CONTINENTALSEQUENCE STRATIGRAPHIC

CONCEPTS -A REVIEW

by

Paul V. Grech & Nick Lemon

ModernAncient

This review was conducted as part of aPhD project, studying the mainly continentalsediments of the Early Triassic in theBowen Basin

Continental successions have been describedby Legarreta et al. (1993) as ‘internally complexrock piles, where consistent correlation isdifficult because of abrupt and often randomfacies changes’.

However, Shanley and Mc Cabe (1993) proposedthat ‘accommodation variations are generallycaused by base level changes’.

This means that continental facies variations andtheir distribution can be predicted by using asequence stratigraphic framework.

Change in accommodation has been used as the basis for sequence stratigraphy in marineenvironments for a long time.

Definition of a sequence:

Mitchum et al. (1977) -‘a stratigraphic unit composed of a relativelyconformable succession of genetically related stratabounded at its top and base by unconformities andtheir relative conformities’.

Posamentier & Vail (1988) and Van Wagoner et al.(1990) -‘it is a product of a cycle of a fall, a rise and asubsequent fall of base level’.

The three main factors controlling accommodationand eventually sedimentary architecture and basinstratigraphy:

In continental environments however, eustaticeffects are limited, and accommodation isincreasingly controlled by:

Shanley & Mc Cabe, 1994

The variation of accommodation with timecan be divided into major tracts and surfaces:

During negligible accommodation:

• Total reworking of flood-plain fines• Lateral amalgamation of channel sands• Formation of sheet-like sands• Well-sorted, coarse-grained, well-rounded sands

During low accommodation:

• Some reworking of flood-plain fines• Lateral and vertical amalgamation of channel sands• Limited development of palaeosols• Sorted, medium to coarse-grained, rounded sands

• Mature sands• Scour-based sandstone beds• Amalgamated channel bodies

Negligible to low accommodationin the Latest Permian, Bowen Basin

During rapid accommodation:

• Minor reworking of flood-plain fines• Formation of isolated channel sands• Unsorted, very fine to fine-grained, angular sands

Rapid accommodation in theEarly Triassic, Bowen Basin

• Immature sands• Gradational to planar-based

sandstone beds• Isolated sand bodies• Well preserved channel sands• Palaeosol development

Rapid accommodation in theEarly Triassic, Bowen Basin

The different fluvial and flood-plain facies start to be preserved as the lateral migration of fluvial system is offset by the vertical migration.

Palaeosols become more mature away from thechannel system, allowing prediction of faciessequences within a stratigraphic section.

During highest accommodation:

• Preservation of flood-plain fines• Formation of isolated channel sands• Possible formation of lacustrine environment and

associated facies• Possible formation of peat swamps

Surface ofMaximum

Transgression

Highest accommodation (Middle Triassic, Bowen Basin)

Flood-plain fines and fluvial facies architecture are well preserved.

Sand to shale ratio becomesvery low.

Channel sands become isolated, thinner and sands are immature.

A lake may form if rapid subsidenceoutstrips sediment supply.

The transgressive surfacemay not necessarily be atthe level of the first lacustrine facies.

Highest accommodation (Middle Triassic, Bowen Basin)

During decreasing accommodation:

• Resumed reworking of flood-plain fines• Increasing lateral and vertical channel sand

amalgamation

In summary, fluvial architecture within the various systems tracts and surfaces:

LST - amalgamated, mature channel sands

TST - minor amalgamation of channel sandsand preservation of flood-plain fines

MST - lowest sand:shale ratio with possiblelacustrine facies

HST - isolated, immature sands becomingamalgamated in the later stages

Proposed sequence stratigraphic model fornon-marine sediments - 1D model

Proposed sequence stratigraphic model fornon-marine sediments - 2D model

Depositional environments over the differentareas of the basin and within the differentsystems tracts:

Implications for hydrocarbon exploration:

Reservoir sands are most mature within the LST and upper HST through reworking due to the lack of accommodation

During the TST, reservoir-quality sands are restricted to the basin margins where accommodation is low

At the maximum rate of accommodation, lacustrine margin facies can form,that are of reservoir-quality, with associated mud facies forming good source rocks and seal

In conclusion:

Non-marine sediments and facies form a coherent inter-related pattern that can be correlated both laterally and vertically

Sequence stratigraphy can be applied to continental environments

Such a model can be used to predict prospective areas in unexplored parts of a basin, and also target new plays in explored areas

Thanks to Oil Company of Australia, Santos Ltd, Petroz, Magellan Petroleumand the APCRC for the financial support and for the provision of data.

ARCADIA (OSL3) - plugged 18/7/57

pioneering through the continentalsediments of the frontier

Bowen Basin