Abstract

In the Salado Formation, a thick evaporite sequence in the Permian Delaware Basin of southeastern New Mexico, primary detrital silicates such as feldspars, quartz and clay minerals have undergone significant diagenetic alteration. Seventy-five samples were collected from argillaceous partings in the lower Salado Formation halite facies, at the stratigraphic level of the Waste Isolation Pilot Plant (WIPP). These were compared with twenty-eight samples from cores of the Vaca Triste member of the Salado, a thin clastic unit at the top of the McNutt potash zone, and with a clay-rich sample from the lower contact of the Culebra Dolomite (in the overlying Rustler Formation). The purpose of this comparison was to evaluate the influence of brine chemistry (i.e., halite and potash facies, normal to hypersaline marine conditions) and sediment composition (clays, sandy silt, dolomitized limestone) on diagenetic processes. X-ray diffraction analyses and electron microscopy identified a clay similar to corrensite from all three stratigraphic horizons. Authigenic quartz and potassium feldspar were found in both Vaca Triste samples and in those from the WIPP. The feldspar occurred in greater abundance in the Vaca Triste samples than in those from the WIPP horizon. Fluid inclusion compositions from halite in the Vaca Triste were compared with those from the WIPP repository horizon. The solution speciation/mineral solubility code EQ3NR was used to calculate ionic activities of relevant species in WIPP and Vaca Triste brines. For these high ionic strength Salado Formation brines, the activity coefficients calculated by this code must be qualified. However, the assumed pH and the computed activities of K (super +) and Mg (super +2) correctly predict a mineralogy that is consistent with our observations of mixed-layer, Mg-rich, corrensite-like clays and microcline. EQ3NR predicts that the WIPP horizon fluid compositions are at or above saturation with respect to minerals characteristic of the halite facies: halite, aragonite, dolomite, magnesite, and huntite. Application of EQ3NR to the Vaca Triste fluid inclusions indicates saturation or supersaturation for these phases plus those typical of potash-facies mineralization (including kainite, sylvite, carnallite, syngenite, and polyhalite). We propose that the original, detrital clays reacted with magnesium-rich brines to produce a suite of Mg-rich, mixed-layer species and another brine depleted in Mg relative to K. By-products of these reactions were authigenic quartz and K-feldspar. The dissolution of plagioclase, kaolinite, and fine-grained, detrital illite probably contributed components precipitated in the authigenic phases. To summarize, a generalized reaction is suggested: brine 1 + detrital silicates --> brine 2 + or - Mg-clays + or - K-spar + or - quartz where the K:Mg ratio in brine 2 is greater than K:Mg in brine 1 .

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