Organic Maturity, Hydrous Pyrolysis and Elastic Property in Shales

Abstract

Organic-rich shales (ORS) are common source rocks for most clastic reservoirs. More recently, they have gained importance as reservoirs. However, the processes of kerogen maturation, hydrocarbon storage, and hydrocarbon transport are still poorly understood. Empirical relations have been developed to relate the increase in acoustic velocity and elastic modulus with increasing maturity. The reason for this increase in velocity also remains poorly understood. We conducted experiments on ORS samples with a range of maturities from the Bakken formation. Our study focuses on investigating methods of predicting maturity of ORS by evaluation of their impedance micro-structure. Our Young's modulus measurements on a nanometer scale help understand variations of Young's modulus of minerals, clay particles, and kerogen matter in naturally matured shales. The samples were re-measured after subjecting them to hydrous pyrolysis. This step helped us investigate the cause for change in modulus with maturity and the mobility of the pyrolysed organic matter. In the naturally matured samples, we find direct qualitative relationships between the Young's modulus of shale samples and its maturity indicators, such as TOC and Transformation Ratio. After hydrous pyrolysis, there is a significant lowering of the Young's modulus in some immature samples. We will present results of elastic property changes before and after hydrous pyrolysis in shales of various maturities. This study improves our current understanding of maturity-related variations by using analysis from nanoindentation. We integrate these measurements with geochemical analysis, and observations from downhole sonic measurements to develop relationships of elastic impedance to shale maturity. These results are critical to help us understand how shales evolve with burial and maturation and how hydrocarbons are stored and transported to sustain large storage even at high overburden stresses.

Raman spectroscopy: an analytical tool for evaluating organic matter

“…Studies have shown, in immature source rocks the organic matter appears to surround other minerals, becoming a load-bearing part of the rock framework. 4,5,48,49 However, by increasing the maturity, kerogen becomes more isolated whereas the grains are more in contact with each other 49,50 and records increase in its Young's modulus. By comparing maturity level of the samples in this study with the Young's modulus values from PeakForce AFM, a general increasing trend can be observed.…”

Nanochemo-mechanical signature of organic-rich shales: a coupled indentation–EDX analysis

“…the heterogeneity of mechanical properties has been studied by Atomic force microscopy 69 (AFM) and instrumented indentation on organic-free shale materials [45, 7, 52, 54, 60-70 61, 3] and organic/inorganic phases in oil and gas bearing shales [2,39,59]. Yet, a 71 characterization method that (1) integrates geochemical and mechanical properties in the 72 assessment of organic-rich shale properties, and (2) is able to handle the high 73 heterogeneity of the rock remains to be developed.…”

On the evolution of the elastic properties of organic-rich shale upon pyrolysis-induced thermal maturation

“…This fracturing of the rock and subsequent nonphysical increases of up to 1.25 in Thomsen's anisotropic parameter epsilon (Thomsen, 1986;Allan et al, 2014) must be minimized (or ideally removed entirely) to more robustly investigate the thermal maturity dependence of elastic anisotropy in organic-rich shale. Additionally, Zargari et al (2011) conduct hydrous pyrolysis experiments on organic-rich Bakken samples; however, the retorting vessels used lacked any separation of the confining and pore spaces. As a result, the experiments were also conducted at zero-effective pressure.…”

“…As a result, the experiments were also conducted at zero-effective pressure. Further, Zargari et al (2011) performed targeted nanoindentation measurements of the pyrolyzed organic matter, which prevents direct comparison of results with the acoustic measurements presented in this study.…”