Oil Formation and Discovery

1. SESSION 1: FORMATION &
DISCOVERY

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2.15 km
7.4 km
500 km
Session 1 – Formation and DiscoverySession 1 – Formation and Discovery

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Learning Objectives
To appreciate:
Where we find oil and gas deposits
Characteristics of oil and gas reservoirs
How we identify possible reservoirs
How the size and productivity of a reservoir
is estimated

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Oil and Gas Deposition Periods
Tertiary: 2­65 million years ago
Cretaceous: 65­136 million years ago
Jurassic: 136­190 million years ago
Triassic: 190­225 million years ago

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-163mil
-135mil

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For Oil and Gas to be Recovered
from the Earth
There must have been an organic­rich source rock to
generate the oil and/or gas
AND the source rock must have been cooked enough
(approx 120 degC)
AND the hydrocarbons must have migrated to a reservoir
rock formation
AND the reservoir and seal must be arranged in such a
way as to trap the hydrocarbon.
AND an impermeable cap rock must seal the reservoir
AND the reservoir must have porosity and permeability

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Porosity & Permeability
A porous rock is one which is full of small
pours or holes which contain the reservoir
fluids
A permeable rock is a porous rock in which
the pours are interconnected, allowing the
hydrocarbon to ‘migrate’ through the rock

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Traps
We need to find the places in the Earth’s crust
where hydrocarbons have not been able to rise
to the surface and escape
Trap requirements:
Impermeable upper barrier
Structure to prevent further upward movement of the
hydrocarbons

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Two types of rock are necessary
to form a hydrocarbon trap……
Reservoir rock ­ porous rock containing
hydrocarbon fluid
Sandstones (90% of reservoirs)
Carbonates
Cap rock ­ Non porous / Impermeable Seal
Clay shales (90% or reservoirs)
Evaporites e.g. salt crystals

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Three types of Trap
Structural Trap - Reverse Fault
Structural Trap - Anticline
Stratigraphic Trap

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Reverse Fault
Oil (black) in the permeable reservoir rock
(green) is trapped by the upper and adjacent
sealing rock (red and white) of zero
permeability

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Anticline
Impermeable Barrier

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Stratigraphic Trap - River
Channel Sand
Oil and gas are
trapped in the
permeable sands
which were
deposited in the
impermeable river
bed and covered by
impermeable
sediments

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Gas, oil and water is trapped and layered
inside reservoir rock according to fluid
density.

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Oil reservoirs are layers of
sedimentary rocks which contain
microscopic globules of oil and
gas, and not underground
lakes…………

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DISTRIBUTION OF PETROLEUM

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Permits and Legislation
Governments allow companies to explore and produce oil and
gas under strict conditions and in return receive all the
information collected about the resources and a large share of
the pre-tax profits.
Permits are issued which regulate all activities associated with
petroleum exploration and production in specifically defined
areas, or ‘license blocks’.
Oil companies then ‘bid’ for the license to explore and /or
produce in these areas
In the case of Bayu Undan, the license areas are 91-12 & 91-13
of the joint zone of co-operation (JDA) which is equally
administered by Australian and East Timorese governments.

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Surveys
Essentially, the task in oil and gas exploration
is to locate sites where there are geological
structures in which oil or gas might have
been trapped, in order to minimise the risk of
the high cost of drilling a ‘dry hole’.
The two types of survey are:
•Geological
•Seismic

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Geological Surveys
The first step for the survey team is to study all
the available geological and geographical
information about the area under investigation
Aerial photographic surveys are often
undertaken, especially in remote areas.
Today, increasing use is made of satellite
images taken from several hundred kilometres
up in space which are able to show features
only a few metres in size.

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N-S Trending Fault
Separating Geological
Regions e.g. Darling
Fault
Separates ‘the hills’
granites
from the sandy Swan
River Plain
Present day drainage System
Colour change
Indicates Change in
Vegetation/Rock Type
Subsidiary Faulting
Relating to Shear along
Major N-S trending Fault

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Potential areas are then chosen for
more detailed survey.
•Geological study of formation
•Analysis of rock samples and fossils
for clues as to their origins and ages.
Much more important these days,
however, is the seismic survey……..

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Seismic Surveys
In this type of survey, sound waves are sent into the
earth where they become reflected by the different rock
layers present.
Seismic surveys can
be carried out
without disturbing
people or damaging
the environment and
indicate what kinds
of rock lie beneath
the surface, and
their depth.

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Before any drilling, a seismic survey is the only way to gather
detailed information from areas lying below water.
The survey is conducted using purpose built ships towing a
number of air operated devices at depth of 6 - 10 metres.
These generate sound
waves by releasing
large bubbles of
compressed air below
the water surface and
are directed at the
ocean bed.
Offshore Seismic

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Seismic Results
The most sophisticated seismic surveys are
three-dimensional, in which the recorded
data is processed in advanced computers to
give a very accurate, 3-D picture of the
formations and structures below the survey
area.

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N

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The next step is to interpret the data to
identify possible reservoir locations
The process is very expensive, @ AU$15,000 per
square kilometre, but time and money spent on
accurate surveys are good investments, since they
help to locate the wells correctly and minimise the
risk of dry holes.
The only sure way to test for oil and gas is to drill
an exploration well into the formation……….

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Purpose of Drilling
Exploration (Wildcat Well)
To determine the contents of the underlying rock
structure
Appraisal
To determine the physical extent, reserves and likely
production rate of the field
Development
To provide a conduit for extracting oil and gas from a
reservoir

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Data Required
Even if the well does not encounter hydrocarbons
important information can be gained from the well
which can aid further exploration:
Permeability
Porosity
Lithology ( Types of rock layers in Formation)
Water Saturation
Downhole Seismic Survey

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Cuttings Analysis
Sample cuttings are routinely
taken (recorded against depth)
from the mud returns and
analysed for:
• Formation lithology
• Rock dating ( by examining
fossils etc)
• Hydrocarbon presence

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Cuttings Analysis
The well site
geologist utilises a
fluoroscope which
uses ultra violet
light to check
cuttings for signs of
hydrocarbons which
will glow brightly

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Core Analysis
A cylindrical rod of the well bore or reservoir rock
is cut and brought to the surface either by:
Using a hollow diamond drill bit to retrieve a 9m to 50
m section of 50mm diameter core
Using a logging tool with a rotary sidewall coring
attachment to retrieve a 75mm long 25mm diameter
slug
Sidewall coring gun to fire catchers into formation
10mm diameter by 20mm long

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Well Logging
By utilising acoustic, electrical, radioactive
and electromagnetic logging tools in the
well bore it is possible to:
Determine lithology
Determine important reservoir parameters
• Porosity
• Permeability
• Reservoir depth (hydrocarbon column)
Determine the reservoir fluid composition

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Well Testing
Purpose - to determine the productivity of the well
and reservoir size by flowing the well through a
fixed choke for a period of time and monitoring
flowrate and pressure decay.
Fluids sampling and analysis – downhole and
surface samples of the reservoir fluids are taken to
the lab for chemical, thermodynamic and physical
property analysis.

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Well Testing
Pressure buildup test
Produce well for around 12 hours at
a high flow rate
Measure
• Stabilised flow rate
• Total production
Shut well in and measure wellbore
pressure increase for an equal period
Can repeat at an increased flow rate
for establishing maximum flow rate

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Summary
In this session we have covered:
Where we find oil and gas deposits
Characteristics of oil and gas reservoirs
How we identify possible reservoirs
How the size and productivity of a reservoir is
estimated.