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ABSTRACT: Porosity estimation of a porous rock using 4-D gravity survey

Kadir, W. G., and D. Santoso , Geophysical Engineering, ITB, Bandung, Indonesia

4-D gravity anomaly is an anomaly that derived from discrepancy between two gravity observed at a station for specific time interval. It shows that the fourth dimension of the anomaly is time.

Mathematical relationship between the anomaly that observed in the surface, and density contrast of porous rock as source of anomaly was derived by assuming that geometry of the rock (and also its volume) is constant. The equation implicitly shows that the anomaly directly relate with density changes which affected by existence of pore filling material dynamics. Therefore, by using forward gravity modeling and least-square inversion technique, porosity and saturation parameter, which related to fluid-filled pore space, of rock could be estimated.

To show advantage of the application, a synthetic model was constructed from two vertical side prisms with 0.5 x 0.5 x 0.05 km3 and located at approximately 0.5 km depth. By assuming that there is an interchange between fluid and gas as pore filling materials with 1.073 gram/cm3 and 0.252 gram/cm3 densities respectively, the results shows that -0.05 gram/cm3 and -0.1 gram/cm3 bulk density changes, which have approximately 65 mGal and 110 mGal maximum anomalies, related with 6% and 12% porosities. The synthetic data also reflected value of saturation parameter that closed to zero. It's means that fluid-filled porous rock was fully replaced by gas-filled porous rock.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia