Best Paper Award

Abstract: The Design and Function of Oil and Gas Traps

By

W. H. Roberts III

It is in traps that oil and gas are found, and thus traps should yield the most positive information. If we can understand well what is going on in the traps, that should enable us to look back along the migration trail with special insight as to what has been happening. That insight could even extend all the way back to the "source."

This study concludes that traps are the most logical places for hydrocarbon (HC) mixtures to be put together as distinct oil and gas fluids. It follows that traps are not just passive receivers or containers of HC mixtures put together elsewhere. Effective oil and gas traps of different well-known styles have a very important feature in common: structurally and stratigraphically, they are designed to discharge waters from depth. Thus they function as active focal mechanisms to gather and process feedstock waters carrying HCs and other organics. It is a forced-draft system. The concept adds an exciting new dimension to the anticlinal theory. It honors all factual observations around oil and gas deposits

Very simply, the most important function of a trap is to leak water while retaining HCs. The water can leak because the enclosing membranes and cover are water-soaked, like a wick. The HCs and other organics are separated from the waters as they pass through the trap. The separation is caused by abrupt changes in pressure, temperature, and possibly salinity; these are related to the basic change in direction of feedstock (water) movement from lateral to upward. Coalescence of HCs makes bubbles or globules which cannot move easily like water. The ultimate composition of a trapped HC mixture depends on the residence times of the various components, which in turn depend on (1) what the water carries, (2) what the trap retains, and (3) the pore-volume exchange rate.

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