裂变径迹热年代学在珠江口盆地渐新统珠海组物源分析中的应用*

渐新统珠海组是南海北部珠江口盆地的主要储集层发育层位之一,但其物源区位置、沉积物搬运路径等关键问题仍存在争议。为此,选取珠江口盆地不同构造单元3口关键井珠海组上段砂岩样品开展锆石裂变径迹(ZFT)、磷灰石裂变径迹(AFT)热年代学研究,进而开展珠海组物源分析。盆地西部西江凹陷XJ28井与白云凹陷LW3井ZFT、AFT年龄组分相似,包含晚三叠世的ZFT年龄组分,且ZFT、AFT主要年龄组分分别为白垩纪、古新世;盆地东部陆丰凹陷的LF7井ZFT、AFT年龄组分明显比前2口井偏年轻,ZFT主要年龄组分为古新世。3口井的ZFT、AFT年龄组成表明其珠海组上段物源均主要来自珠江口盆地北侧的华南地块,但盆地西部与东部显示出明显的物源差异。盆地西部XJ28井以及LW3井物源特征总体一致,均由古珠江水系供源,其物源以珠江流域中生代花岗岩为主,还包括华南地块腹地的中生代沉积岩。盆地东部LF7井的物源主要为陆丰凹陷北侧的沿海中生代花岗岩,物源组成相对简单,沉积物搬运距离相对较短。

Application of fission track thermochronology in provenance analysis of the Oligocene Zhuhai Formation in Pearl River Mouth Basin

The Oligocene Zhuhai Formation is one of the main reservoirs in the Pearl River Mouth Basin(PRMB), northern South China Sea. However,the key issues such as the provenance location and sediment transport path are still controversial. Zircon fission track(ZFT)and apatite fission track(AFT)thermochronology are carried out for sandstone samples from the upper Zhuhai Formation of three key wells in different structural units of PRMB to analyze the provenances of Zhuhai Formation. The ZFT and AFT age components of Well XJ28 in Xijiang sag are similar to those of Well LW3 in Baiyun sag,and include the ZFT age components of Late Triassic. Furthermore, the main age components of ZFT and AFT are Cretaceous and Paleocene respectively. In contrast,the ZFT and AFT age components of Well LF7 are obviously younger than those of the first two wells and the main ZFT age component of Well LF7 is Paleocene. The ZFT and AFT age components of those three wells indicate the provenance of the upper Zhuhai Formation is mainly from the South China block in the north of PRMB,but there are obvious provenance differences between the western and the eastern of the basin. The provenance characteristics of Wells XJ28 and LW3 in the western basin are generally consistent,both of which are supplied by the ancient Zhujiang River water system. The provenance is mainly the Mesozoic granite in the ancient Zhujiang River drainage area,and also includes the Mesozoic sedimentary basin in the hinterland of South China block. The provenance of Well LF7 in the eastern basin is mainly the coastal Mesozoic granite in the north of Lufeng sag. The provenance composition is relatively simple and the sediment transport distance is relatively short.