Introduction

Shale gas is a kind of unconventional natural gas resources, and accumulated in the mudstones, shales, and other fine-grained sedimentary rocks. Natural gas in shale systems can be biogenic, thermogenic, or the mixture of them. Shale gas is stored as (1) adsorbed gas in organic matter and inorganic minerals, (2) free gas in pores and fractures, and (3) possibly dissolved gas in oil and water (Curtis in AAPG Bull 86:1921–1938, 2002). Methane content in shale gas is generally above 85%, and can be up to 99.8%. Sometime, shale gas may contain a small amount of ethane and other hydrocarbons, nitrogen, carbon dioxide, and other non-hydrocarbon components. Shale gas reservoirs are classified as non-buoyancy driven accumulations, continuous hydrocarbon plays, and self-contained source–reservoir systems.