PETROPHYSICAL CHARACTERISTICS OF THE MIDDLE EOCENE LAUMONTITE TUFF RESERVOIR, SAMGORI FIELD, REPUBLIC OF GEORGIA

In the Samgori field, near Tbilisi in the Republic of Georgia, changes in reservoir properties caused by alteration of middle Eocene volcanogenic rocks control the amount of oil and gas these rocks will yield. These changes, which include fracturing, development of vugs, and laumontite zeolitization, impart a vertical and lateral variability, thus giving rise to irregular petrophysical properties of the rock such as filtrational capacity, elastic deformational properties, and productivity. Indicative of such differences is the large variation in initial recovery from one well to another.
According to the reservoir model proposed here, productivity at Samgori is dominantly controlled by fracture porosity and permeability. Fractured bodies of oil-bearing laumontized tuffs are enclosed within a thick layer of andesite-basalt tuffs and tuffites, which act as a seal. As a result, the fractured tuffs are relatively isolated from the action of geostatic and geotectonic loads, which would otherwise result in closure of the fractures and microfractures. In addition, the fluid pressure of the reservoir tends to maintain open fractures. As fluid pressure declines over the producing life of the field, effective stress increases and the fractures close. This results in the isolation of producing zones and the consequent development of reservoir heterogeneity. An understanding of this effect is essential to proper management of the reservoir so as to ensure optimum recovery of oil.
The contribution of matrix porosity to productivity is also recognized. Fluid injection analysis has established that matrix porosity and permeability vary according to lithology, with the highest values in laumontized tuffs: laumontite inter-crystalline voids can also be observed, using scanning electron microscopy.