Re-recognition of “unconventional” in unconventional oil and gas

JIAO Fangzheng

Table 1 Types and formation mechanisms of oil and gas accumulations (modified according to references [1-2]).

Oil and gas accumu- lation type	Lithology	Hydrocarbon type	Pore throat size	Driving force	Knudsen number	Flow mechanism
Conventional oil and gas far from source	Carbonate rock, sandstone and conglomerate etc.	Mainly conventional oil	>100 μm	Capillary force + buoyancy	<0.01	Darcy flow (pipe flow+seepage flow)
Conventional oil and gas far from source	Carbonate rock, sandstone and conglomerate etc.	Mainly conventional gas	>100 μm	Capillary force + buoyancy	<0.01	Darcy flow (seepage flow)
Conventional oil and gas near source	Carbonate rock, sandstone, etc.	Tight oil and gas	0.1-0.5 μm	Capillary force + buoyancy	0.01-0.10	Mainly Darcy flow (seepage flow)
Unconventional oil and gas integrated source and reservoir	Organic-rich shale	Shale oil	20-50 nm	Overpressure + diffusion	0.10-10.00	Non-Darcy flow (transitional flow)
Unconventional oil and gas integrated source and reservoir	Organic-rich shale	Shale gas	5-20 nm	Overpressure + diffusion	>10.00	Non-Darcy flow (Knudsen flow)