CHARACTERIZING MOLECULAR STRUCTURE OF KEROGEN AND ITS HETEROGENEITIES IN MARCELLUS SHALE: IMPACTS ON HYDROCARBON GENERATION AND PRODUCTION

Unconventional shale reservoirs are producing more than 59% and 69% of the total crude oil and dry gas, respectively, and have become one of the biggest contributors to the energy sector in the United States. However, due to inaccurate estimation of gas/oil in place (GIP/OIP), low hydrocarbon (HC) recovery, and steep production decline, the efficiency of HC still remains low. The declining oil and gas prices necessitate improving “per well” recovery of HCs. Kerogen is the primary component of shale that not only generates all the HCs but also holds the most amount of it. Molecular understanding of heterogeneities in kerogen structure and its interaction with fracturing fluids can help in better estimation and recovery of hydrocarbons from individual wells. In this study, we characterized the molecular structure of kerogen isolated from the Marcellus Shale maturity series and compared it with kerogen from Duvernay and Eagle Ford Shale.

The structural parameters of kerogen were determined using ¹³C MultiCP/MAS NMR and MultiCP NMR/DD (dipolar dephasing). The average unit structural models of kerogen at different maturity stages were reconstructed using these structural parameters. The models indicate that with increasing maturity, from oil window to wet gas window, there is a gradual increase in the aromatic C%, bridgehead C%, and a gradual decrease in aliphatic C% and aliphatic carbon chain length (Cn’). However, we observe a drastic increase in the bridgehead C% for the sample lying in the dry gas window, indicating a collapse of kerogen interlayers. Comparison of kerogen structural units of Marcellus shale with Duvernay shale and Eagle Ford shale also shows that large heterogeneities can also exist within a particular kerogen type and maturity in different shales. Our data demonstrates that the current kerogen structural models based on kerogen “type” do not incorporate these structural heterogeneities, and could potentially result in inaccurate estimation of 1) HC generation potential (GIP/OIP) of shale and, 2) physico-chemical properties of shale such as its porosity, sorption capacity, elastic and fracture properties, which controls flow and recovery of HC’s.