Chapter 2

The Geology of the Earth

and Hydrocarbon habitat

Presenter: Leigh Brooks

1

The early Earth (a) was probably a homogeneous mixture originally, with no continents or oceans. In the process of differentiation, iron sank to the centre and light material floated upward to form a crust (b). As a result, the Earth is a zoned planet (c) with a dense iron core, a crust of light rock on the surface, and, between them, a residual mantle.

2

Formation of the layered earth • This differentiation took place as temperatures deep within the earth

increased to >1500 deg C due heating caused by the radioactive decay of U, Th and K. Probably about 4.5 byrs ago, Iron (Fe) began to melt and sink towards to centre of the earth, as it is denser than the other common elements.

• This process caused the interior to become hot enough for material to melt and become soft (viscous) enough for it to move over long periods of time. Convection began to occur as less dense material rose.

• Lighter material rose to form successively less dense layers, culminating in Si and Al rich material (eg granites) in a crust at the surface as repeated lava flows solidified.

• Escape of gases formed the atmosphere and oceans

• This convective flow within the mantle is responsible for sea floor spreading and continental drift in the crust- and the formation of sedimentary basins – where petroleum is generated and found.

12.3 g/cc

13.6 g/cc

5.5 g/cc

3.3 g/cc

2.90 g/cc

10.0 g/cc

Schematic section through the earth, showing the

division into core, mantle and crust

Velocity of P (compressional) & S (shear) seismic waves: one tool used to interpret the interior of the earth

Another is the earth’s gravity field from which the mean density of the earth can be estimated

WAVE VELOCITY (KILOMETRES PER SECOND)

DE

PT

H IN

EA

RT

H (

KIL

OM

ET

RE

S)

CORE

TRANSITION ZONE

LOWER

MANTLE

MESOSPHERE

ASTHENOSPHERE

LITHOSPHERE

4

S wave P wave

Plate Tectonics: driven by mantle convection

eg African Rift

Plate Tectonics: driven by upwelling molten rock via mantle convection

- new igneous rock created along spreading ridges (such as the Mid Atlantic Ridge).

- Crust destroyed at subduction zones (Earth’s diameter believed to have remained fairly constant)

Plate Tectonics: How do we know all this?

-New molten crust formed at the spreading ridge retains evidence of the earth’s fluctuating magnetic field as it solidifies and the age of the crust can be mapped

-Age of the overlying sediments established by drilling

-Earthquake zones concentrated along plate boundaries

Early (1954) map of seismic activity

One large project to gather data on the crust: Deep Sea Drilling Project and Ocean Drilling Program (ODP) sites – sediment and igneous material cored

Magnetic data confirms age of oceans

5

Deep ocean

drilling vessel

system. The

riser drilling

system provides

diversified

possibilities for

scientific drilling

6

The plate tectonic cycle showing some of the geological fluxes and processes that control them

Spreading centres and subduction zones are earthquake belts

9

This cross-section of a subduction zone with an accretionary complex reveals fluid influence on processes at various depths of the zone. At depths of 1 to 5 km, fluids flow through accreted sediments (small arrows) either along conduits such as faults, stratigraphic horizons, and mud volcanoes, or by porous flow. At depths of 13 to 18 km, water from the subducting slab forms serpentinite within the overlying mantle wedge. It erupts because its density is lower than that of the surrounding peridotite (large arrow at green blobs). At depths of about 80 kilometres, water evolves from the slab and initiates mantle wedge melting, causing arc volcanism (large arrow at red blobs).

7

Plate tectonics: mid Atlantic rifting.

Prolific petroleum fields occur in sedimentary basins along passive margins of divergent rifts (as well as in other tectonic settings) ie along the edges of many continents, which mark the location of where the rifts first formed

Mid Atlantic

Ridge Basins eg

Niger Delta

Plate tectonics: mid Atlantic rift passes through the middle of Iceland

fissure

Divergent rift – most common mechanism for formation of sedimentary basins

Spreading centres

Location of current plates – with sense of motion

Rifting Convergence

Australian Plate moving north at ~ 5cm/yr (50 km/myr), colliding with Eurasian and Pacific plates (convergent margin). Subduction and island arc volcanism occurs to the north along the Indonesian archipelago. Collision caused widespread structuring

next slide

Trench - subduction

Continental crust-continental crust collision (convergent margin). Indian plate collides with Eurasian plate causing buckling and uplift of Himalayas. Huge amount of erosion and subsequent deposition in basins to the south – Ganges and Bengal Fan

•Continental positions in

various geologic time

frames:

•It is likely that seafloor

spreading and drift

occurred prior to the

Permian, but is more

difficult to detail in the

geologic record.

Gondwanaland, as formed by the assemblage of the southern continents. Rifting started in the Jurassic approx 160 million years ago

14

The suggested fitting together of Africa and South America. The present coastlines, shown by the thin lines, do not fit as well as the contours of the continents at a depth of about 6,500 feet below sea level (shown by the heavier lines).

15

Diagrammatic cross-section across a continental margin showing sedimentary basin

8

For further info, good basic description of Plate Tectonics

http://pubs.usgs.gov/gip/dynamic/dynamic.html

Sedimentary Basins - The home of hydrocarbons.

Location of sedimentary basins – global sediment thickness

18

$

$

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Perth

Adelaide

Melbourne

Hobart

Sydney

Brisbane

Darwin

AMADEUS BASIN

OFFICER BASIN

CANNING

BASIN

PERTH BASIN

BASS STRAIT

MURRAY BASIN

SYDNEY

BASIN

CARPENTARIA

BASIN

BONAPARTE

BASIN

ARAFURA BASIN

COOPER BASIN

EROMANGA BASIN

GEORGINA BASIN

BOWEN

BASIN

SURAT

BASIN

CAPRICORN BASIN

DUNTROON BASIN

CARNARVON

BASIN

BROWSE

BASIN

$

$

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Australia’s

Sedimentary Basins

Sediments

Weathering by water, sun, wind, ice (freezing and thawing), mechanical (eg faulting) and chemical action breaks down the igneous rock derived from deeper in the earth into smaller grains which are eroded and transported by water and wind to low lying areas where they are deposited to produce sediments. Sand sized grains are often composed of more stable minerals such as quartz

• Sedimentary Basins can cover areas of tens of 000’s of square kilometres to many millions of square kilometres

• Basins of interest usually contain 5 -12 kilometres thickness of sediments, up to a maximum of 15 km

• Basins form in areas where the earth’s crust has subsided continuously for millions of years due to the plate tectonic forces of collision and rifting.

• Rivers, sourced in higher terrain, flow into these subsiding “holes”, which commonly become lakes or seas. Sediments are deposited as floods wane.

• The Black Sea & Caspian Sea are good examples of present-day basins – being filled by Danube & Volga Rivers respectively.

• Sediments such as sand, mud, corals, shells, lime sand & mud, and volcanics keep filling the space caused by the subsiding basin.

• The amount and rate of uplift near the basin and subsidence within the basin, together with the climate and sea level affects the rate of sediment supply and the depositional environments that prevail in the basin

Sedimentary basins

• The thick sequence of sediments is usually saturated with water because it is below lake or sea-level or below the water table

• As the sand & mud get buried more deeply by addition of more sediments above, they are compacted through the expulsion of water, and through the effects of increasing temperature and pressure & cements they become sandstone, siltstone, mudstone, shale and limestone

• Most (~80%) of basins are caused by lithospheric extension, causing rifting eg African Rift, which is an early stage rift. The increase in heat flow associated with rifting is important in “cooking” the organic matter to produce petroleum

• Some basins result from crustal compression, where sediments shed from rising mountain chains cause flexure and continued subsidence of the crust eg Ganges Basin in front of the Himalayas

• A small number result from strike-slip (ie lateral or sideways) movements of the crust eg basins along the San Andreas Fault in California

Sedimentary basins

Divergent rift – most common mechanism for formation of sedimentary basins

West –East cross section through Sth America >4000km section shows basins at high vertical exaggeration

subduction Magmatism,compression Passive margin, extension 200 km

Extensional basin on coast of Brasil

Smaller Basins also form along strike slip faults or lateral movements in the earth’s crust

Oceanic crust

next slide Failed rift

Cross section through Carnarvon Basin and Exmouth Plateau, North West Shelf, Australia, showing the Carnarvon/Exmouth Basin failed rift and Jurassic oceanic crust at the edge of the continental crust

Extensional basin in North Sea – failed rift

20

How do we know when things happened?

Geologic time scale

• The absolute age of some igneous

rocks can be calculated from the

relative abundance of radioactive

elements such as U, Th & K and the

products of their spontaneous decay

• As various life forms died, their

remains were preserved in

sediments, when conditions were

right, to form fossils

• A record of the evolution of life forms

and their extinctions have been

related to the absolute ages of

adjacent igneous rocks to construct

the geological time scale

Global

eustatic

(sea level)

chart

Major drop in

seal level

• Relative sea level

through geologic

time can be

interpreted from

the sedimentary

record

• Wide variations in

sea level (>100 m)

have occurred in

the past

• Knowing the

global Sea level

was high or low at

a particular time in

the past can help

you interpret the

geology

Papuan Basin

stratigraphy and

eustatic cycle chart

- assuming that processes in

the past are the same as

today, we can interpret the

geological record

- the eustatic chart is used to

help interpret regional geology

and paleogeography and to

predict patterns of

sedimentation

Eg Major drop in sea level (major

sequence boundary) results in more

energetic rivers (higher gradient)

carrying a large influx of coarse

clastics (sandstone) further out into

the basin. Commonly results in

submarine fan deposits in deep

water

You could walk from

PNG to Australia and

then to Tasmania.

Climate & SL drop

has huge effect on

sedimentation

Temperature changes of the Earth in last 400,000 yrs

National Geographic

Sedimentary depositional environments in a delta plain and marginal marine setting - prograding

Position of corehole in next slide

Braided streams, Canterbury Plain, NZ

43

A Typical

vertical

Shoreface

Sequence

– formed as the beach

progrades (moves

laterally) into the basin

due to continued

sediment supply

- moderate to high

energy beach

Mid to Upper Shoreface: sandstone

Lower Shoreface: sandstone HCS Hummocky Cross-

Stratification

Upper Offshore: sandstone, siltstone, shale

Turbidites

Lower Offshore Muds, shales

Beach-Foreshore: sandstone 1S

T

AN

GL

E_O

F_R

EP

OS

E

CR

OS

S B

ED

DIN

G

Grainsize incr.

Typical river dominated delta – Mississippi Delta. No reworking by wave energy to form beaches.

A Typical Submarine Fan Model Redhead et al 2000 GCS SEPM Generally form during a drop in sea level as shelf is exposed

EXTREME VERTICAL EXAGGERATION = 21 X

NW

100km

1km

Pleistocene Delta

(1.7 million to 11,500 years ago)

Pliocene Delta (1.7 to ~3

million years ago)

Sand can Also Get “Stuck” on the Slope if there is “Accommodation” Space / Ponding

Offshore Brunei Looking Landward McGilvery & Cook 2003 GCS SEPM

• Many people think that oil and gas occur in pockets or caves beneath the

ground

• But they actually occur within the pores of the rocks or within fractures.

• The pore-spaces are called “porosity” and the porous rocks are called

“reservoirs”

• Sandstones and Limestones are the two most important reservoir rocks

Sedimentary basins Where does the oil and gas lie beneath the ground?

Reservoirs & Porosity Sandstone reservoirs are rocks formed by compaction of sand

which is defined as particles 0.125 -2mm in diameter. The porosity

is the total pore space between the grains in the rock.

Sandstone

(Reservoir)

Mudstone

(Cap-Rock

or Seal)

Sand Grains

Pore

Spaces

Mud Particles

Pore

Spaces

More Pore Space =

Higher Porosity

Less Pore Space =

Lower Porosity

POROSITY question: Which

Has More Porosity – the gravel

or the sand ? What Porosity

do you Guess ?

• The most obvious demonstration of porosity is a bucketful of dry

sand.

• A significant volume of water can be poured into the bucket until it

finally fills all of the pore spaces, and then forms a layer above the

sand.

• Sandstone commonly has 20% to 35% Porosity

Porosity

• Sandstone forms from sand, granules & pebbles (most commonly silica)

washed out of mountains by rivers. The grains are chipped and abraided

and rounded during transport

• The sand is deposited in broad river plains, deltas, beaches and sand

dunes

• The rivers also contain mud and silt, which form mudstones, siltstones

and shales when they are buried and compacted.

• In most places that we see sand to-day, it simply washes down the river,

or along the beach, and gets moved from one place to another

• It may never be buried & turned into rock.

• Burial and conversion into sandstone only occurs in Sedimentary Basins,

where subsidence continues and more sediment is washed in, burying the

older sediment.

Sandstone

• As we drill deeper into the sub-surface, the pressure in reservoirs increases

steadily. In normally pressured sediments, the pore pressure increases by

~1.4 psi/m due to the weight of connected water above (density ~1gm/cc).

• When we drill into trapped oil or gas in specific localised accumulations, it is

usually at pressures much higher than at the surface (atmospheric pressure)

• This pressure forces the oil & gas into the drill pipe and they may flow to the

surface if the reservoir is sufficiently permeable

• In many cases, large volumes of gas are dissolved in the oil, and when the

pressure is released by drilling a well, this gas effervesces like CO2 from soft

drinks, and helps drive the oil & gas to the surface

• In many (most) boreholes only water is present & this can flow to the surface

as artesian water

Reservoirs

The PORE SPACES in the Reservoir Rocks

can be very easily seen under microscopes:

Sand Grains

Pore

Spaces

Pore Spaces

filled with Oil

Sand

Grains

Carbonates:

Another major

reservoir

lithology

Satellite image of

Great Barrier reef

200x200km view

Carbonates Individual reef on Great Barrier Reef is an analogue for many fields, particularly

in SE Asia where they may have high porosity in the subsurface (when they

have been buried by continued sedimentation)

Warrlich et al SEPM Special Publ No 95, 2009

Carbonates: Seismic cross section through Malampaya oil and

gas Field in a Miocene reef, Philippines.

Blue AI (acoustic impedance, a sonic attribute

calculated from the seismic data) indicates low

porosity while yellow and red indicate higher

porosities

Anticline – This ‘arched’ reservoir

is the simplest common

hydrocarbon trap.

Other traps include

Tilted Fault Blocks (B),

Sub-Unconformity (E),

Salt Diapirs (C) and

Stratigraphic Pinch-

outs (D)

Oil and gas traps

Keel

Space gap atkeystone block

Non-rotationalplanar faulting

Fault termination at depth

Rotational planar Faulting

Listric faulting

Asymmetrical listricfault termination

Structure: Extension

Detachment, beds sliding horizontally

Sea level

Syn-deformation sediments (yellow) grow above the hanging wall (HW)

Pre-deformation sediments are roughly constant thickness

Structure: Extension

Low angle normal faults

HW

Extension,

HW beds below

Regional

Compression,

HW beds above

Regional

Detachment

Detachment

Most faults are curved in section. Movement on a curved extension fault produces a rollover to fill the hole

A family of Listric Thrusts is known as imbricates

Fault-bend Folding

3 types of fold-thrust interactions

ramp

ramp Fault-propagation Folding

Detachment Folding

Structure: Compression

Wage

Shelf Kubor-Mendi Trough

Basin Toro

Shelf Toro

Shelf Toro

Kutubu Mubi

NE

PNG Regional Kutubu - Wage - Nembi Section

SW

A

Nembi Valley Wage

Kutubu

Iagifu

Omati Trough Hedinia Iorogabaiu Mubi

0

0

5

5

10

10

km

km

Darai Limestone

Miocene (distal)

Eocene

Ieru Formation

(Miocene)

(Cretaceous)

Toro Sandstone

Jurassic

Basement

(Neocomian)

(Paleozoic)

0 20 km

Darai Plateau

Structure: Original extension (lower cross section),

later suffered compression (upper cross section)

Anticline – compressional The geological structure (inverted half graben) is interpreted from the

seismic section, with particular interest in the shape of the reservoir rock

which is “mapped” in 3 dimensions, depth and area, via a grid of data.

NE SW

0 5

km

0

1

twt

Tilted fault blocks and half grabens-

extensional

2.5km

SE

Top Cau

Top Dua

Reservoir

Top Cau

Top

Carbonate

Basement

Miocene reef

Carbonate reservoir target

Oligocene/E Miocene

Clastics – Primary Oil Target

NW SE

Secondary Objective

GAS

Nam Con Son Fm

Carbonates

1500 mSS

Primary Objective

OIL

Dua Fm

Marginal Marine

Sandstones

1950 mSS

Secondary Objective

OIL

Cau Fm

Fluvial Sandstones

2250 mSS

Tilted Fault Block