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Sequence StratigraphySequence Stratigraphy

A more inA more in--depth look at the data depth look at the data sources and the Cretaceous sources and the Cretaceous Interior Seaway in AlbertaInterior Seaway in Alberta

Unconformities• Unconformities are typically looking for

erosional surfaces• Subaerial erosion will typically accompany

lowstand• Wave-erosion (ravinement surfaces) can

occur with both lowstand and transgressive deposition

• Structural uplift can produce localized erosion that is not related to eustatic sealevel change

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Sudden changes in facies/depositional environment are clues to possible unconformities

Tools used in SeqStrat Analysis

• Seismic– Major reflectors are typically unconformities or

flooding surface– Depositional geometry key in identification in

parasequences– Covers large areas of the basin– Expensive and most data is proprietary– Processing – reprocessing and filtering the

data is a lengthy and expensive job

• Well Logs– Determination of changes in lithology and

provide a basis for lithostratigraphy– Various logging tools can provide information

regarding porosity, conductivity and allow reservoir characterization

– Fairly cheap, drilling the hole is expensive– Does not allow facies analysis

Tools used in SeqStrat Analysis

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• Cores from wells– Typically accompanied by well-logs– Allow observation of sedimentary structures,

trace fossils and bedding contacts– Allow more exact tests for physical properties

(conductivity, porosity) as well as petrographic analyses and values for seismic/well-log processing

– Expensive – and requires planning ahead, post examination prep work and storage.

– Cores not always intact

Tools used in SeqStrat Analysis

Cores generally range from 3 to 10 inches in diameter – sedimentary structures of larger dimension require some luck in observing them in cores.

Cut and polished cores provide an better conditions for observing and determining the trace fossil assemblage for the facies being examined

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• Outcrops and modern analogs– Allow observations of relationships between

sedimentary structures, trace fossils, bedding and lithological units

– Typically easier to see and understand what’s going on

– Not always available, accessible or occur within the same conditions/area that needs to be study

Tools used in SeqStrat Analysis

Back to Alberta

• Closer look at some of the data and models for reservoirs within the Cardium and Viking formations.

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Hornbeck

Carrot Creek

Burnstick

Raven River

Waskanigan

Dismal Rat

Bickerdike

Karr

Nosehill

Kakwa -HKakwa - BKakwa - N

BLACKSTONE FM.

Moreau

AmundsonKarrNE

SW WAPIABI FM.

approx. 200 km

terrestrial depositsincised valley fill depositslowstand shoreface deposits

highstand shoreface depositsoffshore/marine deposits

Cardium Formation

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Hornbeck

Carrot Creek

Burnstick

Raven River

Waskanigan

Dismal Rat

Bickerdike

Karr

Nosehill

Kakwa -HKakwa - BKakwa - N

BLACKSTONE FM.

Moreau

AmundsonKarrNE

SW WAPIABI FM.

approx. 200 km

terrestrial depositsincised valley fill depositslowstand shoreface deposits

highstand shoreface depositsoffshore/marine deposits

Cardium Formation

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Simplistic illustration from Hart and Plint (1993) showing the model for structural control of the transgressive incisions in the Cardium.

Viking Formation – reservoir map

During lowstand the existing river valley is eroded and expanded, while deposition of coarse sediments also occurs

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During the early stages of transgression the sediments within the valley are remobilized, reworked and redeposited as estuary deposits of barrier sands, estuarine muds and fluvial gravels (tripartite assemblage)

Walker and his students’ model for deposition of the Viking reservoirs using high-frequency sealevel changes as the driving force for the various sand-bodies – primarily derived from well-log data – or at least they haven’t published any seismic.

Alternative model by MacEachern and Pemberton (1999) based on facies and trace fossil changes – showing lowstand erosion and subsequent transgressive erosion for creation of the lenticular profiles, but the later sands are likely prograding deltaic tongues of the highstand