Abstract:

The most significant event in the diagenetic history of the carbonate rocks for the 1st and 2nd Members of Triassic Jialingjiang Formation in southern Sichuan might be the dissolution and infilling of carbonate grains in synsedi-mentary and early diagenetic stages.Whenever the carbonate sediments were stationed above sea level,the ool-pellet banks were drenched by meteoric water,the aragonite and high-Mg calcite constituting inner part of the grains were dissolved resulting in the appearance of dissolution hole inside the grains.Later,the underground water-table elevated,the lower holes within freshwater phreatic zone would be fully filled with freshwater calcite,and the upper dissolved holes still in the upper phreatic and vadose zone were only locally and partially filled with freshwater calcites.Then,new ooid and pellet carbonate layer was sedimented upon the previous bank while it was submerged under sea level.Again and again,these courses repeated in cycles,and so,a great deal of layers of the dissolvedholes remained so far in the section of Jialingjiang Formation.Consequently,la-yers of dissolved holes and its local distribution of mica-like calcite in the dis-solved holes might be a diagnostic feature of vadose zone or upper phreatic zone,and a textural set of dissolution and infilling of ooides and pellets represents a hiatus of sedimentation.As for oil and gas reservoirs,the most significant pore type of the 1st and 2nd Members of Jialingjiang Formation is the dissolved holes of ooid and pellet and that of the dedolomitized dolomites,the primary intergranular holes are of second importance,pores inside bioclasts and between crystals are almost negligible,while the existence of bitumen is harmful due to its role in blockage.