Geomechanical Properties of Organic Matter in Fine Grained Source Rocks

Abstract

The mechanical behaviour of shales is controlled by the elastic properties of both their inorganic and organic components. The application of mechanical testing techniques is challenging due to the micrometer size of individual components. Additionally, the initial composition of the organic matter (organofacies) and different degrees of diagenesis and catagenesis change their physical and mechanical properties. Here we present the Young’s of organic matter within shales from different depositional and maturity.

The development and continued improvement of techniques such as nanoindentation and Atomic Force Microscopy (AFM) enable in-depth analysis of features less 1mm in size across polished rock surfaces. The recent application of combining AFM with quantitative imaging techniques to geologic samples allow the measurement of mechanical properties in a non-destructive manner at a resolution of less 100nm. A tip (initial diameter approximately 10nm) of known Young’s modulus is oscillated onto a sample with a similar modulus in a ‘tapping’ motion. The tip-surface interaction and the force required to deform the surface by 1-2nm is recorded. The gradient of force-displacement curve is used to calculate Young’s modulus.

In this project, shales are categorised and related by Probability density functions (PDFs) of organic matter stiffness; relating shales by depositional environment and maturity. Two comparable shales are the Eagle Ford shale with the Tarfaya Oil shale of Morocco, owing to similar ages and depositional environments. Likewise, correlations may exist between Barnett shale and the Bowland shale as both contain significant proportions of type III terrestrially derived organic matter. An increasingly bimodal distribution in Young’s modulus indicates that organic matter becomes increasingly complex with maturity, but little is known about the initial complexity in modulus resultant from variable organic precursors. Knowledge of these relationships will help play assessment and development by enabling predictions of organic matter geomechanical response to the stress exerted during hydraulic fracture. This may initiate complex fracture stimulation models aiming to access hydrocarbons within previously inaccessible intraparticle organic pores.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018