基于火山岩氧化系数分布特征的火山喷发环境判识——以三塘湖盆地马朗凹陷牛圈湖-牛东构造带上石炭统为例

Identification of volcanic eruption environment based on the volcanicrock oxidation index——a case study from Upper Carboniferousigneous rocks in the Malang sag of Santanghu basin

受火山喷发沉积环境(陆上或水下)及岩浆酸度等因素的影响,火山喷发物的地球化学特征有很大差异,准确判识火山喷发环境是火山岩油气储层性质评价的重要地质依据。本文基于三塘湖盆地马朗凹陷牛圈湖-牛东构造带上石炭统58件火山岩样品主量元素地球化学数据,采用Fe;O;与FeO比值作为氧化系数,利用数理统计方法投点,根据上石炭统哈尔加乌组和卡拉岗组火山岩氧化系数分布特征判识其喷发沉积环境。统计结果表明,卡拉岗组34件样品氧化系数介于0.55~5.00,数值主要分布于1.0~3.0,说明火山喷发沉积环境复杂,非单纯的氧化环境;从部分单井垂向分布来看,氧化系数具有随埋深由深至浅呈明显增大的趋势,表示由弱还原环境逐渐向强氧化环境过渡。哈尔加乌组24件岩芯样品氧化系数介于0.25~0.94,数值主要集中于0.2~0.4,表明火山喷发沉积于贫氧的还原环境;从单井垂向分布来看,氧化系数受埋深影响极小,表示火山喷发沉积受水动力条件境影响小,沉积环境具有一定的水体深度。研究调研筛选了国内10个不同盆地108件火山岩样品数据资料,并充分考虑酸度对火山岩氧化系数的影响,建立了经岩浆酸度校正的基于氧化系数分布特征的火山喷发环境判识图版,以此标准判识研究采集的58件火山岩样品的喷发沉积环境;结果显示卡拉岗组火山喷发沉积环境比较复杂,以陆上喷发沉积成因为主,存在频繁水、陆过渡相环境;而哈尔加乌组火山岩则为典型的水下喷发成因,沉积于具一定水体深度的强还原环境;与前人基于微量元素富集特征、淬碎渣状构造、不整合及风化壳、海相化石、充填胶结物等有效判识标志识别的结果对比检验,二者是极为符合的;且在合理校正和规避各项影响因素下,基于氧化系数分布特征的火山喷发沉积环境判识方法具有普遍适用性,可应用于不同地质时代、不同盆地不同地质背景火山岩的喷发沉积古地理环境判别中。

The geochemical characteristics of volcanic eruption vary greatly due to factors including the volcanic eruption sedimentary environment(including submarine eruptive environment and subaerial eruptive environment) and magmatic acidity. Therefore the accurate identification of volcanic eruption environment is an important geological basis for the evaluation of volcanic oil and gas reservoir properties. In this paper, based on the geochemical data of 58 samples of Upper Carboniferous igneous rocks from NiuquanhuNiudong structural belt, Malang sag, Santanghu basin. We use the ratio of Fe;O;to FeO as the oxidation index and mathematical statistics method to cast points to identify the eruptive sedimentary environment of the Halgau Formation(C;h) and Karagang Formation(C;k) of the upper carboniferous according to its distribution characteristics. Statistics show that the oxidation indexes of the 34 samples of the Karagang Formation(C;k) range from 0.55 to 5.00 and the values are mainly concentrated from 1.0 to 3.0, indicating that the volcanic eruption sedimentary environment is complex and not a pure oxidation environment. From the vertical distribution of some single wells, the oxidation indexes have a tendency to increase significantly from bottom to top, indicating that the volcanic eruption sedimentation has gradually transited from a weakly reduced environment to a strongly oxidized environment. The 24 rock core samples of the Carboniferous Halgawu Formation(C;h) have oxidation indexes between 0.25 and 0.94 and the values are mainly concentrated in 0.2~0.4, indicating that the volcanic eruption sedimentation is in a simple oxygen-reduced environment. From the vertical distribution of a single well, the change of oxidation indexes influenced by the depth is not obvious, indicating that the volcanic eruption sedimentation has been little affected by the dynamic water environment and has a certain depth of water. The research used data of 108 volcanic samples in 10 different basins in China and fully considered the effect of acidity on the oxidation index of volcanic rocks to establish a volcanic eruption environment identification map based on the distribution characteristics of oxidation indexes corrected by magmatic acidity. We used the map to identify the volcanic eruption sedimentation environment of the collected 58 rock core samples. The result shows that volcanic eruption sedimentation environment of Karagang Formation(C;k) is complex, which is dominated by land eruption and deposition and featured by frequent water and land transition environment. The HaerjiawuFormation(C;h) volcanic rocks are of typical underwater eruptive origin and were deposited in a highly reductive environment with a certain water depth. The results accord with the previous results obtained by effective identification symbols including the characteristics trace element enrichment, quenched slag-like structure, unconformity and weathering crust, Marine fossils and filling cement.Under the reasonable correction and avoidance of various influencing factors, the method of volcanic eruption sedimentary environment identification based on the distribution characteristics of oxidation index has universal applicability, which can be applied to the ancient geographical environment identification of volcanic eruption sedimentary in different geological ages and different basins with different geological backgrounds.