Shale seismic anisotropy vs. compaction trend

Abstract:

Shales comprise more than 60% of sedimentary rocks and form natural seals above hydrocarbon reservoirs. Their sealing capacity is also used for storage of nuclear wastes. Shales are notorious for their strong elastic anisotropy, so-called, vertical transverse isotropy or VTI. This VTI anisotropy is of practical importance as it is required for correct surface seismic data interpretation, seismic to well tie and azimuth versus offset analysis. A number of competing factors are responsible for VTI anisotropy in shales, namely, (1) micro-scale elastic anisotropy of clay particles, (2) anisotropic orientation distribution function of clay particles, (3) anisotropic orientation of pores and organic matter. On the contrary, silt (non-clay mineralogy grains with size between 0.06 -0.002 mm) is known to reduce elastic anisotropy of shales. Methods developed for calculations of anisotropy in polycrystalline materials can be used to estimate elastic anisotropy of shales from orientation distribution function (ODF) of clay platelets if elastic properties of individual clay platelets are known. Unfortunately, elastic properties of individual clay platelets cannot be directly measured. Recently, elastic properties of properties of individual clay platelets with different mineralogy were calculated from first principles based on density functional theory. In this work we use these elastic properties of individual platelets of muscovite, illite-smectite and kaolinite to obtain correlations between elastic anisotropy and Legendre coefficients W₂₀₀ and W₄₀₀ of different ODFs. Comparison of the Legendre coefficients calculated for more than 800 shales from depths 0 - 6 km (www.rockphysicists.org/data) with those of compaction ODFs shows that compaction has no first order effect on elastic anisotropy. Thus, elastic anisotropy is to large extent determined by factors other than compaction processes, such as depositional environment, chemical composition of fluid, silt fraction, etc.