The tidal-flat evaporation carbonates

widely spread in the Upper Sinian

the Middle Carboniferous and the Middle and Lower Triassic of the Sichuan Basin

are important reservoir rocks

The carbonates are often interbeded or intercalated with evaporites.The constituents ofthe carbonates are mainly lime-mud and algae-cohered

the fossils beingseldom and few in species.The tidal sedimentary features are welldeveloped

and the cyclothem and rhythms are clear.These rocks wereobviously formed under conditions of regional arid climate

restricted andextremely shallow waters and strong tidal activities on a wide and gentleplatform.These reservoir rocks are characterized by:1.Well developed porosity mainly appearing in crypto-algae dolo-mites of algae-flat microfacies in intertidal zone.The combining effectof intercrystal pores and cavities produced by diagenetic and epigeneticdolomitzation and solution

along with algae-supported fenestal and in-tergranular pores

will certainly cause an increase in the porosity;2.Wide spread fractures and fissures greatly improve the permea-bility of reservoir rocks.Because of their thin bedding and pure qualityand of their intercalation with soft evaporites and/or agrillaceous rocks

the carbonates usually yield easily to fracturing when a tectonic eventhappens;3.Weathering in the interval of sedimentation gap

which makes theevaporites expand and dissolve

thus the carbonates may be compressedto form collapse breccia.Obviously

these cavity and fracture-rich rocksmay constitute a collapse breccia type of reservoir generated by dissolu-tion;4.Evaporated tidal flat deposits

intercalated within thick carbona-tes are apt to produce deep-seated karst and thus may form a reservoirwith high productive potentiality.Generally

these reservoirs are usually formed from dolomites withfractures and pores of secondary nuture.Such a sort of reservoir usually contains many productive horizons and is wide-spread

but shows little homogeneity.It is a notable sedimentation type of carbonate platform.