Abstract:

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 ^TTImethod. 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.