Gas Generation at High Maturities (> Ro = 2%) in Gas Shales

Mahlstedt, Nicolaj ¹; Horsfield, Brian ¹
(1) GFZ German Research Centre for Geosciences, Potsdam, Germany.

Shale gas is most often of thermal origin. The secondary cracking of unexpelled oil at temperatures exceeding 150°C over geological time is usually viewed as the major pathway for generating the gas, corresponding to Ro = 1.5%, and kinetic models of secondary gas generation are used as part of GIP estimations to predict the extent of secondary cracking in time and space. Here we present new findings concerning a second later gas charge that is generated from some types of organic-rich shale at high maturity (R0 > 2.0%; T > 200°C). Using a large selection (~100 samples) of putative source rocks and gas shales we have been able to demonstrate that high late gas potentials are associated with heterogeneous type III to type II/III organic matter that is aromatic/phenolic; low late gas potentials are associated with homogeneous Type I to Type II organic matter. This “High Temperature Methane” goes largely unnoticed when evaluation of immature source rocks is based on routinely used open-system pyrolysis screening-methods alone. Here we use a rapid closed-system pyrolysis method, which consists of heating crushed whole rock samples in MSSV-tubes from 200°C to 2 different end temperatures (560°C; 700°C) at 2°C/min, marking the main stage of late gas generation under laboratory conditions.

During natural maturation, chain shortening reactions via β-scission related to hydrocarbon generation might lead to a concomitant enrichment of methyl-aromatics and hence late gas precursor structures within the residual organic matter. Interestingly, late methane yields of various natural maturity series samples increase up to ~40 mg/g TOC by the end of catagenesis stage, e.g. Barnett Shale, indicating that predicted late gas amounts based on immature equivalents are underestimates. This interpretation is corroborated by increasing late gas potentials of pyrolysis residues prepared under both closed- and open-system conditions. Decreasing late gas potentials observed for highly mature source rocks (R0 > 2.0%) demonstrate that dry gas generation takes place under geologic conditions during metagenesis and indirectly confirm previous [1, 2] and new (this study) compositional MSSV-kinetic calculations.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.