Heading 1 · Web viewThe Gage Sandstone reservoir has porosities of 18–25% and permeabilities of 100–1300 mD. It lies 1000–3000 m below the seafloor, which is suitable for injection

CO2 storage potential of the Gage Sandstone. Vlaming Sub-basin, offshore southern Perth Basin: A case study based on seismic facies mapping and well log interpretation

Megan E. Lech, Diane C. Jorgensen, Liuqi Wang, David Lescinsky, Irina Borissova, Chris Southby, Stephen Johnston and Danielle Robertson

Introduction

This National CO2 Infrastructure Plan study assesses the suitability of the Vlaming Sub-basin for CO2

storage. The Vlaming Sub-basin is a Mesozoic depocentre within the offshore southern Perth Basin, Western Australia (Figure 1). It is around 23 000 km2 and contains up to 14 km of sediments. The Early Cretaceous Gage Sandstone was deposited in palaeo-topographic lows of the Valanginian breakup unconformity and is overlain by the regional South Perth Shale. Together, these formations are the most prospective reservoir-seal pair for CO2 storage. The Gage Sandstone reservoir has porosities of 18–25% and permeabilities of 100–1300 mD. It lies 1000–3000 m below the seafloor, which is suitable for injection of supercritical CO2 and makes it an attractive target as a long-term storage reservoir.

Methods and datasets To characterise the Gage reservoir, a detailed sequence stratigraphic analysis was conducted by integrating 2D seismic interpretation, well log analysis and new biostratigraphic data (MacPhail, 2012). Palaeogeographic reconstructions of components of the Gage Lowstand Systems Tract (LST) are also based on seismic facies mapping.

Results

The Gage LST reservoir largely coincides with the Gage Sandstone and is defined by the presence of the lower G. mutabilis dinoflagellate zone. A palynological review of 6 wells led to a significant revision, at the local scale, of the Valanginian Unconformity and the extent of the G. mutabilis dinoflagellate zones (MacPhail, 2012). G. mutabilis dinoflagellates were originally deposited in lagoonal (or similar) environments and were subsequently redeposited in a restricted marine environment via mass transport flows. Mapping of the shelf break indicates that the Gage LST was deposited in water depths of >400 m.

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Figure 1 Location of the Vlaming Sub-basin study area, petroleum wells and Gage LST reservoir thickness map

Intersected in 8 wells, the Gage LST forms part of a sand-rich submarine fan system (Figure 2) that includes channelized turbidites, low stand fan deposits and debris flows (Table 1). This interpretation is broadly consistent with Spring & Newell (1993) and Causebrook et al., (2006). The Gage LST is thickest (up to 360 m) at the mouth of large canyons adjacent to the Badaminna Fault Zone (BFZ) and on the undulating basin plain west of Warnbro 1 (Figure 1).

Palaeogeographic maps depict the evolution of the submarine fan system (Figure 3). Sediment transport directions feeding the Gage LST are complex. Unit A is sourced from the northern canyon (Figure 3a). Subsequently, Unit B (Figure 3b) derived sediment from multiple directions including incised canyons adjacent to BFZ and E-W oriented canyons eroding into the BFZ high. These coalesce on an undulating basin plain west of Warnbro 1. Minor additional input for the uppermost Unit C (Figure 3c) is derived from sources around Challenger 1.

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Figure 2 Block diagram of a sand-rich deep marine submarine fan (modified from Richards et al., 1998)

Figure 3 Palaeogeographic maps of the Gage LST showing changes in sediment input and deposition through time. a) time slice for Unit A, b) time slice for Unit B, and c) time slice for Unit C

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Table 1 Characteristics of the different seismic stratigraphic units between the Valanginian Unconformity to the top of the Gage LST reservior.

Seismic units Facies Depositional Processes

Internal Reflection Pattern (continuity; amplitude; reflector configuration)

Example Seismic line

ValValanginian Unconformity surface

Unconformity created by varying amounts of erosion

Variable amplitude. Contact ranges from a non-conformity to an angular unconformity with significant erosion on the border faults bounding the Gage LST reservoir.

Unit ACanyon-fill on inclined canyon floor

Confined stacked coarse-grained debris flows with silty interchannels; intersected in Mullaloo 1 and Charlotte 1; onlaps canyon walls

Commonly continuous. Can be semi-continuous to continuous; moderate amplitude; parallel, sub-parallel, prograding, downlap onto Val

Unit B; sub-unit 1

Inner-fan confined to canyon

Channel-levee complex that feeds fan; intersected in Mullaloo 1 and Charlotte 1; onlap canyon walls

Commonly semi-continuous. Can be discontinuous to semi-continuous with minor continuous towards canyon margins; low-moderate amplitude; parallel, sub-parallel, downlap onto Val and Unit A

Unit B; sub-unit 2

Middle-fan on rise

Confluence of channelised depositional lobes from multiple sources; sandy middle-fan facies of stacked channelised sands and debris flows; silty debris flows intersected in Warnbro 1

Commonly discontinuous to semi-continuous; some continuous SW of Peel 1 sourced from eastern canyon; moderate amplitudes; parallel, sub-parallel, downlap onto Val and Unit A

Unit B; sub-unit 3

Outer-fan on basin plain

Non-channelised depositional lobe; bedded turbidity currents; no wells intersected

Commonly semi-continuous. Can be semi-continuous to continuous ; moderate amplitude; parallel, sub-parallel, downlap onto Val

Unit C; sub-unit 1

Inner fan on rise

non-channelised, stacked coarse-grained debris flows & high energy turbidity currents that feed middle fan; intersected in Peel 1

Commonly continuous but can be semi-continuous; moderate-high amplitudes; parallel, sub-parallel, downlap onto unit B

Unit C; sub-unit 2

Middle fan on rise

channelised depositional lobes from multiple sources meet; sandy middle fan facies of stacked channelised sands; intersected in Warnbro 1 (cored)

Commonly semi-continuous to discontinuous; moderate amplitudes; parallel, sub-parallel, downlap onto unit B

Unit C; sub-unit 3

Outer fan on undulating basin plain

Depositional lobe with limited channels; bedded turbidity currents; no wells intersected

Commonly continuous but can be semi-continuous; moderate-high amplitudes; parallel, sub-parallel, downlap onto Val and unit B

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Seismic units Facies Depositional Processes

Internal Reflection Pattern (continuity; amplitude; reflector configuration)

Example Seismic line

Unit C; sub-unit 4

Inner fan on moderately dissected palaeo topography

Stacked high energy turbidity currents; channelised flow exhibiting lateral accretion in submarine canyons; minor debrites; intersected in Challenger 1 and Parmelia 1

Commonly continuous but can be semi-continuous; variable amplitudes; parallel, sub-parallel, downlap onto Val

Summary The Gage LST is an Early Cretaceous submarine fan system that began deposition during the G.

mutabilis dinoflagellate zone. It ranges from confined canyon fill to outer fan deposits.

The three units within the Gage LST show multidirectional sediment sources. The dominant supply is via large canyons trending north-south adjacent to the BFZ.

Seismic facies interpretations and palaeogeographic mapping suggest that the best quality reservoirs for potential CO2 storage are located in the outer fan (Unit C sub-unit 3) and the mounded canyon fill (Unit A). These are more likely to be laterally connected.

The defined units and palaeogeographic maps will be used in a regional reservoir model to estimate the CO2 storage capacity of the Gage LST reservoir.

References: CAUSEBROOK, R., DANCE, T. & BALE, K., 2006, Southern Perth Basin site investigation and geological

model for storage of carbon dioxide. CO2CRC Report Number; RP06-0162. MACPHAIL, M., 2012, Palynostratigraphic analyses of samples encompassing the Valanginian unconformity

in Challenger 1, Mullaloo 1, Parmelia 1, Peel 1, Quinns Rock 1 & Warnbro 1: Warnbro & Parmelia groups, Vlaming Sub-basin, Perth Basin (unpublished).

SPRING, D.E. & NEWELL, N.A., 1993, Depositional systems and sequence stratigraphy of the Cretaceous Warnbro Group, Vlaming Sub-basin, Western Australia. The APPEA Journal 33(2), 190–204.

RICHARDS, M., BOWMAN, M. & READING, H., 1998, Submarine-fan systems I: characterization and stratigraphic prediction, Marine and Petroleum Geology, 15, 689-717.

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