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Vivek PriyadarshiM.Sc Petroleum Geosciences Sem. - I

Deltaic Environment

In Context of Petroleum

Geology

ContentIntroductionDelta Formation ProcessDeposition Of DeltaClassification of DeltasIn context of petroleum geology

Introduction :-

The word Delta was coined by a Greek Philosopher Herodotus to describe the triangular shaped ( ∆ )of alluvial plain formed at the mouth of river body with conference of standing water body.

Delta formation processThese are as follows:-River have sufficient load.Velocity of river water is low.Subsidence in area of deposition.(Deltas undergo subsidence because • Gradual sedimentation and consequent

increase in the weight of delta materials.• Compaction of sediments caused by load

sediments• Enormous thickness of sediments. )

Deposition Of Delta:-Sediment size gradually decreasing from

coastal land towards sea.The deltaic deposits can be classified into three

types of depositional bed settings.

Topset beds Foreset bedsBottomset beds

Top Set Bed Top set bed can be classified into two

types:-

Sub Aerial Topset Environment.Sub Aqueous Topset Environment.

Consists of coarse sediment.

Sub Aerial TopsetDeltaic sequence is mostly made up of

swamp deposits.

Mostly these deposits are mixture of clay and silt.

In arid climate extensive plant cover is missing , topsets deposits show algal mats and mud cracks.

Sub Aqueous TopsetThis sub environment is complex in

nature.Characterized by followings :

Distributary channels.Sub aqueous leeve.Distributary mouth bar.Distal bar.

Fore Set BedConsists of two parts:-

Delta Slope (Delta front).Pro delta.

Medium course to medium fine sediments.

Bottom Set BedAlso known as Distal part. Mainly consists of fine sediments. Deposits are the product of slow deposits

of suspended sediments.

Classification of Deltas:-

In context of petroleum geologyDeltas often contain large amount of oil and gas

because the deltaic process injects porous reservoir sands far out into marine basins with abundant source beds.

Deltas contain many different types of trapsA very large proportion of world’s sedimentary

cover is composed of ancient deltaic deposits.Deltaic processes have a built-in sediment

with upward coarsening grain size profiles in which potential source beds underlie potential reservoirs.

Have a high ratio of gas to oil

Hydrocarbon Source Bed Organic matter that are responsible for

generation of oil and gas in deltaic region are basically marine flora and fauna .

Deltaic facies contain abundant source bed gas prone sources within each delta and oil prone sources in marine shales envelopes.

Hydrocarbon Traps In Deltaic faciesDelta are favorable areas for the entrapment

of hydrocarbon.Delta contains both types of traps.

Structural trap( Roll-over Anticlines and diapiric).

Stratigraphic trap (Mouth bars ,Channel bars and barrier bars types ).

SEAL IN DELTAIC FACIESThe fourth parameter needed for an oil

field to occur is cap rock or seal.

Traps must be overlain by an effective seal that prevents the vertical migration of hydrocarbons.

Shales are the most common seals.Evaporites are the most effective seals.

ConditionsDelta facies are major hydrocarbon provincesas

they fulfill all the five conditions necessary for accumulation of oil and gas.

1.Source bed2.Reservoir bed3.Cap rock4.Arrangement of these 3 such that hydrocarbons

can be trapped.5.The source bed must have been heated

sufficiently to have generated hydrocarbon.

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