Oil & gas
How are oil and gas formed?
All of the oil and gas we use today began as microscopic plants and animals living in the ocean millions of years ago. As these microscopic plants and animals lived, they absorbed energy from the sun, which was stored as carbon molecules in their bodies. When they died, they sank to the bottom of the sea. And over millions of years, layer after layer of sediment and other plants and bacteria formed layers of an organic polymer called kerogen.
Sand, clay and other minerals were buried with the kerogen during geological time, eventually becoming sedimentary rocks. Rocks which have abundant kerogen are called oil shales or black shales.
As the sedimentary rocks became buried ever deeper, heat and pressure began to rise. The amount of pressure and the degree of heat determined if the material became oil or natural gas.
After oil and natural gas were formed, they tended to migrate through tiny pores in the surrounding rock. Some oil and natural gas migrated all the way to the surface and escaped. Other oil and natural gas deposits migrated until they became trapped under impermeable layers of rock or clay – what we call 'cap rock'. As these trapped deposits built up, reservoirs were formed. This is where we find oil and natural gas today.
What are the differences between conventional and unconventional resources?
Natural gas produced from shale is often referred to as 'unconventional gas' by contrast to 'conventional gas' produced from other kinds of rock usually sandstones or limestones. In contrast shale gas is produced directly from the source rock.
The key outward difference is flow rate. Drilling into a conventional accumulation of oil or gas would normally result in at least some immediate flow.
An unconventional accumulation has to be stimulated in some way before it will even begin to flow. Gas flows much less freely through shales than sandstones or limestones, so the techniques have to be applied in a different way.
Hydraulic fracturing is simply a method used to stimulate the flow of oil or gas either to begin or continue flowing. A conventional well can also be hydraulically fractured to help extraction by improving flow rates. This does not mean it has become an unconventional well as the nature of the hydrocarbon accumulation has not changed.