Distinct Petroacoustic Signature of Burrow-Related Carbonate Reservoirs: Outcrop Analog Study, Hanifa Formation, Central Saudi Arabia

The Upper Jurassic Hanifa Formation in central Saudi Arabia consists of burrowed and non-burrowed carbonate strata. This study investigated the impact of bioturbation intensity (expressed here as burrow percentage, BP) and burrow-fills on the elastic velocity of these carbonate strata. Burrowed units of these carbonate strata were classified as UB unit with BP > 15% and LB unit with BP range of 2–15%, whereas the non-burrowed strata were classified as NB unit. Seventy-seven core plugs extracted from these three units were characterized using different laboratory techniques including petrography, petrophysical measurements of porosity, permeability, and velocity, as well as computed tomography scanning to estimate BP. The laboratory analysis results were complemented by rock-physics modeling using the differential effective medium (DEM) theory. The permeability–porosity and velocity–porosity relationships of the combined dataset show noticeable scatter with coefficient of determinations (R²) of 0.03 and 0.11, respectively. Such scatter can be explained largely by variations in BP: samples characterized by BP of > 15% (UB samples) have higher permeability (6–400 mD, where 1 mD = 9.87 × 10^–16 m²) and compressional velocity (> 5000 m/s) at any given porosity compared to LB and NB samples. These observations are explained by the stronger cementation of the host microporous matrix (in UB samples) around burrows as well as the different texture and pore structure in burrow-fills compared to those of the host matrix. Burrows are filled by coarse grains with dominant interparticle, moldic, and vuggy pores, which result in stronger stiffness compared to the weakly cemented microporous matrix in non-burrowed samples. As the BP increases, the fraction of stiff (high elastic moduli) material increases resulting in faster velocity for any given porosity. When BP is incorporated into the regression analysis, the R² for the permeability–porosity and velocity–porosity relationships increased to 0.72 and 0.75, respectively. The experimental velocity–porosity data and the pore type observations were consistent with rock-physics modeling whereby variable equivalent pore aspect ratios (EPARs) were incorporated in the DEM theory. The UB samples, which had relatively higher BP and permeability, were characterized by higher EPARs (> 0.22) compared to LB and NB samples. The outcomes of this study suggest that strata with relatively high BP (i.e., > 15%) and high permeability (i.e., > 6 mD) possess a distinct petroacoustic signature in the velocity–porosity domain, which can be identified using the EPAR-based method. Accordingly, the rock-physics scheme presented here can inform the interpretation of sonic data and the assessment of reservoir quality in burrow-related carbonate hydrocarbon reservoirs and aquifers.