羌塘盆地南部古油藏带布曲组白云岩地球化学特征及成因机制

The geochemical characteristics and mechanism of dolomite in the paleo-reservoir of Buqu Formation,south Qiangtang Basin,Tibet,China

探讨羌塘盆地南部古油气藏中侏罗统布曲组白云岩的成因机制。通过对布曲组岩心观察、岩石学研究及微区C、O、Sr同位素分析,确定布曲组白云岩包括保留先驱灰岩原始组构白云岩、晶粒白云岩和白云石充填物。其中保留先驱灰岩原始组构的白云岩Sr同位素值与同期海水相似,C-O同位素值较同期灰岩偏正,属于准同生阶段高位体系域晚期局限环境中蒸发的咸化海水,高度过饱和的白云石化流体快速通过有利于原始组构的保存。晶粒白云石中分别由细晶自形白云石和细晶半自形白云石构成的白云岩Sr同位素值与同期海水相似,C-O同位素值较同期灰岩偏正,较保留先驱灰岩原始组构白云岩负偏,为浅埋藏阶段、高位体系域晚期局限环境中因蒸发咸化的海水回流或下渗扩散交代先驱灰岩而来,过度白云石化是形成细晶半自形白云石的主要原因。由中-粗晶他形白云石构成的白云岩形成于中-深埋藏阶段,Sr同位素值与同期海水相似;但O同位素值较前几种白云岩负偏,可能为早期白云石重结晶的结果;部分样品C同位素值与灰岩相似,可能为不彻底交代灰岩的结果;较高的成岩温度是晶体发生曲面化的主要原因。细-中晶自形-半自形白云石充填形成于浅埋藏阶段末期到中-深埋藏阶段初期,是化学沉淀或结晶作用的产物。鞍形白云石充填物Sr同位素值普遍高于同期海水,C同位素分布范围较宽,O同位素值比中-粗晶他形白云岩偏负,可能与早白垩世末期的剪切背景造成大气淡水下渗及64~78 Ma B.P.的构造热事件有关。

The observation of drilling cores,study of petrology and analysis of C,O and Sr isotopes reveal that the Jurassic Buqu Formation dolomite in the south Qiangtang Basin consist of original dolomite with original limestone,crystal grain dolomite and dolomite filling.The isotopic value of Sr for dolomite is similar to contemporary seawater and the isotopic value of C and O is positive compared to contemporary limestone.Therefore,the original dolomite with original limestone is formed in evaporative salty seawater with limited environment in penecontemporaneous stage of highstand system tracts.The fast-flowing and highly supersaturated dolomitization fluid is favorable for preservation of original dolomite.For crystal grain dolomite,the value of isotopic Sr for fine grained euhedral dolomite and fine grained semi-euhedral dolomite is the same as contemporary seawater.Compared to contemporary limestone,the value of isotopic C and O are positive and compared to original dolomite with original limestone,the value of isotopic C and O are negative.In shallow burial stage with limited environment in penecontemporaneous stage of highstand system tracts,the crystal grain dolomite is formed by metasomatism from original limestone duo to back-flow or infiltration of evaporative salty seawater.Over-dolomitization is responsible for the formation of fine grain euhedral dolomite.The mid-coarse grained none-euhedral dolomite is formed in mid-deep burial stage,and the value of isotopic Sr is the same as the contemporary seawater.However,the value of isotopic O is negative compared to other dolomites,it may be resulted from recrystallization of early dolomite.It shows that the value of isotopic C in the dolomite is the same as limestone in some of samples duo to incomplete metasomatism,and high diagenetic temperature is responsible for the distortion of crystal surface.The study reveals that the fine-mid grained,euhedral and semi-euhedral dolomite filling are formed in late shallow buried stage to early mid-deep buried stage,as a result of recrystallization.The value of isotopic Sr for saddle shaped dolomite filling is higher than that for contemporary seawater,the distribution scope of isotopic C is wide,and the value of isotopic O is negative compared to mid-coarse grained none-euhedral dolomite.These geometrical characteristics are considered to be resulted from atmospheric fresh water infiltration and heating source in 64～78 Ma B.P.,caused by shearing in late early Cretaceous period.