Traps: Hydrocarbon Seals in a Regional Context
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.
During uplift, class 1 traps may be gas-flushed and thus degrade. Class 3 traps should change minimally. Class 2 traps should vent additional, now more buoyant gas, and make more space for oil, improving their economics. A slightly different scenario applies if seal strength decreases during uplift.
In the North Sea, Gullfaks, Snorre, and most of the Ekofisk group of chalk anticlines are class 3, Oseberg is class 2, and Troll East, is class 1. Turner Valley in Alberta, which has a two-phase fill, perpetual gas flare, great closure, and is surrounded by gas fields with less closure, is class 2. The Mahakam Delta in Indonesia has seemingly random distribution of oil and gas fields, but on closer inspection, the class 1 gas fields may have the less-damaged seals and the class 3 oil fields the more-damaged seals. Despite historic interpretation as a class 1 trap, the Little Lost Soldier Field in Wyoming appears to be class 3.
Figures & Tables
Seals, Traps, and the Petroleum System
This memoir provides the information to help explorationists greatly increase their understanding of seals and traps and thereby markedly improve their ability to forecast hydrocarbon occurrences.