ABSTRACT

Brittle deformation, together with grain rearrangement and ductile grain deformation, is a key mechanism of compaction in sandstones. Quartz cementation, by virtue of its impact on sandstone mechanical properties, is expected to affect the compaction progress by these various mechanisms. Sandstone samples of different ages and compositions, taken from two basins with contrasting burial histories, are used to quantify the relationship between brittle deformation and quartz cementation in the context of burial compaction. Exponential trends of increasing deformation by microfracturing are observed in both the lithic-rich Frio Formation from the Gulf of Mexico basin (r² = 0.81) and in the quartz-rich Mount Simon Formation from the Illinois basin (r² = 0.78). The two formations contrast in terms of the observed rate of grain fracture increase with depth. A larger number of quartz grains in the Mount Simon Formation undergo fracturing at shallow burial (< 2 km) compared to the Frio Formation, whereas at intermediate to deep depths of burial (> 3 km) a larger number of quartz grains are fractured in the Frio sandstones.

Cathodoluminescence (CL) imaging conducted on an SEM shows that grain fracturing and subsequent cementation of fractures by quartz can occur before, during, and after normal quartz overgrowth precipitation, although fracturing is generally concurrent with cementation. The intensity of brittle deformation for a single fractured grain is independent of depth, as illustrated by plotting the ratio of the number of particles in fractured grains versus depth (r² = 0.21 for Frio and 0.03 for Mount Simon), suggesting that, once fractured, grains do not tend to become the locus of subsequent fracturing.

Apparent fracture apertures in the Frio grains are slightly wider (average 5 µm) than in Mount Simon grains (average measurable aperture width 4 µm). Quartz grains in the Frio have a variety of fracture morphologies, including wedge-shaped apertures, intense comminution at grain contacts, and grains with exploded fabrics. Apparent fracture apertures in Mount Simon grains are thinner and transect grains as straight, uniform traces. Differences in fracture intensity and pattern are attributed to the fact that cementation in the Frio began at greater depth than cementation in the Mount Simon.

Combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures (here referred to as C_f or volume of intragranular cement) allows calculation of the amount that brittle deformation influences compaction. In the Frio Formation, 0.12 to 8.37% of porosity loss due to compaction can be attributed to brittle deformation, whereas the values for the Mount Simon Formation lie between 0.25 to 2.16%. The larger values within both formations are affiliated with deeper samples in which a majority of grains manifest fracturing.