A regional assessment of CO2 storage potential in the Browse Basin: Results of a study undertaken as part of the National CO2 Infrastructure Plan

Geoscience Australia undertook a regional assessment of the geological CO2 storage potential of the Browse Basin, offshore northwest Australia, between 2013 and 2015 as part of the Australian Government's National CO2 Infrastructure Plan (NCIP). The NCIP program aimed to accelerate identification and development of suitable areas within Australia for long-term CO2 storage proximal to major emission sources. The Browse Basin was selected with two other offshore sedimentary basins and several onshore basins for pre-competitive data acquisition and geological studies under the NCIP funding. The basin is a proven hydrocarbon province that hosts significant reserves of gas and condensate, with the majority of accumulations being characterised by high concentrations of CO2. This study implemented an integrated approach in assessing CO2 storage potential in the context of remaining hydrocarbon prospectivity, in light of the numerous existing hydrocarbon discoveries and a high probability of undiscovered accumulations within the basin. Potential CO2 storage plays were assessed for the likelihood of conflict with exploration for, and access to, existing and remaining hydrocarbon resources in within the basin. As the bulk of discovered, commercial hydrocarbon accumulations are hosted within the Jurassic and the lowermost Cretaceous successions, the study focused on the Cretaceous succession where there is a reduced risk of conflict between CO2 storage and hydrocarbon exploration (Jurassic and Lower Cretaceous units).
The data used for this study include information from over 60 wells, regional 2D and 3D seismic reflection surveys, potential field data, as well as existing and newly acquired pre-competitive geochemical, aeromagnetic and marine environmental data.
A key part of this work was an update to regional-scale structure of the basin, including deep faults associated with PermianCretaceous rifting events, and inversion and recent faulting associated with the Cenozoic collision between Australia and Asia. Another focus of the study was an update to the Cretaceous sequence stratigraphy across the basin. A play fairway mapping approach using the revised tectonostratigraphic framework was applied to assess, identify, risk and high-grade areas for their potential suitability for geological storage of CO2. The main constraints for geologic CO2 storage and containment analysed in this study, other than the distribution of reservoirs, seals and reservoir'seal pairs, were reservoir depth range, fault distribution, and hydrocarbon resource conflicts. Common risk element maps were produced for each supersequence through the overlay of mapped constraints (or risk elements) with the play fairway mapping, thus enabling the high-grading of potential CO2 storage play fairways. The results indicate that the Lower Cretaceous basin margin plays and the Upper Cretaceous (early Campanian) confined submarine fan play are potentially more prospective for CO2 storage. However, these plays are subject to potential resource conflict from up-dip migration of hydrocarbons from Cretaceous and older source rock units in the basin depocentres. This major risk requires further assessment for the high-graded priority areas identified in this study.
This study provides a revised basin framework and a regional-scale preliminary prospectivity assessment for the geological storage of CO2 in the Browse Basin. The results will guide future, targeted, site-specific assessments, and identify the main geologic risks warranting more detailed investigation. The study findings will also assist in reducing the risk of conflict between CO2 storage and exploration and utilisation of hydrocarbon resources, as well as in identifying new opportunities in these activities.

澳大利亚地球科学局（Geoscience Australia）于2013至2015年间，作为澳大利亚政府国家二氧化碳基础设施计划（National CO2 Infrastructure Plan, NCIP）的一部分，对澳大利亚西北海域布劳斯盆地（Browse Basin）的地质二氧化碳封存潜力开展了区域评估。该计划旨在加速识别并开发澳大利亚境内靠近主要排放源的长期二氧化碳封存适宜区域。布劳斯盆地与另外两个近海沉积盆地及数个陆上盆地一同被选中，依托NCIP资助开展竞争前数据采集与地质研究工作。
该盆地是已探明的油气富集区，拥有可观的天然气与凝析油储量，多数油气藏以高二氧化碳浓度为特征。本研究结合盆地内大量已有的油气发现以及未发现油气藏的高概率存在性，采用一体化方法，在剩余油气勘探前景的框架下评估二氧化碳封存潜力。研究针对潜在二氧化碳封存有利区带，评估其与盆地内现有及剩余油气资源的勘探和开发利用之间的冲突可能性。由于已发现的商业油气藏大多赋存于侏罗系及下白垩统地层中，因此本研究将重点放在白垩系地层——该地层中二氧化碳封存与油气勘探的冲突风险更低（侏罗系与下白垩统地层）。
本研究使用的数据包括超过60口钻井的数据、区域二维与三维地震反射勘探资料、位场数据，以及既有及新采集的竞争前地球化学、航空磁测与海洋环境数据。
本项工作的核心内容之一是更新盆地的区域尺度构造，包括与二叠纪—白垩纪裂谷事件相关的深大断裂，以及与澳大利亚和亚洲新生代碰撞相关的构造反转与新近断裂活动。研究的另一重点是更新盆地全域的白垩系层序地层格架。研究采用基于修正后的构造地层框架的有利区带（play fairway）制图方法，对二氧化碳地质封存的潜在适宜区域进行评估、识别、风险分级与优选。本研究分析的地质二氧化碳封存与圈闭的主要限制因素，除储层、盖层及储盖组合的分布外，还包括储层埋深范围、断裂分布以及油气资源冲突。通过将制图得到的限制因素（风险要素）与有利区带制图结果叠加，为每个超层序生成了通用风险要素图，从而实现对潜在二氧化碳封存有利区带的分级优选。
研究结果表明，下白垩统盆地边缘有利区带与上白垩统（早坎潘期）封闭性海底扇有利区带的二氧化碳封存潜力更优。但这些有利区带面临来自盆地沉积中心内白垩系及更老烃源岩单元中油气向上倾方向运移带来的潜在资源冲突风险。针对本研究中分级优选出的重点优先区域，这一重大风险尚需开展进一步评估。
本研究更新了布劳斯盆地的盆地构造格架，并为该盆地的地质二氧化碳封存提供了区域尺度的初步前景评估结果。相关成果将为未来针对性的场址专项评估提供指引，并明确需要开展更详细调查的主要地质风险。研究结果还有助于降低二氧化碳封存与油气资源勘探、开发利用之间的冲突风险，同时为相关活动识别新的机遇。