Abstract

The constant compromise between oil production and reserves requires differentiating gas from oil. For the detection of gas we have used the density porosity vs. neutron porosity plots, including all the environmental correction processes and the influence of the clay content in the evaluated layers. Nevertheless, it is not always an accurate method because of the nature of the stratigraphic column of the San Jorge basin which is composed of fine sand and shale layers of lenticular geometry, with different types of clay, mostly generated in a continental environment with marked volcanic participation that make forecasts unreliable.

With the introduction of NMR technology we have obtained, among other parameters, a total porosity curve. From this data, a successful quicklook technique has been developed using the cross over density porosity vs. total porosity, which allows the identification of gas levels and to figure out the clay content for its use in the log analysis.

This paper presents the method for gas detection at well site and its economic impact in logging costs. In some oilfields of the San Jorge Basin, Argentina, logging costs were reduced by 12 %.

Introduction

Frequently during well completion, it appears the necesity of knowing the productive layers of gas, by using a logging program that includes density and neutron. The continental setting originated clays in lenticular layers with mixed grains distribution, which hides the particular crossover of density porosity and neutron porosity in gas detection.

In this complex environment, the use of SP and GR as clay indicators puts into evidence the lack of reliability on the environmental correction process applied to the porosity tools, in order to determine an effective porosity through the bulk porosity, and then to determine the real water saturation in a porous environment.

At the time of the writing of this article, more than 700 wells of the San Jorge Basin have NMR logs and more than 7000 individual swabbings make up the database.

It is an important reason to develop this methodology.

Geological framework - Reservoir features
Anticlinal Perales Area

The San Bernardo Fold Belt southern prolongation, in Santa Cruz Province, Argentina is composed of several structures with a high hydrocarbon potential, as Los Perales, Las Mesetas, Aguada Bandera and Cerro Guadal among others (fig.1). The sediments that filled the late sag sequence of the San Jorge Basin are controlled mainly by their proximity to source material and the exposure of the basement. Also they are highly contaminated with pyroclastic material provided by the continental arc volcanoes active during the Upper Cretaceous.

Lower Member of Bajo Barreal Formation lithofacies represent proximal fans with wide low relief areas of fluvial deposition (fig.2). Sheet deposits of sand bodies due to non channelized currents of torrential rain origin, are present in this part of the column (2). Lenticular conglomerates and sandstones deposited by a poor drainage system related to tectonically active local areas, are also developed and both lithofacies represent a regional progradation. Thickness of individual sand bodies - 9 to 24 ft - are beneath seismic resolution. The reservoir properties of these volcanic rich sandstones are strongly affected by diagenetic evolution that varies because of the variations in depositional environments and lithofacies.

Anticlinal Perales Area

The San Bernardo Fold Belt southern prolongation, in Santa Cruz Province, Argentina is composed of several structures with a high hydrocarbon potential, as Los Perales, Las Mesetas, Aguada Bandera and Cerro Guadal among others (fig.1). The sediments that filled the late sag sequence of the San Jorge Basin are controlled mainly by their proximity to source material and the exposure of the basement. Also they are highly contaminated with pyroclastic material provided by the continental arc volcanoes active during the Upper Cretaceous.

Lower Member of Bajo Barreal Formation lithofacies represent proximal fans with wide low relief areas of fluvial deposition (fig.2). Sheet deposits of sand bodies due to non channelized currents of torrential rain origin, are present in this part of the column (2). Lenticular conglomerates and sandstones deposited by a poor drainage system related to tectonically active local areas, are also developed and both lithofacies represent a regional progradation. Thickness of individual sand bodies - 9 to 24 ft - are beneath seismic resolution. The reservoir properties of these volcanic rich sandstones are strongly affected by diagenetic evolution that varies because of the variations in depositional environments and lithofacies.

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