Investigating the source of acoustic anisotropy in the Asmari Formation, a fractured carbonate reservoir, using a well in the Southwest Iran

Abstract

Acoustic wave anisotropy in fractured carbonate reservoirs remains a key challenge for accurate reservoir evaluation and well placement. The Asmari Formation, one of Iran's most productive reservoirs, is strongly affected by complex fracture networks and heterogeneity, significantly distorting its acoustic log responses. This study applies an integrated workflow combining facies analysis, borehole image interpretation, borehole geometry assessment, and advanced dipole sonic processing across five reservoir zones in a well in a Southwest Iranian oilfield. Geological controls were constrained through petrography and facies descriptions, while textural and structural features, fracture sets, and in-situ stresses were identified from borehole image logs. Dipole sonic data were processed to evaluate shear-wave splitting, azimuthal velocity variations, and Stoneley wave reflection coefficients, with fracture density used to assess connectivity and transmissivity. Results show that anisotropy is predominantly governed by fracture intensity and connectivity rather than facies variability. Zone 1 records the strongest anisotropy, while Zone 3 remains nearly isotropic. In contrast, Zone 4 demonstrates poor fracture connectivity reduces anisotropy despite higher fracture counts. Borehole effects in Zone 5 generate apparent anisotropy unrelated to the actual reservoir properties. Overall, anisotropy decreases systematically from Zone 1 to Zone 3, with connectivity and borehole artifacts as decisive modifiers. The above workflow highlights the necessity of integrating geological and geophysical datasets to separate genuine reservoir anisotropy from borehole-related effects, enabling more reliable prediction of fracture permeability and improved reservoir development strategies.

Keywords: Acoustic anisotropy; Asmari Formation; Borehole image logs; Dipole sonic logging; Fractured carbonate reservoirs.

Fig. 1

(a) Structural map of the Zagros fold–thrust belt in southwest Iran, showing tectonic subdivisions, major faults, and anticline axes, modified after. (b) Stratigraphic column illustrating the Asmari Formation within the Cenozoic stratigraphic section of the Zagros Basin.

Fig. 2

(a) Rock variations within the carbonate system represented by different facies across the study area. (b) Associated facies identified within the main facies categories, including proximal to middle outer ramp, distal mid-ramp, middle ramp, proximal middle ramp/distal ramp, inner ramp, distal inner ramp, open lagoon, protected to restricted lagoon, shoal, exposure, and crystalline carbonate throughout the reservoir. (c) Key characteristics of each facies across the study area^.

Fig. 3

Workflow for analysing acoustic anisotropy in a fractured carbonate reservoir in southwest Iran, integrating geological, petrophysical, and wellbore information with DSI-derived acoustic attributes.

Fig. 4

The Schmidt stereonet (upper hemisphere) displays dip and azimuth measurements of various reservoir features within this carbonate field. The spatial orientations of key structural and textural elements are depicted on separate stereonets, including: (a) bedding orientation, (b) lithology boundary orientation, (c) crossbedding orientation, and (d) stylolite orientation.

Fig. 5

Fractures contributed to the heterogeneity of this carbonate reservoir. Determination of open fractures in reservoir intervals by interpreting image logs.

Fig. 6

Spatial orientation analysis of fractures in various zones, illustrating dip and azimuth to determine fracture intensity, anisotropy, and stress orientation within the reservoir.

Fig. 7

(a) Breakouts and deformation of borehole shapes in the reservoir to detect minimum and maximum horizontal stresses. (b) The stereonet illustrates the trend of breakouts, representing the minimum horizontal stress at 33 degrees northeast.

Fig. 8

Integrated zonation (Z-1 to Z-5) showing lithology, color-coded facies (using the same color scheme as in Fig. 2), fast-shear azimuth, porosity, permeability, and fracture dip. Zones Z-1 and Z-4 comprise fractured facies alternations, Z-2 a fractured interval with moderate to weak reservoir quality, Z-3 a non-fractured homogeneous facies (crystalline carbonate) interval with good reservoir quality, and Z-5 a non-fractured interval with weak reservoir quality where any anisotropy is not attributed to the formation fabric.

Fig. 9

(a) FD versus depth across five structural zones of the Asmari Formation, expressed as the number of fractures per meter of borehole. (b) Stoneley-wave RC versus depth for the same zones, representing fracture connectivity and fluid transmissivity.

Fig. 10

(a) Mean FD (1/m) for five structural zones of the Asmari Formation, showing variations in fracture abundance. (b) Mean Stoneley-wave RC for the same zones, indicating relative fracture connectivity and fluid transmissivity.

Fig. 11

Integrated analysis of Zone 1 (2878–2955 m) in the Asmari Formation. The image log shows a high density of steeply south-dipping conductive fractures, while dolomite–anhydrite variations introduce only minor impedance contrasts. Dipole sonic data display clear separation between fast and slow shear slowness, with the fast-shear azimuth aligned with the dominant fracture strike and SHmax direction. The 3D borehole profile reveals slight irregularities, indicating that anisotropy in this interval is interpreted as weak to moderate, with fractures as the primary contributor, although local borehole influence is possible.

Fig. 12

Integrated analysis of Zone 2 (2954–3060 m) in the Asmari Formation. The borehole profile is close to gauge, with only minor washouts, and the lithology is dominated by dolomite and calcite, with thin anhydrite interbeds. Image logs show clusters of steeply dipping conductive fractures aligned with the regional stress regime, and dipole sonic data reveal clear fast–slow shear separation with fast-shear azimuths matching the dominant fracture strike. These observations indicate that anisotropy in this zone is primarily fracture-controlled.

Fig. 13

Integrated analysis of Zone 3 (3047–3127 m) in the Asmari Formation. The borehole profile is regular with only minor washouts, and the dynamic image is dominated by bedding and lithological boundaries rather than conductive fractures. Stereonet plots show scattered, low-intensity structures with no dominant fracture set. Cross-dipole sonic data display modest and laterally uniform fast–slow shear separation, and rose diagrams indicate only weak clustering of fast-shear azimuths. Overall, this interval is best described as anisotropy-poor, with weak layering-related transverse isotropy and negligible fracture-controlled anisotropy.

Fig. 14

Integrated analysis of Zone 4 (3133–3248 m) in the Asmari Formation. The borehole profile is smooth and near-gauge, and image logs indicate a moderate number of steeply dipping conductive fractures, grouped in three depth intervals (13, 22, and 25 dips). Stereonet plots indicate E–W to SSW fracture orientations consistent with the regional stress regime, and dipole sonic data display moderate fast–slow shear separation with fast-shear azimuths aligned with fracture strike, pointing to structural control on anisotropy. Persistently low Stoneley reflection coefficients, however, suggest poor fracture transmissivity and a relatively muted anisotropic response.

Fig. 15

Integrated analysis of Zone 5 (3229–3307 m) in the Asmari Formation. The borehole profile exhibits clear enlargements and irregularities toward the base, consistent with breakouts and wellbore instability observed in the dynamic image. Only a few conductive fractures are identified, and stereonet plots indicate sparse, scattered orientations compared with the fracture-rich zones. Cross-dipole sonic data nonetheless display clear fast–slow shear separation, with fast-shear azimuths broadly aligned with SHmax. Overall, the anisotropic response in Zone 5 is interpreted as being primarily controlled by borehole geometry artefacts rather than by the intrinsic formation fabric or fracture system.