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Chemistry of Brine in an Unconventional Shale Dominated Source Bed: Understanding Organic Matter-Mineral Interactions During Hydrocarbon Generation


A geochemical investigation was carried out to have an understanding of interactions between fluid, mineral, and organic material in hydrocarbon generating source beds. Brines collected from unconventional shales allow the study of fluids before any changes in composition that may occur during secondary migration to reservoir rocks or post migration processes. Brines are typically found to be rich in elements such as calcium (Ca) and strontium (Sr), while being poor in potassium (K), rubidium (Rb), and magnesium (Mg). K/Rb ratios of the Woodford brine samples were found to be in the range of 490–600. These ratios are higher than average silicate materials, including clay minerals that are abundantly present in the Woodford. These relatively higher ratios suggest that organic matter could be an influence. Rare earth element (REE) distribution patterns were characterized by heavy rare earth element (HREE) enrichment, superimposed on it middle rare earth element (MREE) enrichment and gadolinium (Ga) and cerium (Ce) positive anomalies and thulium (Tm) negative anomalies. The general trend with HREE is potentially an indication of the complexation of the rare earth elements with carboxylic anions derived from organic materials present in the hydrocarbon source shale bed. The MREE enrichment may be attributed to phosphate complexation linked to the same organic materials. The Ce positive anomalies may be reflecting dissolution of Fe-Mn oxides. The Gd anomalies have been known in many organic materials with an influence of enzyme activity. Plant enzyme activity has also been also suspected for Tm anomalies. Thus the REE in the brines have been carrying signatures of organic sources, potentially they are carrying records of events during oil generation.