The existence of a new gas accumulation type sourced from the source rocks within the Ordovician was confirmed at the lower slope in the west flank of a paleo-uplift in Ordos Basin based on effective source rock assessment results.Analyses of 347 samples taken from the source rock intervals in 11 wells revealed high-quality source rocks developed in marginal slopes of the middle and upper Ordovician platform in the Basin.Measurements showed that the source rocks in the Wulalike and Kelimoli Formations had relative higher TOC values

with marlite TOC averaged at 0.33% and 0.39%

and mudstone TOC averaged at 0.51% and 0.65%

respectively.The accumulative effective thicknesses of source rocks of the two formations were 12 to 40 m and 7 to 14 m

respectively.Areas with thick source rocks were assessed to have a gas generation capacity of 14.0×10

8

to 15.5×10

8

m

3

/km

2

.Gas accumulations in the Kelimoli Formation experienced two charging stages:the early stage in the Middle Jurassic

during which paleo-oil reservoirs were formed;and the late stage in the Late Jurassic to Early Cretaceous

during which the paleo-oil reservoirs were cracked into petroliferous gas reservoirs.Carbon isotopes of gas-bearing inclusions from the later stage were measured to be -38.84‰ in average

indicating late petroliferous gas charging characteristics.Coal-derived gas reservoirs were distributed widely in the upper slope in the west flank of the paleo-uplift.The fact that the massive detrital calcareous berccias formed in the Kelimoli Formation at the lower slope had a close contact with the overlying effective source rocks of the Middle and Upper Ordovician and that they stretched away laterally from the top Ordovician weathering crust

made it possible for the formation of self-sourced gas accumulations

like Yutan1 gas reservoir in the area.Fractured-vuggy karst reservoirs developed in the lower slope of the platform margin were considered to be potential exploration targets for such kind of gas reservoirs.