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D. V. Chitale1,
Peter I. Day1,
George R. Coates1
(1) Halliburton Energy Services, Houston, TX
Abstract: Detrital versus authigenic clays : NMR (Nuclear Magnetic Resonance) and petrographic investigation
Detrital and authigenic clay minerals in a clay-rich sandstone control to a large extent its pore size distribution and porosity that in turn influence the NMR response of the rock. This paper explores the relationship between the NMR T2 distribution versus the pore size distribution, clay mineralogy and clay morphology of a sandstone containing substantial proportions of detrital and authigenic clays.
Mineralogy, texture, pore size distributions, and clay morphology of a water-bearing, clay-rich sandstone were analyzed by standard petrographic techniques. NMR T2 distributions were acquired with inter-echo spacings of 0.3 and 0.6 milliseconds, from both brine-saturated and partially-desaturated samples.
NMR porosity, computed by summing the signal in the T2 spectra, matches favorably with the helium porosity, indicating that the full spectrum of pore sizes is included in the NMR porosity. The NMR T2 spectra obtained from brine saturated samples show distinct T2 classes that appear to be controlled by pore size changes that in turn are influenced by the changes in the composition and morphology of the clays. The < 5-ms components of the T2 spectra apparently correspond with < 5-µm-sized pores typically characteristic of detrital illite. T2 features in the 5-20-ms range are also likely associated with the authigenic aggregates of kaolinite and chlorite that enclose 5-30-µm-size pores. T2 spectra obtained from desaturated rocks support the match of the specific T2 components to the authigenic and detrital clays present in the shaly sandstone.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana