- You'll remember from the last slide that one of the key properties of a good reservoir is sufficient permeability such that hydrocarbons will flow through that reservoir easily. The trap element of a conventional hydrocarbon accumulation is what keeps those hydrocarbons from migrating through the reservoir and thus making them available for production from that reservoir. As an analogy, if we take the sponge we talked about in the last slide as a reservoir and submerge it in a bucket of water, it will be saturated with water. When we take it out of the bucket and set it on an unrestricted, flat surface, the water will start to run out, or migrate, from the sponge due to pressure differences. Now, if instead of setting the saturated sponge on an unrestricted surface, we put it in, say, a plastic container, the water will not be able to leak out. It will be trapped by our container. This scenario is an analog for a hydrocarbon trap. A subsurface trap forms when the buoyancy forces driving upward-migrating hydrocarbons through permeable rock, a reservoir, or the sponge in our analog, cannot overcome the forces of impermeable sealing formations, or our plastic container in our analog. There are two basic types of conventional hydrocarbon traps: structural and stratigraphic. Structural traps are formed as a result of changes in the subsurface structure leading to the formation of anticlines, fault blocks, or folds. Hydrocarbons migrated from the source rock into the reservoir and are then trapped in these subsurface closures. Stratigraphic traps are formed as a result of lateral and vertical variations in lithology. Stratigraphy changes from reservoir rocks that have high values of both porosity and permeability, to lithology that has very low permeability. These stratigraphic changes are most often due to changes in deposition. In the next slide, we'll see a couple of cartoons that will demonstrate what has been presented here. What is depicted in these two cartoons is a structural trap in the top diagram, and a stratigraphic trap in the bottom. The structural trap shows a simple anticline where due to buoyancy differences, oil has migrated to the crest of the reservoir section, and then been trapped by a combination of the fold of the rock layers and the impermeable nature of the sealing formation that overlies that reservoir. The stratigraphic trap in the bottom cartoon shows variations in lithology such that the reservoir quality rock is fully encased by impermeable lithology that surrounds it. A stratigraphic situation that might lead to a model similar to this would be a pinnacle reef. Please realize that both these cartoons are meant to be very simplistic realizations or illustrations. Traps found in the real world would be much more complex than either of these pictures depicts.