Geochemistry of Formation Water and Its Controlling Factors: A Case Study on the Shiwu Depression of Songliao Basin, NE China

摘要

It is a common way to trace the fluid flow and hydrocarbon accumulation by studying on the geochemistry of formation water. This paper focused on the space distribution of the geochemical features of the formation water in Shiwu depression and its indication of the water-rock interaction processes. The fluid dynamic fields control the space distribution of the formation water. Due to the penetration of the meteoric water, the salinity is below 4500 mg/L at the basin margin and the severely faulted central ridge and increased basin-ward to 7000—10000 mg/L. The vertical change of the formation water can be subdivided into 3 zones, which correspond respectively to the free replacing zone (depth < 1250 m), the obstructive replacing zone (1250 m < depth < 1650 m) and the lagged zone (depth > 1650 m) in hydrodynamics. In the free replacing zone, the formation water is NaHCO_3-type fluid with its salinity increased to 10000 mg/L. The formation water in the obstructive replacing zone is Na_2SO_4-type fluid and its salinity decreased to 5000 mg/L—7000 mg/L because of the dehydration of the mud rocks. The formation water from the lagged zone is CaCl_2-type fluid, but its salinity decreased sharply at the depth of 1650 m and then increased vertically to 10000 mg/L. This phenomenon can be best explained by the osmosis effect rather than the dehydration of the mud rocks. The relationships of Cl~--HCO_3~- and Na~++K~+-Ca~(2+) show that the initial water-rock interaction is the dissolution of NaCl and calcium-bearing carbonate, causing the increase of Na~+-K~+-Ca~(2+)-Cl~- and salinity. The succeeding water-rock interaction is albitization, which leads to the decrease of Na~+ and the increase of Ca~(2+) simultaneously and generates CaCl_2-type fluid. The above analysis shows that the geochemistry evolution of the formation water is governed by the water-rock interactions, while its space distribution is controlled by the hydrological conditions. The analyzed water-rock interaction processes have been proved by other geological observations, suggesting that the fluid geochemistry is viable method to trace the fluid-rock interaction processes and has a broad applications in practice.