西沙海域上新统—全新统高伽马地层的发现及地质分析  

Identification and geology analysis of high natural gamma value formation of Pliocene-Holocene in Xisha offshore

作　　者：杨涛涛[1,2] 吕福亮 李林[1,2] 王霞 鲁银涛[1,2] 杨志力 李丽[1,2] YANG Tao-tao;LFu-liang;LI Lin;WANG Xia;LU Yin-tao;YANG Zhi-li;LI Li(Petrochina Hangzhou Research Institute of Geology,Hangzhou 310023,China;Hangzhou Institute of Petroleum Geology Company Limited,Hangzhou 310023,China)

机构地区：[1]中国石油天然气股份有限公司杭州地质研究院,杭州310023 [2]中国石油集团杭州地质研究所有限公司,杭州310023

出　　处：《地球物理学进展》2019年第6期2526-2532,共7页Progress in Geophysics

基　　金：国家科技重大专项“南海中建海域深水油气地质条件及目标评价”(2016ZX05026006);国家自然科学基金“南海北部淹没碳酸盐台地发育演化及混合沉积研究”(91228208)和国家自然科学基金“海洋一次波与多次波联合最小二乘逆时偏移”(41504105)联合资助

摘　　要：高伽马地层是指比泥岩层伽马值相当或更高的地层,主要有四种成因:与同沉积火山事件有关、黏土类型和含量的变化、含有放射性强的特殊矿物和注水开发造成的放射性污染等,对其研究对深化地质评价有帮助.本文根据最新钻井资料首次在西沙海域识别出高伽马地层,并结合高品质三维地震资料对其展布和成因进行研究.高伽马地层纵向上有两套,伽马均值140~174 API,比上覆和下伏泥岩层高出约40%~74%;平面上层段1仅在W2井区发育,层段2两口井均发育且可对比,向W1井方向厚度和伽马值均增加.从三个方面进行成因分析:首先,对钻头尺寸、接单根和钻井参数等分析,可排除钻井工程因素.其次,经钻井液性质和井径等分析可排除随钻伽马测量方面因素.再次,通过地质分析表明高伽马地层是由火山活动引起,主要有五个依据:(1)国内大量实际资料表明中性-酸性火成岩具有高伽马特征;(2)处于拉张环境的南海北部大陆边缘火山活动频繁,基性和中-酸性火成岩都发育;(3)南海北部钻遇的中-酸性火成岩具有高伽马特征;(4)海岛钻井等4类资料均证实西沙海域晚期火山活动较频繁;(5)地层高放射性物质来自邻近的火山岛.The high gamma formation refers to the gamma ray value equal or higher than the claystone layer.There are four main forming reasons:Associated with syn-sedimentary volcanic events,clay type and its content changing,special mineral containing strong radioactive and the radioactive pollution caused by waterflood development,etc.The northern South China Sea has geological development conditions for high gamma formation,which have been found in the Beibu Gulf basin and Ying Gehai basin.In this paper,according to new drillings data and a large number of high quality seismic data,the high gamma formation was identified the first time for Xisha offshore.Longitudinally two sets of high gamma intervals were identified,mean value 140~174 API,higher than the overlying and underlying claystone layer about 40%~74%.On the plane,interval 1 only developed in B well area,and layer 2 developed in both two wells which can be compared.Thickness and gamma ray value were increased to the direction of A well.The distribution and the causes of high gamma formation were studied.Cause was analyzed from three aspects:First,bit size,stem connection,and drilling parameters were analyzed.Drilling engineering factors could be eliminated.Second,drilling mud properties and wellbore diameter analysis can eliminate gamma ray measurement while drilling.Third,geological analysis showed that high gamma ray formation is caused by volcanic activities in Xisha offshore.Five evidences supported this conclusion:(1)a large number of data and articles show that intermediate-acid igneous rocks in domestic multiple basins have high gamma ray characteristics.(2)In the extensional environment of the northern South China Sea continental margin volcanic activities were frequent,intermediate and acidic igneous rocks are development.(3)In northern South China Sea,drilled intermediate-acidic igneous rocks is characterized by high gamma ray value.(4)Island drilling and other four kinds of materials confirmed that late volcanic activities were frequent Xisha offshore

关 键 词：西沙海域 高伽马地层 展布特征 地质分析 

分 类 号：P631[天文地球—地质矿产勘探] P738[天文地球—地质学]