ABSTRACT

Analysis of well logs has been an important contribution to evaluate with efficiency the Yacoraite formation, which is the hydrocarbon producer in Caimancito field, Jujuy Province, Northern Argentina. This rock unit of Cretaceous age is made up mainly of low porosity carbonates. Consideration is given to graphical and analytical solutions for the interpretation method employed to determine the percentages of major lithologic components and porosity values. The information is obtained from the Compensated Sonic log, Compensated Density log and Sidewall Epithermal Neutron log. Each log gives a different response to lithology and porosity. Log interpretation is processed by a computer and the results are automatically represented in a graphical form by a plotter, which enables a continuous and detailed study of long sections. A correlation of Lithology-Porosity carried out in wells of this reservoir shows that where the thickness increases, the proportion of dolomite and the shaliness in the formation also increase. It shows too that the siliceous zones can be correlated. The obtained lithology was verified by X-ray analysis of cores and data derived from well logs have been compared with conventional core porosities. A comparative study was made of the porosities calculated from a combination of Compensated Sonic log, Compensated Density log and Sidewall Epithermal Neutron log with values deduced separately from each single log. The Sidewall Epithermal Neutron log appears to give the porosity values closest to those obtained from the combination of three logs. It is essential to have a correct porosity value to attain a reliable determination of fluid content from log interpretation. It is possible to obtain this value using the abovementioned combination of logs. Thus, an efficient evaluation can be made of rocks with adverse characteristics for log interpretation, which is the case with the Yacoraite formation carbonates. In this way, it becomes possible to distinguish pay zones from water producing intervals, and then, better well completions are feasible.

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