The negative impact of Ophiomorpha on reservoir quality of channelized deposits in mixed carbonate siliciclastic setting: The case study of the Dam Formation, Saudi Arabia

Abstract

Several studies have documented the role of burrows with coarse-grained filling in enhancing the reservoir quality of a tight rock matrix. However, very few studies have shown the inverse case where burrows (i.e., Ophiomorpha) exert a detrimental role on the reservoir quality of permeable strata. Documenting such a scenario in the rock record is essential to fully understand the role of burrows in shaping the reservoir quality of sedimentary strata. This study integrates results from field observation and laboratory analysis to document and contribute to the understanding of the impact of Ophiomorpha on reservoir quality (porosity and permeability) of tidal channel deposits in a mixed carbonate siliciclastic setting (Dam Formation, Saudi Arabia). The results showed that the Ophiomorpha consists of shafts (diameter range from 1 cm to 24 cm) that have porous, sand-dominated fillings (porosity of 25%) with quartz (65%) and feldspars (10%); and a dolomitized wall. In the studied outcrops, Ophiomorpha penetrated the entire tidal channel at the flanks and extended laterally to the channel axis for more than 40 cm. In the computer tomography (CT) scans of the samples collected from the tidal channel deposits, Ophiomorpha exhibited a three-dimensional boxwork pattern that intersected the lamination and cross lamination of the tidal channel deposits. These observations suggest that the textural and mineralogical characteristics of Ophiomorpha walls may negatively impact the petrophysical properties of the tidal channel deposits, as the petrographic analysis showed that the wall of Ophiomorpha has very low visual porosity (<1%). The presence of the Ophiomorpha wall can act as a permeability baffle within the tidal channel sandstone due to its textural and mineralogical contrast compared to the surrounding matrix and burrow-fill. The relationship between the bulk porosity and the percentage of the Ophiomorpha wall in the samples confirms this suggestion. The measured bulk porosity on 1.5-inch plugs showed a strong negative correlation with the volume percentage of Ophiomorpha wall in the sample (Coefficient of determination [R2] of 0.79). The presence of the wall in the samples also influences the measured bulk permeability in three different ways. Firstly, the measured bulk permeability is high (>2800 mD) when the volume percentage of the wall in the samples is low (<5%). Secondly, the measured bulk permeability is moderate (∼100 mD) when the volume percentage of the wall is high (>40%), but the wall does not completely block the permeability pathways. Thirdly, the measured bulk permeability is low (≤0.1 mD) when the volume percentage of the wall in the samples is high (∼40%), and the wall acts as a permeability barrier in the samples and blocks their permeability pathways. These results suggest that due to its textural and mineralogical characteristics, the presence of Ophiomorpha may modify permeability anisotropy of the tidal channel by modifying the relationship between vertical and horizontal permeability. They also provide insight into the potential negative impact of bioturbation in similar biogenically modified reservoirs, where Ophiomorpha is the dominant element introducing heterogeneity in an otherwise isotropic fabric.