Formation evaluation and rock type classification of Asmari Formation based on petrophysical- petrographic data: A case study in one of super fields in Iran southwest

Abstract

The Oligo-Miocene Asmari Formation is one of the most important hydrocarbon reservoirs in the Middle East. The oilfield under study is one of the largest oilfields in the Zagros basin with the Asmari Formation being the major reservoir rock. In this study, petrographic analyses, petrophysical data and neural network clustering techniques were used for identifying rock types in the Asmari reservoir. Facies analysis of the Asmari Formation in the study area has resulted in the definition of 1 clastic lithofacies and 14 carbonate microfacies types. Using petrophysical logs from 43 wells and their correlation with capillary pressure (P_c) curves, led to the recognition of 7 electrofacies (EF1-EF7). Microscopic evidence of Electrofacies group C1 and S1 show that the sedimentary facies of these electrofacies are most commonly found in restricted and shoal facies belts zone. Also, petrographic studies show that the sedimentary facies of C2, C3, C4, S2 and S3 were formed in the open marine, Lagoon, and Tidal flat facies belt zone of homoclinal ramp sedimentary environment during the Oligo-Miocene based on relative sea level changes respectively. The link between electrofacies and geological data indicated that both sedimentary and diagenetic processes controlled the reservoir quality of the Asmari Formation. Porosity, permeability and water saturation were used to estimate the reservoir quality of each electrofacies. EFs 1 and 2 with high porosity and permeability, low water saturation is considered as the best reservoir with regard to sedimentary textures (dolowackestone and dolograinstone) and the effect of diagenetic processes such as dolomitization processes. Vuggy, growth framework and interparticle porosities are major in EF-2, while the intercrystalline porosity is the major type in EF-1. EFs 3 and 4 show low values of porosity, permeability and high percentage of water saturations, which characterizes them as poor reservoir rocks. Finally, EF-5 is the only electrofacies in the siliciclastic parts of the Asmari reservoir, which is composed of rounded and well-sorted quartz grains that are slightly cemented. In sandstone electrofacies, electrofacies EF- 5 (S1), is the best type of sandstone reservoir rock and to move towards electrofacies EF-7 (S3), will reduce reservoir quality. In carbonate electrofacies, also, electrofacies no 1, the best type of carbonate reservoir rock can be observed and move towards electrofacie number 4, lower quality of reservoir rocks is seen.