Abstract:

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.