CO2 Storage Potential of the Gage Sandstone. Vlaming Sub-Basin, Offshore Southern Perth

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

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

2 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.

3 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.

4 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.

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

5 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.

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

6 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

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

Internal Reflection Pattern Seismic Example Facies Depositional Processes (continuity; amplitude; reflector units Seismic line configuration) Variable amplitude. Contact ranges Valanginian from a non-conformity to an angular Unconformity created by Val Unconformit unconformity with significant erosion varying amounts of erosion y surface on the border faults bounding the Gage LST reservoir. Confined stacked coarse- Commonly continuous. Can be Canyon-fill grained debris flows with silty semi-continuous to continuous; Unit A on inclined interchannels; intersected in moderate amplitude; parallel, sub- canyon floor Mullaloo 1 and Charlotte 1; parallel, prograding, downlap onto onlaps canyon walls Val Commonly semi-continuous. Can be discontinuous to semi- Channel-levee complex that Inner-fan continuous with minor continuous Unit B; feeds fan; intersected in confined to towards canyon margins; low- sub-unit 1 Mullaloo 1 and Charlotte 1; canyon moderate amplitude; parallel, sub- onlap canyon walls parallel, downlap onto Val and Unit A Confluence of channelised depositional lobes from Commonly discontinuous to semi- multiple sources; sandy continuous; some continuous SW of Unit B; Middle-fan middle-fan facies of stacked Peel 1 sourced from eastern sub-unit 2 on rise channelised sands and canyon; moderate amplitudes; debris flows; silty debris parallel, sub-parallel, downlap onto flows intersected in Warnbro Val and Unit A 1 Non-channelised Commonly semi-continuous. Can Unit B; Outer-fan on depositional lobe; bedded be semi-continuous to continuous ; sub-unit 3 basin plain turbidity currents; no wells moderate amplitude; parallel, sub- intersected parallel, downlap onto Val non-channelised, stacked Commonly continuous but can be coarse-grained debris flows Unit C; Inner fan on semi-continuous; moderate-high & high energy turbidity sub-unit 1 rise amplitudes; parallel, sub-parallel, currents that feed middle fan; downlap onto unit B intersected in Peel 1 channelised depositional lobes from multiple sources Commonly semi-continuous to meet; sandy middle fan Unit C; Middle fan discontinuous; moderate facies of stacked sub-unit 2 on rise amplitudes; parallel, sub-parallel, channelised sands; downlap onto unit B intersected in Warnbro 1 (cored) Commonly continuous but can be Outer fan on Depositional lobe with limited Unit C; semi-continuous; moderate-high undulating channels; bedded turbidity sub-unit 3 amplitudes; parallel, sub-parallel, basin plain currents; no wells intersected downlap onto Val and unit B Stacked high energy turbidity Inner fan on currents; channelised flow Commonly continuous but can be moderately Unit C; exhibiting lateral accretion in semi-continuous; variable dissected sub-unit 4 submarine canyons; minor amplitudes; parallel, sub-parallel, palaeo debrites; intersected in downlap onto Val topography Challenger 1 and Parmelia 1

8 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.

9 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.