ABSTRACT: Chlorites in Reservoir Sandstones of the Guadalupian Delaware Mountain Group

WALLING, SUZETTE D., Texas A&M University, College Station, TX

Late-stage authigenic clay minerals are pervasive in the very fine-grained, subarkosic sandstones of the Guadalupian Delaware Mountain Group, comprising up to 10% of the bulk rock. Thus, reservoir rock properties are influenced by these minerals. Samples selected from cored intervals, ranging from 600 to 2500 m, were studied using optical and electron microscopy and x-ray diffraction methods to determine the distribution and nature of occurrence of the authigenic clay minerals.

In thin sections, the clay minerals are recognizable as grain coatings, with thickness varying from a few micrometers to tens of micrometers. Details revealed by scanning electron microscopic analyses show three distinct morphologies: featureless to irregular grain coatings; well-developed interlocking platelets that line, fill, and dissect pores; and delicate "hair-like" growths that are sparse and randomly distributed. X-ray diffraction analyses of clay concentrates show chlorite to be the most abundant clay mineral, with lesser and variable amounts of mica-illite. Successive heat treatments of samples reveal varying degrees of interstratification in the chlorites as indicated by differences in both the amount of peak shift and intensification of the 1.4-mm reflection. Differences i the relative intensities of even- versus odd-ordered x-ray reflections combined with energy dispersive spectrometry also suggest variable Fe/Mg ratios among chlorites. A correlation between the morphology and the amount of interstratification has been observed, with more interstratification corresponding to the formless chlorite variety. The structural and morphological evidence suggests that the chlorites represent different stages of development, possibly evolving from a smectite component through an interstratified intermediate, to a more well-ordered form. There appears to be no systematic trends vertically or laterally in clay mineralogy.

The importance of understanding the clay mineralogy and chemistry in these sandstones is evident when considering enhanced recovery procedures. Different clay structures and chemistries may respond differently to production and stimulation techniques. The proposed chlorite diagenetic sequence suggests that drastic changes in borehole fluid chemistry may cause retrogression of chlorite to some expansive forms, which may be water sensitive or inclined to migration.

AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)