Abstract: Detailed Core-Log Calibration for Electrofacies Identification as a Tool for Clastic Reservoir Characterization, Lower Lagunillas Member, Maracaibo Basin

Helena Gamero De Villarroel, Carolina Coll

Sedimentological and petrophysical detailed core to log calibration is required to characterize a clastic reservoir. The lower Lagunillas Member of the Lagunillas Formation (Miocene) in Block IV is a sedimentologically complex interval about 300 feet thick and composed of stacked meter-scale coarsening-upward sequences. These sequences have been interpreted as cyclic progradations of a tide-dominated delta front. The use of old logs with a low vertical resolution has resulted in poor well completion strategies in this field. The commonly available logs are the SP/EL/ML and occasionally GR/CNL. The SP and EL have a low vertical resolution (6 feet) and are not effective for fine-scale, sedimentologically based correlations of the reservoir. Furthermore, the SP responds to changes in gra n size and not to changes in clay content. However, the Microlog has a higher vertical resolution (between 2 to 4 inches), and the differences between the MINV and MNOR curves are sensitive to slightly changes of sedimentary facies. The Microlog response showed a significant correlation with facies changes recognized in the cored wells. The microresistivity curves of the Microlog appear to be the most reliable tool for electrofacies identification in wells which lack core information in the Block IV field.

Integration of sedimentological and petrophysical data was used to recognize electrofacies from well logs. The determination of electrofacies in wells without core data allowed a reevaluation of the reservoir and the distribution of the original oil in place. Such analysis assists in future identification of recompletion, horizontal and infill targets to ensure the optimal exploitation of the field.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994