Rock-Eval Basic/Bulk-Rock vs. Shale Play Methods for Characterization of Unconventional Shale Resource Systems: Application to the Eagle Ford Shale, Texas

Abstract

Artificial thermal maturation of rock samples by open-system pyrolysis is the most widely accepted method to evaluate the hydrocarbon content and the petroleum generation potential of source rocks in oil and gas exploration. Traditional pyrolysis methods (e.g. Rock-Eval Basic/Bulk-Rock®) were developed to evaluate conventional petroleum systems where the source and reservoir rocks are not the same. In contrast, for self-contained source-reservoir rocks in unconventional petroleum systems, it has been demonstrated that classical pyrolysis methods underestimate the amount of retained hydrocarbons (free or sorbed). To solve this issue, the Shale Play® method was recently proposed by IFPEN (France). In this method, a new programmed ramp of temperature during both thermovaporization and pyrolysis stages is set to better quantify the amount of producible hydrocarbon. This new pyrogram provides three main parameters: a) the Sh0 peak, that corresponds to the available free and sorbed low-to-medium molecular weight aliphatic and aromatic hydrocarbons (20); b) the Sh1, that corresponds to the medium and high-molecular weight hydrocarbons (30 aromatics and saturates); and c) the Sh2 peak, that represents the hydrocarbons released by cracking of sedimentary organic matter. In this work, we compared Rock-Eval pyrolysis results obtained using both the Basic/Bulk-Rock® and the Shale Play® methods. 12 core samples from Well #1 and Well #2 drilled in the Eagle Ford Shale Play at Gonzales and La Salle counties (Texas) respectively were investigated. We detected that part of the low to medium-molecular weight hydrocarbon (S1 peak) is lost during the Basic/Bulk-Rock® method because the thermovaporization step starts at 300°C. In contrast, the Shale Play® method showed a more accurate quantification of low to high-molecular weight thermovaporized hydrocarbons (Sh0 and Sh1 peaks) as a result of the new temperature settings, where the starting temperature is 100°C. The increase in hydrocarbon quantification per rock sample with the Shale Play® method ranges between 17.4-40.6 %, averaging 25.7 %. Based on these results, the Shale Play® method allows an improved recovery of hydrocarbons still present in source rocks. The oil crossover effect and potential productive intervals were mainly identified in core samples from the Lower Eagle Ford member. We demonstrated that this method is more adapted to identify potential producing targets in unconventional shale resource systems.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017