Evaluating the Effects of Post-Depositional Alteration of Carbonates on δ13Ccarb and δ18Ocarb in a Mud-Rich Depositional Environment: A Case Study from the Midland Basin, Texas

Stable isotopes of carbon and oxygen in marine carbonates are commonly utilized as indicators of past climate and are thought to record primary sea water conditions. However, these minerals can be altered following deposition and burial, overprinting the primary signal to reflect subsurface conditions. Processes proposed to influence the geochemistry of carbonates at all stages of deposition and burial include bacterial sulfate reduction, interactions with meteoric water, and interactions with brines. These processes impact carbonate geochemistry in different ways, though in some cases, the processes are recorded through similar trends in carbon and oxygen stable isotopes.

This study analyzed carbonate samples from the mud-rich Wolfcamp D shale unit in the Midland Basin to evaluate the impact of post-depositional alteration on δ¹³C_carb and δ¹⁸O_carb values in a mud-rich depositional environment. Here we show the importance of using multiple indicators to fully evaluate the impact of post-depositional alteration on the isotopic composition of carbonates in mud-rich environments. XRD and XRF on bulk samples, trace elements concentrations from electron microprobe analysis, and δ³⁴Sof both bulk and phase-specific sulfur minerals have been integrated to construct a diagenetic timeline for this unit.

These indicators identified three main phases of diagenesis and dolomitization acting on the carbonates: early-burial organogenic diagenesis, deep-burial diagenesis, and brine-induced dolomitization. The extent of diagenesis was primarily driven by fluctuations in benthic redox chemistry which controlled sulfate reduction and the preservation of organic matter. The impact of each successive process was limited by the extent of the previous process. The results of this study were incorporated into a conceptual model which can likely be applied as a framework for other studies in mud-rich depositional environments.