THE ROLE OF DIAGENESIS IN QUALITY RESERVOIR EVALUATION FOR PALEOZOIC SANDSTONES IN THE ORINOCO OIL BELT, VENEZUELA

Orinoco Oil Belt Paleozoic sandstones overlaying the basement of the Guayana shield have been found impregnated with oil of Mesozoic or younger origin. They are attributed to the Hato Viejo Formation. Core samples from three wells of the studied area were used to determine the reservoir quality of these sandstones.

Reservoir rock characterization was carried out using conventional petrography and scanning electron microscopy combined with cathodoluminescence, backscattering and secondary electron techniques. Various methods including modal analysis of petrographic data, image analysis of secondary electron, backscattering and cathodoluminescence response were applied to determine porosity, mineralogy, type and abundance of cements as well as diagenetic processes.

The considered interval of Hato Viejo Formation consists mainly of fine-to coarse-grained, moderately to poorly sorted sandstones with an average porosity of 8%. Compaction and quartz cementation have virtually obliterated primary porosity; however, significant secondary porosity has been generated by dissolution of feldespars, clays minerals and rock fragments. the percentage of quartz cement determined by catodoluminscence is higher than one estimated by petrography. Besides, cathodoluminescence permits to recognize authigenic quartz, post-dating detrital quartz grains. The secondary electron and backscattering images showed that the quartz cement is also susceptible to subsequent diagenetic dissolution and formation of secondary porosity. Based on these results the Paleozoic sandstones of Hato Viejo Formation can be considered potential reservoir rock.