OIL FIELD RESERVOIR

Oil Field Reservoir



Oil Field Reservoir

PART A)

Introduction

This report summarizes the economic analysis of the U.S. Geological Survey's 2010 assessment of oil and gas in undiscovered accumulations in the National Petroleum Reserve in Snorre; the assessment results were released by D.W. Houseknecht and others in October 2010 in USGS Fact Sheet 2010-3102. The Snorre field is located in the northern North Sea and belongs to the prolific hydrocarbon province on the western margin of the Viking graben, which contains the Brent, Statfjord, and Gullfaks fields(Lappegård, 1991b, pp.9). The Snorre field was discovered in 1979. It has two main reservoirs--the Triassic Lunde Formation and the Triassic-Jurassic Statfjord Formation. Each of these reservoirs consists of a network of fluvial sand bodies in a mudstone matrix.

The reservoir properties range from fair to very good. Typical test permeabilities are 2000 md in the upper part of the Statfjord Formation and 125 to 380 md in the underlying units.

The Snorre field contains 490 million standard m 3 (3.1 billion bbl) of undersaturated oil in place, of which approximately 75% occurs in the Lunde Formation. The oil-water contact varies from 2561 m (8400 ft) on the crest to 2599 m (8530 ft) in the western region.

This field is a structural/stratigraphic trap formed by westward tilting and erosion of a major fault block. Both reservoirs are truncated by the Kimmerian unconformity and overlain by Jurassic and Cretaceous shales. Organic-rich Upper Jurassic shales constitute the major source rock (Corbett, 2003, pp. 105).

The Snorre field will undergo a two-phase development. In phase 1, a tension leg platform (TLP) will be located in the southwestern part of the field. Production will start in 1992 with six wells predrilled from a template beneath the TLP. Additional TLP wells and another subsea template will complete phase 1. For phase 2, there are two different options--either relocating the TLP to the northern part of the field or leaving it in its original position and adding two more subsea templates (Underhill, 2009, pp. 291).

Since the time of the 2002 assessment, six Federal lease sales have taken place, and an additional 30 exploration wells have been drilled on Federal and Native lands. U.S. Geological Survey (USGS) geologists have interpreted results of exploratory drilling to show that formations thought to be oil prone are actually gas prone. The new data have also indicated that actual reservoir quality is inferior to the reservoir quality inferred in the 2002 assessment (Houseknecht and others, 2010). The change in paradigm results in a decline in the estimated mean value of undiscovered oil from 10.6 billion barrels of oil (BBO) to 895 million barrels of oil (MMBO). The mean estimate of non associated natural gas (gas in gas fields) was revised from 61.4 trillion cubic feet (TCF) of gas in 2002 to 52.8 TCF in 2010 (Underhill, 2009, pp. 197).

Description of the Geologic Assessment

The geologic assessment method (Schuenemeyer, 2003) and results (Houseknecht and others, 2010) are briefly reviewed here. The commercial value of newly discovered oil and gas accumulations ...