廊固凹陷沙河街组烃源岩沉积环境与控制因素

Depositional environment and controlling factors of source rock in the Shahejie Formation of Langgu sag

通过对廊固凹陷沙河街组三段(沙三段)和沙河街组四段(沙四段)1600多个烃源岩样品的热解有机碳分析,65个样品抽提物的饱和烃、芳烃GC/MS分析,及42个烃源岩样品的微量元素分析,研究了廊固凹陷烃源岩发育的沉积环境与控制因素。结果表明古气候条件及生命营养元素的含量对廊固凹陷烃源岩的发育有较明显的控制作用。沙三段烃源岩沉积时期气候湿润,湖盆水体相对较深,较低的Pr/Ph比值、高的V/(V+Ni)比值指示了还原环境;此时水体中磷(P)等营养物质较丰富,初始生产力较高;较高的4-甲基甾烷/∑C29甾烷比值反映了沟鞭藻等藻类较为发育;同时因气候湿润流域内高等植物发育,较高含量的惹烯、卡达烯、苝、芘等芳烃化合物,指示陆源有机质输入增多,因此整体有机碳含量较高。沙四段沉积时期气候干旱,较高的伽马蜡烷含量、较高的Sr/Ba和B/Ga比值反映了水体盐度相对较高,形成盐度分层,同时较低的Pr/Ph比值、高的V/(V+Ni)比值、及较丰富的噻吩类化合物含量反映水体为缺氧的还原环境;但因气候干旱,流域内高等植物不发育,较低含量的惹烯、卡达烯、苝、芘等芳烃化合物含量反映陆源有机质输入有限,生源构成以水生生物为主;同时水体中磷(P)等营养元素含量较低,导致初始生产力水平不高,因此整体有机碳含量偏低。

The depositional environment and controlling factors for the development of source rock in Langgu sag is studied based on rock pyrolysis and through analysis of organic carbon in more than 1 600 source rock samples,saturated and aromatic hydrocarbon of 65 samples using Gas Chromatograph-Mass Spectrometry（GC/MS）,and microelements in 42 samples,and all these samples are selected from the third（Sha-3） and fourth（Sha-4） member of Shahejie Formation in Langgu sag.The results indicate that the ancient climate condition and nutrient elements have an obvious control effect on the development of source rock in Langgu sag.During the sedimentary period of source rock in Sha-3 member,the climate is moist and water of the lake basin is relatively deeper; there is a reducing environment as indicated by the lower Pr/Ph ratio and high V/（V ＋ Ni） ratio.In this period,the water is rich in the nutrients like phosphorus（P）,creating a high primary productivity; the algae（dinoflagellates） development is reflected by a high 4-methyl steranes/Sigma C29 steranes ratio.Moreover,as indicated by the development of advanced plants within river basin due to moist climate and high content of aromatic compounds such as retene,cadalene,perylene,pyrene and etc.,the amount of terrestrial organic matter is increased and thus results in a high total organic carbon（TOC） content.During the sedimentary period of Sha-4 member,the climate is dry; as reflected by the high gammacerane content as well as high Sr/Ba and B/Ga ratios,the salinity of water is higher leading to salinity stratification.Meanwhile,it is demonstrated by the low Pr/Ph ratio,high V/（V ＋ Ni） ratio and abundant thiophene compounds that the water has an oxygen-deficient reducing environment.However,advanced plants are not developed due to drought climate.Further,there exists limited amount of terrestrial organic matter as demonstrated by the low content of aromatic compounds including retene,cadalene,perylene,pyrene and etc.,and quatic organism is dominant in the biological source composition; the nutritive elements（P） is deficient in the water,leading to a low level of primary productivity and also a low TOC content.