延长组砂岩中碳酸盐胶结物氧碳同位素组成特征

CHARACTERISTICS OF CARBON AND OXYGEN ISOTOPIC COMPOSITIONS OF CARBONATE CEMENTS IN TRIASSIC YANCHANG SANDSTONE IN ORDOS BASIN

碳酸盐胶结物中氧碳同位素组成研究有助于阐明成岩过程中流体—岩石相互作用的机理。为此,综合运用岩石学、矿物学和地球化学方法,对鄂尔多斯盆地三叠系延长组砂岩中普遍存在的不同期次碳酸盐胶结物的成因机理和物质来源进行了系统研究。结果表明,研究区早期碳酸盐胶结物主要为方解石,其碳同位素值相对较重,为-0.3‰～-0.1‰,氧同位素值较轻,分布范围为-22.1‰～-19.5‰,与碱性湖水中碳酸钙发生过饱和沉淀有关。铁方解石的碳同位素值相对较轻,为-8.02‰～-3.23‰,氧同位素值也较轻,分布范围为-22.9‰～-19.7‰,它的形成明显与早成岩阶段晚期—晚成岩阶段早期的有机质脱羧基作用有关,属于成岩阶段中晚期产物,可以作为指示烃类在油气储层中正在发生显著运移的标型自生矿物。晚期铁白云石的碳同位素值相对较重,为-1.92‰～-0.84‰,氧同位素值变化较大,分布范围为-20.5‰～-12.6‰,它的形成与晚成岩阶段中晚期形成的碱性成岩环境及古代海相碳酸盐岩岩屑提供的碳有关。

Carbon and oxygen isotopic compositions of carbonate cements are useful for illustrating the mechanism of fluid-rock interaction in the process of diagenesis.The genesis and origin of the carbonate cements common in the Triassic Yangchang Formation sandstone in the Ordos Basin are systematically studied by using petrological,mineralogical and geochemical methods.The results show that the early carbonate cements in the Yanchang Formation sandstone are mainly composed of calcite with relatively heavier carbon isotope with δ18O ranging from-0.3‰ to-0.1‰,and lighter oxygen isotope with δ18O ranging from-22.1‰ to-19.5‰.Generally,they are closely related to the direct precipitation of oversaturated calcium carbonate from the alkaline lake water.They also indicate that ferroan calcites are characteristic of relatively lighter carbon isotope withδ13C in the range from-8.02‰ to-3.23‰,and lighter oxygen isotope with δ18O in the range from-22.9‰ to-19.7‰,and are obviously related to the decarboxylation of organic matter during the late period of early diage-netic stage to early period of late diagenetic stage.As the products of mid-late diagenesis.ferroan calcites in the study area are considered as the characteristic authigenic minerals for recognizing large-scale hydrocarbon influx and migration within clastic reservoirs.The late ankerite is relatively heavier in carbon isotope with δ3C ranging from-1.92‰ to-0.84‰,and is wide in variation of oxygen isotopic composition with δ28O ranging from-20.5‰ to-12.6‰.It is believed that the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation,and the alkaline diagenetic environment at the mid-late stage aids this process.