Finals - Reservoir Geosciences

A lithologic unit which significantly impedes the flow of hydrocarbons.

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

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

Properties of an Effective Seal

Major Type of Seals

There are three types of geologic 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.

Types of Structural Traps

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

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

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.

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.

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.

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

There are three common fault – petroleum pool associations:

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.

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

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

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

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

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

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

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

The gap in the rock record

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.

Types of Unconformities

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

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

Why is timing important?

Why is retention important?

EFFECT OF FRACTURES ON A SEAL

Thief Beds

ROLE OF FAULTS

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

REQUISITES OF A PETROLEUM SYSTEM

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

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.

sedimentary rocks tracks changes their character through geologic time.

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

He Proposed the Law of Original Horizontality

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

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.

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

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

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.

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.

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

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.

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.

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

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

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

introduced by the German geologist Johannes Walther

He introduced Walthers 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.

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

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

Three branches of 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.

focuses on the composition and texture of sedimentary rocks

focuses on the composition and texture of metamorphic rocks

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

Structure

The basic classification of igneous rocks

formed on the surface of the Earth

formed at considerable depths ( 7- 10 km)

formed at intermediate depths (<2km)

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

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

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

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

Degree of Crystallization

Granularity

Types of Textures: broadly equal in size

Types of Textures: difference in their relative grain size

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

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

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

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

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

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

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

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

Structures due to mobility of magma/lava

Types of Textures

Structures due to cooling of magma

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

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.

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.

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.

Miscellaneous 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

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

Forms of igneous rocks has two types:

Concordant Bodies

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

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

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

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