Effects of Quartz Precipitation on the Abundance and Preservation of Organic Matter Pores in Cambrian Marine Shale in South China

Abstract

To evaluate the effects of quartz precipitation on the abundance and preservation of organic matter pores in marine shale reservoirs, the type of authigenic quartz and the source of silica, as well as the corresponding relation of the Lower Cambrian Shuijingtuo Formation shale in South China were investigated. Quartz in the Shuijingtuo shale occurs as four different types: detrital quartz, replacement of biosiliceous debris, euhedral quartz filled in interparticle pores, and microquartz dispersed in a clay matrix. Euhedral quartz (1–5 μm) and matrix-dispersed microquartz (100–400 nm) are found to be the dominant forms of authigenic quartz. The euhedral quartz accumulates along the interparticle pores, and the porous organic matter fills the interior of the space. Microquartz is mainly wrapped in porous organic matter. Two silica sources were revealed: biogenic silica and clay-derived silica. Biogenic Si is most likely the major source for authigenic quartz in the organic-rich (total organic carbon (TOC) > 2.55 wt.%) samples, which accounts for 23–57 wt.% (average 35 wt.%) of the total Si. Based on petrographic observations, we posit that the precipitation of large-sized euhedral quartz in the interparticle pores most likely originated from biogenic silica in the early stage of diagenesis and that the silica for the clay matrix-dispersed microquartz is provided by biogenic silica and clay-derived silica. The observation of SEM images indicates that the precipitation of early diagenetic euhedral quartz in the interparticle pores enhances rock stiffness, and the buttressing effect can protect the organic matter pores from compaction during the late-stage burial diagenesis. In contrast, the precipitation of late diagenetic microquartz in the clay matrix can lead to a reduction in the capacity of the accommodation space to host retained petroleum, consequently leading to a reduction in the development of organic matter pores and the generation of shale gas.

Sedimentary Environment Interpretation and Organic Matter Enrichment of the Lower Cambrian Shuijingtuo Shale in the Yichang Slope, South China: Insight from Sedimentary Geochemical Proxies with Major/Trace Elements

“…The black, clay-rich siliceous shale presents mainly in interval III of the examined Shuijingtuo Shale and is characterized by moderate to high quartz content (45~48 vol.%, average of 47 vol.%) and moderate clay content (30~34 vol.%, average of 32 vol.%) (Figure 2c1-c3). There is a significant reduction in the radiolarians and sponge spicules in the black, clay-rich siliceous shale compared to the black siliceous shale (Figures 4 and 5 in Wei et al [23]). The TOC content of the black, clay-rich siliceous shale ranges from 2.21 wt.% to 3.27 wt.% (Table 1).…”

“…The black siliceous shale has the highest quartz content and the lowest carbonate mineral content, ranging from 55 to 75 vol.% (average of 62 vol.%) and 4 to 13 vol.% (average of 6 vol.%), respectively. The thin section shows that the black siliceous shale contains an abundance of radiolarians and sponge spicules (Figures 4 and 5 in Wei et al [23]). The range of TOC content in the black siliceous shale varies widely, with high TOC content (average of 6.02 wt.%) mainly distributed in interval I (Table 1, Figure 2a1-a3) and moderate TOC content (average of 3.94 wt.%) distributed in interval II (Table 1, Figure 2b1-b3).…”

“…The black siliceous shale is mainly characterized by high silica content (Figure 2a1-a3, b1-b3). The mineral compositions and contents can be referred to in Wei et al [23]. The black siliceous shale has the highest quartz content and the lowest carbonate mineral content, ranging from 55 to 75 vol.% (average of 62 vol.%) and 4 to 13 vol.% (average of 6 vol.%), respectively.…”

“…It is mainly distributed in the Yangtze block and characterized by a wide sedimentary range, considerable thickness, and high organic matter content [3,20]. Previous studies on the Lower Cambrian Shale have focused on shale pore structure, shale gas adsorption capacity, and organic-inorganic diagenesis [6,20,[22][23][24][25]. Among these investigations, the content of organic matter has been recognized as an important influencing factor.…”

An Overview of Recent Developments and Understandings of Unconventionals in the Vaca Muerta Formation, Argentina

“…From a geological standpoint, Vaca Muerta's significance cannot be understated. Comprising a mix of organic-rich shales and interbedded carbonates, its resource-rich strata present both opportunities and challenges [11]. Detailed seismic surveys and core sampling methodologies have been instrumental in mapping the complex geological heterogeneities that characterize the formation [12][13][14], ultimately guiding drilling and completion activities [15,16].…”