31 rock samples and 20 oil samples have been collected from Niuzhuang subsag and its southern slope for detail GC

andGC-MS analyses.The rocks have been sampled from Es

3

and Es

4

members

which are respecting the two main source rocks being deposited in freshwater lake and brackish to saline lake environments

respectively. The oils have been sampled from Bamianhe

Guangli and Shaziling oilfields in the southern slope of Niuzhuang subsag.Our oil-oil and oil-rock correlations show that there are two genetic types of oils in Niuzhuang subsag and its periphery.They are separately corresponding to the two major source rocks in middle to lower Es

3

and upper Es

4

members.Rock extracts of Es

4

member and the related oil are characterized by Pr/Ph

<

1

high gammacerane

sterane and dibenzothiophene contents

relatively low diasterane content

and high C

35

/C

34

homohopane ratios

suggesting that they have been generated in a reduction and brackish to saline lacustrine paleoenvironment. However

rock extracts of middle to lower Es

3

and the related oil are characterized by Pr/Ph

>

1

and relatively high concentration of hopane and dibenzofuran contents

representing freshwater lacustrine genesis. It is found that methylsteranes including dinosteranes are of importance to indicating sedimentary environment and even specific species of organisms. The alge-rich Es

4

shales deposited in brackish water in the southern slope of Niuzhuang subsag are rich in C

28-30

4-methyl steranes and dinosteranes

and but very small amount of dinosterane in the Es

4

source rock buried at a depth over 2700 m. Similar phenomenon has been observed in Es

3

mudstones deposited in fresh water

i.e.C

28

C

29

C

30

4α methyl-ααα-20 Rsterane of Es

3

mudstone distribute in the shape of “V” in the m/z 231 mass chromatogram

while those in Es

4

source rock have a linear distribution

suggesting that they have apparently different paleoenviornment and related geochemical environment. Analyses of formation water and trace element also show that the sedimentary environments in Es

3

and Es

4

are quite different. The Sr/Ba ratio

boron content

paleo-salinity and saltiness in Es

4

are relatively higher than those in Es

3

.In a word

the development of the Tertiary lake in this area has been closely related to the tectonic movement and paleo-climates. Sedimentary environment and biomarker index show that the oil field in the deeper part of Niuzhuang subsag have mainly been sourced by the Es

3

source rocks.They are actually of lenticular oil reservoirs characterized by short-distance migration and indigenous oil generation and accumulation. The oils generated from Es

4

have mainly been accumulated in the tectonically high position

such as southern slope of Niuzhuang subsag and the adjacent area.They are characterized by relatively long-distance migration. The sedimentary environment indexes indicate that most of the oils discovered in the southern slope have been sourced from Niuzhuang subsag

only Guangli oilfield and the oil pools in northern Bamianhe oilfield have been sourced from Guangli subsag and adjacent area. It can

therefore

be concluded that papleoenvironment is one of the most important factors having controlled oil and gas distribution in the area.