Log Analysis Of Gas-Bearing Fracture Shales In The Saint Lawrence Lowlands Of Quebec
Authors: Aguilera Roberto 
Published: December 2007

Introduction

Black and brown shales are widely distributed all over the world (North and South America, Africa and Europe). Most of them form part of late devonian and early Mississipian ages. In Quebec, black shales are found mostly in the external zone of the Saint Lawrence Lowlands forming part of the Utica Group (Ordovician age). In normal stratigraphic sections, the Utica black calcereous shale has an average thickness of 1000 ft.

When this study was initiated, a literature survey indicated that very little has been published on the subject of evaluating gas-bearing fracture shales from logs. Therefore, an effort was made to find a means of evaluating qualitatively and quantitatively the fractured shales of Quebec. The objectives of this evaluation program from logs were as follows:Determination of water saturationEvaluation of porosity and shale volumeIdentification of fractured zonesQuantitative estimates of fracture porosityQuantitative estimates of water saturation each matrix and fractures.Differentiation between zones which were gasbearing, but had amounts of gas which were commercial or non-commercial.Estimates of formation strength parameters for proper design of stimulation jobs.

It was obvious that it was impossible to warrant exactitude when trying to calculate the previous parameters. However, it was found that the results correlated very well with laboratory data, pressure surveys, and actual stimulation and production information.

Objective 1 was accomplished with the combination sonic-resistivity, which proved to be the most efficient for direct calculation of water saturation due to the fact that shales affected both logs in a similar opposite manner. The log-log cross-plot of sonic response versus resistivity permitted reasonable estimates of water saturation without previous knowledge of the water resistivity, Rw, and the system porosity exponent m. This was very significant as porosity exponent m. This was very significant as the Rw value of the horizon of interest was not known. The cross-plot revealed that m had a relatively low value of 1.54. Objective 2 was accomplished with the use of sonic and neutron logs. Experience indicates that, in general, this combination is not very useful for shale volume analysis because both logs are affected in a similar fashion by shales. For the Quebec consolidated gas-bearing fracture shales, however, this combination proved to be very valuable. Objective 3 was accomplished with the use of sibilation, temperature and dipmeter logs. Quantitative estimates of fractures porosity (Objective 4) were obtained as a function of total porosity, the system porosity exponent discussed previously, and the porosity exponent of the matrix. Objective 5 was accomplished by assuming that the fractures could be represented by a bundle of tubes. Objective 6 was achieved with the use of sonic and resistivity logs which gave an indication of over-pressured zones. For the fractured shales of Quebec, it was found that abnormal subsurface pressures were indicative of small non-commercial reservoirs. pressures were indicative of small non-commercial reservoirs. Objective 7 was accomplished with the use of logs, pressure surveys, and data from stimulation jobs. pressure surveys, and data from stimulation jobs. It was found that Poisson's ratios and fracture gradients were unusually high as anticipated by thrust tectonics of the area.

To date three wells have been drilled and 150 square miles have been mapped from seismic. The findings presented in this paper have led to cautious optimism presented in this paper have led to cautious optimism with regard to the gas potential of the fracture shales of Quebec.

LOG PROPERTIES FOR EVALUATING GAS BEARING FRACTURED SHALES - STATE OF THE ART

Only a few techniques have been published on the subject of evaluating fractured shales from logs.