Reservoir Diagenesis and Quality Prediction of Late Cretaceous PAB Sandstone, Lower Indus Basin, Pakistan

Abstract

The recognition of intervals having promising reservoir quality in a heterogeneous reservoir from well logs is important as coring program is always not feasible as it is time-consuming and expensive. Inaccuracies can be reduced by incorporating the standardized results obtained from the well-core information for continuous and precise estimates. The goal of the study is to identify diagenetic events resulting in reduction of porosity and determining a procedure for rock type identification. In present study, inquiry regarding diagenetic controls of Pab Sandstone is carried out by the help of unified methodology, which includes thin section study, scanning electron microscopy, helium porosity and permeability, which is key hydrocarbon target in Pakistan’s Lower Indus Basin. The sandstones are quartz arenite, deduced from interpretation done through petrography and XRD analysis of core samples have shown abundance of quartz and kaolinite as the major authigenic clay mineral, cross plot analysis also showed the presence of these clay minerals. Permeability exhibited a positive correlation with increasing grain size, in the quartzose sandstones. Porosity showed a reduction with poorer sorting. Main diagenetic processes included compaction, carbonate cementation, quartz overgrowth and kaolinite. Core-derived average porosity and permeability are found to be 6.34% and 69.9 md respectively. For integration between well and core data, core-calibrated petrophysical techniques had been employed for estimation of different reservoir parameters. Characterization of porosity carried out using statistical techniques including root mean squared error and coefficient of variation. Total porosity (NDS ) has minimum root mean squared error and coefficient of variation as compared to other porosity estimates. Therefore, the computed NDS provided more accurate results of different reservoir parameters including effective porosity, water saturation, and hydrocarbon saturation compared to porosities derived from conventional petrophysics. These core-calibrated reservoir parameters are employed in cluster analysis for rock type identification defining reservoir based on quality. Rock types are described by specific log responses, which are ultimately distinguished with the help of electrofacies. The current study used cluster analysis technique for the evaluation of reservoir rock types in the identified sand units. K-means cluster analysis used to define electrofacies, which are ultimately classified into four rock types based on reservoir quality, from bad to excellent. Rock typing using cluster analysis has been done for four wells, and a correlation has been made to depict changes in electrofacies. The applied prediction technique to the studied field provides continuous rock type identification for the entire reservoir. The greater value of Young Modulus shows formation is hard rock and should respond to water frac with high injection rates. Using this methodology in defining rock type is cost-effective, requires less time in the demarcation of zones of interest, and is more accurate than manual analysis of the heterogeneous and thick Pab Formation. This assimilated work would be supportive for investigation and testing of abundant wells where core samples are not readily available. The studied approach is not only useful in the exploitation of the heterogeneous Pab Formation, but it can also be applied to other heterogeneous sandstone reservoirs elsewhere.