羌塘盆地二叠系白云岩油苗地球化学特征及意义

Geochemical Characteristics and Significance of Permain Dolomite Oil Seepages in Qiangtang Basin

通过采集羌塘盆地中央隆起带二叠系展金组白云岩油苗及烃源岩样品,对其族组成、饱和烃、芳烃色谱—质谱以及单体碳同位素进行了研究,并进行了油源对比。研究结果表明:白云岩油苗中饱和烃含量最高,其次为非烃,芳烃和沥青质含量较低,饱芳比大于1;白云岩油苗的正构烷烃主要为前高单峰型分布特征,碳数分布范围n C_(15)~nC_(36),主峰碳为nC_(17)~nC_(20),nC_(21)-/nC_(22)+值为1.0~4.56,C_(27)-C_(28)-C_(29)规则甾烷表现为不规则的"V"型的分布并且C29甾烷占优势,升藿烷从C_(31)~C_(35)均有检出,显示出白云岩油苗的生烃母质主要来源于藻类和细菌等低等水生生物;白云岩油苗的Pr/Ph值为0.91~1.30,伽马蜡烷指数值为0.42~0.88,三芴系列化合物中以硫芴含量最高(平均67.39%)而氧芴含量(平均7.89%)较低,4-MDBT/DBT比值为0.97~4.26,(2+3)-MDBT/DBT值为0.37~1.62,表明白云岩油苗生烃母质为形成于具有一定盐度还原环境为的海相页岩;白云岩油苗的OEP值接近平衡值1,甾萜烷异构参数C_(29)ααα20S/ααα(20S+20R)、C_(29)αββ/(ααα+αββ)和C_(31)22S/(22S+22R)等接近或达到平衡值,芳烃标志物成熟度参数MPI_1、F_1和4,6/1,4-DMDBT计算出的等效镜质体反射率表明白云岩油苗为成熟原油。白云岩油苗的生物标志物和展金组泥岩具有较好对比性,反映了白云岩油苗主要来源于展金组烃源岩;其单体碳同位素与二叠系展金组烃源岩分布形式和变化趋势基本相似,但也有一定的差异,反映了其具有混合来源的特点。

Dolomite oil seepages and hydrocarbon source rocks of Permian Zhanjin Formation were collected systematically from Central uplift belt area in Qiangtang Basin.Based on the analytical data of group component, biomarkers and carbon isotopic compositions of n-alkanes and isopre, the author analyzed the geochemical characteristics of Permian dolomite oil seepages,and the oil-source rock correlation was also studied in the paper. The results indicated that the Permian dolomite oil seepages are high fraction saturated hydrocarbons,followed by non hydrocarbon, aromatics and asphaltene fraction are low. The ratio of saturates/aromatics of dolomite oil seepages are more than 1.0. The alkanes of dolomite oil seepages are composed of nC15 - nC36,with nC17~nC20 as the main peaks,and characteristics of single peak distribution.The nC21-/nC22＋ ratios from 1.0 to 4.56,the regular sterans C27-C28-C29 distributed with irregular “V”shape and predominance of C29,homohopane were detected from C31~C35, indicating dolomite oil seepages of hydrocarbon material sources were in algae and other lower aquatic organisms. The Pr/Ph values range of dolomite oil seepages were between 0.91 to 1.30,and gammacerane/C30hopane values were from 0.42 to 0.88, the content of dibenzothiophene were the highest （average 67.39%） and low dibenzofuran content （average 7.89%）. 4-MDBT/DBT values of dolomite oil seepages were 0.97 to 4.26, （2＋3）-MDBT/DBT values were 0.37 to 1.62, showing dolomite oil seepages hydrocarbon material sources were formed in marine shale with certain salinity reduction environment. The OEP values, C29ααα20S/ααα （20S＋ 20R） values,C29αββ/（ααα＋αββ） values and C3122S/（22S＋22R） values etc., closed to equilibrium values 1, aromatic maturity parameters MPI1,F1 and 4,6/1,4-DMDBT calculated from the equivalent vitrinite reflectance showed that dolomite oil seepages were mature crude oil. The biomarkers of oil seepages are closely correlated to the Zhanjin Formation black mudstone, reflecting that the oil seepages were mainly originated from the Zhanjin Formation. However, carbon isotopic compositions of n-alkanes and isopre exists some differences between oil seepages and Zhanjin Formation black mudstone, indicating a mixture of sources.