 1953 tested gas in a number of wells and discovered small accumulations at Barrow, Meade, Square Lake, and Wolf Creek and adjacent to NPRA at Gubik as shown on the North Slope location map in Figure 1.  Of the three oil accumulations discovered by the Navy, the largest, Umiat, tested gas in a deeper reservoir horizon than the oil accumulation.  Later drilling by the U.S. Navy and the USGS in 1973 to 1981 discovered gas accumulations on the Barrow Peninsula at Walakpa and Sikulik, and tested gas from a number of potential reservoirs at other locations, some at depths greater than 10,000 ft.

            This paper addresses several natural gas resource questions based on a summary of the geochemistry of natural gas across the North Slope of Alaska.  First, is there evidence of distinct gas-rich petroleum systems?  Second, is there any evidence of unique gas systems in the foothills of the Brooks Range?  Third, are there any potential problems with the quality of gas (significant quantities of non-hydrocarbon gases) in undiscovered gas accumulations?

            To address these questions we present evidence for the source and maturity of the known natural occurrences.  In particular we will examine the effects of microbial gas generation, microbial alteration of thermogenic gas, and the role of mixing of gases of various origins in controlling the present day composition of gas.  Finally, we will examine the extent to which non-hydrocarbon gases, carbon dioxide and nitrogen, affect the quality of North Slope gas.  Carbon dioxide is particularly important because a large fraction (5 to 18%) of the gas cap of the Prudhoe Bay field and surrounding satellite fields is CO₂ and may amount to as much as 5 trillion cubic feet (TCF) of gas.  Although this amount of CO₂ is beneficial for current practices of miscible flooding for enhanced oil recovery in Prudhoe Bay, marketable natural gas must contain 4 % or less CO₂.  Therefore, gas from the Prudhoe Bay field gas cap will require processing or dilution with low CO₂ gas before it is marketable.  Costs associated with gas processing or dilution may impact the economics of natural development on the North Slope.