А. Comprehension

Date: 2015-10-07; view: 486.

Stratigraphic traps

A second major class of oil traps is the stratigraphic trap. It is related to sediment deposition or erosion and is bounded on one or more sides by zones of low permeability. Because tectonics ultimately controls deposition and erosion, however, few stratigraphic traps are completely without structural influence. The geologic history of most sedimentary basins contains the prerequisites for the formation of stratigraphic traps. Typical examples are fossil carbonate reefs, marine sandstone bars, and deltaic distributary channel sandstones. When buried, each of these geomorphic features provides a potential reservoir, which is often surrounded by finer-grained sediments that may act as source or cap rocks.

Sediments eroded from a landmass and deposited in an adjacent sea change from coarse- to fine-grained with increasing depth of water and distance from shore. Permeable sediments thus grade into impermeable sediments, forming a permeability barrier that eventually could trap migrating petroleum.

Answer the questions:

1) What are the main characteristics of an oil trap?

2) Can traps be classified on the basis of their size? If not, explain why.

3) What can form a barrier to petroleum migration?

4) What types of cap rocks do you know? Name their main features.

5) How do we call the level between the highest point and spill plane in a trap?

6) What rocks can be used as a seal? What is the main characteristic of clay gouge material?

7) What is the difference between major groups of traps?

8) What kinds of structural traps do you know? Describe the methods of their formation.

9) What are the prerequisites of stratigraphic traps formation?

10) What types of stratigraphic traps? Describe them.