Hydrogeochemistry of formation water with implication to abnormal pressures in the Northern Songliao basin, China

Abstract

Formation water is major component in sedimentary basin. The geochemistry properties of formation water are ought to be the final outcomes of basin evolution due to a number of hydrodynamic and thermodynamic factors. Therefore, hydrochemistry of formation water and its potential relationship with phenomena of abnormal pressures are of considerable interest and importance to study the generation, transport and accumulation of hydrocarbon in sedimentary basin. The Songliao basin is one of the largest Mesozoic continental basins in China related to rifting process, dominated by Cretaceous fluvial and lacustrine strata with volcanic and volcaniclastic rocks. The stratigraphic sequences can be classified into 6 hydrostratigraphic units from bottom to top: the Upper Jurassic Formation (J3), the Lower Cretaceous formation (K1), the Late Cretaceous Quantou Formation (K2q), Qingshankou (K2qn), Yaojia (K2y) Formation, Nenjiang (K2n) Formation. In this paper, the characteristics of the strata pressures, hydrogeology and hydrochemistry in the whole Songliao basin are investigated according to 2433 pressure data and 6270 water sample data collected in the different structural zones and geological formations in the northern Songliao basin. The pressure data shows that, not only the abnormal pressures exist in different formations and structural zones, but also the underpressure coexists with the overpressure in different structural zone within a single geological formation. Besides, the pressure coefficients change gradually from 1.02~1.30 in the western Central Depression to 1.05~0.5 in the eastern Central Depression, and then become 0.65~0.9 in the east Uplift zone, especially in the Quantou formation (K2q). The analysis of hydrochemistry data shows that the geochemistry of formation waters in the Songliao basin shows a distinct variation in different pressured systems. There are several significant vertical and horizontal zones of hydrochemistry characteristics in the whole basin, which have closed relationship with basin hydrodynamics, formation sequences and diagenesis reactions. The salinity of formation water ranges from 1.0 g/L to 20g/L in most formations. Water composition is dominated by water type of NaHCO3 but varies greatly in salinity and ion ratios in the different area. The origin of formation may be referred as the mixing of the evaporation and condensation of connate water, clay dehydration due to compaction, topography-driven meteoric water recharge as well as diagenesis alteration such as the dissolution/precipitation and albitization of plagioclase, and their impacts differ greatly in the different hydrochemical and abnormal pressured environment.