火山岩陆上与水下喷发沉积环境的地球化学判识方法研究

Geochemistry discriminance study of subaerial and underwater eruptive depositional environment of volcanic rocks

陆上、水下两种不同喷发沉积环境火山岩储层特征差异明显,准确判识火山喷发沉积环境是火山岩储层评价的重要依据。研究使用Fe_(2)O_(3)/(Fe_(2)O_(3)+FeO)作为火山岩氧化系数,并充分考虑火山喷发期的地理环境、岩浆性质(岩石酸度)、构造成因环境、流体活动、风化淋溶作用、成岩后表生作用等影响因素,针对不同影响因素提出校正方法和合理的规避思路;建立及改进了基于火山岩氧化系数的喷发沉积环境判别图,得出消减带火山岩(活动陆缘和岛弧火山岩)陆上与水下喷发沉积环境的氧化系数判别标准。运用改进的图版及标准,对新疆三塘湖盆地马朗凹陷上石炭统卡拉岗组(C_(2)k)火山岩喷发沉积环境进行判识,结果表明,39件火山岩样品主要为陆上喷发沉积火山岩和水、陆过渡复杂环境沉积成因的火山岩。实际应用结果与前人基于卡拉岗组火山岩共生岩石与化石组合、风化壳、示底构造等典型标志得到的结果相符合;基于多因素分析法筛选样品,建立改进的判别图对喷发环境的指示意义具有普遍适用性,不受地质时代及区域地质条件的限制。

There are obvious differences in volcanic reservoir characteristics between onshore and underwater eruptive sedimentary environments.Accurate identification of volcanic eruptive sedimentary environment is an important basis for volcanic reservoir evaluation.In this study,Fe_(2)O_(3)/(Fe_(2)O_(3)+FeO)ratio is used as the oxidation coefficient of volcanic rocks,and the influence factors such as rock acidity,tectonic genetic environment,weathering and leaching,alteration and metasomatism are fully considered.This paper establishes and improves the discrimination chart of eruptive sedimentary environment based on the oxidation coefficient of volcanic rocks,and obtains the discrimination standard of oxidation coefficient of volcanic rocks(active continental margin and island arc volcanic rocks)on land and underwater eruptive sedimentary environment in the subduction zone of block B.Based on the improved chart and standard,the volcanic eruption environment of the upper Carboniferous karagang formation(C_(2)k)in Malang Sag,Santanghu Basin,Xinjiang was identified.The results show that 39 volcanic samples are mainly continental eruptive sedimentary volcanic rocks and volcanic rocks originated from the complex environment of water land transition.The practical application results are consistent with those obtained by zhuka et al.Based on typical indicators such as paragenetic rocks and fossil assemblages,weathering crust and bottom showing structure of karagang Formation volcanic rocks.The improved discriminant map based on factor analysis method has universal significance in indicating eruption environment,and is not limited by geological age and regional geological conditions.