Chapter 1 - Origins of Oil & Gas

Question 1

For hydrocarbons to accumulate, three conditions must be met, what’s first?

Answer 1

First, a sedimentary basin must be created-the result of movement of the earth’s crust, which creates large depressions into which sediments from surrounding elevated areas are transported over time.

Question 2

For hydrocarbons to accumulate, three conditions must be met, what’s second?

Answer 2

Second, the sediments laid down in such basins must contain a high level of organic material. This organic-rich matter becomes part of the sedimentary material to create what is called source rock.

Question 3

For hydrocarbons to accumulate, three conditions must be met, what’s third?

Answer 3

Third, over millions of years, the effects of elevated temperature and pressure must be sufficient to convert the material in the source rock into oil and gas.

Question 4

Define maturity.

Answer 4

Maturity describes the degree to which petroleum generation has occurred. Heavy, thick oil is considered immature, having been generated at relatively low temperature. Mature oil – lighter or less viscous – forms at higher temperature.

Question 5

Define migration.

Answer 5

In an important subsequent process called migration, the hydrocarbons must move out of the source rock through cracks, faults, and fissures and into porous and permeable reservoir rock.

Question 6

Petroleum geochemists and geologists widely agree that crude oil is derived what?

Answer 6

Ancient organic matter-ranging from single-celled plankton to more-complex aquatic plants (e.g., algae) and even invertebrates and fish-laid down and then buried and preserved in sediments eons ago at the bottom of ancient oceans.

Question 7

Define kerogen.

Answer 7

As the sediment layer gets thicker, subsequent bacterial processes at work in deep seabed mud (with little or no oxygen present in the mud itself or the water immediately above it) convert the remaining organic matter into a waxy material called kerogen.

Question 8

What is the composition of kerogen?

Answer 8

Kerogen is a complex mixture of large organic molecules whose appearance and characteristics depend on the kind and concentration of materials—such as algae, plankton, bacteria, pollen, resin, and cellulose— of which it is composed. It is from kerogen that oil and gas are generated.

Question 9

Organic matter was not laid down evenly through the various geologic eras (table 1–1). Accumulations were clearly concentrated in a limited number of intervals whose duration was determined by movements in the earth’s crust and by climatic changes. Different periods contributed the following approximate shares of the world’s kerogens:

Answer 9

Middle Cretaceous (about 100 million years ago): nearly 30% Late Jurassic (150 million years ago): 25%
 Late Devonian (350 million years ago): less than 10%
 Silurian (420 million years ago):less than 10% Early Cambrian (550 million years ago): less than 10%

Question 10

The total amount of organic matter stored in the earth’s crust has been estimated to be what?

Answer 10

10 quadrillion (1016) tons, of which approximately 1% (100 trillion [1014] tons) is in organic-rich rocks—principally, shales containing at least 3% organic matter.

Question 11

Kerogen concentrations as low as ____ are generally sufficient to be make source rock (typically shales and limestones) suitable for commercial exploitation of crude oil and natural gas.

Answer 11

1%–3%

Question 12

What is the most common kind of source rock?

Answer 12

Black Shale

Question 13

Define Thermal Gradient.

Answer 13

Temperature plays a key role in the generation of oil and gas from kerogen. As the organic-rich source rock undergoes progressive burial (i.e., as additional sediments are laid down above it), the rock becomes hotter.This phenomenon reflects what is called the geothermal gradient of the earth.

Question 14

For tectonically stable shield areas and sedimentary basins, what is the typical ratio?

Answer 14

1.5-2C per 30.5 meters

Question 15

Define Oil Window.

Answer 15

Oil Window is used to describe the range of temperature or depth within which most of oil’s complex constituents are produced (fig. 1–1).This window is typically 80–220°C (176–428°F) or 2,200– 5,500 meters (7,200–18,000 feet).

Question 16

Answer 16

Did you describe it well?

Question 17

At what temperatures does kerogen being to be transformed into oil?

Answer 17

Below a temperature of about 50°C (122°F), small amounts of kerogen begin to be transformed into oil. Peak conversion occurs at about 100°C (212°F).

Question 18

At what temperature does gas start to produce?

Answer 18

If the temperature rises above 130°C (266°F) for even a brief time, then crude oil itself begins to break into smaller molecules, and gas starts to be produced.

Question 19

Describe they physical process gas goes through as temperature rises.

Answer 19

At first, this will be wet gas and condensate, with high levels of relatively heavy hydrocarbons.With further increases in temperature, the gas will become more dry, containing more of the lighter hydrocarbon gases.

Question 20

Reservoir rock with good porosity and permeability is generally classified as?

Answer 20

Reservoir rock with good porosity and permeability is generally classified as either a clastic or a carbonate system.

Question 21

Define Clastic sediments.

Answer 21

Clastic sediments are formed from fragments of various rocks that were transported and redeposited to create new formations. Sandstones, siltstones, and shales are the most common types.

Question 22

Where are notable clastic depositions found?

Answer 22

Notable clastic depositions are found in river delta regions, such as along the U.S. Gulf Coast, several Venezuelan coastal fields, the Niger River delta in Africa, and the south Caspian Sea.

Question 23

Define Carbonate rock.

Answer 23

Carbonate rock, in contrast, is typically formed by a chemical reaction between calcium and carbonate ions in shallow seas, or by a process called biomineralization.

Question 24

What is the most notable example of carbonate rock?

Answer 24

The most notable example of the latter is the creation of large reefs by marine coral. Carbonate rock also can build up from the constant rain of tiny shell fragments that falls to the sea floor from microorganisms living in the water above. Limestone and dolostone are typical carbonates.

Question 25

Define Traps.

Answer 25

There are several types, all created by prior deformation of the earth’s crust. Highly impermeable rocks above and around the trap seal it in a way that prevents any further significant movement of hydrocarbons upward or laterally.

Question 26

What are the three basic types of hydrocarbon traps?

Answer 26

structural, stratigraphic, and combination

Question 27

Define structural trap.

Answer 27

Structural traps are formed by tectonic processes—movement of the rock plates that comprise the top of the earth’s crust—that deform underlying rock layers.

Question 28

What is the most common structural trap and what is significant about it?

Answer 28

One common type is the anticline, a smooth, archlike fold. By some estimates, nearly 80% of the world’s largest oil reservoirs are found in anticlinal traps.

Question 29

What are the four common traps and describe their formation. (Diagram).

Answer 29

Question 30

Define fault trap.

Answer 30

A second type of structural trap is the fault trap, created by the displacement of rock layers (strata) relative to each other.

Question 31

Define strike-slip.

Answer 31

When rock strata have moved mostly horizontally, a strike-slip fault results (e.g., the San Andreas Fault, in California).

Question 32

Define normal fault.

Answer 32

Movement that is mostly vertical and downward creates a normal fault

Question 33

Define thrust fault.

Answer 33

While vertical upward movement results in a thrust (or reverse) fault.

Question 34

What are the tree types of faults and describe them. Diagram.

Answer 34

Question 35

Define Stratigraphic Traps.

Answer 35

Stratigraphic traps are created when a seal or barrier is formed above and around an oil- or gas-bearing formation by sedimentary deposition of impermeable rock.

Question 36

What are Primary Stratigraphic Traps.

Answer 36

Primary stratigraphic traps include channels and barriers of sandstone in a river delta area, carbonate slopes, coral reefs, and clay-filled channels of dolomite or calcite.

Question 37

What are major oil fields with stratigraphic traps

Answer 37

Major oil fields with stratigraphic traps include Prudhoe Bay (Alaska), East Texas, and a supergiant field along the east coast of Lake Maracaibo, in Venezuela.

Question 38

Define Combination Traps.

Answer 38

Combination traps are formed by a combination of processes that occurred in the sediments during the time of deposition of the reservoir bed. They are also formed by tectonic activity that occurred in the reservoir beds after their deposition.

Question 39

Give an example of a combination trap.

Answer 39

One example of a combination trap is associated with a salt dome—a mass of lighter salt that has pushed upward through heavier surrounding rock and sediments.

Question 40

Define Salt Beds.

Answer 40

Salt beds were formed by the natural evaporation of seawater from an ancient enclosed basin, and the resultant salt layer was then buried by successive layers of sediments over geologic time.

Question 41

Three conclusions can be drawn from this discussion of hydrocarbon generation, migration, and trapping. What’s first?

Answer 41

First, by some estimates, an average of only about 10% of all the gas and oil that forms in a sedimentary basin ever reaches a trap.The remainder never moved from the source rock, is lost during migration, or seeps from the earth’s surface.

Question 42

Three conclusions can be drawn from this discussion of hydrocarbon generation, migration, and trapping. What’s second?

Answer 42

Second, oil may in some cases be found above gas. Even though figures 1–2 and 1–3 show gas accumulating above oil in various traps, the situation can be more complicated.

Question 43

Three conclusions can be drawn from this discussion of hydrocarbon generation, migration, and trapping. What’s third?

Answer 43

Third, as will be discussed in more detail later (see chap. 7, on hydrocarbon production), water is virtually always found in the pore spaces of reservoir rock, intermingled with oil and gas. For this reason, most wells pump not only oil and gas but also mineral-laden water called brine.