Geochemical and Isotopic Evidence of the Genesis of a Condensate in the Eastern Tarim Basin, China: Implications for Petroleum Exploration

摘要

The exploration activities in the eastern Tarim Basin have been thwarted over the last decade after the discovery of several Jurassic gas condensate fields. In this study, the two-dimensional gas chromatography/time-of-flight mass spectrometry (GC X GC-TOFMS) and compound-specific carbon isotope analysis were performed on a Jurassic condensate (Yingnan2) and the associated gas to determine its genesis and the accumulation process in this area. The geochemical and isotopic features suggested that the condensate analyzed was a mixture of the Ordovician cracked paleo-oil with the Jurassic intact oil as evidenced by the concentrated diamondoids and ethanoadamantanes, the high gas generation temperature (similar to 195 degrees C), the heavy whole oil delta C-13 (-28.6 parts per thousand), and the significant variation in the isotopic profile of n-alkanes. As the gas amount was constantly elevated due to both oil cracking and the mixing of the kerogen-cracking gas from the Cambrian source rock, phase transition occurred and thus formed the Yingnan2 secondary condensate. The constant mixing of nitrogen-rich kerogencracking gas complementarily caused an increase in the nitrogen gas content. The accumulation model of the condensate in the eastern Tarim Basin was proposed with consideration of the post-accumulation alterations including thermal cracking and mixing, which complicated the quality and distribution of subsurface petroleums. It was further speculated that abundant gas and condensate resources may be preserved in favorable reservoir-seal assemblages in this field.