Production of natural gas from mud rocks (shale) is not new in the United
States. Gas has been produced from Devonian-age shales in NE US since 1821 while
the first industrial-scale shale gas development (Big Sandy Field in Kentucky) from the
Ohio Shale took place in the 1920s. An exponential growth in shale gas exploration and
production, led by the Barnett Shale in Texas, has occurred since the late 1990s. In
2014, shale gas production in the US reached 9.6 TCF (26 BCF/D), which corresponds
to almost 40% of total gas production. The fast decline curve of shale gas wells
necessitates the drilling of thousands of additional wells in order to keep up with the
demand. Due to a decline in natural gas prices in recent years, the focus has shifted to
shallower shale oil reservoirs.
Thick sequences of shale containing varying volumes of gas are found in many basins
across the US. Shales are extremely heterogeneous in their properties but at a scale not
generally considered. Main challenges include, among others: screening exploration
targets, identifying intervals to fracture stimulate and/or drill horizontal wells, and
predicting production rates and EURs. Developing a Shale Gas Model is very complex
because: a) not two shale rocks are alike, and b) there are many parameters that
influence the oil/gas storage capacity and producibility, some of which are uncontrollable.
The quantity (expressed by TOC content) and quality of the organic matter
(expressed by the S2 and HI parameters from Rock-Eval Pyrolysis) and its thermal
maturity (measured by vitrinite reflectance-VRo) are few very important – and easy to
assess – parameters that influence oil and gas generating/storing capacity in the mostly
microporous matrix system present in shale source/reservoirs, commonly referred to as
‘unconventional’ rocks.
The objective of this chapter is to provide the reader with a better understanding of the
variability in the above parameters in nine oil and gas reservoir shales in the United
States- (referred thereunto as “The Magnificent Nine”). Particular emphasis will be
given to the role that organic petrology plays in predicting the types of hydrocarbon
(oil, wet gas/condensate liquids, and dry gas) that will be produced. These nine US
shale formations were selected based on variations in their thermal maturity, organic
richness, kerogen type(s), depositional environment, age, and mineralogical composition.
They are the following: Utica, Marcellus, Woodford, Bakken, New Albany,
Eagle Ford, Niobrara, and Green River. The ninth formation is the Wolfberry, which is
considered to be a ‘hybrid’ play or a combination of unconventional and conventional.
It is hoped that the contents of this chapter will serve as a useful guide to the reader and
that learnings can be directly applied to basins around the world that contain analogous
types of rocks.
Keywords: Organic petrology, Rock-Eval pyrolysis, Fe-SEM, USA shales,
Unconventional hydrocarbons, Exploration.
