摘要： 硫循环及硫同位素(δ³⁴S)分馏研究对地表圈层的成岩作用具有重要意义,其中多种金属硫化物中硫同位素的分馏程度可以约束成矿热流体温度,进而作为地温计证据约束热液活动。四川盆地龙王庙组储集层内的热液改造影响着该储集层的非均质性,本研究着重讨论目的层中与热液成因白云石所伴生的黄铁矿(FeS₂)-黄铜矿(CuFeS₂)成矿现象: 基于详尽的岩石学证据,应用纳米二次离子探针(NanoSIMS)对金属硫化物内部硫同位素分布进行测定,并基于热力学驱动下的硫化物间平衡分馏程度计算其成矿温度,进而约束层段内热液活动过程。研究发现: (1)微区硫同位素分布显示黄铁矿(FeS₂)与黄铜矿(CuFeS₂)沉淀过程中不仅存在热力学分馏,还存在动力学分馏现象,其中动力学分馏程度可以达到40.1‰,应用NanoSIMS微区测定手段可以有效剔除动力学分馏数据影响,获取热力学平衡分馏数据; (2)黄铁矿(FeS₂)与黄铜矿(CuFeS₂)成矿过程或利用不同的硫源,其中黄铁矿沉淀主要利用下覆筇竹寺组硫源(近似等于未经热化学硫酸盐还原作用[TSR]改造固体沥青 δ³⁴S 值),存在约2.4‰~2.9‰程度的分馏,指示其成矿流体温度为98.2~135.0 ℃,而黄铜矿沉淀主要利用目的层地层水中的硫源(近似等于碳酸盐晶格硫酸盐的 δ³⁴S 值),成矿温度计算结果无实际意义; (3)黄铁矿的成矿温度指示目的层中断裂—热液活动始于生油高峰阶段,并在后期埋藏过程中持续活动,形成目的层中具较高均一温度的铁白云石沉淀现象。

关键词: 四川盆地, 龙王庙组, 硫同位素, 黄铁矿, 黄铜矿, 纳米二次离子探针, 热液活动

Abstract: The study of sulphur cycle and sulphur isotope(δ³⁴S)fractionation is significant for diagenesis of surface sphere,where the fractionation degree of sulphur isotope between multiple metal sulphides can be used to constrain the temperature of ore-forming hydrothermal fluid,which can act as the geothermometer of hydrothermal activity. The hydrothermal transformation in the reservoir of Longwangmiao Formation in the Sichuan Basin affects the heterogeneity of the reservoir. In this paper,the co-precipitation of pyrite(FeS₂)-chalcopyrite(CuFeS₂)associated with hydrothermal diagenetic dolomite in the target formation is studied in detail. The sulphur isotope distribution in the metal sulphides is measured based on the NanoSIMS. The mineralization temperature was calculated based on the degree of equilibrium fractionation between multiple sulphides driven by the thermodynamics,which in turn constrains the hydrothermal process of the specific formation. Results show that(1)the sulphur isotope distribution in the microzone implies that both the thermodynamic fractionation and kinetic fractionation occur during the co-precipitation of pyrite(FeS₂)and chalcopyrite(CuFeS₂),where the degree of kinetic fractionation can reach 40.1‰. The influence of kinetic fractionation can be effectively excluded based on the NanoSIMS to obtain thermodynamic equilibrium fractionation data;(2)there may exist different sulfur sources during the pyrite(FeS₂)and chalcopyrite(CuFeS₂)mineralization processes. The pyrite precipitation primarily utilizes the sulfur sources from the underlying Qiongzhusi Fm.,which is approximately equal to the δ³⁴S value of solid bitumen modified without thermochemical sulfate reduction[TSR],and the fractionation degree is about 2.4‰ to 2.9‰,indicating the mineralization temperature is in the range of 98.2 to 135.0 ℃. The chalcopyrite precipitation mainly utilizes the sulphur source in the formation water,which is approximately equal to the δ³⁴S value of carbonate association sulfate[CAS]),and the mineralization temperature calculation is insignificance;(3)the mineralization temperature of pyrite indicates that the fault activity and hydrothermal activity in studied formation occurred at the peak phase of oil production and it continued in the later burial process,resulting in the ankerite precipitation with high uniform temperature.

Key words: Sichuan Basin, Longwangmiao Formation, sulfur isotope, pyrite, chalcopyrite, Nano Secondary Ion Mass Spectrometry, hydrothermal activity

中图分类号: 

• P618.13