牛庄洼陷“岩性油气藏”油气成因

Origin of the Subtle Oils in the Niuzhuang Sag

为了解析东营凹陷岩性油气藏的成因与成藏机理,采用地球化学途径对岩性油气藏最为发育的牛庄洼陷的原油、不同层位烃源岩进行了详细的烃类化学成分分析与精细油源对比研究.牛庄洼陷以沙三中为主的岩性(含透镜体)及其相关油气藏原油普遍具有低Pr/Ph值(<1)、相对较高的甾烷、伽玛蜡烷丰度等沙四上亚段烃源岩的特征,与沙三中烃源岩(Pr/Ph>1)无相关性,与沙三下亚段烃源岩(Pr/Ph>1、伽玛蜡烷含量低)也有明显的非相似性.沙三中亚段的岩性及其相关油气藏油气并非以沙三中为主力烃源岩,而是表现出沙三下特别是沙四上亚段烃源岩的某些特性,暗示以往所确认的透镜体等岩性油气藏极为发育的牛庄洼陷油气成因复杂,油气成藏模式很难用传统的“自生自储”来描述.沙三中岩性油气藏中混合了相隔数百米的沙四段成因油气指示隐蔽油气运移通道的存在.

Detailed oil-oil, oil-source rock comparison according to the composition and distribution of hydrocarbons in the Niuzhuang sag was performed to unravel genetics and migration mechanism of the oil Results indicate that the oils analyzed are characterized by low ratios of Pr/Ph （〈1） and relative high concentration of steranes and gammaceranes, which are typical features of Es4 source rocks. No genetic relationship was observed between the oil and the middle interval of Esa mudstones characterized by Pr/Ph〉1 and low concentration of gammaceranes, and apparent dissimilarities were also observed between the oil and the low interval of Esa mudstones. We suggested that the oils are mixed oils derived from the Es1/3 and Es4 mudstones, and there is no possibility of the Es3^M to be the primary source rock for the oil. The results show that the traditional hydrocarbons migration and accumulation model for subtle oil pools especially sandy lens-shaped pools featured by short oil migration distance with hydrocarbons sourced from surrounding mudstones is unlikdy fit for the cases of this study. The mixing of the oils source from the Es4 with hundreds of meters under the Es3^M oil pools probably indicates the existence of subtle oil migration pathways .