Organic Petrology Characteristics of Selected Shale Oil and Shale Gas Reservoirs in the USA: Examples from “The Magnificent Nine”

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.