Porosity and permeability correction of laumontite-rich reservoirs in the first member of the Shaximiao Formation in the Central Sichuan Basin

The development of a large amount of laumontite cement in clastic reservoirs generate pseudo fractures at normal temperatures and pressures conditions on the ground surface, causing porosity and permeability values to be significantly higher than the actual values of buried conditions. Taking the first member of the Shaximiao Formation in the Middle Jurassic of the Central Sichuan Basin as an example, this study analyzes the material composition, reservoir porosity and permeability, diagenesis, and other characteristics of reservoirs rich in laumontite using the petrology, geochemistry, porosity and permeability characterization, and overburden pressure porosity and permeability tests and further discusses a method for correcting porosity and permeability at normal temperatures and pressures conditions on the ground surface. The reservoir rich in laumontite in the first member of the Shaximiao Formation in the Central Sichuan Basin primarily comprises lithic arkose sand-stone, with porosity ranging from 2.12% to 17.48%, with an average of 10.28%, and permeability ranging from 0.004 to 120.351 mD, with an average of 2.691 mD. There are various types of diagenesis, including laumontite cementation, calcite cementation, siliceous cementation, clay mineral cementation, laumontite dissolution, feldspar dissolution, etc. Laumontite is mainly exhibited as pore filling cementation, and the extensive cemen-tation and dissolution of laumontite are the key factors affecting porosity and permeability of reservoirs. By comparing the porosity and permeability difference between overburden pressure condition and normal tem-peratures and pressures condition, the influence of granular marginal pseudo fractures on the porosity and permeability of the reservoir is corrected. The corrected porosity and permeability values at normal temperatures and pressures conditions on the ground surface are linearly correlated with those before the correction. In different laumontite content intervals, the permeability of the reservoir rich in laumontite accords with the linear correction correlation. The higher the laumontite content, the smaller the correction coefficient is. Therefore, this correction guarantees accurately predict the porosity and permeability of the reservoir rich in laumontite at normal temperatures and pressures conditions on the ground surface.