Quartz types in the Upper Pennsylvanian organic‐rich Cline Shale (Wolfcamp D), Midland Basin, Texas: Implications for silica diagenesis, porosity evolution and rock mechanical properties

Abstract

Associate Editor -Catherine Reid ABSTRACTThe origin and form of quartz in mudrocks has significant implications for interpretation of depositional environments, diagenetic pathways, mechanisms of porosity reduction and rock mechanical-property evolution. Quartz types in the Upper Pennsylvanian Cline Shale, Midland Basin, Texas, were examined using a combination of field-emission scanning electron microscopy-based energy-dispersive spectroscopy elemental mapping (to determine mineralogy) and scanning electron microscopy-based cathodoluminescence imaging (to determine quartz types) with the goal of elucidating a high-resolution imaging protocol at the micrometre scale for shale petrology. Also, the unconfined compressive rock strength of shale samples with contrasting proportions of different quartz types was measured using Equotip Bambino analyses. The results suggest that extrabasinal detrital quartz, which accounts for an average of 26 vol.% of the rock in all analyzed samples, is the dominant form of quartz in the Cline Shale. The intergranular clay-size microquartz, which accounts for an average of 10 vol.% of the rock in all analyzed samples, is the dominant form of authigenic quartz. Dissolved radiolarians and sponge spicules are likely sources of silica for clay-size microquartz and other authigenic quartz showing pale-mauve to dark greyish cathodoluminescence colour. Some authigenic quartz in the form of intragranular pore filling and mollusc skeletal replacement displays bright-reddish cathodoluminescence colour, which may be associated with silica released at a different time in the rock's diagenetic history, such as during smectite illitization. Porosity reduction in the Cline Shale predominantly resulted from compaction because of extremely low intergranular volume and the general lack of early cementation. Quartz form significantly impacts rock mechanical properties in the Cline Shale: widely distributed intergranular clay-size microquartz cement is a major factor controlling rock strength. This correlation also applies to other mudrock successions of various geological ages, tectonic histories and lithologies.

Sealing Mechanism of the Roof and Floor for the Wufeng-Longmaxi Shale Gas in the Southern Sichuan Basin

“…When the clay layers underwent rapid mechanical compaction, diagenesis and hydrocarbon generation, consequent reorientation of originally flocculated clay flakes into a bedding-parallel planar microfabric (Figure a–g). The compacted clay supporting structure furtherly reduced the porosity (Figure e), pore throat (Figure c,f), and pore connectivity (Figure g,h) and the higher water saturation (Figure f) also enhanced the sealing capacity of mudstone. , Additionally, as lacking protection from rigid minerals (quartz, feldspar, and pyrite), some less plastic organic particles have been molded into clay-related interparticle pore space, thereby further reducing pore throat diameters and enhancing the sealing capacity. ,, …”

“…Microscopically, the sealing character and sealing capacity were attributable to systematic differences in shale texture and fabric. ,, The sedimentary structure, mineral compositions, and arrangements, as well as the diagenesis process of mud shales, can directly influence the porosity, permeability, pore throat, and pore connectivity of the caprocks, thereby affect the sealing performance of the mud shale. ,− Fine-grained, parallel microstratification and high clay minerals content can improve the sealing capacity of mud shale. − , For the roof in the LM2 and LM3 members, the argillaceous shale and silty shale developed multilayers of centimeter-scale muddy bands and silty bands (Figure a,b), the well-developed laminar fabrics appear to enhance the sealing capacity…”

Multiproxy geochemical characterization of Kommugudem Formation, Krishna Godavari Basin, India: Implication on hydrocarbon potential and shale brittleness

“…This matrix‐dispersed clay‐sized quartz cement significantly increased the brittleness of the Barnett Formation. Clay‐sized microquartz cement identified in few samples (Figure 4b,c) may bind silt‐size detrital quartz and increase brittleness by also enhancing intergranular pores preservation for cement precipitation (Peng et al, 2020). Illitization of smectite clay also leads to the formation of recrystallized microquartz cement as can be confirmed by XRD semi‐quantitative results indicating the presence of illite and absence of smectite (Hall, 2019).…”

“…Brittleness is a function of the pore, mineralogical composition, elastic, and mechanical properties and has significant control on hydraulic fracturing of shale gas reserves (Z. Guo, Chapman, & Li, 2012). Brittleness in shales is further also influenced by the mineral texture that is governed to a greater degree by post‐depositional processes leading to complex heterogeneities (Milliken, Esch, Reed, & Zhang, 2012; Peng, Milliken, & Fu, 2020). The present study considered mineralogical brittleness index (MBI) where mineralogical assemblage is used to identify brittle zones.…”

Clay mineral diagenesis in alternating mudstone‐sandstone beds from Late Palaeozoic strata of the Anadarko Basin, United States

“…A great deal of research focusing on clay mineral diagenesis in conventional clastic reservoirs has been published over the last decades, primarily to understand how mineralogical changes in the subsurface may impact the quality of sandstone reservoirs (Bloch et al, 2002; Ehrenberg, 1990; Haszeldine et al, 2000; Nadeau, 2000; Virolle et al, 2022; Weaver, 1960). However, the response of clay minerals to burial diagenesis in fine‐grained sedimentary rocks has, however, been less studied leading to a research momentum in recent years that coincides with the advances in micro‐beam technology and unconventional oil and gas boom (Abou El‐Anwar, 2017; Peng et al, 2020; Środoń, 1999; Uffmann et al, 2012). Moreover, the diagenetic evolution of interbedded mudstone and sandstone is, not understood well, particularly the rate of progressive transformation of smectite to illite (Gier et al, 2015; McKinley et al, 2003), mesogenetic formation of kaolinite (Lanson et al, 2016), and origin of chlorite (Beaufort et al, 2015; Šegvić et al, 2020; Worden et al, 2020; Zanoni et al, 2016).…”