澳大利亚北卡那封盆地中上三叠统Mungaroo三角洲陆源有机质分布特征

北卡那封盆地中上三叠统Mungaroo组发育巨型浅水辫状河三角洲——Mungaroo三角洲。利用钻井、岩心资料,并结合区域地质背景,将Mungaroo三角洲划分为近端三角洲平原、远端三角洲平原、三角洲前缘、前三角洲4个沉积亚相,其中近端三角洲平原与远端三角洲平原亚相分布广泛,三角洲前缘与前三角洲亚相欠发育。陆源有机质分析表明:(1)研究区目的层段沉积时期,在环特提斯洋巨型季风洪水的影响下,泥炭沼泽普遍遭受冲刷改造,有机质类型主要为分散有机质,仅远端三角洲平原发育薄煤层;(2)不同相带受季风洪水的影响差异较大,近端三角洲平原受洪水改造强烈,砂砾岩发育,夹薄层泥岩,泥岩中有机质显微组分主要为惰质组,镜质组大部分遭受氧化;(3)远端三角洲平原的沼泽沉积尽管受到洪水频繁冲刷,但仍发育多套薄煤层,泥岩中陆源有机质最为丰富,显微组分以镜质组为主;(4)三角洲前缘和前三角洲相带窄,沉积物中陆源有机质含量较低。远端三角洲平原烃源岩有机质生烃潜力最大,除发育广泛的薄煤层外,暗色泥岩中平均TOC含量可达4.11％。上述沉积相带与陆源有机质分布规律体现了强季风洪水影响下大型辫状河三角洲的沉积特点。

Distribution of the Upper Triassic terrigenous organic matter in Mungaroo delta of North Carnarvon Basin,Australia

The giant shallow water braided fluvial delta, Mungaroo delta, was developed in the Mungaroo Formation in North Carnarvon Basin during the Middle and Upper Triassic. Based on logging data and core data,together with the regional geological setting,the Mungaroo delta is divided into four sedimentary sub-facies,including the proximal delta plain,the distal delta plain,the delta front and the prodelta. The proximal delta plain and the distal delta plain are widely spread,while the delta front and the prodelta are underdeveloped. Analysis of the terrigenous organic matter in the different sub-facies suggests that: (1)During the deposition of sediments, the peat bog underwent a scouring transformation due to the influence of the circum-Tethys giant monsoon flood,hence the organic matter is mainly dispersed and the thin coal strata is only developed in the distal delta. (2)The influence of monsoon floodvary greatly from sub-facies to sub-facies. The proximal delta plain was transformed intensively by the flood and was rich in glutenite although thin mudstone could also be found in it. The maceral of the mudstone is mainly inertinite because most of the vitrinite was oxidized. (3)Although the marsh sediments in the distal delta plain underwent frequent scour of the flood,there are still many thin coal beds developed in dark mudstone. The content of terrigenous organic matter in the mudstone is highest with vitrinite as the main type. (4)The delta front and the prodelta are narrow. The terrigenous organic matter content is low in these facies. The distal delta plain is the most potential hydrocarbon-generating facies of the four facies and its average total organic carbon(TOC) content can be up to 4.11％. The terrigenous organic distribution reflects that the braided river delta sedimentary features underwent a megamonsoon influence.