Petroleum geology - class 4 Flashcards
What is a source rock?
Source rocks are units that are or were capable of generating significant amount of hydrocarbons. These type of rocks are uncommon in the geologic record.
What determines the effectivity of the source rock?
The effectivity of source rocks not only depends on the capacity of generating hydrocarbons (volume and richness), but also in the maturation history and its geological setting within a sedimentary basin.
What are the types of source rock?
Source rock can be Oil or gas prone.
Preservation and maturation by me
The preservaton of organic matter occurs when it scapes from the carbon cycle. Dead animal and plants usually consume carbon during the whole life and gives the carbon back to atmosphere when they die and their bodies are oxidized. But some of those animals and plants are taken by currents to anoxic environment where there won’t be bacterias to oxidize their body. They will be burried together with fine grained sediments and go through the whole diagnetic, catagenetic and, sometimes metagenetic processes. In diageneses kerogen will be produced. Then in catagenesis it will go through thermal degradation generating oil and gas. If temperature and depht continue increasing, the hydrocarbons will go through metagenesis and there will be only carbon residues left.
What are the basic requirements for the generation and preservation of organic matter in sediments?
High productivity and oxygen deficiency of the water column and the sea bed are the basic requirements for the generation and preservation of organic matter.
The supply of organic matter to any depositional site is controlled by primary productivity and the depth of water through which the material must settle. Preservation beneath the sediment/water interface is a function of the rate of burial and oxygenation of the bottom waters. Both productivity and oxygen deficiency at the site of deposition can combine to produce excellent source rock, although some source rocks may result from a dominance of only one control.
What are the environments of high organic productivity?
continental margins
lagoons and restricted seas
deltas in warm latitudes
lakes
How do hydrocarbons enrich on C and H?
Kerogen is composed of large hydrocarbon molecules that are stable at low temperatures, but will break down into smaller molecules of liquid and gaseous hydrocarbon compounds with progressive exposure to higher temperatures.
With Increasing depth and temperature and Increasing compaction, what changes does the organic matter go through i the following phases?
- Diagenesis
- Catagenesis
- Metagenesis
- The diagenesis of organic matter composed of biopolymers (protein, carbohydrates, lipids) results in kerogen, a type of geopolymer.
- The temperature increases and catagenesis starts and the kerogen will go through thermal degradation producing oil (oil window), and after cracking that will lead to gas production.
- If the temperature is still increasing, metagenesis starts and only carbon residue will be left.
What is TOC?
How can we measure TOC?
TOC is the measuring of rock’s ability to generate hydrocarbons (Richness). Total Organic Carbon (TOC) present in weight% is the most common method.
The methods to measure TOC (Richness) are:
Pyrolisys: is the decomposition of organic matter by heating in the absence of oxygen. This method is used to measure source richness and maturity
LECO: Uses the LECO analyzer to measure TOC by combusting the organic carbon and measuring the resulting carbon dioxide produced
Well logs: Requires calibrating well log intervals to samples measure for TOC using other methods
What do the methods HI and OI evaluate in a source rock?
Hydrogen index (HI): Amount of hydrogen relative to the organic carbon
Oxygen index (OI) : Amount of oxygen relative to the organic carbon
HI/OI are used to determine the source rock quality of inmature source rocks. For mature source rocks the HI/OI is not indicative of the original source rock quality
What are the types of kerogen?
The type of kerogen determines the source rock quality It can be:
- Sapropelic kerogen (algal) - Type I - oil prone
- Lipid rich kerogen (phyto- and zooplankton) - Type II - oil prone
- Humic kerogen (land plants) - Type III - gas prone
What are the conclusions we can get by using the visual method in kerogen?
Woody - Gas prone
Herbaceous - Oil and gas prone
Vitrinite - Gas prone
Inertite - No potential
Amporphous (mostly algae) - Oil and gas prone
What is transformation?
It is the rate at which hydrocarbons are generated from organic matter.
The chemistry of organic matter combined within a sedimentary rock changes over time, reflecting its temperature and burial history, can tell us about the amount of hydrocarbons generated. Using source rock maturity we can estimate transformation.
Transformation ratio = Hydrocarbons formed/amount of kerogen in the rock capable of generating
What are the methods to measure maturation and transformation rate?
•Tmax : Pyrolysis Tmax is the temperature at which the maximum rate of hydrocarbon generation occurs in a kerogen sample
•Vitrinate reflectance: measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock (%Ro)
•Apatite fission track
•Spore coloration
•Hydrogen index
•Production index
•Computer modeling: Time-temperature index (TTI) and level of organic maturation (LOM).
Time temperature index (TTI): Formula that integrated temperature with the time spent in each temperature as a source rock is buried.
Level of organic maturation: Assumption that reaction rate to produce hydrocarbons form kerogen doubles for each 10ºC increment temperature.
Both methods assume that the geothermal gradient has been constant through time.
