ABSTRACT

Stylolites have been reported in basinal, reef, forereef, and backreef carbonates within the Permian Basin of West Texas and southeastern New Mexico. They are described as serrated interfaces between carbonate rock units and are composed of their insoluble residues. Samples of stylolites and adjacent host rock were examined using x-ray diffraction and analytical electron microscopy. The results revealed a very close relationship between the stylolite and its host rock. Stylolites are the markers indicating that considerable thicknesses of strata may have been dissolved away during diagenesis of the carbonate. The extent (thickness) of stylolite seams may affect the porosity and permeability of a carbonate formation. Presence of fine platelets of micas and sulfur along with the organic matter in stylolites can make them impermeable to fluid flow and seal petroleum reservoirs. The comparative study of the mineralogy of the stylolites and the acid-residues of their immediate host rocks directly points to the origin of these stylolite seams in these dolostones. The stylolites analyzed consist mainly of quartz, feldspars, pyrite, mica, native sulfur, and organic material. All these minerals were found to be the impurities in the enclosing host carbonate. It was estimated that about 100 mm of the host dolomite column must have been dissolved to generate a stylolite with a thickness of about 1.0 mm.

Introduction

Stylolites are frequently observed features in carbonate rocks in the Permian Basin. They are described as serrated interfaces between carbonate rock units and are composed of the insoluble residues of the host carbonates. Stylolites are generally considered to have formed by the "pressuresolution" mechanism1,2,3,4,5. Sometimes acidic solutions, related to H2S supply throughout the basin, can also generate such residues. Clays, quartz, feldspars, iron oxides, and pyrite have been reported to make up these residues in stylolite seams in Permian Basin carbonates. A detailed analysis of the materials in stylolites, however, has not yet been carried out. In this report, an attempt has been made to study the insoluble residues in the stylolite seams in core samples selected from back-reef dolomites of the Upper Permian Artesia Group, in the Northwestern Shelf of the Delaware Basin. Figure 1 indicates the location of the core (marked as "A") from which samples were obtained. The stylolite seams have been examined using x-ray diffraction, and analytical electron microscopy. Further, measured quantities of the host rock adjacent to the stylolites were gently dissolved in 1N Na acetate solution buffered to 5 pH with glacial acetic acid, and the resulting residues were studied to determine the mineralogical and volumetric relationship between the stylolites and the host rock. The results are presented here.

Stylolitization

The term ?stylolite' is derived from the Greek word "stvlo?" meaning ?column'6. Stylolites, generally speaking, are serrated boundaries between rock (carbonate) units, consisting of the accumulation of clays, some other minerals, and/or organic matter 7 in them.

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