海水侵入对湖相烃源岩形成的影响——以渤海湾盆地渤中凹陷西南缘沙三段泥岩为例

Effect of marine incursion on the formation of lacustrine source rocks:A case study of mudstones from the third member of the Shahejie Formation in the southwestern Bozhong Sag,Bohai Bay Basin

渤中凹陷是渤海湾盆地最大的生烃凹陷,其沙河街组三段(E2s3)是最重要的烃源岩层段。前人对沙三段烃源岩的母质来源、沉积环境、热演化程度等方面已取得一定认识,但在沉积事件和成烃生物来源上并未开展深入研究。研究通过有机、无机地球化学及有机岩石学的手段对渤中凹陷西南缘沙三中亚段和沙三下亚段的9块烃源岩岩心样品进行了详细分析,结果表明:①沙三中亚段和沙三下亚段均属于优质烃源岩,但整体上沙三下亚段烃源岩品质更好。温湿气候下水体盐度的上升及24⁃正丙基胆甾烷化合物的检出反映了沙三段沉积期海侵事件的发生,且从沙三下亚段到沙三中亚段海侵作用逐渐增强;②样品中4⁃甲基甾烷、24⁃正丙基胆甾烷、2⁃甲基藿烷及奥利烷等化合物的检出,并且存在丰富的藻类体和少量镜质体,表明沙三段烃源岩中成烃生物的多样性;③沙三下亚段沉积期的适度海侵带来的营养物质造成了细菌、藻类的勃发,为优质烃源岩形成提供了丰富的物质基础;而沙三中亚段由于过量海水侵入导致了蓝细菌的消亡,水体富营养化减弱,使烃源岩品质较沙三下亚段有所下降。

Bozhong Sag is the largest hydrocarbon generation sag in Bohai Bay Basin,and the third member of the Shahejie Formation(E_(2)s^(3))is the most important source rocks.Previous studies have a certain understanding of source material,sedimentary environment and thermal evolution of the source rocks from the E_(2)s^(3),but the research on sedimentary events and biological sources is not deep enough.Nine mudstone core samples from the E_(2)s^(3) in the southwestern Bozhong Sag were analyzed in detail based on organic-inorganic geochemistry and organic petrology.The results show that:(1)The middle and lower sections of the third member of Shahejie Formation are high-quality source rocks,but the quality of the source rocks in the lower section of the third member of Shahejie Formation is better.The increase of water salinity in warm and humid climate and the detection of 24-n-propylcholestane compounds reflected the occurrence of transgression events in the E_(2)s^(3),and the transgression gradually increased from the lower sections to the middle sections of the E_(2)s^(3).(2)The abundant presence of 4α-methyl-24-ethylcholestanes,24-n-propylcholestanes,2α-methylhopanes,and oleanane in the studied samples,and the presence of abundant algainite and a small amount of vitrinite indicate the diversity of hydrocarbonforming organisms in the source rocks of the E_(2)s^(3).(3)The nutrient elements brought about by the moderate marine incursion in the lower part of the E_(2)s^(3) caused the flourishing of bacteria and algae,which provided abundant material basis for the formation of high-quality source rocks.However,due to the large-scale seawater intrusion in the middle of the E_(2)s^(3),the water eutrophication is weakened,resulting in a decrease in the quality of source rocks compared with the lower part of the E_(2)s^(3).