四川盆地东部地区寒武系洗象池群碳酸盐岩碳、氧同位素特征及其意义*

四川盆地东部地区寒武系洗象池群碳酸盐岩的碳、氧同位素组成受后期成岩作用影响较小,基本保留了原始海洋的同位素组成,笔者根据采集的111个碳、氧同位素数据,研究、讨论了盆地东部地区中上寒武统碳酸盐岩碳、氧同位素组成、演化及地质意义。研究表明,δ¹³C值变化介于-3.36‰~2.65‰之间,均值为-1.027‰,绝大多数的样品都分布在-2‰~2‰的区间,δ¹⁸O值分布于-11.1‰~-6.01‰之间,均值为-7.991‰,在-10‰~-6‰的范围内波动。通过对碳、氧同位素数据分析研究,认为四川盆地东部地区洗象池群主要发育在盐度较高、温暖—炎热的近岸海相沉积环境;洗象池群沉积早期和中期经历了短暂而快速的海侵后进入缓慢的海退,在晚期缓慢海侵后的快速海退,碳同位素组成反映的海平面变化趋势与沉积相演化一致。洗象池群沉积中期显著的碳同位素正向漂移,标志着较高的生产力和有机碳埋藏率,具有重要的石油地质学意义。

Characteristics of carbon and oxygen isotopes and their significance of the Cambrian Xixiangchi Group carbonate rocks in eastern Sichuan Basin

The carbon and oxygen isotopes of the Cambrian Xixiangchi Group carbonate rocks in eastern Sichuan Basin are less affected by post-depositional diagenesis,and thus provide helpful insights into the original ocean. Based on the C and O isotopic profile of 111 analytic results obtained by the standard method of phosphorolysis,we have discussed the composition and evolution of δ¹³C and δ¹⁸O recorded in the Middle-Upper Cambrian carbonate rocks in eastern Sichuan Basin,as well as their geological implication. The research shows the values of δ¹³C range from-3.36‰ to 2.65‰ with mean value of-1.027‰,most of which are in the range of-2‰~2‰;and the values of δ¹⁸O range from-11.1‰ to-6.01‰, most range in of-10‰~-6‰, with average value of-7.991‰. Carbon and oxygen isotope analysis illustrates that the Xixiangchi Group was formed in a warm to hot onshore marine sedimentary environment with a high salinity. The δ¹³C analysis suggests there were a slow regression after a rapid and short transgression at the early-middle depositional stage,as well as a rapid regression after a slow transgression at the late depositienal stage of Xixiangchi Group. Sea level changes reflected by carbon isotope distribution are consistent with the evolution of sedimentary facies.The positive excursion of carbon isotopes in the Middle Xixiangchi Group in Sichuan Basin indicates relative high productivity and burial rate of organic carbon,which is of great petroleum geological significances.