Finals - Reservoir Geosciences

Seals

A lithologic unit which significantly impedes the flow of hydrocarbons.

Pore throat size

the major control on the ability of hydrocarbons to enter the pore network of a given rock.

Brittle lithologies

tend to develop fractures while ductile lithologies tend to flow plastically under deformation.

1. Small throat space (ie. fine grained)
2. Laterally continuous
3. Maintain uniformity of lithology over large areas
4. Relatively ductile

Properties of an Effective Seal

Shales - 65%
Evaporites - 33%
Carbonate - 2%

Major Type of Seals

Structural
Stratigraphic
Combination

There are three types of geologic traps

Structural Traps

It is created by the deformation of rock strata within the Earth’s crust. This deformation can be caused by horizontal compression or tension, vertical movement and differential compaction, which results in the folding, tilting, and faulting within sedimentary rock formations.

Anticlinal and Dome Trap
Salt Dome
Fault Trap

Types of Structural Traps

Anticlinal and Dome Trap

were originally laid down horizontally the folded upward into an arc or dome. Later, hydrocarbons migrate into the porous and permeable reservoir rock.

Anticlinal and Dome Trap

Necessary conditions: An impervious cap rock and a porous reservoir rock; closure occurs in all directions to prevent leakage.

Salt Dome

A trap created by piercement or intrusion of stratified rock layers from below by ductile nonporous salt. The intrusion causes the lower formations nearest the intrusion to be uplifted and truncated along the sides of the intrusion, while layers above are uplifted creating a dome or anticlinal folding.

Fault Trap

occurs as a result of vertical and horizontal stress. At some point the rock layers break, resulting in the rock faces along the fracture moving or slipping past each other into an offset position.

Fault Trap

formed when the faulted formations are tilted toward the vertical. When a non-porous rock face is moved into a position above and opposite a porous rock face, it seals off the natural flow of the hydrocarbons allowing them to accumulate.

Fault Trap

Necessary conditions: The fault plane must have a sealing effect so that it functions as a fluid migration barrier for reservoir rocks

1. The fault itself makes the trap without an ancillary trapping mechanism such as a fold — normal faults are the most common examples.
2. The fault creates another structure (e.g., a fold or horst) that in turn forms the main trap.
3. The fault may be a consequence of another structure that forms the main trap — e.g., the extensional crestal faults that form above some anticlines.

There are three common fault – petroleum pool associations:

Stratigraphic Traps

formed as a result of differences or variations between or within stratified rock layers, creating a change or loss of permeability from one area to another. These traps do not occur as a result of movement of the strata.

Stratigraphic traps

created by any variation in the stratigraphy that is independent of structural deformation, although many stratigraphic traps involve a tectonic component such as tilting of strata

Primary stratigraphic traps

result from variations in facies that developed during sedimentation. These include features such as lenticular, pinch-outs, and appropriate facies changes.

Secondary stratigraphic traps

result from variations that developed after sedimentation, mainly because of diagenesis. These include variations due to porosity enhancement by dissolution or loss by cementation.

Lenticular Traps

A porous area surrounded by non-porous strata. They may be formed from ancient buried river sand bars, beaches, etc.

Pinch-out or lateral graded Trap

A trap created by lateral differential deposition when the environmental deposition changes up-dip.

Unconformity Traps

resulted from the truncation of reservoir rocks and the subsequent sealing of the subcrop by an unconformable, relatively impermeable, fine-grained, rock unit.

Sandstone Lenses

This kind of trap is also difficult to locate from the surface, and requires subsurface exploration techniques.

unconformity

The gap in the rock record

unconformity

it is identified by the erosional surface between rocks of different ages, and represents a major depositional break between the rocks above and below that surface.

1. Disconformity
2. Parallel unconformity
3. Angular unconformity
4. Nonconformity

Types of Unconformities

combination trap

s where two (or more) trapping mechanisms come together to create the trap.

Combination traps

structural closures or deformations in which the reservoir rock covers only part of the structure.

The trap must have been formed before or during the migration of the hydrocarbons. If no trap is present, the migrating hydrocarbons will just move updip until its movement is constrained.

Why is timing important?

Once trapped, the hydrocarbons can further migrate (tertiary migration) or be altered chemically (biodegraded). Tertiary migration will drain the oil field while biodegradation will destroy the quality of the oil.

Why is retention important?

1. A large field can be easily drained if there are several fractures.

EFFECT OF FRACTURES ON A SEAL

rocks with reservoir qualities that abut the reservoir.

Thief Beds

Faults either aid in the [entrapment] of hydrocarbons or cause [leakage] from the trap. They can be sealing or non-sealing.

ROLE OF FAULTS

Source rocks

rich in organic content that must be buried deep enough in the basin

1. Source rocks rich in organic content that must be buried deep enough in the basin so that the temperature will be sufficient to transform the organic matter into petroleum in a process called maturation.
2. The generated petroleum is expelled from the source rock and migrates into a permeable and porous reservoir rock.
3. A seal must envelope the reservoir rock to prevent it from leaking out to the surface or dispersed elsewhere.
4. A trap should exist so that hydrocarbon can be contained and will accumulate within the reservoir.
5. The timing of migration and trap formation is critical.
6. Once it is trapped, retention is important. Post depositional events should prevent it to further migrate or become biodegraded.

REQUISITES OF A PETROLEUM SYSTEM

Stratigraphy

the science of understanding the variations in the successively layered character of rocks and their composition.

Sequence stratigraphy

a branch of sedimentary stratigraphy, deals with the order, or sequence, in which depositionally related stratal successions (time-Rock) units were laid down in the available space or accommodation.

chronostratigraphy

sedimentary rocks tracks changes their character through geologic time.

Law of Original Horizontality

proposed by the Danish geological pioneer Nicholas Steno (1638–1686)

Nicholas Steno (1638–1686)

He Proposed the Law of Original Horizontality

Law of Original Horizontality

This principle states that layers of sediment are originally deposited horizontally under the action of gravity.

Law of Superposition

states that beds of rock on top are usually younger than those deposited below. This is logical, consider a layered cake or a stack of books, you can’t add another layer unless one already exists to begin with.

Law of Lateral Continuity

suggests that all rock layers are laterally continuous and may be broken up or displaced by later events

Cross-cutting relationships

also helps us to understand the timing of events. Younger features cut across older features.

The Principle of Faunal Succession

states that a species appears, exists for a time, and then goes extinct. Time periods are often recognized by the type of fossils you see in them.

Angular unconformities

represented by an older group of rock layers has been tilted, eroded, and another younger set of rock layers were deposited on top of this erosional surface.

Disconformities

are an erosional surface between two sets of rock layers. Unlike with angular unconformities, there is no tilting of the older rock layers.

Nonconformities

are unconformities that separate different rock types. This is commonly the separation between igneous and sedimentary or metamorphic and sedimentary rocks. These types of unconformities usually indicate that a long amount of time has been eroded away before the younger sedimentary rocks were deposited.

Uniformitarianism

one of the most important unifying concepts in the geosciences that suggests that catastrophic processes were not responsible for the landforms that existed on the Earth's surface.

Uniformitarianism

It suggested that the landscape developed over long periods of time through a variety of slow geologic and geomorphic processes.

Cyclicity

intrinsic feature of marine sedimentary basins, and is controlled by relative sea-level change resulting from the interplay of tectonics, sediment supply, and eustasy.

Hierarchy

basic stratigraphic principles apply across a wide range of space and time scale.

Walther’s Law of Facies

introduced by the German geologist Johannes Walther

Johannes Walther

He introduced Walthers Law of Facies

Walther’s Law of Facies

states that any vertical progression of facies is the result of a succession of depositional environments that are laterally juxtaposed to each other.

Petrology

the branch of geology that studies the origin, composition, distribution and structure of rocks.

Lithology

focuses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography is the speciality that deals with microscopic details.

1. Igneous petrology
2. Sedimentary petrology
3. Metamorphic petrology

Three branches of petrology

Igneous petrology

focuses on the composition and texture of igneous rocks (rocks such as granite or basalt which have crystallized from molten rock or magma). Igneous rocks include volcanic and plutonic rocks.

Sedimentary petrology

focuses on the composition and texture of sedimentary rocks

Metamorphic petrology

focuses on the composition and texture of metamorphic rocks

Texture

refers to the mutual relationship of the different mineralogical constituents in a rock

refers to the large scale features or field characteristics of the rocks

Structure

1. extrusive igneous rocks
2. intrusive igneous rocks

The basic classification of igneous rocks

Volcanic rocks

formed on the surface of the Earth

Plutonic rocks

formed at considerable depths ( 7- 10 km)

Hypabyssal rocks

formed at intermediate depths (

coarse grain size

(> 1 mm) is associated with plutonic, or intrusive rocks. Slow cooling usually causes this texture.

fine grain size

(< 1 mm) is associated with volcanic, or extrusive rocks. Rapid cooling usually causes this texture.

Mafic rocks

richer in Mg, Fe, and Ca. They are also darker in color and denser

Felsic rocks

richer in K, Na, Al and Si, and, compared to mafic rocks, are lighter in color as well as density.

1. Holocrystalline
2. Holohyaline
3. Merocrystalline

Degree of Crystallization

1. Coarse-grained
2. Medium-grained
3. Fine-grained

Granularity

Equigranular

Types of Textures: broadly equal in size

Inequigranular

Types of Textures: difference in their relative grain size

Directive

Types of Textures: exhibit perfect or semi perfect parallelism of crystals or crystallites in the direction of the flow of magma

Intergrowth

Types of Textures: two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up or intergrown

Intergranular

Types of Textures: specifically termed intersertal if the material filling the spaces is glassy in nature

Flow structures

Structures of Igneous rocks due to mobility of magma/lava:
development of parallel or nearly parallel layers or bands or streaks in the body of an igneous roc

Pillow structures

Structures of Igneous rocks due to mobility of magma/lava:
development of bulbous, overlapping, pillow like surfaces in the body of igneous mass

Ropy and blocky lava

Structures of Igneous rocks due to mobility of magma/lava:
surfaces show broken and fragmented appearance, these are called the blocky lava

Spherulitic structures

Structures of Igneous rocks due to mobility of magma/lava:
distinguished by the presence of thin mineral fibers of various sizes arranged in perfect or semi perfect radial manner about a common centre

Orbicular structures

Structures of Igneous rocks due to mobility of magma/lava:
rare type of structure of igneous rocks, rock mass appears as if composed of ball like aggregations

1. Flow structures
2. Pillow structures
3. Ropy and blocky lava
4. Spherulitic structures
5. Orbicular structures

Structures due to mobility of magma/lava

1. Equigranular
2. Inequigranular
3. Directive
4. Intergrowth
5. Intergranular

Types of Textures

1. Jointing structure
2. Rift and grain
3. Vesicular structure
4. Miarolitic structure

Structures due to cooling of magma

Jointing structure

Structures due to cooling of magma:
development of cracks or joints in the rocks formed from these sources, these joints sometimes follow definite patterns

Rift and grain

Structures due to cooling of magma:
indicate two separate directions, often used by quarry men, in which the igneous rocks like granite can be broken from the main rock body with a comparative ease. The equally spaced joints are producing cubical blocks.

Vesicular structure

Structures due to cooling of magma:
escape of gases while cooling is going on leads commonly to the formation of cavities of various sizes and shapes in the cooled mass.

Miarolitic structure

Structures due to cooling of magma:
sometimes small and distinct cavities are formed during the crystallization of magma, these cavities often containing projecting crystals are called miarolitic cavities.

1. Reaction structure
2. Xenolithic structure

Miscellaneous Structure

Reaction structure

Miscellaneous Structure:
characterized by the presence in the rock of some incompletely altered minerals conspicuously surrounded on their borders by their alteration products, often happens that some earlier formed minerals react with the magma during the subsequent stages of crystallization

Xenolithic structure

Miscellaneous Structure:
imposed on the igneous rocks because of incorporation of foreign material, the foreign fragments are termed xenoliths

1. Concordant
2. Discordant

Forms of igneous rocks has two types:

1. Sills
2. Phacoliths
3. Lopoliths
4. Laccoliths

Concordant Bodies

Sills

igneous intrusions that have been injected along or between the bedding planes or sedimentary sequence are known

Phacoliths

small sized intrusives that occupy positions in the troughs and crests of bends called folds

Lopoliths

igneous intrusions, which are associated with structural basins, that are sedimentary beds inclined towards a common centre

Laccoliths

concordant intrusions due to which the invaded strata have been arched up or deformed into a dome