渤海湾盆地车镇凹陷大王北洼陷原油类型及成因分析

根据原油物性、含硫量和族组分组成,将渤海湾盆地车镇凹陷大王北洼陷原油分为中—低密度、低粘度、低含硫原油和中—高密度、中—高粘度、高含硫原油2大类.根据原油碳同位素组成,结合生物标志化合物特征,对2类原油的成因和成藏进行了分析.结果表明,第一类原油以该区淡水—微咸水的半深湖—深湖环境下沉积的沙三段成熟烃源岩为母源,保存条件良好,基本没有发生次生变化;第二类原油为至少2期不同来源、不同保存条件下形成的混源原油,沙四段盐湖—咸水湖沉积环境下形成的烃源岩提供的成熟度较低的高含硫原油为最初来源,成藏之后,由于埋藏较浅,发生了一定程度的生物降解,之后沙三段成熟烃源岩提供的原油再次混入,此外沙一段低熟烃源岩提供的低熟油也有一定贡献,导致了该类原油组成和分布的复杂性.

OIL TYPE AND GENESIS ANALYSIS OF CRUDE OIL IN THE DAWANGBEI SUBSAG OF THE CHEZHEN SAG, THE BOHAI BAY BASIN

Crude oil from the Dawangbei Subsag of the Chezhen Sag of the Bohai Bay Basin can be divided into 2 types,i.e.the medium-low density,low viscosity,low sulfur content oil,and the medium-high density,medium-high viscosity,high sulfur content oil,through study of physical properties,sulfur content and oil group-type.Carbon isotopes and biomarkers are used to study the genesis and accumulation of oil,and the result shows that type Ⅰ origins from fresh-weak salt water,medium-deep water genesis mature source rock of the 3rd segment of Shahejie Formation,which is well preserved and has not experienced distinct secondary change.Type Ⅱ is mixed origin oil of at least 2 origins and is preserved under different conditions.Low maturity and high sulfur content oil from the 4th segment of Shahejie Formation of salt lacustrine-salt water lacustrine environment provides the 1st origin.After accumulation,oil has experienced biodegradation since it is buried low.Then oil from mature source rock of the 3rd segment of Shahejie Formation mixes in.Low-mature oil from the 1st segment of Shahejie Formation also contributes to type Ⅱ.Hence composition and distribution of oil type Ⅱ are complicated.