Microfacies Data: Matrix and Grains

Abstract

MicrofabricGrain size: Is measured by counting the longest diameter of 100 to 200 grains. Allows attribution to grain size-based classifications. 'Sorting' in micrites, as the result of competition between space and grain growth, can be used as measure of the textural maturity of fine-crystalline carbonates. Grain size ranges may reflect differences in the depositional environment (e.g. reef micrites and micrites in limestone/shale sequences). Differences in predominant crystal sizes provide indications of the potential aragonite-dominated or calcite-dominated precursors of calcite micrites. Grain contacts: Grain contacts seen under the SEM are straight, curvilinear or serrate. The percentage of specific grain contacts reflects the degree of diagenetic changes (e.g. recrystallization). Grain shape: Differentiation of anhedral, subhedral and euhedral micrite grains. Anhedral and subhedral grains forming a hypidiotopic fabric appear to be common in micritic rocks composed of grains > 4 μm. Irregular, interlocked grains form an 'amoeboid mosaic', in contrast to block-like grains forming a 'pavement mosaic' common in limestones affected by thermal stress. Most micrite grains are angular to subangular. Subrounded and rounded grains result from intergranular solution and crystal overgrowth. Inclusions in crystals: Liquid and gas inclusion pits are common in microspar crystals as well as in larger spar crystals.The composition of the inclusions indicates the conditions under which recrystallization took place. Microfenestrae: Some micrites contain round voids, 40 to 80 μm in size, which are filled with blocky calcite. The preservation of this microfenestral fabric is used as evidence against a strong mechanical compaction of the sediment prior to lithification. Boundary between calcite matrix and fossils: Diagenetic stages are distinguished by differences in the boundaries between matrix and fossils: (a) Sharp boundaries between matrix and Low-Mg calcite fossils (e.g. brachiopods, bryozoans, ostracods, trilobites) with well-preserved ultrastructures; (b) moderately sharp boundaries between matrix and fossils (recrystallized tabulate and rugose corals, stromatoporoids); and (c) boundaries between matrix and fossils (e.g. mollusks) which are difficult to recognize. Microporosity: SEM studies reveal numerous types of micropores (5-10 μm wide) in micritic limestones, including both primary and solution-enhanced intercrystalline pores, as well as micromolds, microvugs and microchannels (Pl. 7/6; see Chap. 5). Microporosity is developed in many subsurface micritic limestones, including significant petroleum reservoirs in Mesozoic and Tertiary deposits of the Middle East, North Africa, and the North American Gulf Coast. Skeletal grainsA main advantage of SEM studies of micrites is the possibility of recognizing the occurrence and abundance of planktonic nannofossils (Pl. 7/3-5, Pl. 77/23) in mudstones and wackestones in both deep-water and shallow-water settings. SEM differentiation of silt-sized and smaller bioclastic grains ...

Depositional and diagenetic constraints on the quality of shale‐gas reservoirs: A case study from the Late Palaeocene of the Potwar Basin (Pakistan, Eastern Tethys)

“…The presence of miliolids shows exclusively marine conditions during its deposition (Figure 5d), while the abundance of clay minerals elucidates low‐energy conditions (Flügel, 2010; Hanif et al, 2013). The abundance of clay minerals and thin laminations reflect low‐energy conditions (Lazar et al, 2015; Schicker et al, 2021).…”

“…The abundance of calcareous content, scarcity of bioclasts and mudstone texture of this lithofacies reveal low energy sub‐oxic depositional conditions (Flügel, 2010; Hanif et al, 2013). The existence of bioturbation and scarcity of bioclasts shows limited organic influx and insufficiency of nutrients during its deposition (Lazar et al, 2015; Percy & Pedersen, 2020).…”

Characteristics of Early Neoproterozoic Stromatolites from Southern Liaoning, North China: Insights into the Formation of Stromatolites

“…The involvement of microbes is more favorable for dolomite precipitation [46]. microbial origin [47]. The light laminae were the results of the spaces filled by synsedimentary marine cement between dark laminae because of their good inheritance.…”

“…DL-1 and DL-2 might correspond to two different types of microbial communities within the mats. Although the preservation of stromatolite-building microbes is difficult to identify from the geological history, the spotted clots and peloids textures were associated with a microbial origin [47]. The light laminae were the results of the spaces filled by synsedimentary marine cement between dark laminae because of their good inheritance.…”