Back to basics - Rifts

PGK presentation 15 January 2014 Harry Doust

Tectonic setting of the world’s largest petroleum

fields (“giants” with >500mmbbl oil or 3tcf

gas.

The World map shows that most sedimentary

basins are rift –related, at least to start with. Most evolve into other basin

types…

…so I prefer to think

about basins as successions of basin

cycles

From Fraser et al. 2007 Petr. Geosci. 13 (2)

Cycles in the development of some typical sedimentary basin types

..most begin with a rift cycle...

Then pass into a postrift sag cycle

Some begin with a sag cycle, but...

With continental break-up: “Passive Margin” cycle

Without continental break-up: “Failed rift” cycle

With ocean closure or compression: “Foreland” or “Retro-arc” cycle

With minor/strike slip compression: “inverted basins” cycle

“interior sag” cycle

Petroliferous rift cycles • As petroleum geologists we are interested in identifying

where and how petroleum systems are developed in basins.

• Rift cycles host very important petroleum systems, with characteristic source and reservoir/seal rocks and traps.

• Areas we explore may consist of several generations of basin cycles so, in relation to a particular rift cycle of interest, I will call:

– Everything older the pre-rift (may comprise several cycles)

– The cycle of interest the synrift

– Everything younger the postrift (may comprise several cycles)

The Syn-rift is the period of rift development, the time of active faulting and syn-rift sediments are those deposited during this period.

First, let’s look at the main rift geometries

…and we can recognise 3 main types of rift:

• Half grabens

– Where the faulting is concentrated on one side of the rift

• Symmetrical grabens

– Where opposing faults of equal throw form symmetrical rifts

• Distributed grabens

– Where the rift is divided or offset by high blocks into semi-isolated segments

Rift structure and orientation depend on

• Pre-rift crust & lithosphere structure

• Rate & amount of extension

• Location & magnitude of thermal perturbations

• Nature and orientation of stress (orthogonal, oblique, transtensional)

…amongst others…

A typical rift half-graben: North Falklands, South Atlantic Ocean. Synrift fluvio-lacustrine to lacustrine sediments thicken

into the main fault in the early rift history, followed by more symmetric subsidence later on. This basin contains the 1billbbl STOOIP Sea Lion

discovery

From: Richardson & Underhill (2002) Mar. & Petr. Geol. 19: 417-443

North Sea rift system: Symmetrical & distributed

graben geometries

1

Axis of JU rift

Axis of Tr rift prerift

synrift

postrift

Symmetrical Viking Graben

2

Axis of JU rift

Broad shallow Tr rift Permian salt

Distributed Central Graben

Rifts evolve through fault linkage with increasing extension.

• first, short faults of minor throw form, bounding isolated half-grabens.

• with increased extension the faults become linked, leading to broader, usually more symmetrical basin geometries.

• Often segments of these expanded rift areas are offset from each other, separated by “accommodation zones”.

Length – throw relationships vary considerably, especially during early rift stages

From Morley 2002 AAPG Bull 86(6):961-978

(a) Rift initiation stage –faulting

with subsidence & rotation,

producing local half-graben

depressions

(b) Rift climax or maximum subsidence stage, faulting, fault

linkage and fault block rotation are

most active, resulting in the

formation of significant topography

(c) Rift Waning stage - the rate of

extension declines and eventually

ceases

Postrift sag stage – gradual thermal relaxation

Stages in the development of a rift to postrift sag basin like the North Sea

(a) Rift initiation stage

(b) Rift climax stage

(c) Rift waning stage

postrift sag stage

In many rift cycles 3 stages of development can be recognised

Digital elevation model of the East African Rift system (Gregory Rift segment), showing two en echelon broad areas of subsidence, separated by a zone with significant footwall uplift. Note the active faults, volcanoes and drainage pattern

away from the rift.

Sedimentary facies patterns are surprisingly consistent in rift environments, whether they are marine or non-marine. The 3 stages have distinct character:

• In the Rift Initiation Stage sediments mainly consist of locally-derived

coarse clastics such as alluvial fans, variable in development and usually of little significance for petroleum

• In the Rift Climax stage faulting causes the topographic relief to be at

its greatest and, with variations in subsidence /sedimentary supply rate this may result in alternations of deep (marine or lacustrine) and shallow water conditions producing a variety of facies, including coarse clastic aprons adjacent to the main active faults

• In the Rift Waning stage the remaining topography is degraded and

the basinal area may gradually fill up, often with shallowing-upward

deltaic deposits, as the rivers gradually find their way into the rift.

Synrift deposits: Sedimentary facies

Generalized stratigraphic framework of some continental synrift sequences.

From Katz., Geol Soc. Sp. Publ. 80, 1995: 213-242

Rift initiation stage

Rift climax stage

Rift waning stage

Source rocks in rifts

Rift environments are relatively protected and in appropriate climates host source rocks in both marine and non-marine environments:

Many of the best synrift marine source rocks correlate with Mesozoic Ocean Anoxic events, such as in the Late Jurassic.

Kimmeridge source rock in outcrop and map of the North Sea, showing high TOC values in the rift zone

A non-marine half-graben rift, The Phitsanulok Basin, Thailand, rich in lacustrine and fluvio-lacustrine source rocks. Right: a log of the Sirikit field, showing the thin nature of the non-marine fluvio-lacustrine reservoir sands – a great challenge for reservoir modelling!

Flint et al. 1988 AAPG Bull 72 (10): 1254-1269

Synrift reservoir rocks

Reservoir distribution patterns in rift sequences are surprisingly consistent, mainly comprising:

• Talus slope apron fans & fan deltas adjacent to active boundary faults, mainly in climax stage

• Deltaic – shoreface sands on the opposite, less active flanks of rifts

• Turbidites on subsiding slopes and along deep axial portions of rift segments, mainly in climax stage

• Carbonates in areas isolated from sedimentary input and in suitable climates

• Fluvio-lacustrine sands in gently subsiding rifts, thinly inter-bedded with lacustrine shales

• Fluvio-deltaic sands in regressive deltas in the waning stage, when rifts may fill

In many non-marine rifts the sands are ill-sorted

This modelled overlapping structural fault transfer zone demonstrates its strong control on the distribution of sand in a rift zone

Sediment provenance, fan deltas in the Palaeogene of the Bozhong Sag, Bohai Bay, China from Zhu et al 2013 (in press)

(a) Seismic attributes (b) sedimentary facies - showing the distribution of apron-fan

sandy facies along steep slopes where subsidence is greatest contrasted with inter-

bedded fluvio-deltaic facies derived from transfer zones and shallower slopes

Sediment provenance, Palaeogene of Bozhong Sag, Bohai Bay,

from Zhu et al 2013 (in press)

Maximum subsidence

Annotated 3-D seismic profile across the Liaozhong depression, Bohai Bay, showing lacustrine source rock mudstones, fan deltas, braid deltas and sub-lacustrine fans

Depositional facies in the Liaozhong depression, Bohai Bay, East China. There is a gradual upward change from the rift climax stage, with minor deltaics, deeper water fans and fan deltas, to the rift waning stage with increasing fluvio-deltaic fill and axial incised delta sands

Section through South Brae field, northern North Sea, showing reservoir distribution in a marine talus fan adjacent to an active rift boundary fault. Trapping against the pre-rift basement is a main risk in such traps. from GeolSocMem 20: 211

Some characteristic trap geometries in rift sequences

• Structural: – Tilted and rotated fault blocks and horsts affecting pre-rift and early

syn-rift, usually partly eroded – Buried hills of pre-rift rocks situated in rift – Inversion anticlines and related structures

• Stratigraphic – Compacted / rotated talus fans – Axial turbidite fans encased in rift-fill clays – Slope fans on subsiding rift flanks

• Sealing clays surround sand bodies, but except in deep water rifts – Regional seals are usually absent (often the first regional seal lies at

the base of the postrift) – Faulting continues through the synrift cycle, so vertical leakage paths

are created and migration remains typically within the rift

Seismic profiles across important Middle Jurassic tilt blocks in the Northern North Sea Large, rotated fault blocks of uplifted, tilted and eroded Middle Jurassic below Cretaceous unconformity seal, simple (Brent) to complex (Gullfaks).

Millenium Atlas Fig 10.23

Bach Ho field, Cuu Long Basin, Vietnam: A

“Buried Hill” Pre-rift granodiorite reservoir

charged from surrounding lacustrine

shales of the climax stage of the synrift

Below: seismic profile

Slope and basin axial turbidite fans in the Moray Firth, North Sea

Left: Seismic lines showing the Buzzard Field with the updip termination of the slope turbidite fan sand reservoir.

Below: chain of axial turbidite fields south of the Halibut Horst

From AAPG Explorer June 2005,p 13

Inversion • Synrift sequences are often relatively protected from post-

rift tectonic events such as major compression.

• However, they are very susceptible to inversion, which can occur along faults if and when the stress direction changes; very common in transtensional / transpressonal situations (such as pull-apart basins and many rifts in active tectonic provinces)

• Inversion can be very local, affecting synrift sequences adjacent to faults of particular orientations, resulting in partially inverted basins

• Salt in the rift sequence impacts on the “linkage” between faults in the pre-rift and the synrift

• Inversion can lead to deep erosion of the synrift sequence

Marmara Sea Region, Turkey: fault motion is controlled by dextral

strike-slip and synchronous extension (WNW faults) and compression (ENE faults)

From: Pfister et al., Bull. Angew. Geol. 5 (2) 2000: 155-176

Transpression may result in localised inversion affecting part of a rift basin complex, as in these examples from South America.

In inversion the centre of the rift now becomes the shallowest area, and the axis of sedimentation shifts to each side of the uplift

Permo-Triassic

Jurassic-Cretaceous

Cenozoic

Rift cycles may also migrate

through time - as here in southwest

England (to be followed by later

inversion)

Synrift 1

Synrift - postrift

Inversion

Note the position of the Isle of Portland,

indicating the eastward migration

of the rifting

Relationships to the post-rift cycle The Syn-rift corresponds to the time of active faulting and when faulting ceases we move into a post-rift sag cycle. The end of the synrift may vary from place to place in a basin, however, as local minor faulting may continue in places In the south Atlantic, a “transitional” cycle is commonly interpreted at the base of the postrift

New Zealand, Taranaki Basin

Palaeogeography Late

Cretaceous to Pliocene; Syn-

postrift and inversion cycles

synrift

transition

postrift inversion

Continental rifts developed in tropical climates

Continental –marine rifts developed in tropical climates

Continental rifts developed in temperate climates

Continental –marine rifts developed in arid climates Deeper marine rifts developed

on continental platforms

shallower marine rifts developed on continental platforms

Families of petroliferous rift basins

examples

1

2 3

4

5

6

4

7

• The rift cycles in all of these families do not exist in a vacuum - they form part of a basin’s evolution and, from the point of view of petroleum, the pre- and post-rift cycles have important roles to play.

• Sometimes both charge and trapping occur in the rift cycle, but normally there is some interaction with the pre- and post-rift, which frequently actually form essential components of the rift petroleum systems. For instance: – The pre-rift may provide charge to the synrift ( Gulf of

Suez), or contain reservoirs and traps charged from the synrift (North Sea)

– The postrift may be charged from the synrift (North Sea) – The postrift often provides crucial burial to mature

synrift source rocks (South Atlantic) – Rift trap development and geometry are often

dependent on events pre- or post-dating the rift cycle …a brief glimpse at some of the characteristics of the petroleum systems and plays in these basin types…

Continental rifts developed in tropical climates

Continental –marine rifts developed in tropical climates

1

2 3

• Typically rich in lacustrine oil-prone source rocks in the rift climax stage and filled with fluvio-deltaic sediments in the waning stage, relatively independent of the pre- and post-rift (which may lead to continent separation).

• Associated with transtensional and orthogonal extension and susceptible to inversion. • Found in many Mesozoic and Tertiary provinces, forming the bases for rich petroleum

provinces in SE Asia and the south Atlantic. • More distal basins become marine in the synrift – a second family, limited oil-prone source

Early Postrift Marine Petroleum System (Gas/(Oil) prone • Gas prone Intradeltaic SR (type II/III); neritic clays

Early Synrift Lacustrine Petroleum System (Oil prone) • • Non-marine synrift, Pg-MiL • Oil prone deep lacustrine SR (type I/II)- rich!

Late Synrift Transgressive Deltaic Petroleum System (Oil/Gas prone • • Fluvial deltaic coals and coaly shale SR (type II/III) • Regional transgressive marine shale top seals

From Doust and Lijmbach, 1997

1 2

Late Postrift Regressive Deltaic Petroleum System (Oil/Gas prone) • • Fluvial deltaic coaly shale SR (type II/III)

4 3

Continental rifts developed in tropical climates – model of typical SE Asian Tertiary basin with cycles,

stages and types of petroleum system

Early Synrift

Alluvial

Fans

Early Synrift

Lacustrine

Deltas

Early Synrift

Lacustrine

Turbidites

Late Synrift

Transgressive

Deltas

Late Postrift

Regressive Deltas,

Coastal / Shallow Marine

Early Synrift

Volcaniclastics

Late Postrift

Marine Turbidites

Early Postrift

Marine Carbonates

V

prerift

Basement

Postrift

Synrift

V

Bach Ho

Janti Fan

Jatibarang

Sirikit

Tropical continental rifts: important plays in

synrift cycles in Southeast Asia Tertiary

Rift Basins

Widuri

Telaga Said

Today we see these basins at all stages

of development

Some are single cycle, most are multiple cycle. As the cycles and stages have similar sedimentary characteristics, we can compare cycles and stages, even where the tectonic histories of the basins differ. The base postrift regional seal keeps most oil/gas in the synrift

From Doust and Sumner 2007

Bampo

Bampo

Bampo

Bampo

Bampo

Seal

Reservoir/Trap

Source

A more distal SE Asian basin forming a Continental to marine rift developed in tropical environments, North Sumatra: Syn-rift half-grabens were subjected to rapid marine flooding as a result of tectonic subsidence and eustatic sea level rise. Synrift includes reefoid carbonates and bathyal marine turbidite plays

Conjugate South Atlantic marginal salt basins. The synrift is marked in green. Main difference is the amount of postrift burial, which is greater on the African side, where the postrift petroleum system dominates. On the Brazil margin the synrift petroleum system dominates. Salt allows upward migration.

Basin Cycle Characteristic trap types

Synrift

Transitional

Postrift (marginal

sag)

Pre-Rift Horst Block

Rift Fault Block

Carbonate Build-Up

Anticline Stratigraphic

Pinch-Out Turbidite

Anticline Carbonate Build-Up

Stratigraphic Pinch-Out

Turbidite

Carbonate Build-Up

Growth Fault Rollover

Salt-Rooted Anticline

Salt Diapir Flank

Combination Trap

Shale Channel Truncation

LACUSTRINE PETROLEUM

SYSTEM

MARINE PETROLEUM

SYSTEM

In the synrift here we see the same basin cycle type and same petroleum system type as in the SE Asian Tertiary basins

In the postrift here we have a good analogue petroleum system type for other passive margin cycles

West African salt basins, cycles, petroleum systems and play typesSYS

(after Bruhn and Walker, 1995)

Model of a Brazilian passive margin basin showing the lithology and the plays typical of the synrift. From Beglinger

2010

Characteristic Plays: 1. Clastics around horst

blocks/structural highs (coarse),

2. Coquinas on top of structural highs,

3. Facies changes/clastic lenses

4. Anticlinal clastic intervals.

1

2 3/4

3/4

Continental rifts developed in temperate climates

examples

4 4

• Similar to the tropical families, but the rich lacustrine oil-prone source rocks are less well developed or absent (high latitudes) and the synrift often comprises alluvial or deltaic sediments,

• Associated with transtensional and orthogonal extension and susceptible to inversion. • Found in many Mesozoic and early Tertiary provinces, forming the bases for petroleum

provinces in SE Australasia (60degrS) and the southern Atlantic margin (30-60degrS). • Some rich oil and gas provinces, some hardly explored, some barren

Non-marine volcaniclastic

Fluvial, flood-plain & coastal plain

Marine mudstone

Marine mudstone

Marine limestone

inversion / erosion

nearshore

ENVIRONMENT

early 1

early

late

early

late

SYN

RIF

T

PO

STRIF

T

Stratigraphy from Rahmanian et al. 1990

A continental rift developed in a temperate climate: Gippsland

Basin, SE Australia

SR

SR

RR

Se

STAGES

PST

PST

SR RR Major source-rock bearing interval

Major clastic reservoir interval Se

Regional seal interval

Main petroleum system: Latrobe (!),

PST PST Early and late synrift petroleum systems

The rift cycle is similar to that of the tropical family, but with predominant terrestrial facies & little or no lacustrine facies. The postrift contributes burial only Early inversion is critical in creating traps

Non-tropical South

Atlantic with rather

patchy and isolated

synrift development

- the Orange and

Falklands basins,

Deeper marine rifts developed on continental platforms

shallower marine rifts developed on continental platforms

5

6

• Exemplified by rift cycles on broad continental platforms corresponding to periods of extensive trangression and periods of ocean anoxia (OAEs), commonly Mesozoic

• Rich petroleum provinces based on world-class synrift source rocks • The deep water environment means the 3 rift stages cannot be distinguished and

commonly the rift topography not filled until the postrift • In more proximal situations the source potential declines and the rift topography is filled

with shallower water or fluvial sediments • Rift geometry allows migration into pre-, syn- and post-rift cycles

A classic deeper marine rift on a continental platform: The symmetric Northern North Sea rift province

Jurassic pre-rift sandstones are sealed below postrift Early Cretaceous shales. Migration is from Late

Jurassic synrift source rocks into tilted fault blocks on the basin flanks. The excellent combination of essential

parameters and their favourable juxtaposition make this a most prolific petroleum province

prerift

synrift

postrift

Section across the northern North Sea half-graben province with the synrift Brae trend along the western boundary fault & the up-dip accumulations of

the Sleipner Terrace. From Isaksen et al. AAPG Bull 2002, 86(4):557-591

Utsira High Brae Trend Sleipner Terrace

fan-glomerate deposits form

the main reservoirs

in the downthrown (hanging-wall)

traps on the west

side of the southern part of the

Viking Graben – the Brae trend

Late Jurassic syn-rift plays in offshore Norway

From: Spencer et al. First Break 11 (5) 1993: 172

A relatively shallow marine rift on a continental platform, the Central Graben, North Sea. Generally shallower environments & pre-rift salt allow vertical migration into postrift reservoirs through fractures & inversion faults The synrift source (climax stage) passes up into shallow marine – fluvial sands (waning stage)

Netherlands: Late Jurassic Syn-rift

Synrift Upper Jurassic was deposited in a series of halfgrabens with fluvial reservoirs

The synrift marine source dies out southwards into this shallow facies and the sequence becomes less deeply buried

Continental –marine rifts developed in arid climates

examples

7

• Rift cycle stratigraphy is strongly controlled by the arid climate and includes evaporites and carbonates

• Includes excellent wadi and dune reservoirs, but source rocks are interbedded or are found only in deeper water environments, so charge often comes from the pre-rift

• Widespread, including a variety of rift types of Palaeozoic to Tertiary age

Postrift 2

Synrift

2

Late

Early

Postrift 1

Synrift 1

Interior Sag 1

Epeiric Sea

Basin stage sourc

e

rese

rvoir

seal

Petr

ole

um

sy

stem

Desc

ribed

pla

ys

I

Rudeis - Nakhul

Belayim – Kareem

Interior Sag 2

Brown Lst - Nezzazat

Pre

rift

Fault blocks shale-outs

Fault blocks shale-outs, onlap, dip/fault

Fault blocks drape

Fault blocks fractures

Gulf of Suez A continental to marine rift developed in arid climate Has excellent clastic and carbonate reservoirs - the main charge is derived

from the pre-rift In the Southern Permian Basin of NW Europe charge is also prerift

Gulf of Suez rift – charge and play types

Crests of tilted fault blocks

Drape over fault blocks edges/”twist zone”

Fault controlled

Fault intersections

Dip closures (hanging wall anticlines)

Fractures along faults

Reefoid buildups

Reefoid buildups over block crests

Sub unconformity truncations (subcrop trap)

Onlap pinch-out

Updip sand pinch-out

Weathered / fractured basement

Synrift kitchens (Rudeis source rock)

Prerift kitchens (KU Thebes source rock)

Note: most fields have several reservoir intervals

Gulf of Suez plays

Main production is from the synrift, with main charge from prerift (prerift blocks are leaky!).

Syn-rift plays: ramp deltas, basin-floor fans, talus fan carbonates, intra-evaporite sand bodies, post salt diapir and piercement anticlines

Pre-rift: fractured/karstified limestones below unconformities, deep large-throw fault blocks,

Back to Rifts !: Recent successes in

synrift basins based on lacustrine charge and

reservoirs (Sudan, Kenya, Uganda)