Unit 3 - Petroleum Play Flashcards
What is a petroleum play?
- A model or perception of how a specific region of the Earth’s subsurface may be an appropriate target for exploration drilling
What are the 4 big parts of the petroleum play?
- producible reservoir → rock with its connected pore or fracture system
- Petroleum charge system → the source rock for HC and the migrations part to reservoir
- Regional topseal (the capping rock preventing migration out of the reservoir)
- Trap (the geological features defining the physical limits to the reservoir rock in the subsurface)
→ combine to create significant petroleum accumulations at a specific stratigraphic level.
What kind of data do explorationists use to identify the parts of a petroleum play?
- outcrops
- well data
- seismic data
- geological studies by government geological surveys or industry contractors
- infos from discussions with other professionals
What is a prospect?
Identified trap which:
- reasonable chance of containing connected porous rocks containing a fluid (permeable)
- is older than the time in which oil and gas was available because oil/gas migrates from a source rock once it reaches thermal maturity
- Is in location to which oil/gas could move (migrate)
- Is of sufficient potential size with enough confidence (or probability) that oil will be found to warrant drilling an exploration well.
What is the most important task of explorationists after identifying a prospect?
- Provide an estimation of technical risk
What are the key elements of a prospect?
- Reservoir
- Seal
- Source Rock, Maturity and Migration Path
- Timing
- Trap
The analysis of the Petroleum Play leads to?
the estimation of the probability of success for a particular prospect
In prospect appraisal, do explorationists usually focus more on porosity or permeability ?
Usually more on porosity → 10% porosity often equates to a 1mD cut-off which is thought necessary for oil production
The explorationist will therefore risk the occurrence of rock that is greater than 10% porosity in order to define the reservoir
What are the two main elements of textural description of the rock in the petroleum play ?
- grain size
- grain sorting
Why do geologists use the term palaeo- ?
to indicate a feature in the geological record
E.g. palaeo- slope, palaeo-wind
Why do you want to understand the depositional processes through time?
- reconstructions can be made with a certain level of accuracy → even in areas where you don’t have samples
Grain size and sorting are a function of ?
Depositional environment
Porosity and permeability developed in carbonate reservoirs is dominated by ?
Secondary processes
What is a seal?
- a fine grained rock, which prevents oil from migrating to surface
What is a source rock?
a rock unit, that has generated oil or gas in a sufficient amount for migration and accumulation
What do source rocks contain?
biologically-derived organic matter buried as sediment → HC originate from these rocks when the pressure temperature conditions cause changes in original organic matter
How much organic matter do “good” source rocks contain?
5-20%
What are the four main chemical components of organic matter ?
- Protein
- Carbohydrates
- Lipid (marine animals)
- Lignin (land plants)
Why do you need anoxic conditions to preserve organic matter?
- because it restricts the action of organisms, that would otherwise consume organic materials
What are the three main settings, where source rocks are deposited?
- lakes
- deltas
- marine basins
why are lakes good for the deposition of source rocks?
- they usually have a poor turnover of a water column because there isn’t a strong water flow which allows the anoxic accumulation of land-derived (gas prone) or algal-derived (oil prone) organic matter
What is a typical source rock?
- Fine grained → like mudstones → become shale when compressed
Why are source rocks often fine grained muddy sediments?
- are often more likely to be impermeable → largely because of small pore sizes and efficient packing of clay particles
- impermeability prevents oxygenated waters in overlying water column to penetrate the sediment and oxidising the organic matter deposited along with the mud particles
What happens after the organic matter is buried and preserved?
- Organic matter contains various biopolymers → are quickly condensed and converted to insoluble kerogen during diagenesis
- Bitumen is the soluble remnant of this process
- Kerogen has different types and properties depending on the source of organic matter: algal matter, waxy plant fragments, or woody plant fragments.
What is headspace gas ?
Gas given off by drilling cutting samples, when stored in a can
How can you use the relectivity of vitrinite?
The reflectivity of vitrinite to normal light under the microscope increases as the maturity of a coal increases.
How is the vitrinite reflectance measured?
As a percentage of the light reflected back
What is kerogen?
Lipid-rich part of organic matter which is not soluble in common organic solvents
What happens to kerogen during the maturation process?
- it is converted to bitumen, which is the extractable content
What happens to bitumen during migration?
- It turns to petroleum ( the lighter HC migrate more easily and heavier ones are left behind)
- Petroleum is liquid organic substance recovered in wells
- Crude oil is the naturally occurring liquid form of petroleum
What are the most important factors of breaking down kerogen?
- time
- temperature
How come that the composition of the HC produced by maturation processes varies ?
Because initial present organic matter (kerogen) differs in composition
What does TOC stand for?
- total organic carbon
What is the necessary amount of TOC for a shale to be considered a source rock
greater 0.5%
What is the source “kitchen”?
location where oil generation happens
What does it mean that oil gets cracked?
- it turns to gas (heat)
What do you call primary migration?
migration from the source rock into the carrier bed
What is the driver of primary migration?
Pressure build up caused by HC generation
Structural traps are caused by ?
Tectonic, diapiric, gravitational and compactional processes
Why is the timing for a petroleum play so important?
Because timing of trap development relative to the timing of HC- migration is super important
How can you determine the timing of oil migration?
by basin modelling → the time period for each rock unit can be determined using biostratigraphy
What is another word for basin modelling?
Geohistory analysis
How do you do a risk analysis of a Petroleum Play ?
Probabilities are assigned for each of the elements (reservoir, seal, source, trap and timing) from 0 (meaning impossible), to 1 (certain or very likely)
What can you do if one of your probabilities for the main factors is very low?
- more date (e. g., shoot more seismic)
- wait for someone else to do it for you (by drilling a well and making a discovery nearby)
- farm-out
- relinquish the area (thereby saving the rental costs)
What is a conventional play?
- HC are generated in source rock and move/migrate to porous and permeable reservoir rock → there they can be extracted by normal drilling
What is an unconventional play ?
HC are generated in a source rock but some or all of them are trapped within source rock → they will not migrate
- you have to drill into source rock using hydraulic fracturing of rock to provide HC present with pathways to escape into well (“shale gas/oil”)
What is a Lead?
Specific locations, where traps may exist
Play to production ? Key stages
- Play concept
- Leads
- Prospects
- Discovery
- Appraisal
- Development
