东营凹陷北部陡坡带稠油地球化学特征与成因

Characteristics and Genetic Mechanisms of Heavy Oils on the North Steep Slope of the Dongying Depression in the Bohai Bay Basin, East China

对东营凹陷北部陡坡带郑家—王庄地区稠油与相对稀油烃类组成进行了定性与定量分析。发现多数原油遭受不同程度的破坏 ,饱和烃、芳烃馏分呈有序缺失 ,于多数原油中检测到生物降解标志物 2 5 -降藿烷系列。地质与地球化学综合研究认为 ,以生物降解—氧化及相关的生化 /化学作用、以水洗—重力分异—轻质组分逸散为主的物理作用是郑家—王庄原油稠化的重要机理 ;东营凹陷北部“有限后退型”盆地边界发育的风化壳潜山、砂砾岩扇体储层孔渗条件好、油藏埋深浅、盖层条件差和不整合面等通道与外界的连通是郑家—王庄及邻区重质油形成的根本原因。提出生物标志物绝对定量可作为识别原油遭受次生改造作用程度、判断不同类型生物标志物抵抗降解能力大小的有效指标。利用生物标志物 ,发现郑家—王庄局部地区原油具有不同降解级别原油相混的混合现象 ,可能反映两次或多次充注。郑家—王庄及其邻区有望获得相对稀油新增储量。

Considerable heavy oils were discovered on the North Steep Slope (NSS) of the Dongying Depression in the Bohai Bay Basin. Oils from the NSS and adjacent Lijing Sag were collected for detail qualitative and quantitative investigation. It was observed that most of the oils from the NSS were subjected to biodegradation or water-washing, which result in saturated hydrocarbons and aromatics of the oils that are depleted in certain degree and absence of 25-norhopanes in most of the oils. It was observed that the oils with different degrees of deformation were mixed commonly in NSS based on composition and relative distribution of biomarkers, suggesting probably at least two main periods of oil charging in the area. The results also show that biomarkers quantification absolutely is useful for identifying the level of biodegradation and providing information of resistant ability of the biomarkers in the oils against microbial removal. According to detail geological and hydrological investigations combined with geochemistry, we suggest that chemically, biologically and physically such actions as biodegradation, water-washing and oxidation are the main genetic mechanisms resulting in the formation of heavy oils, which make most light components of crude oil escaped while heavy components concentrated. It is concluded that special geological backgrounds such as weathering crust of burial hills on the NSS, high-porosity and high-permeability coarse sands and conglomerates of orderly distributed fan reservoirs, shallow buried oil pools, poor cap rock property and lateral plugging conditions, and development of complex faults and several unconformities are the most essential factors responsible for the formation of heavy oils on the NSS. Coexistence of heavy oils and relative normal oils on the NSS indicates that there is still a good prospect for the exploration of normal oils.