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ABSTRACT: Porosity modelling in a complex carbonate reservoir using geostatistical inversion of seismic data

Sams, Mark S.1 and Volker C. Vahrenkamp2
(1) Jason Geosystems Asia, Singapore, Singapore 
(2) Sarawak Shell Berhad

The reservoir of the Jintan field, offshore Northern Borneo, is part of a complex carbonate platform. The reservoir rocks are practically pure limestone with only minor terrestrial input and insignificant dolomitisation. The platform has built up through various stages of growth and attrition resulting in large variations in porosity both vertically and laterally. Consequently, estimates of hydrocarbon volumes based on the extrapolation of well data have had to be significantly revised on the drilling of new wells.

A 3D impedance model of the reservoir was built through the inversion of a seismic volume. This model greatly improves the understanding of the distribution of porosity. However, analysis of well log and core data indicate that the impedance is not only affected by the pore volume but also by the pore structure and hydrocarbons. This results in rocks with the same impedance having different porosity.

The uncertainty in the relationship between impedance and porosity was handled with geostatistical techniques. A geological framework was interpreted from the impedance model that defined intervals of similar properties. A unique geostatistical model was derived for each interval, which reduced the uncertainty in the porosity distribution. This uncertainty was quantified by generating multiple models of porosity that honoured the well data, the geological model, the geostatistics and the seismic data. The differences between well derived and inversion based porosity models are significant both in terms of volumes and field development (optimum well positioning).

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