Petroleum generation and migration modelling for the Capel and Faust basins, eastern offshore Australia - GNS Science Report 2011-22

The Capel and Faust basins, located on the Lord Howe Rise in water depths between 1,300 m and 2,500 m, were the focus of a series of marine surveys by Geoscience Australia in 2006 and 2007. Their interpretation of high-fold 2D seismic reflection, gravity and magnetic, multi-beam bathymetry, sonobuoy refraction, heat flow and geological sample data suggested the basins have petroleum potential. Analysis of petroleum generation and migration, based on structural maps, lithological and other data supplied by Geoscience Australia, is the focus of this study. Basin models predict that most of the deeper depocentres in the Capel and Faust basins, mapped as containing Jurassic-aged pre-rift and Cretaceous-aged syn-rift source rocks, have the potential to expel oil and gas, and charge nearby syn-rift and post-rift reservoir formations from Cretaceous time to the present day.\nMulti-1D thermal and petroleum generation models predict: \n - Pre-rift (215 - 165 Ma) and Syn-rift 1 (130 - 100 Ma) megasequences within the deeper depocentres are within the oil or gas generation window; \n - Based on the expected presence of petroleum-generative coaly source rocks, total oil and gas expulsion from the major depocentres exceeds 5 MMbbl/km2 and 25 Bcf/km2 respectively from the Pre-rift source rocks, and 20 MMbbl/km2, and 300 Bcf/km2 respectively from the Syn-rift 1 source rocks. In terms of timing, 80% of total hydrocarbon expulsion is predicted by the end of the Eocene, with maximum expulsion taking place between the Late Cretaceous and the Late Eocene (c. 68-36 Ma);\n - A significant increase in paleo-water depth in late Cenozoic time has supressed further heating related to post-Eocene burial. However, modelling predicts post-Eocene expulsion of oil and gas may have been partly enhanced by post-rift magmatism.\nIn this study total expelled oil and gas volumes are 'migrated across' mapped horizons to assess charge of and fill-spill relationships between structural traps. This map-based charge modelling assumes certain reservoir properties with no migration losses and predicts that: \n - Accumulations within potential reservoir facies, such as deltaic, shoreline and turbidite sandstones of the lower Post-rift unit (70 - 68 Ma) are dominantly gas with volumes generally about 5 to 9 Tcf at burial depths of 400 - 700 m; \n - Accumulations within similar sandstones of the upper Syn-rift 2 unit are mixed oil and gas (about 2 to 3 billion bbl oil and 10 Tcf gas) at burial depths of 400 - 800 m; \n - Similar accumulations are present in the lower Syn-rift 2 and Syn-rift 1 fluvial sands; \n - Most of the mapped structural traps are buried to relatively shallow depths and seal effectiveness for containment must therefore be a significant risk. Deeper structures and stratigraphic plays may further contribute to the petroleum potential in the basins.\nThe model presented here illustrates the potential for petroleum charge of structural traps in the Capel and Faust basins and highlights the risks associated with source rock distribution and type, reservoir distribution and quality, and seal effectiveness. Volumetric and charge assessments could be further refined using higher density seismic data and appropriate rock property data for reservoir and seal rocks in combination with 3D modelling.\n\nYou can also purchase hard copies of Geoscience Australia data and other products at http://www.ga.gov.au/products-services/how-to-order-products/sales-centre.html

位于豪勋爵海隆（Lord Howe Rise）、水深介于1300米至2500米之间的开普尔海盆（Capel）与福斯特海盆（Faust），是澳大利亚地质调查局（Geoscience Australia）2006年与2007年一系列海洋地质调查的重点研究对象。该局对高覆盖次数二维地震反射、重磁、多波束测深、声纳浮标折射、热流及地质样品数据的解译分析表明，这两个海盆具备油气远景。本研究以澳大利亚地质调查局提供的构造图、岩性及其他数据为基础，开展油气生成与运移分析。盆地模拟结果预测，开普尔与福斯特海盆内多数被圈定为发育侏罗纪裂谷期前与白垩纪同裂谷期烃源岩的深凹陷带，均具备排烃能力，可自白垩纪至今为邻近的同裂谷期及裂谷期后储集层提供油气充注。

多一维热史与油气生成模拟结果预测：
- 深凹陷带内的裂谷期前（215~165 Ma）与同裂谷期1（130~100 Ma）巨层序均处于生油或生气窗范围内；
- 基于预期发育的生油气煤系烃源岩，主要凹陷带的总排油量与总排气量分别为：裂谷期前烃源岩的5 MMbbl/km²与25 Bcf/km²，同裂谷期1烃源岩的20 MMbbl/km²与300 Bcf/km²。就排烃期次而言，总烃类排烃量的80%预计发生在始新世末期，最大排烃期为晚白垩世至晚始新世（约68~36 Ma）；
- 晚新生代古水深的显著升高抑制了始新世后埋藏相关的进一步增温，但模拟结果显示，裂谷期后岩浆活动可能部分增强了始新世后的油气排驱。

在本研究中，总排油气量通过跨圈闭标定面运移的方式进行核算，以评估构造圈闭间的充注过程与满溢关系。该基于图件的充注模拟假设了特定的储层物性且未考虑运移损失，并预测：
- 下裂谷期后单元（70~68 Ma）的三角洲相、滨岸相及浊积砂岩等潜在储集相带内的油气聚集以天然气为主，在400~700米的埋藏深度下，储量规模普遍约为5~9 Tcf；
- 上同裂谷期2单元的同类砂岩储层内则为油-气混合聚集（约20~30亿桶原油与10 Tcf天然气），埋藏深度为400~800米；
- 下同裂谷期2与同裂谷期1的河流相砂岩中也存在类似的油气聚集；
- 多数已圈定的构造圈闭埋藏深度相对较浅，因此盖层封闭有效性是一项重要风险。更深部的构造与地层圈闭成藏组合或可进一步提升这两个海盆的油气远景。

本次研究提出的模型阐明了开普尔与福斯特海盆内构造圈闭的油气充注潜力，并凸显了与烃源岩分布及类型、储集层分布与品质、盖层封闭有效性相关的风险。若采用高密度地震数据、储层与盖层的岩石物性数据，并结合三维模拟技术，可进一步优化体积与充注评价结果。

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