Both conventional and unconventional resources begin the same way — in an organic-rich source rock. What separates them is what happened next: did the hydrocarbons leave the source rock, or not?
Conventional resources migrated into a discrete, permeable, trapped reservoir and flow easily. Unconventional resources stay in the tight, low-permeability source rock and are economic only with horizontal drilling and hydraulic fracturing.
Conventional resources
In a conventional accumulation, the full petroleum system played out as described in the classic model. Hydrocarbons were generated in the source rock, migrated upward through carrier beds, and collected in a porous, permeable reservoir rock held within a trap and capped by a seal.
Because the reservoir is permeable and the oil and gas are concentrated in a discrete trap, the fluids flow readily to a well under their own pressure — at least initially. Conventional reservoirs are the traditional targets of exploration and the source of most historical production.
Unconventional resources
In an unconventional resource, that migration step never effectively happened. The hydrocarbons remain in the rock where they were generated — typically a shale or tight formation — because that rock is too low in permeability to expel them. The source rock and the reservoir are, in effect, the same rock.
This tightness is exactly what makes the resource hard to produce. The hydrocarbons are present, often over vast areas, but they cannot flow to a conventional vertical well at economic rates. Two technologies changed that:
- Horizontal drilling — turning the wellbore sideways to run for thousands of feet through the thin productive layer, exposing far more rock to a single well.
- Hydraulic fracturing — pumping fluid at high pressure to create a network of cracks that give the trapped hydrocarbons pathways to reach the wellbore.
Only with both of these together do shale and tight resources become economic to produce. Their steep early production decline is a direct consequence of producing from such low-permeability rock.
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Frequently asked
Conventional resources migrated out of the source rock into a discrete, permeable reservoir held in a trap, so they flow easily to a well. Unconventional resources stayed in the tight, low-permeability source rock and require horizontal drilling and hydraulic fracturing to produce.
The shale or tight rock holding the hydrocarbons has very low permeability, so the oil and gas cannot flow to a conventional well at economic rates. Hydraulic fracturing creates cracks that give the trapped hydrocarbons pathways to reach the wellbore.
In an unconventional play, shale is both. The hydrocarbons were generated in the shale and never migrated out, so the source rock and the reservoir are effectively the same rock — produced directly with horizontal wells and fracturing.