Oxygen and hydrogen isotopic composition of diagenetic clay minerals in sandstones: A review of the data and controls

Analyses of O and H isotopes (δ¹⁸O and δ²H) in diagenetic clay minerals of sandstones have been used increasingly to decipher primarily: (i) the palaeoclimatic conditions that prevailed during near-surface diagenesis (eo- and telodiagenesis), (ii) precipitation temperature of the clay minerals, and (iii) the isotopic composition, origin and geochemical evolution of formation waters. However, achieving these goals is fraught with a number of uncertainties, including the level of accuracy of the temperature-dependent, isotopic fractionation equations between clay minerals and water at temperatures encountered during diagenesis, degree of isotopic resetting subsequent to crystallization, and difficulties in obtaining pure, monophase clay minerals. In this paper, we address these potential uses and uncertainties based on isotopic data on clay minerals (mainly kaolin, illite, chlorite and mixed-layer types) in sandstones compiled from the literature, and decipher the links between the isotopic evolution and equilibrium state between clay minerals and formation waters during basin evolution. There is evidence indicating that, once formed, diagenetic clay minerals may preserve their original isotopic composition, unless subjected to dissolution-reprecipitation reactions (i.e. transformation into other clay minerals and recrystallization). Each group of diagenetic clay minerals has relatively wide O and H isotopic values both on global and basinal scales, implying wide variations of crystallization temperatures and/or in the isotopic composition of fluids involved. Kaolin has δ¹⁸O values that range from +5‰ to +28.5‰ (75% between +12.5‰ and +18.5‰) and δ²H (δD) values from -140‰ to -30‰ (50% between -70‰ and -40‰). The illitic clay minerals have δ¹⁸O values that range from +50‰ to +26‰ (75% between +11.0‰ and +17.5‰) and δ²H values from -110‰ to -30‰ (65% between -70‰ and -45‰). The chloritic clay minerals have δ¹⁸O values that range between +0.0‰ and +20.0‰ (75% between +7‰ and +15‰) and δ²H values between -110‰ and -10‰ (40% between -80‰ and -60‰). Gulf Coast sandstones have clay δ¹⁸O values that decrease and formation water δ¹⁸O values that increase with depth, approximating to the maintenance of isotopic equilibrium during burial. In contrast, North Sea basin sandstones have widely variable clay δ¹⁸O values despite having formation water δ¹⁸O values that, like the Gulf Coast basin, increase with depth. The variability of North Sea clay δ¹⁸O values is possibly related to (i) localized meteoric influx, inducing clay growth in exotic but transient water and (ii) initial equilibrium between water and clay that grew at moderate depth but failed to maintain equilibrium with the ambient water during continued burial. North Sea basin sandstones and waters have widely variable clay δ²H values and do not show any simple patterns. Extremely low clay δ²H values may be the result of localized petroleum-clay interactions.