Sandstone bodies in incised valleys can be good hydrocarbon reservoirs.
Incised valleys form on the coastal plain and/or continental shelf during a fall in relative sea-level by a combination of fluvial and marine processes (e.g. fluvial erosion, headward erosion). Here's some coastal plain incision by the Orari River in New Zealand — look at those sand bars!
With the caveat that incised valleys can be filled with mud (non-reservoir) too, there are a few reasons why lots of hydrocarbon fields have been discovered in incised valleys:
- Incision spatially focuses sediment delivery to the shelf-edge and slope. So if coarser sediment (needed for porosity preservation and therefore a reservoir) is stranded on the shelf, there'll be more of it the incision.
- Sediment only stays where there is 'accommodation'. A big hole that wasn't there before (like a new valley) is accommodation.
- The valley acts as a container, so channels tend to have a high ratio of thickness to width.
- For these reasons, valleys sometimes fill with multi-storey sand/mud successions. Lots of reservoirs!
- After the sea-level fall comes sea-level rise, or 'transgression'. Everything is flooded, and you're back to a marine environment. There's mud everywhere and so now you have a top seal.
- It's relatively easy to connect a valley to a marine source rock, either directly updip via porosity, or via faults or other conductors.
- Unlike basin-floor fans, which can also be good reservoirs, incised valleys are on the shelf and might still be beneath the modern shelf. This means there are many more options for drilling wells targeting them, and it's much cheaper and easier.
- Compared to unincised systems, incised valleys are relatively easy to spot in well logs and even on seismic. So they are conspicuous, and easily diagnosed.
- Since the start of the incised valley craze in the 1990s, incised valleys have been deliberately targeted, so there's probably some sample bias: lots of wells means lots of discoveries!
The last little gem is that, even if there's no luck exploring on the shelf, incised valleys might mean that coarse sediment has been delivered to the basin floor, opening up a promising play there too.