A New Approach To Calculate Gas Saturation in Shale Reservoirs

In recent years, China's shale gas exploration and development has achieved immense development, but the evaluation of fluid saturation in shale gas reservoirs is still not well documented. The Mesozoic Chang 7 Formation is one of the main source rocks in the Ordos Basin and has also proven to have good shale gas accumulation conditions. At present, there are a few methods to directly obtain gas saturation in shale reservoirs, and the conventional method using Archie's equation (which is mainly designed for pure sandstone reservoirs) is still accepted by most researchers. However, because shales usually contain some amount of clay minerals, the accuracy of the calculated gas saturation is reduced. To compensate for the presence of clay minerals, the Waxman-Smits (W-S) model was proposed, with high applicability in the quantitative calculation of water saturation. In this study, we calculated the oil saturation values of the Chang 7 Formation and compared the results with laboratory-measured values. Furthermore, we complimented our results with the support vector machine (SVM) machine-learning algorithm to make up for sparse data from the laboratory measurements. Our results suggest that gas saturation in shale reservoirs can be obtained by the calculation of oil saturation first, using laboratory-measured litho-electric and pyrolysis data, as well as using the W-S model. When the results were applied to other locations in the study area, the results were consistent with the calculated gas saturation values using previously proposed (laborious) methodologies. This suggests that the methods described here can be applied to calculate gas saturation in unconventional reservoirs.