Log responses,  anhydrite detection and reservoir property implications in the anhydritic carbonate of the Khuff Formation

Clerke, Edward A.

Saudi Aramco - [email protected]

The Permian  Khuff Formation is a varying lithology, low porosity gas reservoir in Saudi Arabia and is a gas and oil reservoir in other areas of the Gulf. Lithological complexities, regionally and locally include: (1) reservoir can be either limestone and/or dolomite; (2) anhydrite in all of its modes (bedded and coalesced nodular, nodular, pore filling and matrix replacement) is present in both the limestone and the dolomite, often in significant volumes, as well as beds of various thicknesses (2 to 35 ft thick) of 100 percent anhydrite, extending over large areas. A visual estimate of anhydrite content may be misleading as some of the  anhydrite may be microscopically disseminated. The varying lithology of the Khuff Formation has been addressed by performing slab-veneer-homogenize-powder X-ray diffraction mineralogic studies of 600 ft of core from two wells in the Ghawar area. The goal of our bulk mineralogy study is to quantify the Khuff Formation minerals for improved reservoir modeling and log calibration. Anhydrite volumes are especially important for the modification of reservoir quality by cementation on the depositional rock fabrics and for the significant effect on the log analytical determination of porosity. Certain combinations of well logs and borehole drilling mud can be used to overcome the log analytical problems. In particular, the ECS (Elemental Capture Spectroscopy) well log yields excellent determinations of anhydrite as compared to the veneer XRD data. When a reliable indicator of anhydrite volume is available, improved diagenetic models of the reservoir can be constructed and more complete petrophysical interpretations are enabled. One of these, the Anhydrite Boost, computes the reservoir carbonate porosity rather than the total formation porosity. This approach grounds the comparison of reservoir quality in rocks with the same amount of porosity but with different amounts of anhydrite; since the porosity is a property of the carbonate and not the anhydrite cement.

Clerke, Edward A, 2004, Log responses,  anhydrite detection and reservoir property implications in the anhydritic carbonate of the Khuff Formation: Geo 2004, Bahrain, Gulf Petrolink.