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SEG/AAPG Distinguished Lecturer,
Fall 2012

Manika Prasad

Colorado School of Mines, Golden, Colorado

Shales and imposters: understanding shales, organics, and self-resourcing rocks

 
Abstract

Manika Prasad

Shales are very commonly occurring sedimentary rocks. They are alternatively described by their grain size (less than 2 micrometers), by their mineralogy (hydrous aluminosilicates), or by an assemblage of sedimentary features (fissile, fine-grained, and clay-rich). Generally, rocks with clay contents larger than 50% are termed as shales. The presence of clay minerals, hydrous aluminosilicates that are smaller than 2 µm can alter the elastic and plastic behavior of materials significantly. Load-bearing clays form weak links between the stronger mineral components. Knowledge about the elastic properties of clay is therefore essential for the interpretation and modeling of the seismic response of clay-bearing formations. However, due to the layered structure, small grain sizes, and reactive nature of clay minerals, it has proved very difficult to investigate their elastic properties. Precise measurements of the static and elastic properties of clay minerals are rare with little agreement between theoretically derived and measured values of clay moduli. For example, elastic moduli values reported in the literature range between 10 GPa and 400 GPa. This discrepancy is mainly due to various amounts of water adsorption by the clay minerals: The nano-sized clay minerals are very reactive and reactions with free radicals, such as hydroxyls, can alter their physical properties.

Rock physics knowledge is essential to evaluate mechanical strength, fluid content, fluid flow, and recovery rates. Information about subsurface formations is generally gathered at different scales, which vary in resolution, spatial coverage, and number of parameters measured. There is a need to up- or downscale to increase reliability of prediction. I will discuss applications of rock physics and experimental data to calibrate observations made on the field. I will analyze experiments results on shales in controlled environment and show the different petrophysical controls on seismic properties, for example, on porosity, permeability, cementation, pore-filling, saturation, and compaction.

A special focus will be on "imposter" shales: organic-rich "shales" (ORS) that are gaining popularity as complete reservoir systems; they form the source, seal, and reservoirs. These rocks are erroneously termed as shale – they often do not have any clay minerals. The organic-rich "shale" (ORS) prospects of today are in reality fine-grained organic-rich rocks. The increasing importance of ORS as hydrocarbon reservoir rocks require better understanding of the processes that lead to shale maturity. Successful exploration and production programs for unconventional petroleum systems need reliable identification of their physical properties, maturity, and changes in mechanical, elastic, and flow properties through indirect methods.

The processes that generate extractable hydrocarbons from kerogen are fairly well understood. Detection of the maturity levels of kerogen in organic-rich shales is not as well known. Current methods determine kerogen content by geochemical analysis of core samples or through empirical methods. For example, the maturity of shales at in- situ conditions may be inferred from empirical relationships between shale pressures and downhole resistivity and sonic logs. The ability to determine maturity by the use of indirect measurements such as seismic is still the subject of research. The properties of kerogen are poorly understood and, so, predictions about maturity and rock-kerogen systems remain a challenge. Assessment of maturity from indirect measurements can be greatly enhanced by establishing and exploiting correlations between physical properties, microstructure, and kerogen content. Successful exploitation of the hydrocarbons also requires significant understanding of the natural fracture systems and the ambient stress conditions. This would allow improved project economics in exploration and development of shale oil fields.