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

2

and

1

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/C

30

hopane ratio and high C

29

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.