摘要
A common way to trace fluid flow and hydrocarbon accumulation is by studying the geochemistry of formation water. This paper focuses on the spacial distribution of the geochemical features of the formation water in the Shiwu Rifled Basin and its indication of the water-rock interaction processes. The hydrodynamic field controls the spacial distribution of formation water. Due to the penetration of meteoric water, the salinity is below 4,500mg/L at the basin margin and the severely faulted central ridge and increases basin ward to 7,000-10,000mg/L. The vertical change of formation water can be divided into 3 zones, which correspond respectively to the free replacement zone （〈1,250m）, the obstructed replacement zone （1,250m-1,650m） and the lagged zone （〉 1,650m） in hydrodynamics. In the free replacement zone, the formation water is NaHCO3-type with its salinity increased to 10,000mg/L. The formation water in the obstructed replacement zone is Na2SO4-type with its salinity decreased to 5,000mg/L-7,000mg/L because of the dehydration of mud rocks. The formation water in the lagged zone is CaC12-type, but its salinity decreases sharply at a depth of 1,650m and then increases vertically downward to 10,000mg/L. This phenomenon can be best explained by the osmosis effect rather than the dehydration of mud rocks. The relationships between Cl^--HCO3^- and Na^＋＋K^＋-Ca^2＋ show that the initial water-rock interaction is the dissolution of NaCl and calcium-beating carbonate, causing an increase of Na^＋-K^＋-Ca^2＋-Cl^- and salinity. The succeeding water-rock interaction is albitization, which leads to a decrease of Na^＋ and an increase of Ca2＋ simultaneously, and generates CaCl2-type fluid. The above analysis shows that the geochemical evolution of formation water is governed by the water-rock interactions, while its spacial distribution is controlled by the hydrological conditions. The water-rock interaction processes are supported by other geological observations, suggesting that formation water geochemistry is a viable method to trace the fluid-rock interaction processes and has broad applications in practice.
A common way to trace fluid flow and hydrocarbon accumulation is by studying the geochemistry of formation water. This paper focuses on the spacial distribution of the geochemical features of the formation water in the Shiwu Rifled Basin and its indication of the water-rock interaction processes. The hydrodynamic field controls the spacial distribution of formation water. Due to the penetration of meteoric water, the salinity is below 4,500mg/L at the basin margin and the severely faulted central ridge and increases basin ward to 7,000-10,000mg/L. The vertical change of formation water can be divided into 3 zones, which correspond respectively to the free replacement zone （〈1,250m）, the obstructed replacement zone （1,250m-1,650m） and the lagged zone （〉 1,650m） in hydrodynamics. In the free replacement zone, the formation water is NaHCO3-type with its salinity increased to 10,000mg/L. The formation water in the obstructed replacement zone is Na2SO4-type with its salinity decreased to 5,000mg/L-7,000mg/L because of the dehydration of mud rocks. The formation water in the lagged zone is CaC12-type, but its salinity decreases sharply at a depth of 1,650m and then increases vertically downward to 10,000mg/L. This phenomenon can be best explained by the osmosis effect rather than the dehydration of mud rocks. The relationships between Cl^--HCO3^- and Na^＋＋K^＋-Ca^2＋ show that the initial water-rock interaction is the dissolution of NaCl and calcium-beating carbonate, causing an increase of Na^＋-K^＋-Ca^2＋-Cl^- and salinity. The succeeding water-rock interaction is albitization, which leads to a decrease of Na^＋ and an increase of Ca2＋ simultaneously, and generates CaCl2-type fluid. The above analysis shows that the geochemical evolution of formation water is governed by the water-rock interactions, while its spacial distribution is controlled by the hydrological conditions. The water-rock interaction processes are supported by other geological observations, suggesting that formation water geochemistry is a viable method to trace the fluid-rock interaction processes and has broad applications in practice.
