Basin Modeling - the Key for Unconventional Play Assessment
Schneider, Frederic J.
The objective of this paper is to demonstrate how a basin modeling study can help in the evaluation of unconventional shale gas / oil resources. We present an improved expulsion model that accounts for the evolution of retention capacity as a function of maturity.
Part of the gas / oil generated is expelled from the source rock and then migrates to conventional reservoir through carrier beds. The other part is retained by the source rock. Hydrocarbons composition and volumes generated and retained depend on the initial organic matter properties and maturity. Expulsion model in the past have been designed in order to quantify the expelled amounts. These models have to be revisited and improved in order to better quantify the retained gas/oil.
The source rock retention capacity is tightly linked to the TOC content, the organic matter type and the maturity. Hydrocarbons can be stored in source rocks by adsorption (this process concerns mainly the gas) and as free hydrocarbons inside the porosity.
The porosity in mature source rock can be of two types: an effective porosity inside the non-organic fraction of the rock, and porosity inside the organic matter itself. This second porosity is created consequently to the loss of kerogen mass resulting of the transformation of organic matter into hydrocarbons.
To assess the potential for shale gas/oil in a formation, it is so necessary to evaluate:
- The initial TOC, the organic matter distribution, type and quality;
- The formation maturity;
- The storage capacity in the non-organic porosity, the organic porosity and the adsorption;
- The generated HC quantity and quality, in order to quantify how much is retained in the source rock intervals.
Our experience proves that these properties can vary significantly laterally and vertically, and that the shale gas potential of a formation can then not be simply deduced from a local evaluation that would be extrapolated throughout the basin.
The methodology has been successfully applied in a number of formations over the world, including Silurian in Algeria, Abu Gabra formation in Sudan, Los Monos formation in Argentina, or Lower Barnett Unit in the Fort Worth Basin. Most of the formations studied present significant differences with US plays, in terms of thickness, organic matter type, present day TOC, adsorbed gas proportion, volume of gas per ton of rock.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013