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The Atlantic coastal plain of North Carolina contains a complex stratigraphic sequence of Mesozoic and younger rocks. Sidewall cores were taken in the shale sequences of 2 closely spaced wells for analysis of their mineralogy and interstitial waters. In both wells, interstitial waters from all shales are less saline than sea water--even in those shales that are distinctly marine. Typical samples from the first well are mostly NaCl; sulfate is commonly high--around 4,000 ppm or more, and calcium also is above its seawater concentration. The waters in the adjacent sands, which are calculated from logs and assumed to be NaCl, have salinities that range from 30,000 to 40,000 ppm. There is no regular relation between salinities in shales versus the associated sands.
In the second well, interstitial waters in corresponding shales are of equal or lower salinities, in some only half as saline as in the first well, and contain more nearly pure NaCl. However, the salinity in the adjacent sands is always greater than 40,000 ppm and increases with depth to over 100,000 ppm. Thus, the correlative shale and sand beds in these 2 wells contain very different interstitial waters. There is no obvious geologic explanation for the different diagenetic history. The mineralogy of all shales is similar, with kaolinite, montmorillonite, and quartz predominant, illite always present, and calcite sparingly present. Neither salt nor gypsum was present in the second well, although many samples in the first well are nearly saturated with gypsum. Temperatures are not esp cially high, and there is no evidence for extensive mineral diagenesis.
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