Petrophysical implications of stromatolite morphologies in the Dam Formation, Saudi Arabia

This study examines how stromatolite morphologies influence the porosity and permeability of carbonate strata, focusing on columnar (CF) and laterally linked (LL) forms. By emphasizing the critical role of morphology in shaping porosity, permeability, and pore system architecture, the research integrates field observations with laboratory analyses of 40 core plug samples collected from the Miocene-aged Dam Formation in Saudi Arabia. These analyses include petrographic and Nuclear Magnetic Resonance (NMR) techniques to assess rock texture, laminae characteristics, pore types, and pore size distribution. Porosity and permeability measurements were analyzed in conjunction with detailed rock characterization derived from both field and laboratory data. The findings reveal that both CF and LL stromatolite forms can be classified as coarse-grained stromatolites, comparable to examples from both modern and ancient settings. Both forms exhibit alternating dense, micrite-rich laminae and highly porous, grain-rich laminae. The grain-rich laminae are primarily composed of ooids, peloids, skeletal grains, and quartz, with well-preserved interparticle, intraparticle, and moldic porosity. In contrast, the micrite-rich laminae are characterized by clotted micrite, clotted peloids, and sinuous biofilms preserved on lamina tops. Fenestral and vuggy porosity are present in both lamina types. Despite having relatively narrow porosity ranges—33.93%–48.04% for CF stromatolites and 32.28%–54.90% for LL stromatolites—both forms exhibit wide permeability ranges. Permeability in CF stromatolites ranges from 1.38 mD to 1900.50 mD, whereas LL stromatolites range from 13.31 mD to 2017.78 mD. Notably, LL stromatolites display less variable permeability compared to the highly variable CF forms. Although petrographic and NMR analyses provided valuable insights into the factors influencing permeability, the results demonstrate that these techniques alone cannot fully explain the observed permeability variability especially in the LL form. The findings suggest that additional factors, such as pore connectivity and laminae orientation, might play a significant role in controlling permeability. Proposed scenarios of laminae configuration in the samples suggest that these configurations may be the most critical factor influencing the measured permeability in stromatolites. This research has significant implications for reservoir characterization, providing a deeper understanding of fluid flow behavior in carbonate systems containing stromatolites.