Lithofacies Estimation by Multi-Resolution Graph-Based Clustering of Petrophysical Well Logs: Case Study of South Pars Gas Field of Iran

Abstract

Abstract South Pars gas field is the largest gas field of the world, located at the boundary between Iran and Qatar in the Persian Gulf in a distance of 100 kilometres from the southern shores of Iran. Its gas-in-place is estimated to be about 14.2 trillion square meters while the amount of its condensate-in-place might be around 18000 million barrels. South Pars gas field also has an oil layer containing about 6 billion barrels oil-in-place. Since South Pars field is a heterogeneous carbonate system, lithofacies characterization is the best solution for overcoming the problem of heterogeneity in determining the petrophysical properties of the reservoir rock, reservoir modeling and identifying producing zones, but coring which is the most robust method of lithofacies identification is very expensive, time consuming and is limited to few number of wells. Therefore this study is focused on determining the lithofacies of the understudy formations from available well logs. For this purpose multi-resolution graph-based clustering (MRGC) technique which is a dot-pattern recognition method based on non-parametric K-nearest neighbor and graph data representation is applied on sonic, density, neutron porosity and gamma ray well logs to define electrofacies similar to core-derived facies that are determined as twelve distinct ones. The cluster of the MRGC method is defined from model that has specific character associated with the group of lithofacies. Then, Kernel representative index is used to calculate the optimal number of clusters. Small facies groups are formed based on utilizing the neighboring index to determine a K-nearest neighbor attraction for each point. At last, final clusters are constructed by combining the small clusters which lead to identifying 12 facies of South Pars gas field from well logs by high accuracy. Method used in this study has obviated the need of extensive coring in the South pars field which caused saving large amounts of money and time.