Organic maturity, elastic properties, and textural characteristics of self resourcing reservoirs

Abstract

Organic-rich rocks have long been recognized as source rocks for clastic reservoirs, but more recently they have gained importance as reservoirs. However, the processes of kerogen maturation and hydrocarbon transport and storage are still poorly understood. Some empirical relations have been developed to relate the increase in elastic modulus with increasing maturity. A systematic study of the cause for this increase in elastic modulus is still lacking, and information about seismic and mechanical properties of kerogen and its alteration products is scarce. Consequently, any rock models must rely on anecdotal or extrapolated data about various types of kerogen. Our experiments address this paucity of data by grain-scale modulus measurements coupled with careful field emission scanning electron microscopy (FESEM) microstructural assessments on organic rich Bakken formation shale samples with a range of maturities. Carefully acquired and detailed FESEM images help to understand the microstructural controls on the reduced (nanoindentation) Young’s modulus of minerals, clay particles, and kerogen matter with maturity in naturally matured shales. Using hydrous pyrolysis, we further investigate the cause for change in modulus with maturity and the mobility of the pyrolized organic matter. In naturally matured shale samples, we find a direct relationship between the reduced Young’s modulus and the total organic content or hydrogen index. Significant lowering of Young’s modulus is observed after hydrous pyrolysis due to bitumen generation. We measured modulus of the extruded bitumen to be less than 2 GPa. The presence of bitumen comingled with the organic matter also reduces its modulus, in excess of 30%. These results are critical to help understand how organic-rich sediments evolve with burial and maturation. The modulus measurements can be used for modeling modulus variations during maturation.

Changes in microstructure and mineralogy of organic-rich shales caused by heating

“…The mean Young's moduli obtained in these nanoindentation experiments decreased with maturity. Zargari et al (2013) explained this by suggesting that bitumen expelled during pyrolysis has lower moduli than kerogen. Uvarova et al (2014) studied variations in chemical composition and microstructure of organic-rich Kimmeridge Shale with a TOC content of 23%.…”

“…To understand relations between thermal maturation and microstructural changes, Zargari et al (2013) measured the distribution of Young's moduli on the surfaces of natural samples at different maturity levels and of samples subjected to the hydrous pyrolysis process. The mean Young's moduli obtained in these nanoindentation experiments decreased with maturity.…”

Quantitative Evaluation on the Elastic Property of Oil-Bearing Mudstone/Shale from a Chinese Continental Basin

“…Most void (pore-fractures) space is saturated by fluids, including hydrocarbon and the formation of water, of which hydrocarbon may be the softest part within the MS rocks. Zargari et al (2013) reported that extruded bitumen exhibit Young's modulus values as low as 2 GPa. In this study, residual hydrocarbon was extracted through the use of trichloromethane.…”

“…As proposed in previous works, the dynamic Young's modulus calculated by acoustic velocities also needs to be converted into the static Young's modulus based on the relationships between the two Young's moduli (Eissa and Kazi, 1988). Recently, some researchers have investigated the nano-micron mechanical properties of shale by using nanoindentation (Ulm and Abousleiman, 2006;Bobko and Ulm, 2008;Ahmadov et al, 2009;Zargari et al, 2013) and atomic force microscopy (AFM) techniques (Trtik et al, 2012;Eliyahu et al, 2015). To date, the factors impacting the MS elastic property have been revealed, primarily including micro/macro-structures (Josh et al, 2012), inorganic mineralogical composition (Mondol et al, 2007;Ding et al, 2012;Eliyahu et al, 2015), organic matter (Eliyahu et al, 2015), fluid media (Valès et al, 2004;Al-Bazali et al, 2008a;Abousleiman et al, 2010;Zhang et al, 2012b), confining stress (Sarout et al, 2007;Abousleiman et al, 2010), strain rate (Cook et al, 1990;Al-Bazali et al, 2008b), temperature (Masri et al, 2014), pore pressure (Al-Bazali et al, 2008b), porosity (Lashkaripour, 2002;Mondol et al, 2007), as well as others.…”

Nanomechanics of organic-rich shales: the role of thermal maturity and organic matter content on texture

“…One of the sources of complexity in organic-rich shales is the intricate interplay between micro-texture evolution and kerogen maturity [1,18,20,23,24]. There is increasing evidence that the maturation of organic matter changes the micro-texture of organic-rich shales [18-20, 23, 25] contributing to the changes in mechanical properties of the rock.…”