1.1- Petroleum Geology

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1.1- Petroleum Geology

Dr. M. Watfa

Role of a geologist, role of a geophysicist, forming of

continents, petroleum system elements, sedimentary

basins, source rocks, reservoir rocks, expulsion and

migration, traps, Timings

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2 Copyright 2001-2011 NExT. All rights reserved Material: M. Watfa

Why Are Petrophysical Rock Types Important To You And How Are They Different Than Lithology Based Rock Types ?

1. Please list as many reasons why Petrophysical rock types are

important in reservoir analysis?

2. Please list as many tools you can use to determine

petrophysical rock types?

3. Please discuss the difference between lithology based rock

types and petrophysical based rock types ?

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Role of Seismologist in Field Development Planning

Conduct Structural and Stratigraphic Interpretation of

3D Seismic Data to Build

Structural Maps/ Models that

form Field Framework-Joint

with PG

Builds Synthetic Seismograms and Velocity

Models for Field-Joint with PG

and PP

Advises on Planning and Provides Quality Control for

Seismic Acquisition and

Processing

Works with Quantitative Interpretation (QI) Group to

Conduct Advanced Seismic

Analysis for Reservoir

Property Prediction from

Seismic and Petrophysical

Data -Joint with PP

Helps Build Field Static Models and compute

Reserves with Uncertainties-

Joint with FDP Team

Helps Plan Well Drilling Trajectories and Targets-Joint

with FDP Team

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Role of Geologist in Field Development Planning

Builds Structural Maps/ Models that form Field

Framework-Joint with PS

Interprets and Describes Field Lithologies,

Stratigraphy, and Facies

Correlates Field Stratigraphy from Well Bore

Data to define Layering and

Heterogeneity

Conducts Analysis of Rock Properties from Drill

Cuttings, Core and Logs-

Joint with PP

Builds Property Grid Block Models for Reservoirs-Joint

with PS, PP and RE

Builds Field Static Models and computes Reserves

with Uncertainties-Joint

with FDP Team

Plans Well Drilling Trajectories and Targets-

Joint with FDP Team

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Overview

The story of oil and

natural gas begins far

back in time as long as

500 million years ago.

Over the 4.5 billion years

of it's lifetime the earth

has been in an extremely

slow but constant process

of change.

Forming of Continents

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The progressive separation of

the continents

Started around the Permian age

The continents took their

present shape around the

Jurassic age

Present day Tectonic plate

movements are the remaining

fingerprints.


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Overview

Present day continents were

once joined together in a

super-continent called

Pangaea and have been

drifting apart over the past

150 million years.

Explained by a theory called

plate tectonics - states that

the continents are on

enormous moving plates.

As these plates move in

response to stresses in the

earth's mantle they collide or

spread apart or slide past

each other.


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Overview

In the petroleum industry you will

hear oil described as found in the

late Cretaceous or the late

Jurassic.

These terms refer to the time on

geologic time scale when these

layers were deposited.

Over the course of millions of

years a fortunate timing of

geological events can bring the

right elements together at the

right time in order to produce

economic accumulations of oil.


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Overview

As these plates move over

the surface of the earth, in

response to stresses in the

earth's mantle they collide or

spread apart or slide past

each other.

Mountains are often formed

where plates collide.

And more important to the

industry these deformations

of the earth's crust also form

the basins, traps, and fluid

movement that make

petroleum possible.


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Sedimentary Basin- Presence of a sedimentary rock

Source Rock: a rock with abundant hydrocarbons-prone organic matter

Reservoir Rock: a rock in which oil and gas accumulates

Porosity: space between rock grains in which oil and gas accumulates

Permeability: ease with which oil & gas can move through the pore space between the grains

Seal Rock: a rock through which oil and gas cannot move effectively (such as mudstone or anhydrite)

Migration Route: avenues through which oil and gas move from source rock to trap

Trap: the structural and stratigraphic configuration that focuses oil and gas into an accumulation.

Timing of Events.

Elements of a Petroleum System

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From John Armentrout


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Generation, Migration, and Trapping of Hydrocarbons

Seal

Reservoirrock

Seal

Migration route

Oil/watercontact (OWC)

Hydrocarbonaccumulation

in thereservoir rock

Top of maturity

Source rock

Fault(impermeable)

Seal


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From John Armentrout


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Sedimentary Basin

A depression in the crust of

the Earth formed by plate

tectonic activity in which

sediments accumulate up to

15,000 meters

600 major sedimentary basins

in the world today

onshore basins in green

offshore basins in purple

1000m / 3300ft water depth contour

Sedimentary Basins

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Sedimentary Rock

Characteristics

Principle of superposition

a sedimentary rock layer in a tectonically undisturbed

stratum is younger than the

one beneath and older than

the one above it

Principle of original horizontality

deposition of sediments occurs as essentially

horizontal beds

Sedimentary Basins

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Petroleum SystemGeneration:

Source Rocks

and Maturation

As sedimentary layers accumulate in

basins the stage is set for the

generation of hydrocarbons.

Source Rock- HC Generation

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Petroleum System: Source rock

Organic material is often preserved

with sediments during deposition

The organic material is the carbon-rich

remains of plants and algae

Kerogen is formed from the organic

material with the increased pressure

and temperature at depth

Oil & gas subsequently forms by the

maturation of the kerogen


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Generation:

Source Rocks and

Maturation

When sediments are

deposited, they often

include carbon-rich

remains of many

different organisms

including plants and

algae. This material is

referred to as organic.


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Generation: Source

Rocks and Maturation

Layers accumulate and in time the overburden weight compresses the

sediments.

Temperature increases with depth and the combination of pressure

and heat begins a slow

transformation of the organic

material into petroleum.

Organic material is changed into intermediate kerogens, and then

into oil and gas in a process called

maturation.


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Petroleum System: Maturation

At 2100 m / 7000 ft depth

(65C) the minimum

temperature for the formation of

oil is reached

At deeper levels, at

temperatures above 150C,

organic matter will be

transformed to natural gas

At deeper levels the organic

material gets carbonised


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Temperature

Window

If the temperature is

too low, the organic

material cannot

transform into

hydrocarbon.

If the temperature is

too high, the organic

material and

hydrocarbons are

destroyed.


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Generation: Source

Rocks and Maturation

Both of these

substances are called

hydrocarbons, because

they consist mainly of

the elements hydrogen

and carbon in various

combinations.


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Petroleum System:

Expulsion and Migration

Movement of hydrocarbons from their

source into reservoir rocks

Primary migration / Expulsion

movement of newly generated hydrocarbons out of the source

rock

Secondary migration

The further movement of the hydrocarbons into reservoir rock in

a hydrocarbon trap


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Surface Oil

Seepage

Surface oil lakes

were very

common in Persia

and Iraq. This oil

was used for a

variety of

applications

dating back

thousands of

years.


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Gas Seepage

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Expulsion and Migration:

Because oil and gas are less

dense than water, buoyant forces

cause them to flow up along the

path of least resistance toward

the surface.

The paths newly generated

hydrocarbons take can be along

faults or fractures, or directly

through other permeable rock

layers, called carrier beds.

This is known as secondary

migration.


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Fluids are able to flow through certain rock beds, such as sandstones, due to

their high permeability.

At this point, migrating hydrocarbons can either find their way to the surface and

create seeps Or they can become trapped in porous

reservoir rock - another of the main

requirements of an economic petroleum

accumulation.

Because of migration, oil and gas can end up accumulating far from their source.

Expulsion and Migration: Expulsion and Migration


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Rocks and Rock Types:

Rock Types

Oil is in general generated

and stored in sedimentary

rocks - rocks formed by the

deposition of particles on

the surface or the bottom of

rivers, deltas, and oceans.

The next section will

discuss the characteristics

of sedimentary rocks and

different ways they can be

formed

Reservoir Rocks

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Rocks and Rock Types:

Rock Types

There are three main types of rock which are classified as igneous,

metamorphic, and sedimentary.

This table summarizes some of the attributes of these three types.

Igneous rocks, such as granite, and metamorphic rocks, such as

gneiss, are called basement rocks.

Only when highly fractured can these rocks serve as a reservoir.

Reservoir Rocks

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Rocks and Rock Types:Sedimentary CharacteristicsThere are two main types of

sedimentary rock - Clastic and

carbonate.

Clastic rocks are formed from weathered particles of sand, silt,

or clay - deposited at places

such as river deltas, beaches, or

on the bottom of the ocean.

Some of the Clastic sedimentary rocks commonly seen are

sandstone and shale.

Reservoir Rocks

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Clastic Sedimentary Rocks

Examples

Breccia

Sandstone

Conglomerate

Shale

Some sedimentary rock types

Breccia - Coarse-grained, angular

fragments - little transport;

Conglomerate - Coarse-grained,

mixture of rounded pebbles and sand

ranging widely in size; well rounded

pebbles imply some transport in a high

energy system

Sandstone - commonly quartz,

feldspar, or rock fragments; deposited

in many environments

Shale - very fine grained; composed

primarily of clay; deposited in low-

energy environments such as lakes,

bays, lagoons, of deep marine settings

Reservoir Rocks

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Clastic depositional systems are

those in which the facies are

composed primarily of

transported rock and mineral

fragments. Siliciclastic

Depositional Systems

Alluvial fan

Fluvial (river)

Barrier / strandplain

Deltaic

Submarine fan

Lacustrine (lake)

Eolian (wind)

Paludal (swamp)

Reservoir Rocks

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Rocks and Rock

Types: Sedimentary

Characteristics

The other kind of sedimentary rock is

carbonate rock.

These are often formed from chemical

precipitation of calcium

carbonate or in organic

reefs.

A common carbonate rock is limestone.

Reservoir Rocks

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Upper Thamama (Mid-Cretaceous) Formation Outcrop

Str

ati

gra

hic

Co

lum

n

Reservoir Rocks

Courtesy Schlumberger

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LST Sequence

Deep Sea-Level

Reservoir Rocks

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Carbonate Reef System

Cross section

showing complex

facies relations in a

carbonate reef

setting.

Reservoir quality

varies with facies.

Permeability could

vary in the range 10-

1000 mD for the

same porosity

Reservoir Rocks

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Reservoir Rocks


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A reservoir rock is any rock that has

enough:

o porosity

o permeability

To allow invasion, accumulation and

later, production of hydrocarbons

As mentioned earlier, the vast

majority of reservoir rocks worldwide

are either sandstone or carbonate

rocks

Reservoir Rocks

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Reservoir Rock:

POROSITY

A good reservoir

rock must also have

sufficient porosity to

hold an economic

volume of oil or gas.

Reservoir Rocks

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Reservoir Rock:

PERMEABILITY

Permeability we have already defined as

the measure of ease with which fluid

flows through a rock.

This is necessary so that hydrocarbons

can initially enter the reservoir, and then

later be produced through a well.

In formations with large grains, the

permeability is high and the flow rate

larger.

In a rock with small grains the

permeability is less and the flow lower.

Grain size has no bearing on porosity,

but has a large effect on permeability.

Reservoir Rocks

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Petroleum System: Traps &

Seal

Structural Traps

anticlines

faults

Stratigraphic Traps

angular unconformities

pinchouts

Reefs

Salt Domes

The opposite of a reservoir rock, but

a rock just as necessary for the

formation of an economic petroleum

accumulation is a seal.

Petroleum System: Traps & Seal

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A seal or cap rock can be any rock

with a relatively low permeability

(10-6 to 10-8 Darcies) such that

fluids cannot migrate beyond the

reservoir

Shales and salts such as anhydrite

commonly act as seals

Seals can also be the result of a

barrier to flow such as a fault


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Anticlines

compressive forces act on a section of the crust that is

ductile, causing arch-like

folds to form

Faults

when the tectonic forces are greater than the mechanical

strength of the rock the rock

fractures along a fault

Structural Traps


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Anticline Trap: is an arch shaped fold in

rock in which rock layers are upwardly

convex.

Fault Trap: is a joint or a fracture

has driven an impermeable layer

above permeable layer.


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Eroded top of an anticline in the Zaggaros

Co

urt

esy S

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lum

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rger

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Drilling in the Zaggaros range in the early 70s- before horizontal drilling became a reality

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Normal Fault

Fault Plane analysis from

Borehole imaging:

Depth of the Fault Strike of the fault Angle of the fault Azimuth of the fault Sealing of the fault



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Micro Fault

TD: 62/304

Normal fault

Striking:

N25E-S25W

Down to WNW



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Major Fault example

Dragging effect produced

by a major fault



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Faults on Dipmeter

Growth fault (Louisiana) Drag fault (Mississippi)

Reverse fault with drag (far east)



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Reverse

Fault



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Angular unconformities

a section of sedimentary rock containing reservoir rock tilted and

eroded, and subsequently covered by

further sedimentary rock layers

Pinchouts

sediments are deposited with a slight dip down in the basin

Reefs

Stratigraphic traps


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Unconformity Trap: where a

permeable reservoir rock has been

converted by an impermeable layer

following by nondepositional period

or a time of erosion.

Pinchout Trap: is a channel

through a flood plain has been filled

with permeable sand that was then

surrounded by less permeable clays

or silts when the channel moved.


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Unconformity Dip motif

Angular unconformity Angular unconformity in Algeria



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Unconformity



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61

UnconformityTaconic Unconformity

with Irregular , non planar

bedding contact

Change of Bedding

orientation below the

unconformity



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Salt Domes

Under pressure at depth, salt acts like a slow-moving

liquid. Pressed by the weight

of neighboring rocks, salt will

flow upwards, creating plug-

shaped diapirs or domes of

salt


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Petroleum System: Timing

Timing of geologic events within

the petroleum system is critical.

trap formation before migration

reservoir seal must remain intact

1 in 3 wells drilled actually will find

significant amounts of petroleum

We now need techniques to

predict the potential traps in the

subsurface


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(From John Armentrout)

Petroleum System: Timing of Events

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Figure 9. Petroleum System for North Oman Huqf -- Shuaiba (!), Fahud Salt Basin

(From Pollastro,

USGS Bulletin, 1999)

Petroleum System: Timing of Events

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Time and Scale ProblemsIf a course rock of a given volume generates 1 gm of oil per year,

how many barrels would be generated in one million years?

Oil Density = 0.85 gm/cc

1 barrel [UK] = 163 659.24 cubic centimeter

1 barrel [US, petroleum] = 158 987.29 cubic centimeter

7.185 Barrels (UK)

7.397 Barrels (US)

I gm occupies: 1/0.85 cc = 1.176 cc

In one million years we generate: 1,176,000 cubic centimetre of oil

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Time and Scale Problems

If an oil droplet moves 1 cm in a year through a carrier bed,

how far could it move in a million years?

= 1,000,000 cm in one-million years

= 1,000,000 / 100 /1000 = 10 km

To meters To kilometres

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Documents

1.1- Petroleum Geology