Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland

The Triassic to Jurassic Clarence-Moreton Basin presented many challenging problems when it was first proposed for study in the Onshore Sedimentary Basins Program of the then Bureau of Mineral Resources (since renamed Australian Geological Survey Organisation, AGSO). Covering about 27 000 km2 , and containing up to 3 km of dominantly continental sediments (see Fig. 1 of Wells & O'Brien, this Bulletin, p. 4), the basin was little understood before this study. No comprehensive basinwide synthesis had ever been undertaken, and the regional stratigraphic framework of the basin was unknown. The predominantly fluvial sediments include important coal measures. These were considered to be a potential hydrocarbon source, although there had been no up-to-date assessment of petroleum source rocks and petroleum potential, and data on which to base a meaningful sedimentological analysis were limited. The main aim of the project was, therefore, to understand the evolution of the basin. This was to be achieved principally through sedimentological research, and by interpretation of industry seismic surveys, and an AGSO deep-crustal seismic study. Most of the objectives of the project were achieved, and the major results are documented in the papers in this Bulletin. Other papers reporting results of the research are listed in the references at the end of each paper. A folio of maps, which includes isopachs, structure contours, potential fields, and locality maps, is being produced as a separate publication. A major research interest to be addressed during the project was the interaction of tectonics, climate and sea level, and its effect on patterns of sedimentation, sediment provenance, dispersal and composition in the basin. Distinguishing the effects of eustatic sea-level change on the history of sedimentary basins from those caused by tectonic events is a major problem in many sedimentary provinces. The succession in the Clarence-Moreton Basin includes an important marker bed of ferruginous oolite, which in some sections includes acritarchs. The origin of the oolite was an enigma, and it was hoped that research into its origin would substantiate its usefulness as a marker zone, and help in understanding the relative merits of eustatic sea level changes versus tectonic events in stratigraphic interpretation. The geological investigations of the Clarence-Moreton Basin concentrated chiefly on the Bundamba Group, a Late Triassic to Early Jurassic succession; the name had been originally used for the largely coal-barren sediments sandwiched between the Ipswich Coal Measures below and the Walloon Coal Measures above. The Bundamba Group covers a large area, and posed major problems in nomenclature and correlation from one end of the basin to the other, besides containing potential petroleum source rocks. Of the overlying formations, the Kangaroo Creek Sandstone and Grafton Formation were not studied in detail, as they have little petroleum potential and no particular problems of nomenclature. However, the Walloon Coal Measures, although they have almost no natural outcrop, contain petroleum source rocks, and were included in the organic geochemistry program.

三叠纪至侏罗纪的克拉伦斯-莫顿盆地（Clarence-Moreton Basin）最初被纳入时任矿产资源局（后更名为澳大利亚地质调查局，AGSO）的陆上沉积盆地计划开展研究时，就面临诸多棘手难题。该盆地面积约27000平方千米，沉积地层最大厚度可达3千米，主体为陆相沉积物（详见本通报中Wells与O'Brien的文章，第4页图1），在本研究开展前几乎未被深入认知。此前从未开展过全盆地范围的综合研究，盆地的区域地层格架也尚未明确。盆地内以河流沉积为主的沉积物中赋存重要的含煤地层，曾被认为是潜在的烃源岩，但始终缺乏针对石油烃源岩与油气潜力的最新评估，且支撑有效沉积学分析的基础数据也十分有限。因此，本项目的核心目标为阐明该盆地的演化历程，主要通过沉积学研究、工业地震资料解译以及AGSO深地壳地震研究来实现。本项目的多数目标均已达成，主要研究成果已收录于本通报的各篇论文中，其他相关研究成果的报告则列于各篇论文末尾的参考文献列表。一套包含等厚线图、构造等值线图、位场图与点位分布图的活页地图集，将作为独立出版物出版。本项目重点研究的科学问题之一为构造作用、气候与全球海平面变化的相互作用，及其对盆地内沉积作用模式、沉积物源区、搬运路径与沉积物组分的影响。区分全球海平面变化与构造事件对沉积盆地演化的影响，是多数沉积盆地域面临的核心难题之一。克拉伦斯-莫顿盆地的沉积序列中赋存一层重要的铁质鲕粒岩标志层，部分剖面中还含有疑源类化石。该鲕粒岩的成因长期以来是未解之谜，研究其成因既可验证其作为标志带的有效性，也有助于厘清地层解释中全球海平面变化与构造作用的相对影响权重。本次针对克拉伦斯-莫顿盆地的地质研究主要聚焦于晚三叠世至早侏罗世的班达巴群（Bundamba Group）；该群最初被用于指代夹于下伏伊普斯威奇含煤地层与上覆沃隆含煤地层之间、基本不含煤的沉积物。班达巴群分布范围广泛，在全盆地范围内的地层命名与对比均存在较大难题，且其本身也赋存潜在的石油烃源岩。在上覆地层中，袋鼠溪砂岩组与格拉夫顿组未被详细研究，原因在于二者油气潜力极低且不存在特殊的地层命名问题。不过，尽管沃隆含煤地层几乎无天然露头，但其中赋存石油烃源岩，因此被纳入了有机地球化学研究计划。