Two sets of Cretaceous source rocks

Dalazi and Changcai Formation

develop in the Songjiang Basin.In this study

we discussed the geochemical characteristics of source rocks and crude oil and the oil-source correlation based on the conventional organic geochemical analysis and biomarker compound test on Cretaceous source rocks and crude oil in the basin.The organic geochemical analysis indicates that Cretaceous source rocks are rich in organic matters

with the major organic matter being TypeⅡ

1

-Ⅱ

2

and the source rock maturity ranges from low-mature to mature degree.And the Dalazi Formation has the greatest potential of hydrocarbon generation

followed by Changcai Formation.The analysis of biomarker compounds show that some of the Cretaceous source rocks and crude oil are subjected to a certain extent of biodegradation.Besides

source rocks of the Changcai Formation can be divided into three groups:the organic matters of source rock Group Ⅰ and Ⅱ are mainly from higher plants

which are deposited in partially oxidized fresh and brackish water

and source rock Group Ⅲ contains large amounts of gammacerane and β-carrotane

deposited in saline water.Source rocks of the Dalazi Formation are mainly deposited in reductive-brackish water environment

and their parent materials come from mixed sources.The oil-source correlation reveals that the crude oil generated from the Changcai Formation can be divided into two categories:the K

1

c

1

oil originates from Group Ⅰ source rock and the K

1

c

2

oil from the Group Ⅱ source rock of the source formation.Meanwhile

the reductive-prone oil from the Dalazi Formation originates from its K

1

d

2

section.