--> ABSTRACT: Characterization of Lithology and Reservoir Rock for Deep Gas Drilling in Siljan Impact Structure, Sweden, by John R. Castano, Robert M. Sneider, and George W. Bolger; #91030 (2010)

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Characterization of Lithology and Reservoir Rock for Deep Gas Drilling in Siljan Impact Structure, Sweden

John R. Castano, Robert M. Sneider, George W. Bolger

The Gravberg 1 well in the Siljan impact structure in Sweden has been drilled entirely in granite and dolerite intrusions of Precambrian age to a present depth of 6.6 km. Study of the cuttings at the well site included a lithologic description, with emphasis on quantifying mineralogical, textural, and pore-space parameters that affect porosity and permeability and that can indicate the potential presence of a fractured reservoir. Important mineral parameters are epidote, reddish feldspar, chlorite, and other alteration products. Textural parameters include the presence of fractures, cataclastic and brecciated grains, vugs, and drusy crystals. Although rare, the presence of drusy crystals clearly shows that open fractures are present. The study is also geared to identify minerals that affect wireline logs (e.g., alteration products such as chlorite, sericite, and illite, and heavy minerals such as pyritic and magnetite). Cuttings examination is crucial as relatively few cores or sidewalls have been recovered.

Laboratory studies incorporating capillary pressure tests, SEM, and thin-section petrography (in polarized and fluorescent light) reveal that the samples contain a bimodal pore structure composed of larger pores representing the microfractures and smaller pores representing the intercrystalline microporosity developed within altered grains. Porosity in the granites averages 0.96 % (typical Swedish granite is 0.5 %), and the permeability is 0.003-0.027 md.

The most promising zones identified by wireline logs generally coincide with fractured intervals identified by lithologic study. The overall approach used in the Gravberg well is very similar to the one proven successful in evaluating low-matrix porosity rocks in sedimentary basins.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.