Influence of stromatolites on petrophysical properties within stratigraphic contexts: A case study from the Dam Formation, eastern Saudi Arabia

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Stromatolites, ancient microbial structures, are significant in geological history and important component of carbonate systems. Despite their widespread occurrence, the relationship between stromatolite morphology, stratigraphic context, and petrophysical properties remains underexplored. Based on field observations, and laboratory analysis of 40 samples, this study provides insights into the variation of petrophysical characteristics of stromatolites from the Miocene Dam Formation in eastern Saudi Arabia within different stratigraphic contexts. The results revealed that the stromatolites above and below the sequence boundary (SB) show marked differences in porosity, permeability, and pore system architecture due to varying depositional and diagenetic conditions. Above the SB, stromatolites formed in a transgressive system tract, where fluctuating energy levels influenced sediment types. High-energy conditions in the initial flooding of the transgressive system tract trapped grain-dominated sediments, promoting interparticle and moldic porosity, resulting in higher permeability (up to 1900 mD). In contrast, low-energy conditions after the initial flooding in the transgressive system tract led to the accumulation of mud-dominated sediments, producing microporosity and reducing permeability (as low as 1.8 mD). This variability in rock texture and pore system significantly affects fluid flow dynamics, indicating that stromatolites in these settings could exhibit a wide range of reservoir qualities. Below the SB, stromatolites were deposited in restricted environments that favored lateral growth and the entrapment of mud-supported rock fabrics. Diagenesis modified pore systems of these stromatolites by cementing original porosity while generating vuggy porosity through dissolution, resulting in a more uniform pore structure with narrower permeability ranges compared to those above the SB. The unimodal NMR distribution below the boundary, compared to the bimodal distribution above, reflects this more homogeneous pore system, indicating potentially lower but more predictable reservoir quality. This study underscores the importance of stratigraphic context in influencing the petrophysical properties of stromatolites, providing crucial insights for reservoir characterization and predicting fluid flow behavior in carbonate systems.