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.

LEVELS OF PETROLEUM INVESTIGATIONS Sedimentary Basin Investigations Petroleum System Investigations Play and Prospect Investigations Magoon and Beaumont

Petroleum System Investigations • Petroleum–petroleum geochemical correlation • Petroleum–source rock geochemical correlation • Burial history chart • Petroleum system map • Petroleum system cross section • Events chart • Table of hydrocarbon accumulations • Determination of generation–accumulation efficiency

Identifying a Petroleum System Magoon and Beaumont

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

Magoon and Beaumont

the level of certainty Magoon and Beaumont

A petroleum system has three important temporal aspects Age • Critical moment • Preservation time

age The age of a system is the time required for the process of generation–migration–accumulation of hydrocarbons.

critical moment The critical moment is the time that best depicts the generation–migration–accumulation 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.

critical moment Magoon and Beaumont

preservation time The preservation time of the petroleum system begins immediately after the generation–migration–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.

preservation time Magoon and Beaumont

Mapping a Petroleum System A petroleum system is mapped by showing the geographic, stratigraphic, and temporal extent of the system.

Examples of Two Petroleum Systems The Mandal–Ekofisk(!) 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 Cretaceous–Tertiary 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 oil–source rock correlation indicates a known system.

Geologic setting Comford,1994

Geologic setting Comford,1994

Burial history chart Magoon and Beaumont

Petroleum system cross section Magoon and Beaumont

Oil–source rock correlation Magoon and Beaumont

Petroleum system events chart Magoon and Beaumont

Petroleum Systems- Elements Source Rock Migration Route Reservoir Rock Seal Rock Trap

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

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

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

Primary Migration Solution: Micelles:are molecules that behave like soap, attaching themselves to a hydrocarbon molecule on one end and to an OH- at the other end. Diffusion:of 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.

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))

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)

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

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.

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

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

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)