The Ordovician has been the only strata with no comericial gas discoveries from the Sinian to Jurassic in Sichuan Basin.The Honghuayuan Fm.and the Tongzi Fm.of the Lower Ordovician share a similar sedimentary environment with the Longwangmiao Fm of the Cambrian and the Jialingjiang Fm of the Triassic

and the latter two formations were proven to contain large gas fields in their carbonates of grain beach facie.Based on an integrated analysis of drilling

seismic and regional geological data of the basin

we obtained the following conclusions.1)The lower Ordovician had the geological conditions needed for forming grain beach.The mound reflections on seismic profiles of southeastern Sichuan Basin are the responses of grain carbonate interbedded with gypsum and mudstone in the Tongzi Fm and the Honghuayuan Fm.2)There are two types of source rock-reservoir combinations in the Lower Ordovician

namely lower source rock-upper reservoir combination and upper source rock-lower reservoir combination.3)According to the exploration expe-riences in the Cambrian and Triassic of central Sichuan Basin

study should be performed in respects of sedimentary facies

paleo-geomorphology and hydrocarbon accumulation and evolution history with carrier system as the focus for exploration in the Ordovician.An exploration well should be emplaced in area with prominent anomaly reflections.