长岭断陷下白垩统湖相烃源岩形成古环境及主控因素

Generation Paleoenvironment and Its Controlling Factors of Lower Cretaceous Lacustrine Hydrocarbon Source Rocks in Changling Depression,South Songliao Basin

恢复烃源岩形成时期的古环境,落实优质烃源岩发育主控因素对确定湖相烃源岩发育层系及刻画烃源岩非均质性意义重大.利用岩石学、元素地球化学对长岭断陷早白垩世火石岭期至营城期的古环境进行恢复,并对反映古环境的微量元素含量及其比值与有机碳含量(Total Organic Carbon,TOC)进行相关性分析,确定控制湖相烃源岩发育的主要环境因素.研究表明,从火石岭期至沙河子期,再到营城期,长岭断陷经历了古水深从变深到变浅,古气候从潮湿-半潮湿气候到半干旱气候,古盐度从淡水相到半咸水相,湖泊水体还原性从增强到减弱的古环境演化过程,早白垩世不同时期长岭断陷古环境表现出明显的分带性,古气候和氧化还原条件是控制长岭断陷下白垩统湖相烃源岩发育的主要环境因素.长岭断陷沙河子组二段烃源岩形成于潮湿与缺氧的古环境,该时期湖盆演化处于均衡补偿阶段,且烃源岩古生产力较高,因此研究区沙河子组二段优质烃源岩十分发育,分布广.

Heterogeneity study of lacustrine source rocks relys heavily on a deeper understanding of paleoenvironment and the major controlling factors on their formation.Based on element geochemical and petrological analyses of the paleoenvironment of the Early Cretaceous（from K_1h to K_1yc）in Changling depression.,the main controlling factors of organic source rocks development were analyzed according to the characteristics of its tectonics and sedimentary and the primary productivity of the source rocks in this study.Results show that from K_1h period to K_1shstage to K_1yc stage,Changling depression experienced paleo-water depth change from deep to shallow,palaeoclimate from humid-semi-humid to semi-arid,palaesalinity from fresh water to brackish water,and reduction condition from enhanced to weakened.From K_1sh period to K_1yc,the paleoenvironment of Changling depression shows obvious zonation;The correlation between trace element indexs indicating the paleoclimate and water reduction and TOC（Total Organic Carbon）of the source rock samples is significant,while the correlation between the trace element indexs indicating the paleosalinity and TOC of the source rock samples is small.So the paleoclimatic and redox conditions are the main environmental factors of the development of Lower Cretaceous lacustrine source rocks in Changling fault depression;K_1sh2 Formation high quality source rock which developed in equilibrium compensation stage,humid climate,hypoxia and high initial productivity is widely distributed.Followed by K_1sh1 Formation and K_1h2 Formation,the organic source rock of K_1yc Formation is relatively developed in the south and east,and the distribution is relatively limited.