Traps and Seals

The existence of reservoir rock alone does not create a reserve, because if hydrocarbons can flow easily into a reservoir rock, they can also flow out. If oil or gas escapes from the reservoir rock and ultimately reaches the Earth’s surface, where it leaks away at an oil seep, there will be none left underground to pump. Thus, for an oil reserve to exist, oil and gas must be trapped underground in the reservoir rock, by means of a geologic configuration called a trap. A field contains one or more traps.

There are two components to an oil or gas trap. First, a seal rock, a relatively impermeable rock such as shale, salt, or unfractured limestone, must lie above the reservoir rock and stop the hydrocarbons from rising further. Second, the seal and reservoir rock bodies must be arranged in a geometry that collects the hydrocarbons in a restricted area. Geologists recognize several types of hydrocarbons trap geometries, four of which are described in Text 4.

Note that when we talk about trapping hydrocarbons underground, we are talking about a temporary process in the context of geologic time. Oil and gas may be trapped for millions of years, but eventually they will manage to pass through a seal rock, because no rock is absolutely impermeable. Also, in some cases, microbes eat hydrocarbons in the subsurface. Thus, innumerable oil fields that existed in the past have vanished, and the oil fields we find today, if left alone, will disappear millions of years in the future.

Task 2. Read the text and answer the following questions.

1. What is a hydrocarbon system?

2. What process is called hydrocarbon generation?

3. Where do the largest oil fields occur?

4. What are the main features of oil reservoir rocks?

5. What do the porosity and permeability of a sedimentary rock depend on?

6. What rocks make the best reservoir rocks?

7. Why do hydrocarbons migrate?

8. Why the existence of reservoir rock alone does not create a reserve?

9. What geologic configuration is called a trap?

10. What are the main distinguishing features of a seal rock?

11. Does a field contain only one oil and gas trap?

12. How many types of hydrocarbons trap geometries are recognized by geologists?

Task 3. Describe and explain the processes which are represented in Fig.7.

Task 4. Discuss in groups the main characteristics of hydrocarbon systems and the making of a reserve.

Task 5. Write a summary of the text.

Text 4

Task 1. Guess the meaning of words and word combinations from the context. If there are some problems, find their definitions in the dictionary.

Underground trap, anticline trap, fault trap, salt-dome trap, stratigraphic trap, sedimentary beds, mountain building, adjacent rock, impermeable layer, sedimentary basin, overlying strata, surrounding strata, bulbous salt dome, pinch-out.

Types of Oil and Gas Traps

Geologists who work for oil companies spend much of their time trying to identify underground traps. No two traps are exactly alike, but we can classify most into the following four categories.

• Anticline trap: In some places, sedimentary beds are not horizontal, as they are when originally deposited, but have been bent by the forces involved in mountain building. These bends, as we have seen, are called folds. An anticline is a type of fold with an arch-like shape (Fig. 8a). If the layers in the anticline include a source rock overlain by a reservoir rock, overlain by a seal rock, then we have the recipe for an oil reserve. The oil and gas rise from the source rock, enter the reservoir rock, and rise to the crest of the anticline, where they are trapped by the seal rock.

• Fault trap: A fault is a fracture on which there has been sliding. If the slip on the fault crushes and grinds the adjacent rock to make an impermeable layer along the fault, then oil and gas may migrate upward along bedding in the reservoir rock until they stop at the fault surface (Fig. 8b). Alternatively, a fault trap develops if the slip on the fault juxtaposes an impermeable rock layer against a reservoir rock.

• Salt-dome trap: In some sedimentary basins, the sequence of strata contains a thick layer of salt, deposited when the basin was first formed and seawater covering the basin was shallow and very salty. Sandstone, shale, and limestone overlie the salt. The salt layer is not so dense as sandstone or shale, so it is buoyant and tends to rise up slowly through the overlying strata. Once the salt starts to rise, the weight of surrounding strata squeezes the salt out of the layer and up into a growing, bulbous salt dome. As the dome rises, it bends up the adjacent layers of sedimentary rock. Oil and gas in reservoir rock layers migrate upward until they are trapped against the boundary of the salt dome, for salt is not permeable (Fig 8c).

• Stratigraphic trap: In a stratigraphic trap, a tilted reservoir rock bed “pinches out” (thins and disappears along its length) between two impermeable layers. Oil and gas migrating upward along the bed accumulate at the pinch-out (Fig. 8d).

FIGURE 8.14.9

(a) Anticline trap. The oil and gas rise to the crest of the fold.

(b) Fault trap. The oil and gas collect in tilted strata adjacent to the fault.

(c) Salt-dome trap. The oil and gas collect in the tilted strata on the flanks of the dome.

(d) Stratigraphic trap. The oil and gas collect where the reservoir layer pinches out.