Oil and Gas Traps

There are many types of traps associated with hydrocarbon accumulations, here are a few examples.

A structural trap formed by the folding of the strata into an anticline. (image1) The hydrocarbons permeate up through the porous rock below until trapped by the denser rock above. One of the first types of traps to be identified, as they are sometimes associated with anticlines on the surface.

This type of structure is readily visible on a seismic section, but, as seismic sections are normally “time-sections”, the actual structure may be obscured (or even artificially produced) by velocity changes above the anticline.

A structural trap formed around a fault. (image2) Many combinations are possible, requiring the impermeable cap rock to be present on the other side of the fault.

Under certain conditions the hydrocarbons will actually migrate along the fault plane, and so move several kilometres from the source rock.

May be difficult to identify on the seismic data due to raypath problems around the fault.

Some of the possible traps associated with a salt intrusion. (image3) The actual trap may form above the salt plug, in which case it may be easier to image on the seismic data than those on the sides of the plug.

Another early discovery; the first seismic reflection surveys were used to identify salt intrusions in and around the Gulf of Mexico and southern USA.

A more complex structural trap, where the reservoir rock was first folded and eroded (image4), then sealed by an impermeable rock which was deposited later over the eroded structure.

Although this type of structure may be visible on the seismic data, it may not be obvious that this is a trap.

A stratigraphic trap formed by lateral changes within one (apparent) rock layer.  (image5)  In this case, the reservoir rock may, for example, have come from river sands which were deposited amongst clays from surrounding swamps. The clays have solidified into an impermeable seal, so trapping the hydrocarbons within the sandstone.

The most difficult type of trap to find on seismic data. May require additional processing (tied to information from wells) to identify.

There are obviously many more types of trap possible (some still to be found) but you should now have a basic idea of the kinds of structures that may be “interesting” on a seismic section. We’ll now move on to some very basic mathematics!