BANARAS HINDU UNIVERSITY

1. Deltaic Environment
In Context of Petroleum
Geology
Vivek Priyadarshi
M.Sc Petroleum Geosciences Sem. - I

2. Content
 Introduction
 Delta Formation Process
 Deposition Of Delta
 Classification of Deltas
 In context of petroleum geology

3. 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.

4. Delta formation process
These 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. )

5. 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 beds
 Bottomset beds

8. Top Set Bed
Top set bed can be classified into two types:Sub Aerial Topset Environment.
Sub Aqueous Topset Environment.
Consists of coarse sediment.

9. Sub Aerial Topset
Deltaic 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.

10. Sub Aqueous Topset
This sub environment is complex in nature.
Characterized by followings :
Distributary channels.
Sub aqueous leeve.
Distributary mouth bar.
Distal bar.

11. Fore Set Bed
Consists of two parts:Delta Slope (Delta front).
Pro delta.
Medium course to medium fine sediments.

12. Bottom Set Bed
Also known as Distal part.
 Mainly consists of fine sediments.
 Deposits are the product of slow deposits of
suspended sediments.

14. Classification of Deltas:-

15. In context of petroleum geology
 Deltas 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 traps
 A 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

16. 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.

17. Hydrocarbon Traps In Deltaic
facies
Delta 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 ).

18. SEAL IN DELTAIC FACIES
The 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.

19. Conditions
 Delta facies are major hydrocarbon provincesas they fulfill
all the five conditions necessary for accumulation of oil and
gas.
1.Source bed
2.Reservoir bed
3.Cap rock
4.Arrangement of these 3 such that hydrocarbons can be
trapped.
5.The source bed must have been heated sufficiently to have
generated hydrocarbon.

20. Thank u….