Lecture 1 - Seals

Question 1

What kind of rock forms a seal? Give examples

Answer 1

Typically crystalline, low permeability materials

• shale/Murdock/flagstone
• Cemented limestone
• Charts
• Evaporates (halite, anhydride, etc.)

Question 2

Why do leaks occur?

Answer 2

Oil and gas less dense than water so buoyantly rise within the sedimentary column, need seal to stop this

Question 3

What happens to oil if it leaks to surface?

Answer 3

Destroyed by bacterial action

Question 4

What is meant by the term membrane seal?

Answer 4

Membrane Seals: Hydrocarbons cannot migrate - force its way through largest pores

Question 5

What is meant by the term hydraulic seal?

Answer 5

Hydraulic Seal: hydrocarbon cannot breach unless they fracture these materials

Question 6

What are the four key characteristics of good seals?

Answer 6

Small pore size
High ductility
Large thickness
Wide lateral extent

Question 7

What is trap fill?

Answer 7

Effective seal + reservoir + hydrocarbons => trap fill

Question 8

What is meant by underfill and overfill?

Answer 8

Underfill - Lack of hydrocarbons or seal failure

Overfill - recharge during extraction, with introduction from deeper reservoirs