Development of a Geologic-Diagenetic Exploration Model Based on Petrophysical Evaluation of Yucal-Placer Gas Field in Eastern Venezuelan Basin

John I. Reistroffer, Dominique Laurier

An exploration model has been developed based on the petrophysical evaluation of the Yucal-Placer gas field which will optimize the development of this field and assist in the exploration of adjacent areas.

Yucal-Placer is a large structural-stratigraphic trap with updip pinch-outs of low-porosity gas sands from the Tertiary Roblecito and La Pascua Formations. Thick, clean sands are prevalent throughout the section, but exploration has been hindered as a result of low-porosity, low-permeability reservoirs. The key to exploration in the area is to locate sands with high porosity.

Water saturation, shaliness, and porosity are calculated for each sand. Effective porosity and shaliness are mapped for these horizons. Graphs of 0 versus initial production and S_w are constructed for each sand. Production cutoffs of 6.5% porosity and 25% S_wirr are defined.

Laboratory analysis of cores reveals evidence of paleo-oil/water contacts in La Pascua gas-bearing sands. This has led to the development of a geologic-diagenetic model, where formation of siliceous cement is reduced above paleo-oil/water contacts, giving rise to porosity trends in ancient oil reservoirs.

The Guarico subbasin has undergone structural rotation. Beds initially dipping north-northwest now dip to the northeast. Porosity trends should be expected above paleo-oil/water contacts in stratigraphic traps at the southern margin of lower Roblecito and La Pascua marine bars.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.