preprints.org > environmental and earth sciences > atmospheric science and meteorology > doi: 10.20944/preprints202010.0299.v2

Preprint Article Version 2 Preserved in Portico This version is not peer-reviewed

Version 1 : Received: 13 October 2020 / Approved: 14 October 2020 / Online: 14 October 2020 (10:47:42 CEST)
Version 2 : Received: 29 November 2020 / Approved: 1 December 2020 / Online: 1 December 2020 (09:24:32 CET)

The grain-to-grain stress vertically in sediments is given by the overburden less the pore fluid pressure, σ, divided by the fraction of the horizontal area which is the supporting matrix , (1 − φ), where φ is the porosity. It is proposed that the fractional reduction of this ratio, Λ, with time is given by the product of φ 4m/3 , (1 − φ) 4n/3 , and one or more Arrhenius functions A exp(−E/RT ) with m and n close to 1. This proposal is tested for shale sections in six wells from around the world for which porosity-depth data are available. Good agreement is obtained above 30-40 C and porosities less than 0.5. Single activation energies for each well are obtained in the range 15-33 kJ/mole, close to pressure solution of quartz, 24 kJ/mol. Values of m and n are in the range 1 to 0.8, indicating nearly fractal pore-matrix spaces and water-wet interfaces. Results are independent of over- or under-pressure of pore water. This model explains shale compaction quantitatively. Given porosity-depth data and accurate activation energy, E, one can infer paleo-geothermal-gradient and from that organic maturity, thus avoiding unnecessary drilling.

shale compaction; kinetics, activation energy; pore interfaces; grain interfaces; fractals

Environmental and Earth Sciences, Atmospheric Science and Meteorology

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