青东凹陷沙河街组稠油成因及影响因素

青东凹陷西部和西南部边缘部位沙河街组广泛存在稠油油藏，目前在这些稠油的成因机制方面未开展深入研究，没有取得明确的认识，影响了研究区资源量评价和油气勘探进程。根据原油物理性质和生物标志物组成特征对研究区稠油的成因进行了综合分析。结果表明，研究区稠油中既存在有原生的低熟油，又分布有由次生作用（生物降解作用）产生的稠油，其中，低熟油主要分布在凹陷西南部的青东5井区；生物降解油主要分布在凹陷西部斜坡带及青东-青南过渡带，原油的降解等级为2~8级。原油的生物降解程度呈现出下轻上重、北轻南重的规律性变化。研究区稠油的形成与分布主要受有机质早期生烃、古地形、构造背景等因素影响。

Factors Influencing Formation of Heavy Oil in the Shahejie Formation of the Qingdong Sag

The Shahejie Formation, located at the western and southwestern margins of the Qingdong Sag, contains extensive reservoirs of heavy oil. At present, there is a lack of in-depth research into and clear understanding of the formation mechanism of this oil. This knowledge gap has affected the evaluation of resources within the study area, as well as oil and gas exploration processes. A comprehensive analysis of the formation of heavy oil in the area was made based on the physical properties of the crude oil and composition characteristics of the biomarkers. The results indicate that the heavy oil contained native immature oil and distributions of heavy oil produced by secondary effects, such as biodegradation. The former was mainly distributed within the Qingdong Well 5 area, located in the southwestern section of the sag; the latter, biodegraded oil, was found at the western slope of the sag and Qingdong-Qingnan transition zone. Degradation of the crude oil was at levels 2-8, and generally had the following characteristics:heavier above and to the south, and lighter below and to the north. The formation and distribution of heavy oil in the study area was mainly affected by factors including the early hydrocarbon generation of organic matter, paleotopography, and tectonic setting.