Abstract: Regional Porosity Distribution in Sandstones of the Maracaibo Basin

Pieter J. Pestman

Sandstone porosity data for 185 wells from the Maracaibo Basin, chiefly from the Eocene Misoa Formation, were analysed in order to describe and explain the porosity distribution throughout the basin.

Porosity (^phgr) appears to be related mainly to thermal maturity (expressed as vitrinite reflectance, R₀):

[EQUATION (1)]

Areas with low geothermal gradients (determined comparing Ro to burial depth) include the axis Boscan-Urdaneta-southern Lake Maracaibo-La Ceiba, which coincides with the stable part of the Paleogene Maracaibo platform. Here, porosities are good relative to depth (e.g., 18% at 13,000^prime with R₀=0.62% in Block V). Northeastward, geothermal gradients increase and porosities (relative to depth) decrease. Thermal maturities are anomalously high and porosities very low in the northern part of the basin, due to faults and uplift.

For sandstones at maximum burial depth, the average relationship between porosity (^phgr) and depth (z, in thousands feet) is as follows:

[EQUATION (2)]

In areas with a lower-than-average geothermal gradient, porosities are higher than those predicted by equation (2). In areas with higher-than-average gradients, the opposite holds true.

Local porosity variations can be related to differences in: grain size and sorting, dissolution of unstable grains (causing secondary porosity), depositional environment (influencing early diagenesis), proximity of major unconformities (which may enhance porosity by up to 8%), and hydrocarbon migration (early entrapment prevents cementation).

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela