ABSTRACT

Shale compaction is generally considered to be an irreversible process resulting from the mechanical rearrangement of sediment grains in response to overburden stress. Under ideal conditions, the sonic log provides a sensitive measure of sediment porosity (O) change with depth. However, compositional variations in the pore fluid and sediment matrix can affect sonic transit time (t) readings and distort the normal compaction trend. The Beaufort-Mackenzie Basin (BMB) provides a unique opportunity to study shale compaction, because it contains shales with rather uniform physical and chemical properties, deposited under highly variable sedimentation rates.

Previous compaction studies have been biased by variations in sediment composition and the incorrect assumption that t is linearly related to O. A new t-O equation, calibrated using mudstone and shale core porosity measurements from the BMB, is characterized by an Acoustic Formation Factor (2.19) and matrix (220 µs/m). This t-O transform, which is valid for organically-lean shales (< 2% avg. TOC), was used to calculate O from t data.

Compaction trends in the BMB are strongly correlated with structural features and depositional history. Data are coherent over broad geographic regions and show a significant change from undercompacted, overpressured sections in the offshore, to normally-pressured sections onshore. A linear compaction rate of approximately 1%/100m in normally-pressured Tertiary shales of the BMB and in Devonian and Cretaceous-Paleocene shales of the Ft. Norman area, Northwest Territories, indicate that compaction is insensitive to temperature. These data suggest that shales may approach a steady-state compaction profile, a result that has broad applications for quantitative basin analysis, particularly with regard to stratigraphic reconstructions of eroded sections.

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