Chapter 5

Seal Strength vs. Trap Closure--A Fundamental Control on the Distribution of Oil and Gas

by

John K. Sales¹
Mobil Oil Corporation (retired), U.S.A.
¹Currently at RR-1, Box 197, Cabot, Vermont, 05647

Prediction of hydrocarbons ahead of the drill is our highest goal. Every factor relating to it has been studied extensively, except the role that traps play. I suggest that this role is important, and in some cases pivotal. Traps act as "valves," controlling what they retain, leak, and spill. Closure (vertical distance between spillpoint and top of the trap) vs. seal strength (height of the hydrocarbon column the seal can retain before leaking) controls this. Three classes of trap are essential to cover the possibilities. Two key hydrocarbon columns are used as thresholds for the classification: the highest possible gas column and the highest possible total column the seal of the trap will allow. The spillpoint may be above (class 1), below (class 3), or between (class 2) these two thresholds. The three classes distribute hydrocarbons differently. Class 1 traps spill rather than leak gas, finally spilling oil from the trap. Class 3 traps spill neither fluid, but accumulate oil until it balances seal strength; thereafter, gas plus excess oil leaks. Class 2 traps leak gas but spill oil, and have gas/oil contacts suspended in midtrap. The argument that oil has a lower interfacial angle and entry pressure applies only until leakage starts. Then gas's higher position, smaller molecules, and lower viscosity make it more mobile. Traps equilibrate based on this principle: full of gas (class 1), partly full of oil (class 3), or filled with a mixed charge (class 2).

To the extent that the best provinces are exposed to an excess of both fluids, this principle controls world distribution of oil and gas. More practically, in an economy-favoring oil, higher-class traps leak gas, preserving oil in themselves and traps updip. The trap class of a discovery has implications that may modify drilling sequence.