陕北长6段高自然伽马砂岩地球化学特征及意义

摘要

陕北地区上三叠统延长组长6段是勘探开发的重要层位,该段普遍发育高自然伽马砂岩.通过对高伽马砂岩及邻近常规砂岩的主元素、痕量及稀土元素、电子探针微区元素、能谱测井放射性元素分析认为,高伽马砂岩中Th、U、K放射性元素含量比常规砂岩都高.其中Th元素含量上升最为显著,主要存在于独居石、金红石、铁泥质黏土、钾长石、部分锆石、磷灰石、黑云母及岩屑中;U元素主要存在于大多数的锆石、部分磷灰石、黑云母及含镁方解石中;K元素主要存在于钾长石、黑云母、金红石、铁泥质黏土、部分钠长石及岩屑中.火山凝灰岩对高伽马砂岩的影响是次要的,高伽马砂岩与来自于盆地东北方向的物源有一定的亲缘性;但该方向上的物源不是高伽马砂岩形成的主要控制因素,热液活动可能对高伽马砂岩的形成有着重要的影响.%Member 6 of Upper Triassic Yanchang Formation (Chang 6) in North Shaanxi is an important horizon for petroleum exploration and development.The high natural gamma ray sandstones generally exist in this member.Through analysing the major elements and trace elements,electron microprobe microscopic elements,spectrum well logging radioactive elements in the high natural gamma ray sandstones and the nearby conventional sandstones,people can conclude that the content of Th,U,K in the high natural gamma ray sandstones increases compared with the conventional sandstone.Among them,the content of Th increases significantly.Th mainly exist in monazite,rutile,iron muddy clay,feldspar,some of the zircon,apatite,biotite and debris.The radioactive element U mainly exists in most of zircon,some of apatite,biotite and magnesium calcite.The radioactive element K is mainly in K-feldspar,biotite,rutile,iron muddy clay,and some of albite and debris.The affection of volcanic tuff is secondary in forming the high gamma ray sandstones.To a certain degree,the high gamma ray sandstones have an affinity to the material source from the northeast of the basin.But the northeast material source is not the main controlling factor for the formation of the high gamma ray sandstones.The hydrothermal activity maybe the important factor in forming the high gamma ray sandstones.