June Gidman, Frank J. Conner, Dennis J. Fischer
The Upper Cretaceous Lago Reservoir in the Malongo North field consists of thoroughly mixed siliciclastic/carbonate lithologies. A problem in building simulation models for this reservoir is that there are significant differences between porosities calculated from logs and those measured from cores. This results in large uncertainties in reserves and remaining oil-in-place estimations.
Reservoir characterization, as part of a waterflood feasibility program, resulted in the identification of lithofacies and mappable reservoir zones. The lithofacies that makes up the majority of the reservoir is burrow-mottled dolomitic siltstone. The burrowed lithofacies has poorly cemented, porous and permeable, silt-filled burrows surrounded by tight, silty dolomite. The fabric of the rock results in heterogeneous distributions of porosity and permeability on a centimeter scale. Minipermeameter data across slabbed cores illustrate this heterogeneity. Similarly, measurements made on each end of routine core analysis plugs show variations between the two ends of up to three orders of magnitude. Comparison of full-diameter and plug data shows that plugs underestimate permeability belo 80 md. A relationship between full-diameter and plug permeabilities was used to scale-up plug data below 80 md. Porosity is not scale dependent, and no significant differences were found between plug and full-diameter data.
The outside of the cores and the borehole wall have significant small-scale rugosity caused by washout of poorly cemented burrow-filling siltstone. This rugosity is measured by logs as porosity. The density log, while performing normally, does not record data representative of the formation.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994