Remote sensing exploration of gas hydrates on the Alaska North Slope (ANS): pingo mapping in relation to the Eileen Fault Zone.

Abstract

As conventional oil and gas reservoirs deplete, the demand for unconventional oil and gas resources grows stronger. The North Slope of Alaska has enormous potential for unconventional oil and gas in the form of viscous oil, coalbed methane and gas hydrates. Trapping and utilizing these resources will help to meet the growing energy needs of the United States. This study uses remote sensing and field based observations to identify surface features which may be associated with migrating methane gas along the subsurface Eileen Fault Zone in parts of the Alaska North Slope (ANS). The presence of methane gas may be an indicator for gas hydrate formation (300-700 m below the surface). Surface features such as ice core pingos are identified as one of the promising gas hydrate indicators. Remote sensing techniques provided spectral and spatial analysis for this study, including Principal Component Analysis, Image Ratios and Density Slicing. An integrated analysis of the processed images, digital elevation models (DEMs), and structural features in the study area was performed with GIS software. These techniques were used to generate a map showing surface distribution of pingos in relation to the projected surface trends of the subsurface Eileen Fault Zone.

Introduction

Many countries are shifting their focus to unconventional resources, as conventional oil and gas resources are depleted (Rogner 1997). The Alaska North Slope (ANS) holds enormous potential for the recovery of unconventional oil and gas resources in the form of viscous oil, coalbed methane and gas hydrates. Unconventional resources are those resources not exploitable by current technology, and therefore not currently economically feasible (Rogner 1997). Effectively delineating and tapping these unconventional oil and gas resources is important for the State of Alaska in order to strengthen the local economy and help the nation meet the growing energy demands.

Figure 1.  Location map of the study area, southwest of Prudhoe Bay, Alaska.  This image is a standard false color composite (FCC) from Landsat Thematic Mapper (bands 4,3,2 coded in red, green and blue), acquired in July 1997.

Figure 1. Location map of the study area, southwest of Prudhoe Bay, Alaska. This image is a standard false color composite (FCC) from Landsat Thematic Mapper (bands 4,3,2 coded in red, green and blue), acquired in July 1997.

The study area lies in the ANS, southwest of Prudhoe Bay (Figure 1). The distribution of pingos, identified from aerial photos and Landsat TM, are compared to the projected surface expressions of the subsurface Eileen Fault Zone. The fault zone lies 8000 ft. below the surface, intercepting the top of the gas hydrate stability field. Migrating water and gases, left over from the formation of gas hydrates within the hydrate stability field, may continue to migrate towards the surface by way of the Eileen Fault Zone. Permafrost features, such as pingos, could potentially incorporate the left over water and gases as they form. Remote sensing offers a way to identify pingos proximal to the surface projection of the Eileen Fault Zone, which may then act as indirect indicators for gas hydrates on the Alaska North Slope.

Background

Gas hydrates are crystalline solids, in which water molecules form an ice-like framework that traps gas molecules. Natural gas hydrates are limited to polar continental and deep sea environments (outer continental margins) (Kvnevolden 2001).

The United States Geological Surveys (USGS) estimates 590 trillion cubic feet of gas hydrate reserves on the Alaska North Slope (Jones 2003). Forty-four trillion cubic feet of this total estimate are along the Eileen Fault Zone, southwest of the Prudhoe Bay oil fields. The Eileen Fault Zone coincides with a known region of shallow (

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