chapter 2 geology of the earth and hydrocarbon habitat
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Chapter 2
The Geology of the Earth
and Hydrocarbon habitat
Presenter: Leigh Brooks
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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.
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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.
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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
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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
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Plate Tectonics: driven by mantle convection
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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)
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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
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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
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Deep ocean
drilling vessel
system. The
riser drilling
system provides
diversified
possibilities for
scientific drilling
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The plate tectonic cycle showing some of the geological fluxes and processes that control them
Spreading centres and subduction zones are earthquake belts
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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).
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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
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Plate tectonics: mid Atlantic rift passes through the middle of Iceland
fissure
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Divergent rift – most common mechanism for formation of sedimentary basins
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Spreading centres
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Location of current plates – with sense of motion
Rifting Convergence
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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
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Trench - subduction
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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
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•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.
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Gondwanaland, as formed by the assemblage of the southern continents. Rifting started in the Jurassic approx 160 million years ago
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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).
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Diagrammatic cross-section across a continental margin showing sedimentary basin
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For further info, good basic description of Plate Tectonics
http://pubs.usgs.gov/gip/dynamic/dynamic.html
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Sedimentary Basins - The home of hydrocarbons.
Location of sedimentary basins – global sediment thickness
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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
$
$
$
$
$
$
$
$$
$$
Australia’s
Sedimentary Basins
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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
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• 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
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• 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
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Divergent rift – most common mechanism for formation of sedimentary basins
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West –East cross section through Sth America >4000km section shows basins at high vertical exaggeration
subduction Magmatism,compression Passive margin, extension 200 km
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Extensional basin on coast of Brasil
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Smaller Basins also form along strike slip faults or lateral movements in the earth’s crust
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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
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Extensional basin in North Sea – failed rift
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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
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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
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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
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You could walk from
PNG to Australia and
then to Tasmania.
Climate & SL drop
has huge effect on
sedimentation
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Temperature changes of the Earth in last 400,000 yrs
National Geographic
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Sedimentary depositional environments in a delta plain and marginal marine setting - prograding
Position of corehole in next slide
Braided streams, Canterbury Plain, NZ
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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.
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Typical river dominated delta – Mississippi Delta. No reworking by wave energy to form beaches.
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A Typical Submarine Fan Model Redhead et al 2000 GCS SEPM Generally form during a drop in sea level as shelf is exposed
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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)
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Sand can Also Get “Stuck” on the Slope if there is “Accommodation” Space / Ponding
Offshore Brunei Looking Landward McGilvery & Cook 2003 GCS SEPM
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• 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?
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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
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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
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• 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
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• 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
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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
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Carbonates:
Another major
reservoir
lithology
Satellite image of
Great Barrier reef
200x200km view
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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)
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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
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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
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Keel
Space gap atkeystone block
Non-rotationalplanar faulting
Fault termination at depth
Rotational planar Faulting
Listric faulting
Asymmetrical listricfault termination
Structure: Extension
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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
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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
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Fault-bend Folding
3 types of fold-thrust interactions
ramp
ramp Fault-propagation Folding
Detachment Folding
Structure: Compression
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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)
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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.
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NE SW
0 5
km
0
1
twt
Tilted fault blocks and half grabens-
extensional
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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