This a pretty colored model of a 3-D seismic model one would see if sitting at a 3-D work station in say, downtown Houston processing seismic data on enormous, very fast computers . Or wherever. It is actually of a drilling prospect off the coast of South Africa.

The blue at the bottom of the image is salt related, a piercement dome that has puked up out of the basement for whatever reason. When the pinnacle was growing upward it was pushing clastic sandstones deposited on the sea floor prior to, or in conjunction with the salt event. Some of these sandstones pinched off against the salt wall and provide possible traps for hydrocarbons, some of the later sandstone deposits actually puckered up over the top of the salt anomaly and formed other possible traps for hydrocarbons, see in red.

Over geological time hydrocarbons were cooked in resource beds and migrated sideways, upwards, along fault planes, etc. using paths of least resistance, like porous, permeable sands and. Some of this oil then got trapped in the drape, so to speak.

Some stayed, some leaked out, from improper seals, and continued their migration upward. The leaks are called chimneys (yellow). Chimneys vent very high-end volatile hydrocarbons and natural gas and are a sort of real time image of hydrocarbon migration.

This vented hydrocarbons then wander up into shallower sandstones deposited in later geological time (later is younger, earlier is older). The shallower interval, also red, offered a better trapping mechanism, perhaps the overlying seals were better, and a lot of hydrocarbons got trapped and stayed. That's the drilling prospect, most likely.

In spite a seal integrity, methane and volatiles will still sometimes leak upwards thru smaller, minor chimneys (yellow) and actually leak to the surface, in this case the mud representing the sea floor. You can sometimes see methane leaking thru the sea floor in video imaging and in the Gulf of Mexico, for instance, oil will leak thru the mud sea floor, weather with cooler climates and in sea water, etc. and form tar balls that float to the surface then wash up on the beach and get between your toes and on your shorts. People think this is spill related, curse the oil business, it's just Mama Nature and all quite natural.

Almost all the world's hydrocarbon producing basins leak to the surface in some form or fashion and very early in exploratory work oil and gas "seeps" were very important.