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Oomolds in a Marine Realm? A Case Study From the Permian Basin’s Happy Spraberry Field


Oomolds in a Marine Realm? A Case Study from the Permian Basin’s Happy Spraberry Field

Due to the complex nature and importance of porosity in carbonate systems, it has been extensively studied and characterized. Many porosity types and textures have been strongly associated with certain chemical and physical conditions that they became cliché indicators of those conditions. Such associations exist for moldic and oomoldic porosity in carbonate rocks. The selective dissolution of metastable (Aragonitic or HMC) allochems has been all but accepted to indicate diagenesis by fresh waters. In this study, oomoldic porosity in the Permian Happy Spraberry Field reservoir is closely examined, in an effort to decipher the conditions that led to its creation. It is suggested that oomolds were created in a marine phreatic to shallow burial environment, with no influence whatsoever of meteoric/fresh waters.

In order to accurately characterize the conditions that led to the creation of oomolds, cement types were characterized and ranked using optical microscopy and SEM, bulk rock minerology by XRD, δ18O and δ13C isotope analysis, and trace element analysis by laser ablation on select samples that represent the oomoldic-rich reservoir unit of the Happy Spraberry Field Reservoir. Cements identified were mostly equant blocky LMC cements filling in primary pore spaces between ooid and bioclast molds, and some bladed isopachous LMC cements. These cements have been interpreted to be precipitated as early initial-stage cementation, and synchronous to the dissolution of the aragonitic ooids. The prominent equant LMC cement texture mimics textures associated with meteoric water diagenesis, however, further analysis proves otherwise. The LMC cements have δ18O (VPDB) ratios that range between -2‰ and -3.5‰, and δ13C (VPDB) ratios that range between 4‰ and 5‰. These values conform with typical low latitude Permian marine carbonate values, and further signify diagenesis within a marine water realm. Trace element analysis of those early cements show slightly elevated Sr content, and low Fe and Mn, which is typical of marine water chemistry. Although oomolds and equant blocky cement textures are typically associated with meteoric water diagenesis, it is believed that marine waters can mimic and produce similar, if not identical textures. To explain this process, we suggest, as a working hypothesis, that the dissolution of aragonite ooids and precipitation of early cements may have occurred below the aragonite compensation depth (ACD).