Chemical Characteristics of Ordovician Formation Water and Its Relationship with Hydrocarbons in Halahatang Depression, Tarim Basin, NW China

Abstract

The chemical characteristics of Ordovician formation water and its relationship with hydrocarbons in the Halahatang depression (Tabei Uplift, Tarim Basin, NW China) were analyzed on the basis of the detailed formation water test data. The formation water in the Halahatang depression can be characterized as CaCl2 type with high total dissolved solids (TDS) generally. The TDS concentration has a weak negative relationship with the depth, and is above 200 g/L in the North Region (north of the pinch-out line), then gradually decreases to the south, but is still greater than 50 g/L. The ion-proportionality coefficients of formation water, including the sodium-chlorine coefficient, desulfurization coefficient and metamorphic coefficient, reflect that the present strata are well sealed and had once experienced strong water-rock interactions. Furthermore, the source and evolution of the formation water presents a closed relationship with the hydrocarbon accumulation. The meteoric source of the formation water indicates the denuding by the Ordovician formation and the damage from the previous oil and gas reservoirs. The reservoir quality was also improved due to the strong karstification during the denudation, which was beneficial for hydrocarbon accumulation. The distribution of the TDS concentration is controlled by the caprock (Sangtamu Formation) and the high salinity fluids from overlying strata and adjacent regions. A geological model was established, the high salinity fluids penetrated the Ordovician strata resulting in the TDS increases in the northern part. Whereas, the South Region (south of the pinch-out line) was less affected due to the shielding layer of the O3s. The favorable preservation conditions reflected by the high TDS and ion-proportionality coefficients correspond to the stable subsidence of strata since the Triassic era, the oil and gas reservoirs formed in the Himalayan can be preserved.