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Where do most geochemists believe petroleum is derived from?
What is the process of recovering kerogen from sedimentary rocks by dissolving most of the rock away with HCl or HF?
Kerogen Recovery
What is the term used to describe the vertical movement of hydrocarbons forming a continuous oil-wet pathway?
Petroleum migration
Petroleum could come from purely abiogenic processes deep in the lower crust.
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What are the different types of kerogen based on their H/C and O/C ratios?
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During petroleum accumulation, hydrocarbons will first be confined to zones of the highest _______ and _______ .
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What are the three primary sources of organic matter in sediments in a marine depositional environment?
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Type I kerogen is primarily sourced from marine deposits.
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The longer the organic matter takes to reach the sediment-water interface, the more opportunity it has to be degraded by _chemical processes.
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Is it true that remigration occurs when oil and gas leave a trap and migrate to another trap?
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Type III kerogens are also known as _____ kerogens.
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Match the organic matter preservation characteristics with sediment types:
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In stratigraphic traps, _ _ _ _ _ _ _ _ _ _ _ _ may take place due to a semipermeable seal.
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Study Notes
Petroleum Generation, Maturation, and Accumulation
Introduction
- Petroleum could come from abiogenic processes deep in the lower crust or mantle, but most geochemists believe it comes from the transformation of preexisting organic matter of biological origin.
- The Fischer-Tropsch reaction is a possible mechanism for abiogenic processes, but it cannot account for the volumes of petroleum observed in sedimentary basins.
Incorporating Organic Matter into Sediments
- Source rocks are fine-grained sedimentary rocks containing relatively high concentrations of organic matter deposited in aqueous depositional settings.
- Organic matter in sediments can come from three primary sources:
- Autochthonous organic matter: product of biological activity in the water column above the depositional site.
- Allochthonous organic matter: transported from elsewhere, often from terrestrial higher plants.
- Recycled or reworked organic matter: derived from the erosion and redeposition of older sedimentary rocks.
Formation of Source Rocks
- Autochthonous and allochthonous organic matter need to be rapidly transported to the sediment-water interface, buried in sediment, and converted into a stable form.
- Fecal pellets from zooplankton can rapidly transport autochthonous organic matter to the seafloor.
- The preservation of organic matter is dependent on the amount of oxidant available, the consumer organism population, and the burial rate.
Kerogen Formation
- The organic matter that initially goes into the sediment may have experienced some alteration, but it undergoes a major transformation into geological organic matter.
- Chemical processes and microbial activity break down the large molecules and biopolymers into smaller organic compounds.
- The resulting compounds can follow one of two main pathways, forming either solvent-soluble or solvent-insoluble sedimentary organic materials.
Types of Kerogen
- Kerogen is a complex material with a variable composition and structure that may be converted into oil and gas under the proper subsurface conditions.
- Kerogen can be classified into four main types:
- Type I: sapropelic or algal/alginite kerogen, with high initial hydrogen to carbon atomic ratio and low oxygen to carbon atomic ratio.
- Type II: characterized by a relatively high H/C ratio and a low O/C ratio.
- Type III: humic kerogen, with a relatively low H/C ratio and low O/C ratio.
- Type IV: inert or residual kerogen, with the lowest H content and mainly polycyclic aromatic systems.
Source Rock Deposition
- The optimum conditions for source rock deposition include:
- High primary biological productivity in and around the depositional environment.
- Organic matter rich in hydrogen with major contributions from algal/bacterial material, spores, pollen, and leaf cuticle.
- Oxic/anoxic boundary in the depositional environment near or above the sediment-water interface.
- Fine-grained sediments, such as very fine silt to clay or carbonate muds.
Stages of Kerogen Maturation
- Metagenesis is the stage where kerogen has very little hydrogen remaining and forms methane as its only hydrocarbon product.
- Throughout metagenesis, the residual carbon network takes on an increasingly ordered structure.
- The completion of metagenesis occurs at vitrinite reflectance values around 4% and Tmax values above 510°C.
The Oil Window
- The formation of oil from kerogen depends on the amount and type of source material present in the sediments and the thermal history of the kerogen.
- Temperature is the most important factor affecting the generation of oil and gas from organic matter.
- The oil window is the depth range over which oil generation occurs, and it is usually different for most sedimentary basins.### The Oil Window
- The oil window is the temperature range at which an organically rich, oil-prone source rock generates oil.
- Accurate determination of the oil window is crucial in searching for petroleum reservoirs.
- Geochemical methods, such as Tmax, bitumen extraction, gas chromatography, and optical methods like vitrinite reflectance, are used to determine the oil window.
Petroleum Generation and Migration
- Petroleum is found from the Precambrian to the Pleistocene, but it is increasingly abundant in younger sediments.
- The continental split during the Jurassic caused an increase in continental margins and restricted basins, leading to more oil generation.
- Older oil fields are increasingly destroyed over time, making younger sediments more likely to have commercial quantities of oil.
Primary Migration
- Primary migration refers to the process of moving generated petroleum from the source rock to the reservoir/trap.
- It begins with oil moving from interstitial spaces in the source rock to a porous and permeable carrier system, such as sand or silt, or a fault or fracture zone.
- Oil is expelled from the source rock and moves as a liquid phase, displacing pore water, and eventually forms a contiguous oil-wet migration pathway.
Secondary Migration
- Secondary migration refers to the continued movement of hydrocarbons in the subsurface after leaving the source rock and entering a carrier system.
- The main processes governing secondary migration are buoyancy and capillary pressure.
- Hydrocarbons accumulate in the carrier system and are held in place by capillary forces, but as more hydrocarbons accumulate, the buoyancy force increases, allowing them to move upward.
Petroleum Accumulation and Remigration
- Hydrocarbons will at first be confined to zones of highest porosity and permeability in the reservoir, filling may proceed episodically.
- As filling progresses, hydrocarbons occupy areas of lower porosity and permeability, eventually displacing formation water, and the oil-water contact becomes more uniform.
- Once a reservoir is filled, any oil and gas that leaves the trap and migrates to another trap is called remigration.
- Remigration can occur in structural traps when the trap is filled to the spill point or in stratigraphic traps due to a semipermeable seal.
- All seals are imperfect and leak to some extent, resulting in tertiary migration or seepage, which can be classified as microseepage (low-concentration) or macroseepage (high-concentration).
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Description
This quiz covers the processes of petroleum generation, maturation, and accumulation, including abiogenic processes and Fischer-Tropsch reactions.
