摘要
【目的】海相碳酸盐岩中常见选择性溶蚀现象,前人多认为是大气淡水成岩作用的标志,其他可能的成因缺乏探讨和相关报道。【方法】基于岩石矿物学、无机碳氧同位素和元素地球化学特征,对南羌塘盆地GK-1井布曲组海相选择性溶蚀成因进行综合分析。【结果】岩石矿物学特征表明:GK-1井布曲组发育8个沉积旋回(自下而上为C1到C8),每个旋回下部为灰岩,上部为白云岩。根据旋回中碳酸盐岩组分变化,这8个旋回可分为A、B两类,A类旋回(C1~C4)中灰岩为生物碎屑泥粒灰岩和颗粒灰岩,B类旋回(C5~C8)中灰岩为生物碎屑粒泥灰岩和泥粒灰岩,从A类旋回到B类旋回,灰岩中文石质生屑含量减少,似球粒增多;两类旋回中白云岩均为晶粒白云岩,内部可见鲕粒颗粒幻影,并在幻影内部发育选择性溶孔,其中A类旋回选择性溶孔体积明显高于B类旋回。沉积旋回地球化学数据显示:白云岩碳氧同位素相对灰岩偏正;白云岩和灰岩稀土元素含量低(0.440×10^(-6)~7.254×10^(-6)),Y/Ho值(35.63~75.55)基本位于现代海水范围内,其中白云岩稀土元素PAAS标准化后配分模式类似海水,表现为轻稀土相对亏损,重稀土相对富集的“左倾”样式,而灰岩稀土元素PAAS标准化后配分模式相对平坦;白云岩的Ce/Ce^(*)介于0.55~0.78(平均值为0.63),灰岩的Ce/Ce^(*)介于0.80~0.88(平均值为0.84);氧化还原敏感元素U、Mo、V含量很低,V/(V+Ni)比值介于0.01~0.39;单个旋回中生产力指标Cu、Zn含量在白云岩中表现为高值,且白云岩的Cu、Zn含量从A类旋回到B类旋回有减小的趋势。【结论】GK-1井布曲组为海相颗粒滩沉积,沉积水体从A类旋回到B类旋回局限性增强;沉积旋回中选择性溶蚀被认为在早期海相成岩阶段,由有机质降解产生的有机酸选择性溶解文石形成,并非淡水和深埋藏成因。选择性溶孔的发育受控于文石含量、古生产力及氧化还原界面的波动:单个旋回中,白云岩相对于灰岩孔隙水更加氧化,古生产力更高,从而有利于产生有机酸,形成选择性溶孔;不同类旋回之间,A类旋回相对B类旋回文石含量更高,古生产力更高,早期海相成岩中氧化还原界面更低,因而发育更多选择性溶孔。
[Objective]Selective dissolution is common in marine carbonates,and its origin is typically related to meteoric fluids,although other possible origins lack investigation.[Methods]In this study,the origin of the marine selective dissolution of the Buqu Formation of the well GK-1 in the South Qiangtang Basin of China is investigated based on petrography,stable carbon and oxygen isotopes,and elemental geochemistry.[Results]Eight sedimentary cycles(C1 to C8 from bottom to top)were recognized from the studied Buqu Formation.Each cycle has limestone in the lower part and dolostone in the upper part.These eight cycles were divided into two types(A and B)according to different carbonate components.The limestone in the type A cycle(C1-C4)is dominated by bioclastic packstone and grainstone,while that in the type B cycle(C5-C8)is dominated by bioclastic wackestone and packstone.Upwards from the type A cycle to the type B cycle,the content of aragonite fossils(gastropods and bivalves)in the limestone decreases and the content of peloids increases.The dolomite in both type A and type B cycles is crystalline dolomite,with ooid ghosts showing selective dissolution pores.Importantly,the overall percentage of selective dissolution pores in type A cycles is significantly higher than that in type B cycles.In terms of geochemistry,the carbon and oxygen iso⁃topes of dolostone are higher than those of the limestone in each cycle.Rare earth elements and yttrium concentrations(0.44×10^(-6)-7.25×10^(-6))of dolostone and limestone are extremely low,and the Y/Ho ratios(35.63-75.55)are basically within the range of modern seawater.Dolostone exhibits a seawater-like PAAS-normalized REY pattern,showing a left-leaning style with relatively depleted LREE and enriched HREE,while the PAAS-normalized REY pattern of the limestone is relatively flat.Furthermore,Ce/Ce^(*)values of the dolostone range from 0.55 to 0.78(average 0.63),and the Ce/Ce^(*)value of the limestone ranges from 0.80 to 0.88(average 0.84).The concentrations of redox-sensitive elements(U,Mo,V)are very low,and the V/(V+Ni)ratios range from 0.01-0.39.In each cycle,Cu and Zn in the dolostone are higher in content than in the limestone,and type A cycles have higher Cu and Zn content overall than type B cycles[.Conclusions]Based on lithology and sedimentary components,the Buqu Formation of well GK-1 was likely deposited in shallow marine grain-shoal settings,with seawater restricted from type A cycles to type B cycles.Based on comprehensive petrological and geochemical analysis,the selective dissolution herein is interpreted to be produced by early marine diagenesis,rather than meteoric diagenesis or deep burial.During early marine diagenesis,aragonite may be selectively dissolved by undersaturated pore fluids via organic matter decomposition.In the sedimen⁃tary cycles,the differential development of selective dissolution is probably controlled by aragonite content,paleopro⁃ductivity,and the early marine diagenetic redox boundary.In a single cycle,the dolostone interval is characterized by higher paleoproductivity and more oxic pore water than the limestone interval during early diagenesis,favoring the production of undersaturated fluids and the formation of selective dissolution pores.Compared to type B cycles,type A cycles have higher aragonite content,higher paleoproductivity,and a lower early marine diagenetic redox bound⁃ary,thus resulting in the better development of selective dissolution.
作者
王茂德
付小东
张建勇
刘思琪
葛毓柱
甯濛
夏攀
文华国
WANG MaoDe;FU XiaoDong;ZHANG JianYong;LIU SiQi;GE YuZhu;NING Meng;XIA Pan;WEN HuaGuo(State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,China;Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China;Key Laboratory of Deep-time Geography and Environment Reconstruction and Applications of Ministry of Natural Resources,Chengdu University of Technology,Chengdu 610059,China;Key Laboratory of Carbonate Reservoirs,CNPC,Hangzhou 310023,China;PetroChina Hangzhou Institute of Petroleum Geology,Hangzhou 310023,China)
出处
《沉积学报》
北大核心
2025年第6期2068-2087,共20页
Acta Sedimentologica Sinica
基金
中国石油杭州地质研究院项目(RIPED-2022-JS-681)
四川省天然气成藏物质基础青年科技创新团队基金项目(2022JDTD0004)。
关键词
南羌塘盆地
布曲组
海相碳酸盐岩
选择性溶蚀
South Qiangtang Basin
Buqu Formation
marine carbonate rocks
selective dissolution
