Energy resources: Oil

Text chapter 13

Oil (petroleum) and natural gas (pages 391-395)

Petroleum =Latin petra (rock) + oleum (oil)

Necessary conditions for formation:

• Biological productivity (biomass) of the surface water must be able to generate a settling “rain” of dead organisms

• The bottom waters must have low oxygen to prevent decay and scavengers

Formation process (page 392, fig. 13.8):

• Remains of microscopic marine organisms (phytoplankton, zooplankton and larvae of larger animals) get deposited and buried under marine sediments on the sea floor (DEPOSITIONAL BASINS)

• Burial depth is at least 500 m

• Because of burial, pressure and temperature increases

• Ideal temperature for converting organic matter to crude oil is 50-100ºC

• Chemical reactions take place SLOWLY to transform organic materials into hydrocarbon molecules present in oil

For formation and accumulation of an exploitable oil deposit, there must be:

–A SOURCE ROCK for oil generation

–A RESERVOIR ROCK for storage

–A CAPROCK to confine it

–A GEOLOGIC STRUCTURE to “TRAP” the oil

SOURCE ROCKS are:

–Sedimentary rocks, mostly shales and limestones

–Fine grained (<1/16 mm in diameter)

–Almost all source rocks are dark colored (high organic content)

The increased pressure and temperature in the source rock “squeeze” the oil out of the source rock and make it migrate to a lower pressure RESERVOIR ROCK

Pores = small open spaces between grains of sedimentary rocks

Connected pores give a

rock permeability

Reservoir rocks are usually sandstones, limestones or fractured shales (porous and permeable)

Why do reservoir rocks need to be both porous and permeable?

A CAPROCK keeps the oil from escaping to the surface

Are caprocks porous or permeable or both?

Geological structures that “traps” Oil and gas (page 394, Fig. 13.9)

• ANTICLINE TRAP: Convex-upward fold in a layered rock (Fig. 13.9a)

• FAULT TRAP: When rock layers break (by FAULTING), impermeable layers may lie next to permeable layers and “trap” the oil (Fig. 13.9b).

STRATIGRAPHIC TRAP: When a porous and permeable STRATUM (layer) changes laterally into an impermeable layer

Example: ancient coral reefs covered by shale

Oil fields in the Middle East are of this type.

SALT DOMES: Rising “fingers” of impermeable salt traps oil against it as well as in the overlying dome

Example: “Old Spindletop”, Beaumont, TX

OIL PRODUCTION (page 392)

Modern-day oil prospectors use sound waves to locate oil. In one technique, (1) a signal is sent into the rock by a vibrator truck (2) the reflected waves are received by geophones, and, (3) the data is transmitted to a laboratory truck.

Animation courtesy of Elf Aquitaine. http://fossil.energy.gov/education/energylessons/oil/oil2.html

• The first well drilled is called “WILDCAT WELL”.

• Chance of a wildcat well being successful is 1 in 50 (less favorable than the odds in roulette)

• Initially a successful oil well may not need pumping as oil under high reservoir pressure “gush” to the surface (PRIMARY RECOVERY)

• High reservoir pressure may develop from water pressure or dissolved gas.

Eventually the oil flow under natural pressure falls off and pumping becomes necessary.

Up to 75% of the total oil may still remain in the trap even after pumping

SECONDARY RECOVERY methods are applied at this point

Require INJECTION WELLS for injecting fluids and EXTRACTION WELLS for oil removal

SECONDARY RECOVERY methods:

WATERFLOODING

– Pump water through injection wells into the reservoir to wash some of the oil out

– Can generate only 10-15% of the remaining oil (leaves 65-70% of the total oil in the reservoir)

ENHANCED RECOVERY methods:

(most of these methods are still in the experimental stage)

1. THERMAL

– Injection of steam to make the oil less viscous and make it flow

– Injection of air in the well to set fire to the oil to produce gases and heat to increase the flow

http://fossil.energy.gov/education/energylessons/oil/oil4.html

2. CHEMICAL– Water mixed with large molecule

compounds (called SURFACTANTS) to wash the oil

– POLYMERS to break down large oil molecules in smaller parts

– CO2 to generate pressure to push the oil up

– Fluids that mix with oil (miscible) are used to remove “stuck” oil from the reservoir

3. Using MICROBES (biological)

IMPACTS OF FOSSIL FUELSIMPACTS OF FOSSIL FUELS(pages 397- 404)(pages 397- 404)

Impacts of oil drilling/recoveryImpacts of oil drilling/recovery• Disruption of the land and/or ecosystemsDisruption of the land and/or ecosystems

http://www.explorenorth.com/library/photosofweek/caribou_no_impact.htmlhttp://www.explorenorth.com/library/photosofweek/caribou_no_impact.html

• SubsidenceSubsidence

• Water pollution due to leakage from Water pollution due to leakage from pipelines, oil byproducts (brine) pipelines, oil byproducts (brine) and from recovery fluidsand from recovery fluids

http://energy.usgs.gov/ak-cns-images.htmlhttp://energy.usgs.gov/ak-cns-images.html

Oil pipeline on central North Slope coastal plain. Photo by Dave Houseknecht (USGS).

• Oil seepage and oil spills under water Oil seepage and oil spills under water for offshore drilling rigsfor offshore drilling rigs

• Release of toxic drilling mud Release of toxic drilling mud

http://www.rigjobs.co.uk/oil/semisub.shtmlhttp://www.rigjobs.co.uk/oil/semisub.shtml

• Air pollution Air pollution

• Aesthetic degradationAesthetic degradation

http://www.carlmaples.com/Sunset_Rig5.htmlhttp://www.carlmaples.com/Sunset_Rig5.html

In-class assignment (3/31/10)• Work in a group, hand in ONE answer sheet

with the names (first and last) of all group members

• Describe “POROSITY” and “PERMEABILITY” in your own words. Sketches are welcome

• Will every porous rock always be permeable as well? Why or why not?

• What can be the general relationship between the (a) grain size and (b) sorting of sedimentary rocks and permeability? Talk it out in your group

In-class assignment contd. (3/31/10)

Complete the following table as a group using the samples in your tray

Is it porous? Is it permeable?

Sandstone

Shale

Limestone