ABSTRACT

Litharenites of Paleocene to Oligocene age are well consolidated at the onshore margin of the Beaufort-Mackenzie Basin. At comparable depths, litharenites of the basin centre offshore are much less consolidated and more porous. Petrographic analysis of cores and drill cuttings from 21 wells ranging in depth to 4,900 m, and of outcrop samples, shows fairly uniform detrital composition. Ductile grains are very abundant: syndepositional mudclasts, argillaceous sedimentary/micaceous metamorphic/altered volcanic rock fragments, mica, and carbonaceous debris. Quartz, chert and other competent grains average two thirds of the grain framework.

Deformation of ductile grains destroyed most of the primary intergranular porosity at paleoburial depths of less than 2 km. During the progressive destruction of primary porosity, dissolution of detrital and authigenic constituents created considerable secondary porosity. Virtually no primary porosity is preserved at the basin margin because of burial followed by erosion.

Postdating the destruction of most primary porosity, the following overlapping diagenetic events modified porosity and altered composition:

1. multiple episodes of extensive cementation and replacement by calcite and dolomite
2. several phases of fracturing
3. extensive dissolution of chert, matrix, labile grains and authigenic carbonate creating mainly secondary intergranular pores
4. chemical compaction at grain contacts, and subordinate mechanical compaction reducing secondary porosity
5. progressive cementation by quartz; and
6. episodic cementation and replacement by kaolinite and minor pyrite.

Secondary dissolution porosity is best developed in the geopressured basin centre. It is suggested that this exceptional porosity enhancement resulted from maturing organic matter that generated acid reactants and methane, possibly facilitating overpressuring and fluid convection.

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