Influence of anhydritisation on the reservoir quality of the Butmah Formation in north-western Iraq

Carbonates and evaporites in the Butmah Formation (Lower Jurassic) were deposited in restricted marine (inner ramp) environments in north-western Iraq. The carbonates are composed of ooidal, peloidal and fossiliferous limestone in shoal and lagoon facies, while syndepositional dolomite with early anhydrite cement is found in tidal flat facies. Study of anhydrite intervals shows different structures represented by bedded, massive, chicken-wire and nodular structures which consist petrographically of fibrous, equant, felted, spare crystal, needle and lath shape textures. Two phases of anhydrite cementation were recognised in this study: Phase I is represented by syndepositional anhydrite cement that affected the reservoir quality of the tidal flat facies, and Phase II as late anhydrite cement that affected the reservoir quality of the lagoon and shoal facies. Extensive petrophysical measurements were carried out and have shown that differences in lithology, anhydrite cement type, and the degree of anhydritisation control the reservoir quality of the Butmah Formation. The Butmah Formation was divided into three rock types according to differences in their petrophysical properties and the anhydrite cement fraction. Quantitative and qualitative observations combine to indicate that post-depositional uplift may have affected the study area during the Middle-Late Jurassic and tended to dissolve some of the anhydrite cement in the tidal flat facies, creating sulfate-rich brines, and then movement of those brines toward the lagoon and shoal facies led to saturation and deposition as late anhydrite cementation (Phase II). This mechanism has affected the reservoir quality of the Butmah Formation considerably, altering its petrophysical properties.