珠江口盆地珠一坳陷古近系高自然伽马砂岩形成机制及油气地质意义

运用放射性元素寻找油气是一种非常规油气勘探手段。近年来,在珠江口盆地珠一坳陷富烃凹陷周边古近系钻遇高自然伽马(GR)砂岩,其GR值(100~300 API)甚至大于同区泥岩的GR值(100~200 API)。为了弄清该特殊现象背后的地质意义,对珠江口盆地珠一坳陷古近系高自然伽马砂岩开展了铀(U)、钍(Th)、钾(K)等3种元素含量与GR值的相关趋势线分析,从井震特征、岩性特征及矿物成分特征等入手分析了砂岩GR值增高的主要原因及成因机制,探讨了放射性元素聚集的条件、运移通道、驱动力以及油气意义。结果表明:西江、惠州地区由U含量增高导致砂岩GR值偏高,恩平、番禺地区由K,Th含量增高导致砂岩GR值偏高;砂岩GR值增高有两大成因机制,一是地下流体带来的放射性元素离子U4+在氧化-还原面处富集后导致地层GR值偏高,这种特殊现象说明在具有连通基底大断裂旁的圈闭中,U4+的富集指示了曾经油气的存在,证实了研究区油气运移通道的有效性,对于油气藏的预测有着非常重要的指导性意义,二是地表流体带来的含放射性元素的矿物大量沉积后导致地层GR值偏高,含放射性元素矿物性质不稳定,可指示近源供给的存在,对于判断物源及沉积环境有着非常重要的意义。该研究成果为预测研究区油气成藏有利区带提供了依据。

Formation mechanism and petroleum geological significance of Paleogene sandstone with high natural gamma value in Zhuyi Depression,Pearl River Mouth Basin

It is an unconventional means of oil and gas exploration to use radioactive elements to search for oil and gas.In recent years,sandstones with high natural gamma(GR)value(100-300 API)have been drilled in Paleogene around the hydrocarbon-rich depression in Zhuyi Depression,Pearl River Mouth Basin,and the GR value is even higher than that of mudstone in the same area(100-200 API).In order to understand the geological significance behind this special phenomenon,the correlation trend line analysis of the contents of uranium(U),thorium(Th)and potassium(K)with GR value was carried out for the Paleogene sandstone with high GR value in Pearl River Mouth Basin.The main reasons and genetic mechanism of the increase of GR value of sandstone were analyzed from the characteristics of well seismic,lithology and mineral composition,and the accumulation conditions,migration channels,driving forces and hydrocarbon significance of radioactive elements were discussed.The results show that the GR value of sandstone is higher in Xijiang and Huizhou areas due to the increase of U content,and higher in Enping and Panyu areas due to the increase of K and Th content.There are two genetic mechanisms for the increase of GR value of sandstone.One is that the enrichment of radioactive element ion U4+brought by underground fluid at oxidation-reduction surface leads to the high GR value of formation.This special phenomenon shows that the enrichment of U4+indicates the existence of oil and gas in traps near large faults with connected basement,which confirms the effectiveness of oil and gas migration channels in the study area and has very important guiding significance for the prediction of oil and gas reservoirs.The other is that the massive deposition of minerals containing radioactive elements brought by surface fluids leads to the high GR value of strata and unstable properties of minerals containing radioactive elements,which can indicate the existence of near-source supply,and is of great significance for judging the provenance and sedimentary environment.The research results provide a basis for predicting favorable zones for hydrocarbon accumulation in the study area.