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

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.