陆相烃源岩的沉积环境及其对生烃潜力的影响一以准噶尔盆地侏罗系烃源岩为例

SEDIMENTARY ENVIRONMENTS OF THE LIMNIC SOURCE ROCKS AND THEIR INFLUENCE ON HYDROCARBON-GENERATING POTENTIALITY: AN EXAMPLE FROM THE JURASSIC SOURCE ROCKS IN THE JUNGGAR BASIN,XINJIANG

关键词:  陆相烃源岩  暗色泥岩  煤岩  生烃潜力

Key words:  limnic source rocks  dark mudstone  coal-measure rocks  hydrocarbon-gen-erating potentiality

摘要:

本文以准噶尔盆地侏罗系烃源岩为例,从沉积角度出发,结合有机地球化学分析,探讨陆相烃源岩的沉积环境及其对生烃潜力的影响。陆相烃源岩的沉积不仅控制了烃源岩的发育与展布,而且还由于其中原始生物丰度和种类不同而直接影响着烃源岩的有机质丰度和类型。在陆相河流-三角洲-湖泊沉积环境中,半深—深湖及前三角洲中的暗色泥岩最具生烃潜力,这类暗色泥岩发育的原生有机物主要为水生低等生物和较少的高等植物,水介质条件以还原—强还原为主,所以其有机质丰度高,类型好(以Ⅰ、Ⅱ型为主)。三角洲平原及曲流河泛滥平原岸后湖泊中的暗色泥岩也具较好的生烃潜力,这类暗色泥岩的有机质母源主要来自高等植物,水介质条件为弱氧化—弱原还,因此其有机质丰度较高,但类型以Ⅲ型为主。
煤岩作为烃源岩,其生烃潜力取决于煤岩体积及富氢组分含量和演化程度。而煤岩体积和富氢组分含量又主要受控于成煤环境的古气候、古地理、成煤物质(古生物)及介质条件等。湖湾、滨湖沼泽煤最具生烃潜力,这类成煤环境不仅面积大、发育稳定,而且覆水程度高,底水介质条件多为还原—强还原;原生生物虽以高等植物为主,但发育较多的水生生物,因此其煤层不仅厚度大、分布连续,而且煤岩中的富氢组分较高。相比之下,三角洲平原、曲流河泛滥平原及网结河湿地沼泽煤的生烃潜力次之,辫状河泛滥平原沼泽煤的生烃潜力最差。

Abstract:

Sedimentary environments of the limnic source rocks have an important effect on the hydrocarbon-generating potentiality of the source rocks.Exemplified by the Jurassic rocks in the Junggar Basin,Xinjiang,in combination with sedimentary and organic geochemistry, the present paper discusses the sedimentary environments of the source rocks and their influence on the hydrocarbon-generating potentiality.
Sedimentary environments of the limnic source rocks not only control the development and distribution of the source rocks but also dircetly affect the abundance and type of the organisms because of the discrepancies in the abundance.and type of the primitive organisms.In the continental-river-delta-lake environments,the dark mudstones in the semideep and deep lakes and prodeltas have the greatest hydrocarbon-generating potentiality. The primitive organisms in this kind of dark mudstones are mainly made up of aquatic lower organisms and minor higher plants.The aqueous medium conditions are dominantly composed of reduction to strong reduction conditions.The organisms are higher in abundance and good in type(Types Ⅰ and Ⅱ).The dark mudstones in the backshore lakes of delta plain and meandering flood plain occupy the second position of hydrocarbon-generating potentiality.The organisms in this kind of dark mudstones mostly come from higher plants.The aqueous medium conditions consist of weak oxidation and weak reduction.For this reason,the organisms are typical of higher abundances and Type Ⅲ.
The hydrocarbon-generating potentiality of coal as a kind of the source rocks tends to depend on the volume of the coal-measure rocks,the contents of hydrogen-rich components and evolutionary degrees,the former two of which are,in turn,controlled by palaeoclimates,palaeogeography,coal-forming organisms (bionts)and aqueous medium.The coal in lake estuary and swamps have the greatest latent capacity.This kind of coal-forming environments not only are distributed extensively and developed steadily,but also covered by water on a wide range of scales.The aqueous medium conditions are dominated by reduction to strong reduction conditions.The primitive organisms are mainly made up of higher plants,with significant aquatic organisms.The coal beds are characterized by greater thickness,laterally extensive distribution and high hydrogen-rich components in coal.By comparison,the hydrocarbon-generating potentiality of the coal in delta plains,meandering flood plains and wet swamps of anastomosing rivers has the second position,and that of the coal in the braided flood plain and swamps is the worst.