Keywords: 
• Key words formation water /
• obstructive replaeing zone /
• Lagged zone /
• waterrock interaction /
• western Sichuan Depression

Abstract: Formation water is an important component of pore fluid. The space and time evolution of formation water is controlled by the hydrodynamic field and by fluidrock interaction, respectively, therefore, it is feasible to invert water  rock interaction with the geochemical changes in ions and the combination relationships of different ions. In this paper, by systematical analysis of more than 300 formation water data from Upper Triassic Xujiahe Formation and Jurassic Formation in the middle area of West Sichuan Depression, the vertical geochemical variation characteristics of formation water and the corresponding waterrock interaction processes recorded by chemical characteristic was discussed preliminarily 
The geochemical characteristics show that the cations of formation water are mainly K+, Na+, Ca2+, Mg2+, Fe2+, Fe3+, and the anions are mainly Cl, SO24, and HCO-3 in the study area. Main type of formation water with supplement of Na2SO4 and NaHCO3type is CaCl2 type.
From Jurassic to Xujiahe Formation in West Sichuan Depression, the formation water can be divided into 2 chemical zones: Obstructive replacing zone and Lagged zone. The zone affected by atmospheric water was only above 530 m.The formation water concentration decreased with depth as a result of largescale dehydration of clay minerals in obstructive replacing zone, but the main reasons for HCO-3 incensement with depth in this zone is addition of CO2 related to maturation of organic matter. The formation water in lagged zone has a significantly higher Ca2+ content than in obstructive replacing zone, which may be caused by albitization of feldspar. In addition, the dissolution of carbonate minerals may also have supplied some Ca2+ to lagged zone. From T3x4 to T3x2 HCO-3 concentration in formation water gradually become smaller, there may be two main reasons for the phenomenon above: First, T3x2 hydrocarbon source rocks had a relatively earlier hydrocarbon generation peak, the amount of CO2 injected into reservoir fluid was relatively smaller compared to T3x4; The other more likely reason was that stronger carbonate mineral dissolution in T3x4 than in T3x2 lead to major changes in HCO-3 concentration which was proved by relatively higher Ca2 + concentration in T3x4.
Na deficit ~ Ca2+ excess relationship showed that most of the Jurassic formation water samples were Na+ excess, indicating that Na+ was from dissolution of sodiumbearing minerals, and Ca2+ excess in formation water was caused by illitization of smectite and carbonate dissolution. Albitization of anorthite in Xujiahe Formation was an important waterrock interaction process, the other diagenetic types occurred in Xujiahe Formation included chloritization, dissolution of carbonate mineral and albite.