AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Estimation of Gas Volumes in Shales Gas: Kinetic and Isotope Modeling for Methane and Ethane Generation
(1) Geochemistry, IFP Energies Nouvelles, Rueil-Malmaison, France.
(2) Worldwide Geochemistry Company, Humble, TX.
(3) Cemagref, Antony, France.
(4) Petrolia Company, Québec, QC, Canada.
The aim of the present study is to elaborate a compositional kinetic model predicting the different thermal cracking reactions occurring in source rock for methane and ethane generation. Then, these main reactions will be coupled to an isotope model to predict the 13C signature of methane and ethane in natural conditions with increasing thermal maturity from Ro at 0.5 to 4.0%.
In a first step, 3 immature and mature kerogens representative of the 3 main types of organic matter were studied on which a common compositional kinetic schema was elaborated. It demonstrated that hydrocarbons are generated through 3 successive reactions; first kerogen is decomposed into asphaltenic compounds (NSOs 1) and a first source of hydrocarbons is observed. At the same time, most of the asphaltenic compounds undergo secondary and contribute for a second source of hydrocarbons. These heavy compounds produce a solid residue and resin compounds, the cracking of which generates third source of hydrocarbons. After the oil window, the residual mature kerogen (kerogen 2) and the prechar undergo secondary reaction to generate a very dry gas with minor contribution of ethane. This late gas generation occurs in the Ro range between 1.5 and 4%.
This compositional kinetic schema was then, successfully applied to 2 immature kerogens from the Barnett Shale (US) and from the Posidonian Shale (Germany). From this kinetic schema; it is possible to calculate, the generated amount of both methane and ethane during catagenesis and metagenesis respectively. Results show that, when associated to oil generation, the amount of C1-C4 gas does not exceed 20-30 mg/g C and the gas dryness is between 50 to 70%. On contrast, the absolute yield of late methane is much higher whatever the kerogen type with values between 40 and 70 mg/g C. The corresponding gas dryness is always higher than 95% for the complete maturity stages of Ro from 1.6 to 4.0%.
In terms of 13C signature, the same range of values are observed for methane whatever the studied samples : a first range: between (-40 to - 46 ‰) during catagenesis and (-30 to - 40 ‰) during metagenesis. So far, data acquisition for ethane was done only on the Type I sample on which the range of values was observed between (-40 to - 25 ‰) during catagenesis and (-25 to - 10 ‰) during metagenesis. Data acquisition for the other samples are in progress.