Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Abstract

Figures

Introduction

Top seal

Seal capacity

Seal mineralogy

Seal potential

Conclusions

References

Introduction

The Gippsland Basin is one of Australia’s most prolific hydrocarbon basins and is located about 200 km east of the city of Melbourne, Victoria, southeastern Australia. World-class oil and gas fields are located in the offshore Gippsland Basin, whilst extensive brown coal deposits and several coal-fired power stations are located predominantly in the Latrobe Valley (Figure 1). Given the strategic nature of geologic sequestration for the long term use of coal in Victoria, the State Government has funded the Victorian Geological Carbon Storage Initiative (VicGCS) to investigate the geological carbon storage potential of the Gippsland Basin. In addition, five exploration tender areas (Figure 1) have been gazetted in the Gippsland Basin by the Federal Government (three offshore blocks) and the State Government (two onshore blocks). The assessment of CO2 containment is important if geologic sequestration is to be considered in these areas.

The Lakes Entrance Formation, a succession of calcareous marine claystones deposited at the base of a thick carbonate succession during the Oligocene, provides the regional top seal for oil and gas accumulated at the top of the underlying reservoirs of the Latrobe Group. It will also provide the ultimate top seal to any injected carbon dioxide, and its top sealing potential needs to be understood. This investigation focuses on the physical attributes of the top seal (i.e. seal geometry, seal capacity and mineralogy) and with the results of other studies on hydrocarbon migration models and evidence for seal failure (O’Brien et al, 2008), has been integrated to produce a regional scale qualitative interpretation of CO2 sealing potential for the Gippsland Basin.

Top Seal Geometry

The top, base, thickness and limit of the Lakes Entrance Formation was identified in 320 exploration wells and water bores in the Gippsland Basin (Figure 2). The thickness of the regional top seal generally increases from its margin towards the Central Deep offshore, reaching thicknesses of over 400 m. The base of the regional top seal is also at its deepest at 2,000 and 3,200 m sub-sea in the Central Deep. The seal thins on the flanking Southern and Northern terraces and pinches out on the Northern and Southern platforms. Here, depths to top seal base of 500 to 1,000 m are typical. Onshore, the seal attains its maximum thickness in the Lake Wellington Depression (247 m). As in the offshore portion of the basin, its maximum thickness correlates with the greatest depth to seal base (1,000 m). Seal thicknesses of less than 50 m are recorded across the onshore Gippsland Basin near the seal’s ‘zero-edge’, on the Lakes Entrance Platform, across the Baragwanath Anticline and in the Alberton Depression. On the Lakes Entrance Platform and the Baragwanath Anticline, the depth to seal base is relatively shallow (100 to 200 m). In summary, the top seal attains its greatest thickness where the seal base is at its greatest depth and then thins at the basin margins. The geometry of the seal is therefore consistent with deposition in an early post-rift setting where marine sediments filled early post-rift paleo-topographic lows; it also pinches out against paleo-highs such as the Baragwanath Anticline and a number of the large hydrocarbon-charged anticlines offshore.

Seal Capacity

Seal capacity is an important factor in the evaluation of seal potential of oil and gas traps and for the geological storage of CO2 (Kaldi and Atkinson, 1997). Seal capacities are expressed as the maximum vertical column height of a specified fluid that can be contained by a given seal and are routinely derived from Mercury Injection Capillary Pressure (MICP) analysis of sealing lithologies. Maximum column retention capacities for oil, gas and CO2 were determined using standard Weatherford Laboratories methods and the methods of Daniel (2005) for CO2. The combined MICP seal capacity results for CO2 for the regional top seal are summarised in Figure 3.

The MICP (CO2) top seal capacity of the Lakes Entrance Formation in the offshore Central Deep is excellent, with column heights ranging from 250 m to 947 m. On the Northern Terrace, sealing capacity is fair (i.e. 54 m CO2) and on the offshore Southern Platform it is variable (246 to 13 m of CO2), with seal capacity reduced towards the basin margin. Onshore, MICP (CO2) column heights are greatest in the nearshore and central Lake Wellington Depression with a maximum height of 957 m. From this central area, seal capacities decrease out towards the margin of the Lake Wellington Depression. A similar pattern is discernable in the Seaspray Depression (Goldie Divko et al, 2009). Low sealing capacities are characteristic of the Alberton Depression and the Lakes Entrance Platform, although an anomalous value (i.e. 164 m of CO2), north of Lakes Entrance is the result of local diagenetic effects.

Quantitative Mineralogical Analysis of Seals

Quantitative mineral analysis uses integrated back scattered electron microscopy (BSEM) and energy dispersive X-ray analyses to identify mineral groupings. The use of this technique known as AMA (Automated Mineral Analysis; AMMTEC Ltd) has provided new insights into the mineralogy of the regional top seal. Color-coded and grey-scale mineral maps of samples aid in the visualisation of porosity distribution and textural relationships between mineral phases (Figure 4). The results obtained via this technique illustrate that the Lakes Entrance Formation is primarily a smectite-rich claystone. Of the 18 mineral groupings identified across the basin, smectite was the most abundant mineral recorded, followed by calcite and quartz. In over 70% of the samples, smectite accounted for at least 70% of the rock volume.

Where top seal samples are from depths below 700 m and have a high smectite abundance (above 70%), high MICP CO2 capacities (exceeding 150 m column heights) (Figure 4). At depths greater than 700 m, it is likely that sediments are more compacted and this compaction could be contributing in part to the higher MICP capacities. Samples at shallower depths are more likely to have poor MICP seal capacities and to have more visible porosity. It is probable that these shallower samples have come into contact with the fresh groundwater wedge (Kuttan et al, 1986), which has infiltrated through the onshore and near shore regions during the Neogene. Freshwater-induced diagenetic processes have resulted in the partial dissolution and/or modification of carbonates and other components, thereby compromising seal capacity.

Seal Potential

An interpretation of top seal potential for the containment of CO2 in the Gippsland Basin is presented in Figure 5. This interpretation is based largely on the depth to the base of the regional seal and MICP capacity data with input from the direct indicators of seal failure (O’Brien et al, 2008). Top seal potential is excellent in the Central Deep and the Lake Wellington Depression as the depth to the base of the regional top seal is intersected either at or below 800 m. Below 800 m, seal capacities are generally sufficient for the containment of CO2 or hydrocarbons (i.e. vertical column heights above 100 m and commonly around 200 m). Very good seal potential is indicated on the southern margin of the offshore basin, where seal attributes are similar to those of the Central Deep (i.e. excellent potential) but where there is a degree of uncertainty due to limited data coverage. Areas of good seal potential are attributed to part of the Northern Terrace, the Seaspray and Lake Wellington depressions (as depth to seal base between 700 and 800 m and variable seal capacity ranging from 53 to 285 m). The seal has moderate potential above 700 m, with variable seal capacity (10 to 187 m) and a greater likelihood of the occurrence of gas chimneys, soil gas anomalies and seeps (see O’Brien et al, 2008). In areas with poor seal potential (at the limit of the seal), very low seal capacities are characteristic (i.e. below 20 m), the base of the seal is quite shallow (around 300 to 400 m) and the seal is thinning towards its zero edge. There are also more seal failure indicators in areas with poor sealing potential than in any other areas (see O’Brien et al, 2008).

Conclusions

The Lakes Entrance Formation provides the regional seal for hydrocarbons at the top of the Latrobe Group reservoirs in the offshore Gippsland Basin and will also act as the ultimate barrier to the migration of geologically sequestered CO2. The potential of the regional top seal over the Central Deep, Southern Terrace, central eastern Lake Wellington Depression and the southern to central near shore areas in the Seaspray Depression are most suitable for the containment of supercritical CO2. Further towards the margin of the regional seal in both onshore and offshore areas, containment of supercritical CO2 is less likely. Particularly towards the basin margin, the mineralogical and hence, lithological composition of the Lakes Entrance Formation varies to the extent that the formation is no longer able to function as a barrier to vertical fluid and gas migration; seal capacity and automated mineral analysis results confirm this relationship.

The techniques employed in this basin-scale evaluation of the regional top seal demonstrate the ability of the Lakes Entrance Formation to contain hydrocarbons and CO2 in the Gippsland Basin. This evaluation highlights areas in the basin where future GCS explorers might consider detailed site assessment for geological carbon storage. An understanding of the seal potential in the Gippsland Basin, when integrated with CO2 migration studies, will provide the fundamental basis for the development of GCS in the Gippsland Basin.

References

Cooperative Research Centre for Greenhouse Gas Technologies, RPT05-0035, 85 p.

Goldie Divko, L.M., M.J. Campi, P.R. Tingate, G.W. O’Brien, and M.L. Harrison, 2009, Geological Carbon Storage Potential of the Onshore Gippsland Basin, Victoria, Australia, VicGCS Report 2, Department of Primary Industries, 75 p.

Kaldi, J.G. and C.D. Atkinson, 1997, Evaluating seal potential: Example from the Talang Akar Formation, Offshore Northwest Java, Indonesia, in R.C. Surdam, ed., Seals, Traps and the Petroleum System: AAPG Memoir 67, p. 85-101.

Kuttan, K., J.B. Kulla, and R.G. Newman, 1986, Freshwater influx in the Gippsland Basin: Impact on formation evaluation, hydrocarbon volumes and hydrocarbon migration: Australian Petroleum Exploration Association Journal, 26, p. 242-249.

O’Brien, G.W., P.R. Tingate, L.M. Goldie Divko, M.L. Harrison, C.J. Boreham, K. Liu, N. Arian, and P. Skladzien, 2008, First Order Sealing and Hydrocarbon Migration Processes, Gippsland Basin, Australia: Implications for CO2 Geosequestration, in J.E. Blevin, B.E. Bradshaw and C. Uruski (editors) Eastern Australasian Basins Symposium III, Petroleum Exploration Society of Australia Special Publication, p. 1-28.

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