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Roland Gaschnitz1, Bernd M. Krooss1, Carsten Buker2, Ralf Littke2
(1) Aachen University of Technology (RWTH), Aachen, Germany
(2) Institute of Petroleum and Coal, Aachen University of Technology

Abstract: Migration and Accumulation of Gas in OM rich Strata during Basin Evolution: A Model of dynamic Sorption

Gas adsorption on organic matter can be a crucial factor affecting both gas migration through and gas storage in organic matter rich strata. The most remarkable examples are gas accumulations in coal bearing basins (coal bed methane, CBM). Gas adsorption also appears to be relevant process in TOC-rich shales (e.g. Antrim shale, Michigan Basin). It is predominantly controlled by pressure and temperature both of which increase with depth and result in a maximum adsorption capacity between 1500 and 2000 m. Due to pressure and temperature variation over time, the gas adsorption capacity of a given strata changes during basin evolution. Once the pressure and temperature dependence of the gas adsorption capacity of OM has been established it can be combined with P/T data from numerical basin simulation to balance adsorption and desorption processes within a sedimentary column over time. This dynamic sorption model is applicable beyond coal bearing strata for dispersed OM. It may be used as an add-on to numerical basin simulation and offers new means of interpretation for the enigma of "old" gas in "young" reservoirs. Results from the Ruhr Basin (NW Germany) indicate a temporary buffering effect due to adsorption in the Upper Westfalian strata: Gas generated in the late Carboniferous was predominantly stored by adsorption during subsequent basin inversion. Phases of re-burial that occurred - probably during Triassic/Mid-Jurassic and - in Upper Cretaceous times remobilized adsorbed gas due to rising temperatures within the strata. As a result a free gas phase accumulated within and below the Mesozoic sediments. Such accumulations, though volumetrically relatively small, are well documented for the southern Ruhr Basin. However, producing gas reservoirs further north may also be explained by this concept.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana