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BASIN-CENTERED GAS SYSTEM (BCGS)

Asit Baran Mahato

Roll No - 07306003

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Basin Centered Gas System (BCGS)

Characteristics

Types of BCGS

Development of BCGS

System Elements and Processes

Source Rock, Reservoir rocks and Seals

Hydrocarbon Generation, Expulsion and Migration

Trap Development

Global distribution

Summary and Conclusion

Outline of the talk

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Basin-Centered Gas System

gas accumulation in deep basin syncline

Defined as

abnormally pressured, gas-saturated accumulation in low

permeability reservoirs lacking a downdip water contact

...(Law)

Characterized by

(i) low-permeabil ity reservoir rocks,

(ii) commonly occur structurally down dip from more permeable water-filled

reservoirs,

(iii) are almost always either over-pressured or under-pressured,

(iv) have no obvious structural and stratigraphic trapping mechanism, and

(v) cut across stratigraphic units

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(i) Direct BCGS : having gas prone source rocks (Kerogen type III)

(e.g. Greater Green River Basin)

(ii) Indirect BCGS : having oil prone source rocks (Kerogen type I or II)

(e.g. Appalachian Basin)

Downdip from waterDowndip from waterOccurrence

Highly variable>0.7%R0Thermal maturity top of BCGS

Bedding parallelCuts across stratigraphyNature of upper boundary

GoodVariableSeal quality

Lithologic / capillaryCapillarySeal

Thermal cracking of oil to gasHydrocarbon gas generationPressure Mechanism

Over-/underpressuredOver-/underpressuredReservoir pressure

Short/longShortHydrocarbon migration distance

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Development of BCGS

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Development of BCGS continued

Phase I

Normally pressured and 100% water saturated.

With further burial

- compaction of framework grains

- increase in formation pressure

- decrease in water saturation

- rise in temperature

Oil & gas generation starts for indirect system

Overpressured may develop because of

- rapid sedimentation

- Compaction disequilibrium

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Development of BCGS continued

Phase IIDirect systems:

source rock begins to generate more and more gas

rate of gas generation > gas loss,

reservoir pressure exceeding water-wet capillary pressure in pores

water is expelled from the pores; developing gas saturated

overpressured reservoir

Indirect systems:

oil and gas are generated, expelled and migrated

accumulate in structural and stratigraphic t raps with downdip water

undergoes thermal cracking to gas

water expelled from the reservoir

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Development of BCGS continued

Phase III

Phase of pressure loss

Reservoir experiences a period of upli fting, erosional unloading

and/or heat flow perturbations

Gas started to shrinks and may escape from system

System became underpressured

Phase IV

Theoretical phase and probably more applicable for direct system

Gas loss continued

Water reenter into the system

System may evolve to normally pressured, water bearing reservoir

completing the pressure cycle

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System elements and processes

Source Rocks

Direct System:

Commonly humic-type coal beds and carbonaceous shale

Characterized by kerogen type III

e.g. Cretaceous rocks in Rocky Mountain basins

Indirect System

Hydrogen-rich shale

Characterized by kerogen type I and II

Ordovician shale in Appalachian basin

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System elements and processes continued

Reservoir Rocks

Are regionally pervasive

May be single reservoirs of few feet thick (indirect type) or

vertically stacked reservoirs of several thousand feet thick

(direct type)

Low porosity (

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Direct system

Indirect system

System elements and processes continued

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System elements and processes continued

Trap Formation

Secondary importance for direct system

Necessary for accumulation of oi l and gas in indirect system

Because disseminated oil can not generate overpressured situation

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Resources and Spatial Distribution

Global resource and distr ibution is poorly known

In United States 15% of their annual gas production comes from BCGS

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Summary and Conclusion

BCGS is self-sourced, self-sealed, and self-overpressured petroleum

system.

BCGS reservoir is regionally pervasive accumulations of gasencompassing hundreds or thousands of square miles.

Four main distinguishing characteristics are low permeability

reservoirs, abnormal pressure (over- or underpressured), gas

saturation and no downdip water leg.

Two types of BCGS are recognized: direct and indirect having

different types of source rock characteristics. In direct system source

rock is mainly gas-prone and in indirect system it is liquid-prone.

In future it can be considered as a good substitute of conventional

energy resources.

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Thank you

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