Diagenetic processes and reservoir properties in the ooid grainstones of the Asmari Formation, Cheshmeh Khush Oil Field, SW Iran

Abstract

Original mineralogy of ooids affects both their diagenetic history and, therefore, the reservoir quality of ooid grainstones in the Oligo-Miocene Asmari Formation in the Cheshmeh Khush Oil Field. In the Asmari ooid grainstones, the primary fabric is dominantly affected by dolomitization and selective dissolution. Therefore, determination of primary fabric in this type of ooid grainstones is impossible. Nevertheless the concentric fabric of some ooids can be distinguished by petrographical studies. The aragonitic composition of the studied ooids seems to be responsible for selective dissolution, intra-cortical and moldic porosity development. On the other hand, presence of some un-dissolvable coats on the ooids may be the cause of primary fabric preservation during the aragonite replacement by calcite.Integration of petrographic and petrophysical data shows that these ooid grainstones with different mineralogical composition (dolostone, dolomitic limestone and limestone) have different reservoir properties. The ooid grainstones of the studied well were grouped into 6 reservoir rock types.The ineffective moldic porosity, although considerable, is associated with low permeability in both dolomitic limestone and dolomitic types. Anhydrite cementation has played a significant role in the reducing of porosity and the size of pore spaces, accordingly the permeability. Mechanical compaction has also reduced porosity and permeability of some limited samples. Abundant inter-granular porosity (up to 200μ in size) and lack of anhydrite cement in one of the oolitic intervals have resulted in average permeability up to 523mD. Such characteristics make this interval as the best reservoir rock type in the studied well. The calcite-cemented lime ooid grainstones have minor effective porosity and low permeability, so dolomitization has played a significant role in controlling the reservoir characteristics. Most rock types in which common pore spaces are around or less than 7μ in size show insignificant permeability, but rock type with pore spaces greater than 7μ show significant permeability.