Abstract: A progressive increase in dolomite content over a certain range is usually accompanied by an increase in poro-sity of dolostone.It is challenging to clarify whether dolomitization is capable of creating new pores or not,and this deserves special attention.We assessed this process in the oolitic shoal reservoirs in the Lower Triassic Feixianguan Formation in the northeastern Sichuan Basin,China,through petrographic observations and comparisons of petrologic and mine-ralogical features of the limestones and dolostones.The results show that the content of intergranular calcite cement of phase 3,the product of pressure solution and re-precipitation in oolite,continuously decreases with the increase of dolomite content after the content of dolomite exceeds 40%.Then the intergranular pore space,which was supposedly occupied by the calcite cements in the precursor lithologies,would have increased and developed into the present porosity.This observation explains well the trend of porosity increasing with dolomite content.That is,rather than creating new pores,dolomitization in the oolitic grainstone layers was more likely to have helped to preserve the early pores.We also found that the variation of petrophysical properties of the dolostones in different parts of the present oolitic shoal bodies originated from the differential evolution of dolostone fabric,which was initiated by the difference in lithologies and their distribution patterns in one depositional cycle.The effects of eogenetic freshwater infiltration on oolitic fabric,mineral composition and stability have exerted substantial influence on preservation of the original textures of the oolites.Recrystallization and deep buried dolomite dissolution further obliterated the oolites and the original fabrics of the precursor lithologies,resulting in the final crystallized dolomites.

Key words: oolitic shoal, dolostone fabric, dolomitization, porosity preservation, Feixianguan Formation, Sichuan Basin