塔里木盆地塔中地区下奥陶统白云岩的成岩流体演化:来自团簇同位素的证据

Diagenetic fluid evolution of dolomite from the Lower Ordovician in Tazhong area, Tarim Basin:Clumped isotopic evidence

基于碳酸根中13 C-18 O键的相对丰度与温度的关系,碳酸盐团簇同位素(Δ47)具有独特的温度指示特征,而且不受碳酸盐沉淀时流体的化学和同位素组成的影响,是成岩流体研究中很好的温度指标。应用该指标可以更好地解决与温度相关的成岩流体来源及演化的问题。论文主要针对塔里木盆地塔中地区下奥陶统鹰山组白云岩样品,在详细的岩矿观察基础上,选取了基质(孔隙发育处)、孔洞内充填物和裂缝内充填物进行了团簇同位素测试分析,解析了深层-超深层碳酸盐岩储层的流体性质、演化与其对储层发育的影响。测试结果表明,塔中地区下奥陶统鹰山组白云岩主要经历了3期成岩流体改造:第一期流体为浅-中埋藏,为粉晶-细晶结构,团簇同位素形成温度T(Δ47)分布范围为69~94℃,流体氧同位素值(δ18 O w,SMOW)分布范围为+1.81‰^+5.19‰,为奥陶系改造海水;第二期流体为晚期深埋藏卤水,多表现为细晶与中-粗晶结构,T(Δ47)为111~113℃,δ18 O w(SMOW)值为+8.46‰^+8.95‰,指示进一步增加的水-岩相互作用;第三期为热流体,T(Δ47)为130~147℃,δ18 O w(SMOW)值为+7.93‰^+9.42‰,矿物氧同位素较第二期明显偏负,且鞍状白云石发育。孔洞和裂缝内充填碳酸盐岩的碳、氧同位素分布范围较宽,δ18 O carb(VPDB)值为-17.38‰^-5.84‰,δ13 C carb(VPDB)值为-3.57‰^-1.33‰,也揭示了多期次流体活动的叠加改造。岩心和显微观测结果显示,白云岩具有很好的储集空间,埋藏溶蚀作用和断裂带附近的热流体活动叠加是鹰山组白云岩储层发育的主要建设性成岩作用。

Based on the relationship between the relative abundance of 13 C and 18 O bonds and temperature,carbonate clumped isotope(Δ47)has the special characteristic of indicating temperature even in the absence of knowledge about the chemical and isotopic composition of the fluid from which the carbonates precipitated.This property hence makes the technique an excellent temperature proxy in the study of diagenetic fluids.As such,this study employs clumped isotopes to examine the origin of diagenetic fluids in deep and ultra-deep subsurface settings and we assess their impact on the development of reservoir quality.This study considers both the matrix and associated cements.Three episodes of diagenesis are revealed.In the first stage,the fluids formed in a shallow-to-medium burial environment and the matrix was altered to have a powder-crystal to finely-crystalline texture.The range ofΔ47 derived temperatures ranges from 69 to 94℃,and theδ18 O w values are from+1.81‰to+5.19‰SMOW.These data suggest that the fluid is modified seawater.The second diagenetic episode was influence by a brine after the sequence was deeply-bured.Here,the matrix was altered to a me-dium-crystalline texture and theΔ47 derived temperatures range from 111 to 113℃,and theδ18 O w values range from+8.46‰to+8.95‰SMOW,indicating that a substantial water-rock interaction occurred.The third diagenetic episode was induced by hydrothermal fluids,likely travelling along faults and fractures,which yielded abundant saddle dolomite.Here,Δ47 derived temperatures varies from 130 to 147℃and theδ18 O w values from+7.93‰to+9.42‰SMOW.The vug-and vein-filling carbonates have a wide range ofδ18 O carb andδ13 C carb values.Theδ18 O carb values span-17.38‰to-5.84‰VPDB,and theδ13 C carb values from 3.57‰to-1.33‰VPDB,indicating multiple stages of fluid modification.Both the cores and photomicrographs demonstrate excellent reservoir quality for the Lower Ordovician Tazhong dolomites.