Chapter One - Gas and Oil in Tight Formations

Abstract

Natural gas and crude oil from shale and tight formations offer additional energy-producing resources that can be recovered from shale formations that typically function as both the reservoir rock and the source rock. In terms of chemical makeup, shale gas is typically a dry gas composed primarily of methane but some formations do produce wet gas while crude oil from tight formations is typically more volatile than many crude oils from conventional reservoirs. The shale formations that yield gas and oil are organic-rich shale formations that were previously regarded only as source rocks and seals for gas accumulating in the strata near sandstone and carbonate reservoirs of traditional onshore gas development. On the other hand, the crude oil from shale formations is a light highly volatile crude oil that contains more of the volatile hydrocarbons (C1–C4) than many conventional crude oils. In addition, the gas and oil storage properties of shale are quite different to conventional reservoirs. In the case of shale formations, the natural gas and crude oil present in the matrix system of pores—similar to that found in conventional reservoir rocks—are accompanied by gas or oil that is bound to, or adsorbed on, the surface of inorganic minerals in the shale. The relative contributions and combinations of free gas and oil from matrix porosity and from desorption of adsorbed constituents is a key determinant of the production profile of the well.