Page 13 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Introduction to shale and tight reservoirs 3

The pore sizes are also used to define shale and tight formations. Zou
et al. (2012) defined the pore throat diameters: shale gas 5e200 nm, tight
oil limestone 40e500 nm, tight oil sandstone 50e900 nm, tight gas sand-
stone 40e700 nm. Some authors classified shale formations as the rocks
where hydrocarbons were generated in situ (source rocks) (Aguilera,
2014), or migrated within a very short distance (Yang et al., 2015), and tight
formations as the formations near source rocks (oil migrated in a short-
distance) (Jia et al., 2014) or source rock-storage reservoir interbedded
reservoirs (Zheng et al., 2017). Actually, a shale formation does not have
to be a source rock. Strictly speaking, shale oil comes from shale formations
like source rocks and mud shale rocks; tight oil comes from
low-permeability sandstones, silty sands, and carbonates. However, in prac-
tice, there seems no clear or agreed difference between these two terms, and
they are used synonymously. Apparently, the term tight formation is
commonly used in China, while the term shale formation is commonly
used in the rest of the world, especially in the United States. More discussion
or review of the subject is provided by Zhou and Yang (2012).
Recently, Zhao et al. (2018) listed some differences between shale and
tight formations which are summarized in Table 1.1.
Despite the above discussions about shale and tight oil reservoirs, the
term tight oil does not have a specific technical, scientific, or geologic defi-
nition. Tight oil is an industry convention that generally refers to oil
produced from very low-permeability shale, sandstone, and carbonate
formations, with permeability being a measure of the ability of a fluid to
flow through the rock. In limited areas of some very low-permeability
formations, small volumes of oil have been produced for many decades
(EIA, 2018a).
However, shale and tight formations should be defined. Table 1.2 may
be used as preliminary definitions.

Table 1.1 Differences between shale and tight formations.
Tight formation Shale formation

Types of hydrocarbon Converted (oil and gas) Converted (oil and gas)
migrated from nearby and unconverted
source rocks organic materials
Rocks Reservoirs (oil and gas) Source rocks
Porosity >6% <3%
Permeability <1 mD (air) <1 nD (probably typo,
should be < 1 mD)