Unit 1 - Introduction to the Earth Flashcards
What is a transgression?
- shoreline moves towards land → coastal plain gets drowned → seen in onlap on unconformity surface
Types of sedimentary rocks?
- Clastic - Chemical - Organic
A Clastic sedimentary rock consists of?
Pre-existing rock/mineral particles
How does a Chemical sedimentary rock develop?
By chemical precipitation of minerals
A Organic sedimentary rock consists of?
Biological Detritus (shells, skeletons, leaves etc.)
The grain compositon is determined by?
By the source rock composition - it is called the provenance of the sediment
What is progradation?
a high rate of sediment supply causes shoreline to shift in direction of basin (example: a delta) Accomodation < Supply of Sediment
What is aggredation?
Vertical stacking → shoreline is stable
The genesis of Igneous rocks? (+Examples)
Formed through crystallisation and cooling of molten magma - Volcanic: Lava rock, small crystals, gas holes - Plutonic: Deep intrusion → cooling and the process is slower → large crystals
Description of Sedimentary Rocks?
Bits or clasts, that have been eroded from earlier rocks or shell fragments or calcium or carbonate material and deposited or laid down in layers and buried. Maybe also crystallized from aqueous solutions
What is the effective porosity?
The interconnected pore volume
How are basins often created?
By Crustal extension → creates among other things basins
What is the Meaning of the “Law of Superposition”?
If strata (layers) are not overturned, then the oldest layer is on the bottom and the youngest at the top of the succession
What is Stratigraphy?
The study of sedimentary layers
A description of metamorphic rocks?
They are recrystallized igneous/sedimentary rocks → due to pressure and heat → the mineral orientations are often planar and/or linear
How are Sandstones classified?
Based on the composition, grain shapes, grain size
How are sediments deposited?
In layers → strata
What are reasons for sea-level changes ?
- Glacial ice melting and ice ages (global) - Quick ocean ridge spreading - Local uplift or subsidence of crust → more hot rocks get to the surface, subsidence doesn’t take place quick enough → water gets displaced
What are reasons for the Anisotrophy of Permeability?
- Lamination, cross bedding - Sand body connectivity - Discontinuous shales
Why do mudrocks have more initial porosity than a sandstone, but in depth a lower one?
- Because claystones strongly compactable since they have flat initial grain shapes, due to grain rotations and grain breakage - Sandstones → rounder grains → harder to mechanical compact
What is primary porosity?
Pore space as a result from primary depositional texture
What is secondary porosity?
New pore space due to post depositional processes (dissolution, fracturing)
How can you measure porosity?
-with cuttings, core plugs, wireline logs, seismic measurements
What controls the distribution of porosity and permeability (petrophysical properties) in sedimentary rocks?
The Transport processes and deposition
Which permeability is usually bigger ? Vertical or horizontal ?
Usually horizontal → due to barriers, grain orientation, packing Kv/Kh with anisotrophy between 0-1
What are the main textural components of sediments?
grain- size, sorting, shape, surface, texture, fabric (packing, arrangement)
What is the principle of uniformitarianism?
The assumption that processes happening today, happened in the past (Present is key to past)
The three basic types of unconformity in sedimentary sequences?
- angular unconformity - disconformity - paraconformity
What is a basin?
- a regional depressioin where sediments are actively accumulating → a simple sediment deposition is not enough for a basin!
The original laws of stratigraphy?
- original horizontality : sediments deposited on “flat” surface - superposition: older sediments are on bottom - original continuity: laterally extensive strata
Why do most carbonate reservoirs rely on secondary porosity?
Because early dissolution and cementation with carbonate minerals destroys the primary porosity
What is regression?
The shoreline moves towards the basin, more land gets exposed → seen in offlap → younger beds are deposited in a more restricted area than older ones
What is retrogradation ? What happens and why?
Accomodation > Supply of Sediment small sediment supply → erosion of shoreline because of basin processes → move landwards → cliffs or narrow gravelly beaches
What happens to porosity and permeability if the contact area of grains increases?
Decrease of porosity and permeability
Types of basins?
Rift valleys, passive margins, transform faults, foreland
Igneous Rock Examples?
Granite, Obsidian, Basalt, Gabbro
What is the relative permeability?
The part of permeability for oil
What is paleoflow?
The flow direction of transport mechanism, that was moving the sediment during the deposition (river, wind, sea current) → permeability is usually highest in that direction
How do you call a sequence, that has no unconformity?
Conformable succession
How do you know if sequences in the sedimentary strata are missing?
- Shown by unconformities
Why are PE-Geologists interested in the last 500 Ma years instead of the whole history of the earth?
-Creation time of the source- and reservoir rocks for HC
Which types of rocks are in the crust?
- Igneous - Sedimentary - Metamorphic
What is a dike and what is a sill ?
Dike = vertical magma intrusion Sill = horizontal magma intrusion → into existing rocks
Examples for metamorphic rocks?
Schist, Gneiss, Marble
Why does burial of sediments happen?
Because of extra accomodation → rise of heat and pressure → chemical cementation processes → precipitation of minerals → diagenesis → creation of sedimentary rocks
Grain characteristics ( size, shape, etc.) is determined by ?
Transport/Deposition
What is the property that has the most important control on reservoir properties?
Sediment texture
Grain size range of sand?
0,0625mm - 2mm
What are sediments?
particles that have been or are accumulating in layers
Was ist der hydrostatische Druck oder auch Gravitationsdruck?
Druck, der sich innerhalb eines ruhenden Fluids, das eine Flüssigkeit oder ein Gas ist, durch den Einfluss der Gravitation einstellt
When is a rock considered an organic rock?
organic material > 50%
What are the two main elements in the crust?
- oxygen 2. silicon
Where do trenches form?
Where oceanic crust is being subducted under other oceanic crust → forming deepest parts of the oceans
How are volcanoes created?
As subducted plate descends into asthenosphere, increase in heat causes water within rocks to be released → rises and causes melting of part of the mantle above subducted plate → rising melt creates volcanoes at surface
What is an evidence for sea-floor spreading?
magnetic anomalies (are parallel to spreading axis)
What is stratigraphy?
The spatial and temporal distribution of rock units
What are lithostratigraphic units
rock types
What are biostratigraphic units?
Fossil types
Describe Disconformity!
- strata have the same dip above and below the unconformity
→ no tectonic deformation of the older succession
- there is evidence for erosion at the unconformity level
Describe Paraconformity!
- similar strata at same dip above and below unconformity → no evidence for erosion → difficult to detect, usually requires careful age dating of sequence
Describe Angular Unconformity!
- older strata have been tilted or folded prior to deposition of the newer strata
How do you call a rock unit that looks similar but was deposited at different times in different locations ?
Diachronous → a sediment, that crosses time lines
What is very important to consider about lithologies ?
They are environmentally sensitive, not time sensitive → certain lithologies are deposited in certain environments
Geological Time Scale?
Describe Walter’s Law
Lithologies, that occur in conformable vertical successions of strata, must occur in laterally adjacent environments
What is Biostratigraphy?
- the study of temporal relationships established by occurrence of the same species of animals or plants in the rocks as fossils
How do biostratigraphers define zones?
They use the first appearance level (evolution) and last appearance level (extinction) of one or more species in a succession
