1. Formation Evaluation Fundamentals
Petroleum Geology
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2. Objectives After completing this module, the student can:
Describe theories of origin of hydrocarbons.
Describe how oil migrates from source rocks through carrier rocks to reservoir rocks.
Identify structural and stratigraphic traps.
Describe lithological properties of various types of reservoir rocks.
Differentiate between porosity and permeability.
Describe fluid saturation
Compare an undisturbed formation with an invaded formation in terms of fluid contents.

3. Hydrocarbon Sources
Inorganic
Organic

4. Inorganic Hydrocarbon Sources
Assumes that some combination of chemical reaction occurs naturally to form petroleum and coal.
Receives little support today.

5. Organic Hydrocarbon Sources
Claims that fossil fuels were formed from remains of plants and animals.
Plant and animal remains were transformed into petroleum by:
Bacterial action
Heat and pressure
Catalytic reactions
Radioactive bombardments.

6. Organic Hydrocarbon Sources

7. Migration and Accumulation
Migration is the natural movement of oil from source rock to reservoir rock.
Migration influences include:
Buoyancy
Hydrodynamics
Capillary action

8. Buoyancy Basic mechanism involved in migration of hydrocarbons.
Occurs because petroleum is lighter than water.
Oil will naturally rise to set on top of water.
Gas will naturally rise above oil.

9. Buoyancy

10. Hydrodynamics
Hydrodynamics are the forces by which one fluid flows past another carrying the second fluid with it.

11. Hydrodynamics

12. Capillary Action
Capillary action is the tendency of a fluid to rise in a small tube or the way a sponge soaks up water.

13. Capillary Action

14. Accumulation
Buoyancy, hydrodynamics, and capillary action cause the fluids to migrate from source rock to reservoir rock.
Reservoir rock must have a cap rock or a confining bed next to it to seal the reservoir.

15. Accumulation

16. Reservoir Traps Structural Traps Stratigraphic Traps
Result from rock layer deformation.
Stratigraphic Traps
Result when a reservoir bed is sealed by other beds
Result when porosity or permeability change within a reservoir bed.

17. Structural Traps A few examples of structural traps are: Anticline
Syncline
Fold
Fault
Salt Dome

18. Structural Traps
An anticline is an elongated fold in which the sides slope downward from the crest.

19. Structural Traps
A syncline is a depressed fold between two crests.

20. Structural Traps
A fold is a bend or flex in a layer or layers of rock.

21. Structural Traps A fault is a break in the earth’s strata.
Often strata on one side of the fault line have been displaced relative to their original positions.
Upward
Downward
Laterally

22. Structural Traps
An example of a fault.

23. Structural Traps
A salt dome (diapir) is an anticlinal uplift of sedimentary rocks caused by the pushing up of a body of salt.
The uplift is typically spherical.

24. Stratigraphic Traps
Stratigraphic Traps: Result when a reservoir bed is sealed by other beds or when porosity or permeability change within a reservoir bed.
A few examples of stratigraphic traps are:
Lens
Unconformity
Pinchout
Porosity Trap

25. Stratigraphic Traps
A lens is an isolated body of sedimentary rock, such as a reef or a sand bar.

26. Stratigraphic Traps
An unconformity is a surface that separates one set of rocks from another younger set.
An unconformity represents a period of non-deposition, weathering, or erosion, either sub-aerial or sub-aqueous, prior to the deposition of the younger set.

27. Stratigraphic Traps
A pinchout is the natural ending of a sedimentary rock
regressive sands
transgressive sands

28. Stratigraphic Traps
A porosity trap is created because of variations in porosity within rock.
It can also be created with secondary porosity.

29. Reservoir Rock Almost all reservoir rocks are sedimentary.
Sedimentary rocks include:
Sandstone
Conglomerate
Limestone
Dolomite

30. Reservoir Rock Sand Sandstone
Loose, uncompacted (unconsolidated) quartz or feldspar.
Sandstone
Compacted (consolidated) quartz or feldspar.

31. Reservoir Rock Cementation can decrease or increase porosity.
Example: cementation by fluid movement

32. Reservoir Rock Conglomerate
A cemented clastic rock containing rounded rock fragments of gravel, pebble size.

33. Reservoir Rock Limestone
Formed chiefly by accumulation of shells and coral
Consists mainly of calcium carbonate

34. Reservoir Rock
Water as a dissolving agent.

35. Rock Properties
Lithology is a description of the mineralogy and rock type present.
Lithological types (and oilfield abbreviations) include:
SS: sandstone
LS: limestone
DOL: dolomite
SH: shale
SALT: crystalline salt

36. Porosity
Porosity is the ratio of the void space in a rock to the bulk volume or size of the rock.

37. Porosity Porosity varies relative to grain shape and arrangement.
Normally porosity varies between 10% and 30%.

38. Porosity Cubic porosity provides the maximum possible porosity = 47%
When grains are stacked in a rhombohedral fashion, porosity = 26%

39. Porosity
If formation consists of varying sized grains in a closely packed arrangement, porosity = 10%
If grains are bricklike and loosely stacked, porosity = 1%

40. Porosity Types of porosity include: Total Porosity:
Ratio of total void space in a rock to the bulk volume of the rock.
Effective Porosity:
Portion of total pore space in saturated permeable rock in which movement of fluids takes place.
Ratio of interconnected pore space in a rock to bulk volume of the rock.

41. Permeability
Permeability is the ability of a rock to transmit a fluid.
It is measured in millidarcies (md).
Permeability is a function of area, length, pressure, viscosity, and flow rate of the fluid.
Formations cannot be produced if permeability is less than 50 md.

42. Permeability
Fluid flow in permeable sand

43. Fluid Properties
Gas, oil, and water are the most commonly found fluids in a reservoir rock.
Fluid saturation
Ratio between the volume that a specific fluid occupies in the reservoir’s void space
Fluid saturation of the formation’s void space of a porous rock is always 100%.
If the void space is said to be half filled with oil and half filled with water, then the rock would have a 50% oil saturation (So) and a 50% water saturation (Sw)

44. Fluid Saturation
Water saturation (Sw): percentage of formation fluid that is water.
Oil saturation (So): percentage of formation fluid that is oil.
Gas saturation (Sg): percentage of formation fluid that is gas.

45. Fluid Density Fluid density describes the mass of formation fluids.
Fluid density determines the location of different fluids in a reservoir.
Gas has the lowest (lightest) density.
Water has the highest (heaviest) density.
Oil lies between gas and water in density.

46. Effect of Fluid Densities

47. Effect of Formation Water

48. Comparison Between Undisturbed and Invaded Formations