Origin and role of organic matter in limestone stylolitization: petrographic and geochemical evidence

This study utilizes inorganic and organic petrography, programmed pyrolysis, carbon and oxygen isotopes, and fluid-inclusion microthermometry of the Lower Cretaceous Thamama Group limestone reservoir in the United Arab Emirates to constrain: (i) the composition, origin, and potential role of primary organic matter in stylolitization, and (ii) the origin of diagenetic kaolin, sphalerite, anhydrite, fluorite, and dolomite in the vicinity of stylolites. The organic matter along the stylolites consists predominantly of marine (alginite), with a smaller contribution from terrestrial macerals, including vitrinite and inertinite. The mean random vitrinite reflectance (Ro) ranges from 0.6% to 0.7%, suggesting early stages of the oil window. Low-molecular-weight (LMW) aqueous organic acids, generated through the thermal cracking of organic matter, may have played a role in facilitating the stylolitization process. Stylolitization was accompanied by the dissolution and cementation of host limestones, marked by abundant calcite micro-overgrowths, and blocky calcite cement. The generation of organic acids is supported by the presence of diagenetic kaolin booklets near the stylolites, along with the significant positive correlation between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> ¹³C carb and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> ¹³C org values. This study provides new insights into the mechanism of stylolite formation, highlighting the potential role of organic matter in this process.