Evaluation on sealing ability of caprocks for gas reservoirs in Hangjinqi area

Hangjinqi area is located in the north of Ordos basin, China. The capping strata are mainly comprised of argillaceous rocks of the upper Shihezi and Shiqifeng formation. In this paper, the thickness and displacement pressure of the cap layers are analyzed by logging data, the sealing ability of the cap layers is comprehensively estimated based on the geochemical characteristics of the formation water.

 Based on logging data and well logging interpretation, The cumulative thickness of mudstone cover in the upper Shihezi formation is 32~112m, the highest thickness is 112m in Jin51, and 108m in Jin29. The thickness has the characteristics of being thicker in the east, which is bounded by the junction of Boerjianghaizi and Wulanjilinmiao fault. According to the statistical results of maximum thickness of monolayer, in upper Shihezi formation is generally 4~40m, the maximum in Jin91 is 40m. The spatial distribution shows that the maximum thickness is r in the central Jin72-Jin71-Jin 99 and in the western Jin29. The cumulative thickness of Shiqianfeng formation caprock is relatively thick, 80~201.5m. The maximum thickness of monolayer generally is 2~44m, the maximum is 44m in Jin83.

Based on the logging data of 69 Wells, the displacement pressure data of the upper Shihezi and the Shiqianfeng formation were calculated according to the calculation formula between the measured displacement pressure and the acoustic time difference, the statistics of maximum displacement pressures were conducted. The maximum displacement pressure of the upper Shihezi and Shiqianfeng formation changes from 23MPa in the west to 15MPa in the east, which shows the characteristics of higher displacement pressure in the middle west and lower in the east and northeast.

The results of the comparative analysis of the plane distribution characteristics of the geochemical parameters of formation water and the gas production of natural gas show that, in Jin30 and Jin63 near the Wulanjilinmiao and the Sanyanjing fault, the salinity and metamorphic coefficients of the formation water are relatively high, while the sodium-chloride coefficient, desulfurization coefficient and carbonate rock equilibrium coefficient are relatively low, which indicate that the stratigraphic sealing conditions are good and beneficial to the preservation of oil and gas reservoirs. To the east, the sodium chloride coefficient of the formation water increased obviously, while the salinity and metamorphism coefficient decreased, which indicated that the formation sealing condition became worse. Further to the east, due to the influence of the Boerjianghaizi fault, the fault zone is characterized by low values of salinity and metamorphic coefficient, high values of sodium-chlorine coefficient, desulfurization coefficient and carbonate rock balance coefficient, indicating that the formation has poor sealing conditions, which is not conducive to the preservation of oil and gas reservoirs. But in Jin53-JIn72 and Yishen1-Jinping1-Jin33 located in the south and north sides of fault zone, the salinity and metamorphic coefficient is relatively high, and sodium chloride, desulfurization and carbonate balance coefficient are relatively low, indicating stratigraphic sealing and preservation condition in the south and north areas near the fault zone are in favor of the preservation of the gas reservoirs.