Pore pressure in deep bore-holes: Its measurement and relation with seismic activities

Abstract

The method of measuring the pore pressure of stratum fluid in deep bore-holes is presented. Taking the sedimentary basins in North China as an example, the variation law of pore pressure measured in strata within the depth range of 0 to 4 km is analyzed. Furthermore, the relation between the regional distribution of abnormal pore pressures in 3 000 m depth and seismic activities is discussed in connection with the earthquakes of M _S5.0 that occurred after 1 900 in the studied region. The study result indicates that the strata pore pressure P ₀ measured in deep bore-holes is not exactly the same as the hydrostatic pressure PH. Ultra pore pressure does exist, though not commonly. The relation that the pore pressures (P ₀₁, P ₀₂ and P ₀₃) measured in exploration, evaluation and exploitation wells in Oil-fields and the hydrostatic pressuree (P _H) of the above wells is as follows: 1 P ₀₁>P ₀₂>P _H>P ₀₃ in areas with ultra pore pressures; 2 P _H>P ₀₁>P ₀₂>P ₀₃ in areas where the pore pressures are normal or on the lower side. The relation between the regional distribution of ultra pore pressures and the activity of earthquakes of M _S5.0 has been analyzed. The result shows that, with latitudes 36.0°–36.5°N as a demarcation, pore pressures are ultra-high in the south and are normal or lower in the north. In the south, the measured pore pressure is obviously higher than the hydrostatic pressure below the depth of about 2 000 m and it increases as a power function with increasing depth; meanwhile, the earthquake activity there is weaker. In the north, however, the measured pore pressure increases as a linear function with increasing depth; meanwhile, the earthquake activity there is stronger.