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The quantity of water contained in rocks depends upon porosity. We can define: Total porosity Effective porosity Total porosity Total porosity is the ratio of the voids volume in the rocks to the total volume of the rock. It is a dimensionless quality expressed as a percentage. We differenciate between primary and secondary porosity. Primary porosity, created during the deposition of the sediment, is an intergranular type. Its magnitude depends on the degree of sorting and the shape of the grains. It does not depend on the grain size. Primary porosity, which is mainly observed in clastic rocks, generally decreases in time due to the effects of cementation and compaction. Secondary porosity includes vuggy porosity acquired as a result of dissolution, fracture porosity and intergranular porosity, due to weathering. Effective porosity Pores, to allow the circulation of fluid should be connected. Effective porosity can be defined as: It can be much lower than the total porosity when the pores in a rock are not connected ( pumice stone) or when the size of the pores is such that the fluids cannot circulate (silts) or even when a part of the