羌塘盆地中侏罗统砂糖状白云岩流体包裹体碳—氧同位素分析及白云岩成因机制讨论

Carbon and oxygen isotopes analysis of the fluid inclusions in Middle Jurassic saccharoidal dolostone of Qiangtang Basin and discussion on the genesis of dolostone

羌塘盆地隆鄂尼—昂达尔错古油藏带中侏罗统布曲组砂糖状白云岩储集层成因一直备受关注和争议。白云岩成因及其储层预测对于碳酸盐岩油气藏勘探具有重要意义,但是关于羌塘盆地布曲组白云岩的成因并没有一个相对统一的认识,争议核心聚焦在白云岩化流体性质。笔者等试图在包裹体测温和盐度分析的基础上,利用同位素分馏方程系统研究白云岩包裹体碳—氧同位素,探讨羌塘盆地侏罗系砂糖状白云岩白云岩化流体特征。研究表明白云岩白云石包裹体和灰岩方解石包裹体均一温度分布范围皆为50~220℃,但白云石的包裹体温度峰值分布区间明显要高于方解石的峰值温度,多数样品的温度集中在150℃到220℃区间;另外,与同层灰岩相比,白云岩具有明显的重碳同位素异常富集特征,该特征是高温流体交代石灰岩或者说是水岩反应的标志;此外,通过碳—氧同位素分馏方程计算出白云岩包裹体流体碳、氧同位素分别为-1.30‰~1.53‰和5.81‰~12.50‰,具有高盐度的卤水特征。综合包裹体温度、盐度,以及包裹体碳、氧同位素特征,本文认为羌塘盆地布曲组砂糖状白云岩是热液交代白云岩化产物。

Objectives:As a potential strategic reserve base for oil and gas exploration in the Qiangtang Basin,the discovery of the Longeni—Angdar Co paleoreservoir marked a new climax of the oil and gas exploration.The genesis of dolostone and its reservoir prediction are critical to the exploration of carbonate reservoirs in this case.However,the genesis of the saccharoidal dolostone reservoir has been under debate and attracted much attention.and there is no unified understanding of the genesis of dolostone in Buqu Formation in Qiangtang Basin,and the argument mainly focuses on the dolomitic fluid properties.This paper discusses the genesis of dolostone through geochemical characteristics and dolomitic fluid properties.Methods:In addition to the analysis of the temperature and salinity of the fluid inclusions,the values of carbon and oxygen isotope in fluid inclusions of Jurassic saccharoidal dolostone of Qiangtang Basin were calculated by the isotopic fractionation equation,and the dolomization fluid characteristics of the Jurassic saccharoidal dolostone in the Qiangtang Basin are discussed.Results:The temperature of the dolostone and limestone inclusions ranges from 50 to 220℃.The temperature of dolomite inclusions is significantly higher than that of calcite vein,and most samples are concentrated 150~220℃.In addition,compared with the limestone within the same layer,dolostone is more enriched in heavier carbon isotopes,which is a sign of high-temperature fluid—rock reaction.The carbon and oxygen isotopes of the dolomite inclusions were calculated by the carbon and oxygen isotopes fractionation equation,which were between-1.30‰~1.53‰and 5.81‰~12.50‰respectively,potentially results of high salinity brine.Conclusions:This study integrates comprehensive temperature,salinity and carbon—oxygen isotope analysis of inclusions,reveals that the saccharoidal dolostone of Buqu Formation in Qiangtang Basin is hydrothermal metasomatic origin.