中扬子京山地区方解石脉成因及其对油气保存的指示意义

野外观察中扬子京山地区逆冲推覆带和冲断褶皱带内奥陶系、二叠系和三叠系发育多期沿裂缝分布的方解石脉。通过稀土元素（REE），电子探针，氧同位素，流体包裹体分析及盆地模拟对方解石脉成因进行了研究。奥陶系和三叠系方解石脉REE呈现轻稀土元素（LREE）富集，重稀土元素（HREE）亏损，明显的正铕（Eu）异常，说明成脉流体受到了热液流体的影响；二叠系方解石脉REE总量低，LREE低度富集，HREE配分曲线平坦，中等程度负铈（Ce）异常，显示成脉流体为大气淡水成因。根据电子探针Fe，Mn元素分析，部分奥陶系方解石脉成因与其围岩碳酸盐溶解有关；二叠系方解石脉成因主要与大气淡水淋滤碳酸盐成分有关，三叠系方解石脉成因与其他层位海相碳酸盐成分有关。利用氧同位素温度和流体包裹体均一温度，结合研究区虚拟井的埋藏史和热史模拟结果，推测3套地层裂缝方解石脉体形成深度约在2 000~2 500 m，形成时间约为距今150~60 Ma。综合方解石脉地球化学特征分析认为，燕山期构造挤压抬升作用伴随的热液流体、成岩流体及地表水下渗作用使京山地区油气保存条件遭受了一定程度的破坏。

Genesis of calcite vein and its implication to petroleum preservation in Jingshan region,Mid-Yangtze

Field observation shows that multiple phases of calcite veins occur along fractures in the Ordovician,Permian and Triassic of overthrust belts and fold-thrust belts in Jingshan region,Mid-Yangtze.Using rare earth elements(REE),electron microprobe,oxygen isotope,fluid inclusions and basin modeling,this paper analyzed the genesis of these calcite veins.The REE of calcite veins in the Ordovician and Triassic are characterized by enrichment of LREE,loss of HREE and obvious positive Eu anomaly,indicating that their vein-froming fluids were affected by hydrothermal fluids.Calcite veins in the Permian feature in low REE,only slight enrichment of LREE,flat HREE patterns,and the presence of a moderate negative Ce anomaly,implying that their vein-froming fluids originated from meteoric water infiltration.Analysis of Fe and Mn through electron microprobe indicates that some calcite veins in the Ordovician were derived from the dissolution of surrounding carbonate rocks,the calcite veins in the Permian were derived from leaching and dissolution of carbonates by meteoric water,and those in the Triassic were related to the dissolution of carbonate derived from other horizons.Combined with burial and thermal history modeling of pseudo-wells,analysis of oxygen isotope temperatures and homogenized temperature of fluid inclusions show that the calcite vines filling fractures in these 3 sequences were formed at 150~60 Ma ago at burial depth in the range from 2 000 m to 2 500 m.Based on a comprehensive analysis of geochemical behaviors of the calcite veins,it is believed that hydrothermal fluids,vein-froming fluids and meteoric water infiltration associated with tectonic compression and uplifting during Yanshan Orogeny may have damaged petroleum preservation conditions in Jingshan region to some extent.