The Petroleum System- From Source to Trap

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The Petroleum System-From Source to Trap

• A petroleum system encompasses a pod of active
  source rock and all genetically related oil and
  gas accumulations.
• It includes all the geologic elements and
  processes that are essential
• if an oil and gas accumulation is to exist.

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LEVELS OF PETROLEUM INVESTIGATIONS

• Sedimentary Basin Investigations
• Petroleum System Investigations
• Play and Prospect Investigations

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Petroleum System Investigations

• Petroleumpetroleum geochemical correlation
• Petroleumsource rock geochemical correlation
• Burial history chart
• Petroleum system map
• Petroleum system cross section
• Events chart
• Table of hydrocarbon accumulations
• Determination of generationaccumulation
  efficiency

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Identifying a Petroleum System
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Naming a Petroleum System

• The name of a petroleum system contains several
  parts that name the hydrocarbon fluid system
• 1. The source rock in the pod of active source
  rock
• 2. The name of the reservoir rock that contains
  the largest volume of in-place petroleum
• 3. The symbol expressing the level of certainty

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the level of certainty
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A petroleum system has three important temporal
aspects

• Age
• Critical moment
• Preservation time

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age

• The age of a system is the time required for the
  process of generationmigrationaccumulation of
  hydrocarbons.

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critical moment

• The critical moment is the time that best depicts
  the generationmigrationaccumulation
• of hydrocarbons in a petroleum system. A map
  and cross section drawn at the critical moment
  best show the geographic and stratigraphic extent
  of the system. The burial history chart below
  shows the critical moment and the essential
  elements for the fictitious Deer-Boar(.)
  petroleum system.

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critical moment
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preservation time

• The preservation time of the petroleum system
  begins immediately after the generationmigration
  accumulation process occurs and extends to the
  present day. It encompasses any changes to the
  petroleum accumulations during this period.
  During the preservation time, remigration,
  physical or biological degradation, or complete
  destruction of the petroleum may take place.
  During the preservation time, remigrated
  (tertiary migration) petroleum can accumulate in
  reservoir rocks deposited after the
  petroleumsystem formed.

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preservation time
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Mapping a Petroleum System

• A petroleum system is mapped by showing the
  geographic, stratigraphic, and temporal extent of
  the system.

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Examples of Two Petroleum Systems

• The MandalEkofisk(!) petroleum system in the
  Central Graben of the North Sea contains 21.4
  billion bbl of oil and 39.4 trillion ft3 of gas
  in 39 fields (Cornford, 1994). The age of the
  reservoir rock ranges from Devonian to Tertiary
  age with about 85 of the petroleum in rock
  adjacent to the CretaceousTertiary boundary,
  specifically the Ekofisk Formation of Late
  Cretaceous age. Based on geochemical evidence,
  the Upper Jurassic (Kimmeridgian) to Lower
  Cretaceous source rock is the Mandal Formation. A
  positive oilsource rock correlation indicates a
  known system.

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Geologic setting
Comford,1994
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Geologic setting
Comford,1994
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Burial history chart
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Petroleum system cross section
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Oilsource rock correlation
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Petroleum system events chart
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Petroleum Systems- Elements

• Source Rock
• Migration Route
• Reservoir Rock
• Seal Rock
• Trap

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Petroleum System Summary

• Source (Material and Rocks)
• Generation (Maturation)
• Migration
• Trap
• Reservoir

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Source Material

• Kerogen Types
• Type I Algal (oil prone) sapropelic
• Type II Mixed
• Type III Woody (gas prone) humic

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Petroleum System Summary

• Source (Material and Rocks)
• Generation (Maturation)
• Migration
• Trap
• Reservoir

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Primary Migration

• Solution
• Micellesare molecules that behave like soap,
  attaching themselves to a hydrocarbon molecule on
  one end and to an OH- at the other end.
• Diffusionof most hydrocarbons through rocks is
  also exceedingly slow.
• Gas phase migration- Compressed gas can dissolve
  liquid hydrocarbons. Migration of hydrocarbons
  dissolved in the gas phase can facilitate the
  movement of hydrocarbons through the source rock,
  as the gas phase migrates into shallower regions
  where temperature is lower, the liquid
  hydrocarbons come out of solution.
• Oil Phase Migration- The expulsion of the oil out
  of the source rock is a dynamic process driven by
  the oil generation itself.

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Secondary Migration

• Driving forces for migration
• Buoyancy (This force acts vertically and is
  proportional to the density difference between
  water and the hydrocarbon so it is stronger for
  gas than heavier oil)
• Hydrodynamic flow (water potential deflect the
  direction of oil migration, the effect is usually
  minor except in over pressured zones (primary
  migration))

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Resisting forces

• Capillary pressure (opposes movement of fluid
  from coarse-grain to fine- grain rock, also the
  capillary pressure of the water in the reservoir
  resists the movement of oil)

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Petroleum System Summary

• Source (Material and Rocks)
• Generation (Maturation)
• Migration
• Trap
• Reservoir

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Entrapment Style

• High-impedance entrapment style is characterized
  by( laterally continuous seals coupled with a
  moderate to high degree of structural
  deformation.
• LOW-impedance entrapment style is characterized
  by either a high degree of regional seal
  continuity and a low degree of structural
  deformation, or a low degree of regional seal
  effectiveness coupled with a high or low degree
  of structural deformation.

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Sedimentological traps

• Point bar sands surrounded by flood-plain clays
  in a fluvial system.
• Distributary channels within deltaic muds.
• Reefs within lagoonal and marine shales
• Barrier island sands also within lagoonal and
  marine shales

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Petroleum System Summary

• Source (Material and Rocks)
• Generation (Maturation)
• Migration
• Trap
• Reservoir

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seal

• Regional seal (determines migration pathway)
• Local seal (seals the trap)
• Best seals gas hydrates, evaporites (salt),
  organic rich shales, clay rich shales, tight
  carbonates
• Important characteristics of seal rocks
• Low permeability
• Ductility (otherwise they are easily fractured
  during deformation)