New Seismic Attribute for Determination of Lithology and Brittleness
Sharma, Ritesh; Chopra, Satinder
The discrimination of fluid content and lithology in a reservoir is an important characterization that has a bearing on reservoir development and its management. For the unconventional reservoirs, such as shale gas formations, besides other favorable considerations that are expected of them, it is vital that reservoir zones are brittle. Brittle zones frac better and fracing of shale gas reservoirs is required for their production. Amongst the different physical parameters that characterize the rocks, Young's modulus (E) is a measure of their brittleness and can characterize such stiffer pockets in shales and some practitioners have demonstrated the determination of Young's modulus from seismic data by way of inversion. One limitation of such an approach is the requirement of density which as stated above is difficult to derive from seismic data, unless long offset information is available.
Considering the importance of an attribute that could yield information on the brittleness of a reservoir as well as be a good lithology indicator, we propose a new attribute, Eρ, which is the product of Young's modulus and density. This is different from the conventionally used attribute, μρ, where μ is the shear modulus. We begin by first comparing the derived μρ and Eρ curves for a well in northern Alberta and showing how the Eρ curve emphasizes the variation corresponding to lithology change more than in the μρ curve.
For implementation of this analysis on seismic data, we considered a gas-impregnated Nordegg member of the Jurassic Fernie formation of the Western Canadian Sedimentary Basin. It consists of predominantly brownish, greyish and black shales. These "shales" vary from siliceous rich cherts and dolomites to carbonate rich shale. Due to the complex geology of the reservoir in the Nordegg, differentiating the lithology and fluid content is a challenge. Thus, as the first step, simultaneous impedance inversion was run on the pre-conditioned 3-D seismic data to obtain P-impedance and S-impedance volumes which are then transformed into μρ and Eρ volumes as discussed above. We notice that not only does Eρ attribute have a higher level of detail than the μρ attribute, the sandstone presence exhibits lower Eρ values, whereas the availability of dolomitic siltstone exhibits higher values. The new attribute (Eρ) should not only be a good lithology indicator, but one which intensifies the variation in lithology as well.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013