Controls of geomechanical attributes on the development of stylolites in carbonate hydrocarbon reservoirs; case study from the South Pars gas reservoirs

Carbonate sequences are strongly affected by chemical compaction during their burial diagenesis. In the Persian Gulf, mechanically and chemically-compacted Permian–Triassic reservoirs host the World’s largest gas reservoir. Statistical analysis of sedimentological, geomechanical and petrophysical parameters of stylolite-bearing rock samples from drilled cores has been used to unravel the controls of geo-mechanical properties on the formation and distribution of stylolites and solution seams in these rocks. The textural and mineralogical composition of samples has been defined based on petrographic studies. Geomechanical tests (i.e., Schmidt hammer, Brazilian, Uniaxial compressional and Triaxial tests) of drill core and plug samples have been integrated with petrophysical logs (i.e., sonic, density, and gamma-ray) and, accordingly, seven geomechanical units (GMUs) have been defined. For the morphological analysis of stylolites, their amplitude, wavelength, insoluble residue thickness, length of insoluble materials along the stylolite, density, spacing, intersection type and connectivity have been measured. These data allow defining the statistical relationships of stylolite morphology with the textural-mineralogical classes and their GMUs. The results revealed that there is a negative correlation between rock strength, stylolite frequency and amplitude. The GMUs with lower rock strength and stylolite number (and higher porosity) show higher stylolite amplitude. Stylolites have larger wavelengths in limestone and limy dolomites and, textually, in mudstone/ wackestone and packstone facies. The thickness of insoluble materials shows a power-law distribution, with a weak correlation with stylolite amplitudes. There is a negative relationship between stylolite intensity and matrix porosity, which is related to the dissolution of host rock materials along the stylolite and then precipitation in adjacent pore spaces. Consequently, deciphering these relationships will help to model the stylolite distribution at well- to field scales, and to make more reliable predictions of Permian–Triassic reservoirs in the Persian Gulf and in other carbonate sequences all around the World.

کلیدواژه ها:

Geomechanical units ، Stylolite’s morphology ، Triaxial compression ، Sealing capacity ، Permian–Triassic carbonate rocks