潍北凹陷孔店组烃源岩评价及油源分析

Evaluation of source rocks in Kongdian Formation and analysis of oil sources in Weibei sag

潍北凹陷孔店组二段既有好、较好、较差烃源岩,也发育有机质富集层烃源岩(又称为优质烃源岩)。有机质富集层烃源岩沉积环境为亚热带型湿热古气候条件下的滨浅湖相淡-微咸水沉积,古生物以被子、裸子植物孢粉组合为主,含有介形类及少量藻类化石,干酪根类型为Ⅲ,Ⅱ2及Ⅱ1型,岩性为碳质泥岩及深灰色泥岩,有机碳含量大于4.5%,氯仿沥青“A”含量大于0.17%。依据原油的物理性质将潍北凹陷原油分为稠油、中密度油和低密度油,根据原油的生物标志物特征又可将本区原油分为低熟油和成熟油。大部分低熟油样品具有高Pr/Ph比值,低伽马蜡烷/C30藿烷比值及高C29甾烷含量的特征。稠油及大部分中密度油属于低熟油,油源对比表明主要来源于孔店组二段有机质富集层烃源岩。部分中密度油及低密度油属于成熟油,其油源与低熟油相同。

Source rocks of various qualities, including very good （with rich organic matters）, good, relatively good and relatively poor source rocks, are developed in the 2^nd member of Kongdian Fm in Weibei sag. The source rocks with rich organic matters were deposited in fresh-brackish water of coastal shallow lake under humid subtropical palaeoclimate. Fossils are mainly sporopollen assemblage of angiosperm and gymnosperm, and there are also ostracoda and a few algae. The kerogens are of types Ⅲ, Ⅱ2 and Ⅱ1 and the lithologies are carbonaceous mudstone and dark grey mudstone, with TOC over 4.5% and chloroform bitumen “A” over 0.17%. Crude oils in Weibei sag can be classified as heavy oil, medium density oil and low density oil, according to their physical properties; and they can also be classified as low-mature oil and mature oil, according to the characteristics of biomarkers. Most of the low-mature oil samples have high pristane/phytane ratio, low gammacerane/C30 hopane ratio and high C29 sterane content. The heavy oil and most of the medium-density oil are of low-mature oil, and oil-to-source correlation show that they have been sourced mainly by the source rocks in the 2^nd member of Kongdian Fm with rich organic matters. Low density oil and some medium density oil are of mature oil, and they have the same oil source as that of low-mature oil.