Geology Flashcards
Define lithosphere and mention 3 plate boundaries
The rigid/solid outer part of the earth, consisting of the crust and upper mantle is the lithosphere. 100-200km thick. Plate boundaries are either divergent, convergent og transform faults.
Define mineral and rock. Mention some minerals.
A mineral is a natural occurring (inorganic) matter
with a given composition and an ordered arrangement of atoms/componds in a crystal structure. A rock is a cohesive aggregate of minerals. Quartz, Feldspar, Calcite, Pyrite.
What are the three main categories of rock iht. formation? (One has 2 subidivisions)
Igneous rocks come from cooling of hot magma. Plutonic happens at the subsurface where temperature and pressure is higher (slow cooling, Gabbro, Granite).
Volcanic happens at the surface and results in glassy rock-types and ash. (rapid cooling, Basalt, Obsidian).
Sedimentary rocks are formed by erosion and deposition of sediments. Layers of sediments are deposited on top of older layers for thousands or millions of years. This takes place in a subsiding basin. The lowest layers are the oldest. This is the basis for stratigraphy. Sediments were buried and consolidated to rocks, later uplifted by tectonic activity and finally exposed to new erosion.
Metamorphic rocks form by solid-solid reactions between minerals or solid-fluid reactions. This metamorphic reactions form in respone to changes in temp, pressure and fluid conditions. Metamorphosm is commonly associated with ductile rock deformation like folds on the mesopic scale and mineral alignment on the microscopic scale.
Granite: Plutonic igneous
Basalt: Volcanic igneous
Schist: Metamorphism
Sandstone: Sedimentary
Shale: Sedimentary
What is meant by absolute and relative age?
Relative age is the age of a rock layer (or the fossils it contains) compared to other layers. It can be determined by looking at the position of rock layers. Absolute age is the numeric age of a layer of rocks or fossils. Absolute age can be determined by using radiometric dating.
Describe the origin of sedimentary rocks
Sandstones and shale are sedimentary rocks. Sediments form by weathering and erosion of exposed rocks. After transport, deposition, and burial (due to increasing overburden) they become compacted and consolidated to rocks. Sandstones are made of sand-sized grains, and shale of fine-grained particles such as in clay and mud.
Define sandstone, limestone, shale, chalk
Sandstone is a sedimentary rock consisting of usually quartz sand united by some cement (such as silica or calcium carbonate). Sandstone has intergranular porosity, meaning it has well connected pores and good permeability.
Limestone is a rock that is formed chiefly by accumulation of organic remains (such as shells or coral), consists mainly of calcium carbonate. Limestone has diverse porosity types and is therefore hard to predict.
Shale is a fissile rock that is formed by the consolidation of clay, mud, or silt, has a finely stratified or laminated structure, and is composed of minerals essentially unaltered since deposition. Shale has only microporosity with very low or no permeability, unless fractured.
Chalk, a sedimentary rock, is a soft form of limestone that is not well cemented and thus is often powdery and brittle.
What is the difference between folds, faults and fractures? How are they formed? Explain what a normal fault is (se bilde)
Folds are formed by compression by ductile deformation. They differ in size, geometry.
Faults are planes of failure displacing rock relative to each other along the fault plane. Form in response to stretching, squeezing or shortening of the crust. Normal Fault: In the field of geology, a normal fault is a type of dip-slip fault where the hanging wall moves downwards from the footwall. The average dipping angle of a normal fault ranges from 45 to 90 degrees. Normal faults are the opposite of reverse faults.
Fractures are planes of brittle failure not associated with displacements. They affect hard or competent layers rather than ductile and incompetent layers.
Define source rock, reservoir rock and cap rock
Source rocks are rocks that contain sufficient organic material to create hydrocarbons when subjected to heat and pressure over time. Source rocks are usually shales or limestones (sedimentary rocks). To be a productive source rock, the rock needs time to mature (time to form the oil and/or gas) and the hydrocarbons need to be able to migrate to a reservoir or seep. Source rocks are usually a separate layer from the reservoir rock layers but occasionally they can be both source and reservoir. Used for generation
Reservoir rocks are dominantly sedimentary (sandstones and carbonates); however, highly fractured igneous and metamorphic rocks have been known to produce hydrocarbons, albeit on a much smaller scale. The three sedimentary rock types most frequently encountered in oil fields are shales, sandstones, and carbonates. Used for accumulation.
Cap rocks form the top of reservoirs and create a layer that is impervious to fluid flow. By preventing fluids from flowing through it, the fluids are prevented from escaping the reservoir.
Why is sandstone a common reservoir rock and shale commonly acting as a seal?
Sandstone has high porosity and permeability for the petroleum to flow through, shale on the other hand has low or no permeability.
What is meant by hydrocarbon maturation and what causes it?
For there to occur any hydrocarbons the source rocks have to mature, meaning they need to generate and expel hydrocarbons. Sediment burial and basin subsidence lead to heating of the sediments. Petroleum generation from the organic matter (kerogen) starts at 50C with peak conversion at 100C.
What is meant by hydrocarbon migration, where does it migrate from and where does it accumulate?
Hydrocarbons migrate from the deeper sediments through permeable strata and accumulate upward because they are lighter than water (boyoncy).
Primary migration is from the low permeable source rocks. The generation of petroleum is accompanied by volume increase from the transformation of kerogen to liquid and gaseous hydrocarbons → Pore pressure increases. When pore pressure exceeds the fracture pressure → microfractures form, and petroleum migrates out of the source rock along the fractures, then pore pressure drops, and the fractures heal.
→ As more petroleum is formed, pressure increases, fractures re-open, and more petroleum can move out.
Secondary migration is along permeable layers into the reservoir. Secondary migration occurs along permeable layers such as sandstone bedding planes and faults into the reservoir structure. → It can occur over long distances both vertically and laterally in a sedimentary basin. → Secondary migration is buoyancy-driven. For migration to take place the buoyancy pressure acting on the petroleum droplet must be larger than the capillary pressure. In general gas will migrate faster and with greater efficiency than oil.
What kind of geological structures can form hydrocarbon traps?
Anticlinal, fault traps and stratigraphic traps.
What is meant by porosity and permeability? Why is permeability commonly higher in sandstone than in shale?
Porosity is the void space in the reservoir rock(percentage of the total volume of rock). Porosities of potential reservoir rocks range from 10% to more than 30%. Intergranular porosity can be between the grains or intragranular which is within the grains.
Permeability is the measure of the degree to which fluids can be transmitted through a reservoir rock. Unit is Darcy. The permeability is closely related to the porosity and the texture of the rock.
Shale actually has a lot of porosity (often much more than sandstone, may be 30% or so), but extremely low permeability due to the tiny grain size, which reduces the paths that the hyrdocarbons can follow. That means shale has historically been a poor producer of hydrocarbons.
What is the effect of geological parameters like grain-size, sorting, cementation on porosity and permeability?
When grains get bigger, so do permeability and porosity.
When sorting gets better, so does permeability and porosity.
Cementation decreases porosity and permeability
Hardness (H) and specific weight are important physical property of minerals. Explain what determines the hardness (H) and specific weight (“density”).
H: strong (covalent, ionic) or weak (electrostatic) bounds between atoms in the structure.
Specific weight: average atomic number (weight) in the formula and packing.
