--> Effective Stress, Porosity, and Abnormal Pressure in the Eugene Island Block 330 Field, by A. Deshpande and P. B. Flemings; #90986 (1994).

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Abstract: Effective Stress, Porosity, and Abnormal Pressure in the Eugene Island Block 330 Field

A. Deshpande, P.B. Flemings

We find that porosity of shales is an exponential function of effective stress (Overburden-Fluid pressure) in the E.I. 330 Field. We use this method to predict fluid pressure from sonic logs. We calibrate the porosity-stress relationship in the hydrostatic pressure zone and find:

[EQUATION]

where ^phgro = 0.51, ^lgr approximates 5.8 e-5 psi-1 and ^sgr = effective stress (psi)

The abnormal pressure gradient, predicted from porosity data, are approximately 10% lower than those predicted from mud-weight data. These data are at a much higher resolution than mud-weight data, although there is some noise in the vicinity of oil-bearing sands.

The E.I. 330 field is found on the downthrown and low-pressure side of a pressure-sealing growth fault. We find that the porosity profiles on the upthrown (footwall) side of the fault actually decrease with depth as thick, shallow shales are penetrated. In contrast, geopressures in the hanging wall are much deeper, and although the same mud-weight gradients are reached, the porosity profile reverses only slightly in the geopressured zone. In the hanging wall, the transition from hydrostatic to geopressures occurs in two phases. The major reservoir interval is found in the transition from 12 to 14 lbs/gal; beneath this interval, hard geopressure is marked by an abrupt jump to almost 16 lbs/gal. as a thick shale is penetrated.

These results suggest that the primary cause of geo-pressures, in this region, is undercompaction. The association of the transition to geopressure with the transition to a shale-prone stratigraphic section reveals that the pressure-seals are stratigraphic; regional, low permeability shales, seal pressures in this basin. In addition, the abrupt jump in fluid pressures across the fault plane implies that the fault is itself also sealing.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994