Hydrochemical characteristics of Ahe Formation in eastern Kuqa depression, and its hydrocarbon response

Objective As the carrier during the oil and gas migration, formation water plays an important role in oil and gas accumulation. The oil-water distribution of the Jurassic Ahe Formation (J1a) oil and gas reservoirs in the eastern Kuqa Depression of the Tarim Basin is complicated, and the characteristics of formation water and its relationship with oil and gas reservoirs are lack of systematic study.Methods In order to clarify the chemical characteristics of formation water and its relationship with reservoirs, based on the selection among formation water samples, the test data of Ahe Formation in eastern Kuqa Depression are obtained to study the distribution of water type, TDS and ion-proportionality coefficients of formation water and its significance to reservoirs.Results The results show that the formation water of J1a is mainly NaHCO3 type, followed by CaCl2 and MgCl2 type, and has relatively low salinity. The TDS and ion-proportionality coefficients of formation water are controlled by tectonics. In the structure opening area and the pinch-out area of the strata, the formation water is characterized by NaHCO3 type with low TDS, the sodium-chloride coefficient and desulfurization coefficient are high and the metamorphic coefficient is negative. Nevertheless, the formation water in the closed strata has the opposite hydrochemical feature.Conclusion There is a good coupling relation between the hydrochemical characteristics of formation water and the plane distribution of oil and water, with Dibei oilfield as an example, although the strata north of the Yiqikelike fault had been charged, thegenerated oil and gas had escaped along the fault. The low TDS, high sodium-chloride coefficient, high desulfurization coefficient, low metamorphic coefficient and high kaolinite content indicate that the strata was affected by the meteoric water that entered along the fault. On the contrary, the hydrochemical characteristics and kaolinite content in the strata south of the fault reflect that the formation was well sealed and less affected by meteoric water, which corresponds to the distribution of gas layers. The connection between the hydrochemical characteristics and the distribution of reservoirs provides a basis for subsequent exploration, development and evaluation of tight oil and gas reservoirs.