Hydrogen (H) Isotope Composition of Type II Kerogen Extracted by Pyrolysis-GC-MS-IRMS: Terrestrial Shale deposits as Martian Analogs

Friday, 19 December 2014
Richard Socki1, Denet Pernia2, Kadry K. Bissada2, Joseph Anthony Curiale3, Michael Evans4, Qi Fu2 and Paul B Niles4, (1)Jacobs Technology, NASA Johnson Space Center, Houston, TX, United States, (2)University of Houston, Houston, TX, United States, (3)Chevron Corporation Houston, Houston, TX, United States, (4)NASA Johnson SFC, Houston, TX, United States
Described here is a technique for H isotope analysis of organic compounds pyrolyzed from kerogens isolated from gas- and liquid-rich shales. Application of this technique will progress the understanding of the use of H isotopes not only in potential kerogen occurrences on Mars, but also in terrestrial oil and gas resource plays. H isotope extraction and analyses were carried out utilizing a CDS 5000 Pyroprobe connected to a Thermo Trace GC interfaced with a Thermo MAT 253 IRMS. Also, a split of GC-separated products was sent to a DSQ II quadrupole MS to make qualitative and semi-quantitative compositional measurements of these products.

Kerogen samples from five different basins (type II, II-S) were dehydrated (heated to 80°C overnight under vacuum) and analyzed for their H isotope compositions by Pyrolysis-GC-MS-TC-IRMS. This technique takes pyrolysis products separated via GC and reacts them in a high temperature conversion furnace, quantitatively forming H2. Samples ranging from ~0.5 to 1.0mg in size, were pyrolyzed at 800°C for 30s. and separated on a Poraplot Q GC column.

H isotope data from all kerogen samples typically show enrichment in D from low to high molecular weight. H2O average δD = -215.2‰ (V-SMOW), ranging from -271.8‰ for the Marcellus Shale to -51.9‰ for a Polish shale. Higher molecular weight compounds like toluene (C7H8) have an average δD of -89.7‰, ranging from -156.0‰ for the Barnett Shale to -50.0‰ for the Monterey Shale. We interpret these data as representative of potential H isotope exchange between hydrocarbons and sediment pore water during basin formation. Since hydrocarbon H isotopes readily exchange with water, these data may provide some useful information on gas-water or oil-water interaction in resource plays, and further as a possible indicator of paleo-environmental conditions. Alternatively, our data may be an indication of H isotope exchange with water and/or acid during the kerogen isolation process. Either of these interpretations will prove useful when deciphering H isotope data derived from kerogen analyses. Understanding the role that these H-bearing compounds play in terrestrial shale paleo-environmental reconstruction may also prove useful as analogs for understanding the interactions of water and potential kerogen/organic compounds on the planet Mars.