The main sedimentary facies types in Ordovician are supratidal zone subfacies(mainly mudstone-dolomite and gypsum-dolomite flats)

intertidal and subtidal zone subfacies

lagoon-gypsum salt lake subfacies

restricted gulf and open shelf-shallow sea subfacies(includes storm deposit) and shoal subfacies

of which the gypsum-dolomite flat

lagoongypsum salt lake and open shelf-shallow sea storm deposits are more tipical. In order to discuss the conditions of transforming organic matter into oil and gas

the authors have set up burial-thermal evolution models of Ordovician in the east and west parts of the study area by restoring the original thickness of sedimentary stratu with

TTI

method. Both of the models show that during Ordovician and after

there existed two or three major burial periods that have been confimed by exploration in Ordos area and the west part of the study area

and have potentially favourable conditions for natural gas.Therefore

while paying attention to self-generated gas in Ordovician

it is worth to notice coal-formed gas from overlying Carboniferous-Permian which may migrate into Ordovician erosional karst zones to accumulate again. It has been shown that karst zones with a lot of pores

vugs (or cavities) and fractures are widespread in top of Ordovician

and has provided the main pore spaces for gas migration and accumulation. Besides

multiple ancient erosional haitus and karst zones were formed inside the Ordovician following the crustul vibration and the phreatic fluctuation during sedimentution

thus providing more reservoir spaces for natural gas.