PREDICTION OF ABNORMALLY HIGH FORMATION PRESSURES (AHFP) IN PETROLIFEROUS SALT-BEARING SECTIONS

Abstract

Prediction of overpressures [Abnormally High Formation Pressures (AHFP)] in the evaporite sequences is indeed a challenging problem. Several techniques are suggested here for the solution of this forecasting problem. For a reliable AHFP forecast it is necessary to delineate the salt plugs. A method of accomplishing this is presented here. In many cases there is evidence of vertical gas migration along flanks of salt plugs and associated fractured zones (and faults). The composition of gases in the secondary traps in the sealing evaporite sequence is the same as that of gas in the massive, major accumulation below. Predictive precursors of the AHFP include lithological barrier (e.g., anhydrite) with decreasing drilling rates and appearance of “gas clay” in circulating mud (i.e., plastic greenish-gray and brown clay with gas bubbles).