Based on pressure and temperature data, a comprehensive analysis was carried out for the pressure and temperature regimes in four Tertiary formations of the western Qaidam basin, northeast Qinghai-Tibet plateau. The pressure data were obtained from well pressure tests and the temperature data from systematic steady-state temperature measurements in 10 wells, temperature tests in over 200 wells and calculation for 60 theoretical calculated points. In general, the geothermal gradient in the four Oligocene-Pliocene formations decreases with depth. The Pliocene Upper Youshashan Formation has the highest gradient of 33degreesC/km. The Miocene-Oligocene Upper and Lower Ganchaigou Formations have lower gradients of 25-27.5degreesC/km in most areas but also several lower gradients down to 20degreesC/km. Abnormally high pressures were found at depths of 2650-4350 m, beneath the regional seal in the western Qaidam basin. The abnormal high-pressure is mainly confined to the Lower Ganchaigou Formation with a pressure coefficient up to 1.9. Most parts of the Pliocene Upper and Lower Youshashan Formations are in normal and low abnormal high pressure (r < 1.3) conditions. The relationship between temperature and pressure, together with the areal variation of total salinity values (TSV) of formation water, provide the evidence of the different characteristics of reservoirs distributed in the Tertiary succession. The thermal gradient increases with the pressure coefficient in the Pliocene. The areal variation of TSV, corresponding to the pressure regime, and the dispersed distribution in the histogram of the TSV of formation water indicates that the chemical environment of fluid flows in these strata varies widely. This wide variation implies that the Pliocene succession, with relative low abnormal high pressure values, acts as a vertically open system in the petroleum system. The reservoirs distribution in the Pliocene are isolated distributed and do not correlate with the pressure coefficient. However, the thermal gradient decreases with an increase in the pressure coefficient in the Oligocene Lower Ganchaigou Formation, and its constant TSV of 15-21 X 10(4) mg/l indicates the formation was in a closed chemical environment of fluid flows over the majority of geological time. The reservoirs in this formation all occur in an abnormally high-pressure zone with a pressure coefficient of more than 1.5. (C) 2003 Elsevier Ltd. All rights reserved.
