Coal-measure formations in the Upper Paleozoic mainly distributes in central-south part of Dagang Oilfield and covers an area of nearly 10 000 km

2

.Source rocks there consist of coal beds

carbonaceous mudstone and mudrock

and have a gross thickness of 100 to 450 m.Vertically

they distribute mainly in the Taiyuan and Shanxi Formations with some developed in the Benxi Formation.The coal beds are rich in organic matter (more than 85%) and are high to very high quality source rocks

while the organic matter content in mudstone varies greatly from place to place.Vitrinite dominates the maceral of the source rocks with a content of more than 50%

indicating favorable conditions for gas generation during the mature stage of the rocks.Meanwhile

affected by coal-forming environment

the coal-measure strata in the main coal-forming centers are also contain some hydrogen-rich macerals

hinting large oil generation capacity during low maturity to mature stage.The Upper Paleozoic strata experienced multiple phases of tectonic activities

so were the coal-measure source rocks

which went through two stages of hydrocarbon generation.The secondary hydrocarbon generation was cha-racterized by large area and high resource potential and has been the focus of study during the buried hill exploration.Four types of burial history can be recognized in the oilfield based on the tectonic evolution:the early-burial and late-lift type

the early-lift and late-burial type

the continuous deep-burial type and the steady-burial type.Except for the first type

all the other three types possessed favorable conditions for secondary hydrocarbon generation.Years of exploration in the area of secondary hydrocarbon generation yielded primary oil and gas discoveries in buried hills one after another:Kongxi

Wumaying

Chenghai and Wangguantun

to name just a few.