Stability of Organic Carbon Components in Shale: Implications for Carbon Cycle

Abstract

Stability and mobility of organic matter in shale is significant from the perspective of carbon cycle. Shale can only be an effective sink provided that the organic carbon present is stable and immobile from the host sites and, not released easily during geological processes such as low pressure-temperature burial diagenesis and higher pressure-temperature subduction. To examine this, three Jurassic shale samples of known mineralogy and total organic carbon content, with dominantly continental source of organic matter, belonging to the Haynesville-Bossier Formation were combusted by incremental heating from temperature of 200 to 1400 • C. The samples were analyzed for their carbon and nitrogen release profiles, bulk δ 13 C composition and C/N atomic ratio, based on which, at least four organic carbon components are identified associated with different minerals such as clay, carbonate, and silicate. They have different stability depending on their host sites and occurrences relative to the mineral phases and consequently, released at different temperature during combustion. The components identified are denoted as, C-1 (organic carbon occurring as free accumulates at the edge or mouth of pore spaces), C-2 (associated with clay minerals, adsorbed or as organomineral nanocomposites; with carbonate minerals, biomineralized and/or occluded), C-3(a) (occurring with silicate minerals, biomineralized and/or occluded) and C-3(b) (graphitized carbon). They show an increasing stability and decreasing mobility from C-1 to C-3(b). Based on the stability of the different OC components, shale is clearly an efficient sink for the long term C cycle as, except for C-1 which forms a very small fraction of the total and is released at temperature of ∼200 • C, OC can be efficiently locked in shale surviving conditions of burial diagenesis and, subduction at fore arc regions in absence of infiltrating fluids. Under low fluid flux, C-3(b) can be efficiently retained as a refractory phase in the mantle when subducted. It is evident that the association and interaction of the organic matter with the different minerals play an important role in its retention in the shale.

Best Practices for Shale Core Handling: Transportation, Sampling and Storage for Conduction of Analyses

“…The loss is strongly governed by the TOC content of the shale, with high TOC (>11%) significantly limiting the loss over time. Based on our analyses of samples extracted from the central portions of three slabbed cores stored in a warehouse in North Wales, after storage of samples for over a year under refrigerated conditions in Teflon containers, their isotopic measurements (δ 13 C organic~− 27% ) demonstrate consistent results, in agreement with previous studies [62,65]. The porosity measurements show a depth trend with noble gas isotopic ( 40 Ar/ 36 Ar) and elemental ( 4 He/ 40 Ar) ratios, thereby ruling out any alteration and gas losses due to handling and storage post-retrieval of the cores [63].…”

“…It is advisable to discard the outer 1-2 mm layer of sediment that has been in contact with the plastic or metal liner for potential contamination, especially for isotopic and elemental analyses. Extracting samples from the central portion of any drill core for geochemical analyses can minimise the superficial effects of core retrieval and contamination from handling e.g., [15,60,[62][63][64]. Hammering should be avoided as it can damage the core.…”

Opportunities in Measuring Multiscale Pore Structure of the Continental Shale of the Yanchang Formation, Ordos Basin, China

“…Unlike sandstone or carbonate rock conventional reservoirs, the pores of shale have a complex structure. It is deemed that pores play a vital role in the gas storage and enrichment in shale [3][4][5][6][7]. Investigation on the geometry and connectivity of shale pores plays a vital role in gas exploitation in shale reservoirs [8,9].…”

Editorial: Deep Carbon Science

“…Shales are known to be large carbon sinks in low pressure settings. Basu et al (2019) ask whether shales can retain significant carbon during low pressure-temperature and high pressure-temperature processes during the subduction of Earth's crust. In a custom-built high vacuum line, they incrementally heat shale samples from 200 to 1,400°C in the presence of O 2 gas and record the carbon and nitrogen abundances, δ 13 C values, and the atomic C/N ratios for the gas at each stage of heating.…”

“…In a custom-built high vacuum line, they incrementally heat shale samples from 200 to 1,400°C in the presence of O 2 gas and record the carbon and nitrogen abundances, δ 13 C values, and the atomic C/N ratios for the gas at each stage of heating. Basu et al (2019) propose that carbon silicate minerals, biomineralized and/or occluded, can be efficiently retained as a refractory phase and transferred into Earth's mantle through subduction.…”