准噶尔盆地吉木萨尔凹陷凝灰岩型致密油特征与成因

Characteristics and origin of tuff-type tight oil in Jimusar Depression, Junggar Basin, NW China

通过岩心和薄片观察,结合X光衍射、电子探针和地球化学分析,对准噶尔盆地吉木萨尔凹陷二叠系芦草沟组凝灰岩型致密油特征与成因进行综合研究。芦草沟组沉积岩中发育凝灰岩、沉凝灰岩和凝灰质白云岩夹层。凝灰岩类中碱性长石、石英均表现出富Sr特征,矿物地球化学分析显示其来自碱性—过碱性岩浆岩。白云石87Sr/86Sr值与地幔值非常接近,表明凝灰岩类组成矿物形成于与岩浆作用相关的热液环境。白云石Sr/Ba值较高,δ13C较重,扫描电镜下发现石盐晶体,表明芦草沟组沉积于较高盐度的环境。与火山作用、地幔热液有关的富Mg2+、Fe2+和高盐度湖泊环境促进了白云石的沉淀。致密油分布、烃源岩有机质丰度和储集层物性表明,火山活动提供的丰富营养物质控制了高丰度烃源岩形成。剩余粒间孔和溶蚀孔发育的凝灰岩、凝灰质白云岩为有利储集层发育层段。凝灰岩类控制了致密油的形成与分布。

Based on core and thin section observation, and combined with X-ray diffraction, electron probe and geochemical analysis, the characteristics and origin of tuff tight oil of the Permian Lucaogou Formation in the Jimusar depression, Junggar Basin were examined. There are tuff, tuffite and tuffaceous dolomite interlaid in the sedimentary rocks of the Lucaogou Formation. The alkali feldspar and quartz in the tuff are rich in strontium, which, according to mineral geochemical analysis, are originated from alkaline-peralkaline magmatic rocks. The Sr isotope ratio（87Sr/86Sr） of dolomite is similar to that of mantle, which shows minerals of tuff were formed in volcanism-related hydrothermal environments. High Sr/Ba and δ13C of dolomite, and occurrence of halite under microscope suggest that the Lucaogou Formation was deposited in high salinity environment. High content of magnesium and iron ions related to volcanism and hydrotherm, and high salinity environment contributed to the precipitation of dolomite. The distribution of tight oil, organic matter abundance of source rock and reservoir physical properties show that it is the rich nutrients provided by volcanic activity that controlled the formation of high abundance source rock; tuff, tuffaceous dolomite with plenty of residual inter-granular pores and dissolved pores are favorable reservoir sections; and tuff controls the formation and distribution of tight oil.