塔里木盆地北部沥青、干酪根Pb-Sr-Nd同位素体系及成因演化

Pb Sr Nd ISOTOPIC SYSTEMATICS OF KEROGEN AND BITUMEN IN THE TARIM BASIN AND THEIR IMPLICATIONS FOR ITS ORIGIN AND EVOLUTION

Ｐｂ、Ｓｒ、Ｎｄ同位素体系研究表明沥青与寒武纪、奥陶纪地层中的干酪根具有明显的同位素组成差异，表明来自异源。沥青Ｐｂ同位素组成具有明显的幔源与壳幔混合特征，而干酪根显示出壳源特征。沥青与干酪根的Ｎｄ同位素组成也存在着明显的差别，反映出老地壳源与新地壳源的明显差别。来自柯坪隆起大湾沟沥青的Ｎｄ－Ｓｒ同位素体系给出了晚元古的相关等时线，年龄分别为８７２Ｍａ与１１９５Ｍａ，可能代表了沥青的最早形成年龄。但来自塔北隆起的沥青其Ｓｍ－Ｎｄ体系显示出介于大湾沟沥青与干酪根之间的混合相关，表明具有壳源与幔源的二元混合成因。Ｒｂ－Ｓｒ和Ｐｂ－Ｐｂ等时线趋向和Ｐｂ模式年龄还给出加里东期（～４４０Ｍａ）和印支期（～２５０Ｍａ）的原油运移与沥青沉淀年龄信息。Ｎｄ模式年龄还表明柯坪－阿克苏地区存在太古宙基底（３．２－２．５Ｇａ）。塔里木油藏受深断裂控制。

There is a huge reservoir of solid bitumen in the Silurian sandstones in the Tabei Central and the Kalpin Uprift of the Tarim Basin.Nd Sr Pb isotopic evidence shows that there exists a remarkable difference between kerogen from the Cambrian and Ordovician strata and bitumen,indicating their different sources.The Pb isotopic compositions of bitumen yielded mixing features between crust and mantle,whereas those of kerogen show features of crustal sources.There are also evident differences of Nd isotopic compositions between kerogen(0512 225-0512 262) and bitumen(0511 632-0512 079),evidently reflecting younger and older crustal source respectively.The Sr Nd isotopic systematics of bitumen from Dawangou,Kalpin uprift,yielded isochron correlations of the Late Proterozoic,872 Ma and 1195 Ma respectively,which may represent the formation age of the bitumen.The εNd(t)-εSr(t) correlation lies in the uppart of third quadrant,indicating a genetic environment of the lower crust close to the upper mantle.But the Sm Nd systematics of the bitumen from the Tabei Uprift show a mixing correlation between the kerogen and the bitumen from Daawangou,which indicated its binary genesis between mantle and crustal sources.The isochron trends of Rb Sr and Pb Pb as well as Pb model ages also gave age signatures for oil migration and bitumen deposition in Calidonian(～ 440 Ma) and Indochina(~250 Ma).The Nd model ages show that there exist an Archaean basement in the central uprift of the Tarim Basin(3.2-2.5Ga).Thus,the oil reservoirs in the Tarim Basin should be controled by the deep faults,and petroleum prospect is not only buried in the Paleozoic strata but also in the Precambrian strata.