Predrill Geophysical Method For Geopressure Detection And Evaluation

The existence of geologic formations with pore pressure in excess of hydrostatic pressure is well known in petroleum industry. The phenomenon of geopressure is essentially stratigraphic in nature and in most cases its occurence correlates strikingly well with some mappable geologic characteristics, such as its regional correlation with high shale and low sand content’ and with its relationship with sediment deformation and faulting. In this paper, we will present an integrated methodology to detect pore pressure from seismic information. It not only can assist drilling engineers to design mud weight and casing programs before drilling it also aids in the interpretation of stratigraphic traps located in geopressure environments. We use available well logs to construct both a normal compaction trend and a pressure calibration curve for the area. The pressure calibration curve converts the deviation of observed interval transit times from the normal compaction trend to pore pressure. We then conduct seismic velocity analyses spaced every 80 to 100 feet to produce a high density interval velocity cross section. The compaction trend and the pressure calibration data are used to compute a 2-D high resolution pore pressure cross section from the interval velocity cross section. We have successfully constructed several seismic pore pressure cross sections. From the seismic derived pore pressure cross sections we observe the lateral variablility of pore pressure and relate this to the lithology changes in the sedimentary rocks.