Difference of Microfeatures among Diagenetic Facies in Tight Sandstone Reservoirs of the Triassic Yanchang Formation in the Midwestern Region, Ordos Basin

Table 2

Quantitative calculation method of sandstone porosity evolution.

Porosity evolution parameter	Formula	Remarks
Initial porosity	(i)	Φ1 represents the original porosity of the unconsolidated sandstone, %; Sd represents the Trask sorting coefficient, Sd = (P25/P75)^1/2 (in which P25 and P75 represent the corresponding granular diameter of 25%-in-content particles and 75%-in-content particles in the accumulative granularity curve, respectively, mm)
Porosity after compaction	(ii); (iii)	Φ2 represents the porosity after compaction, %; C represents the cement content, %; P1 represents the surface porosity of intergranular pores, %; P2 represents the surface porosity of original micropores, %; P3 represents the measured porosity of the sample, %; P4 represents the total surface porosity, %; Pa represents the porosity loss due to compaction, %; Fa represents the ratio of porosity loss due to compaction, %
Porosity after cementation	(iv); (v)	Φ3 represents the porosity after compaction and cementation, %; Fb represents the ratio of porosity loss due to cementation, %
Dissolution increases porosity	(vi); (vii)	Φ4 represents the increased porosity due to dissolution, %; P5 represents the total dissolution pore surface area, %; Fc represents the ratio of porosity loss due to dissolution, %