Case Study: Understanding the Reservoir heterogeneity in Cretaceous Carbonates with Borehole images and NMR measurements: A Methodology from Natih A, North Oman
Authors: Al. Harthy Arafa , Kato Yusuke , Neumann Steve , Lawatia Redha H. , Shoman Ahmed M. , Shrivastva Chandramani 
Published: February 2011

The carbonate reservoirs pose various challenges in hydrocarbon exploration, development and production phases. Primarily, syn and Post-depositional processes like diagensis and bioturbation alter the original fabric of the rock to account for the variations in reservoir properties and rock qualities which impact the decision making for optimum production and development. Therefore, it is very important to understand the link between the geological controls and reservoir heterogeneity, quality and performance to better characterize, quantify and predict carbonate reservoir quality variation across the field.

An attempt has been made to apply a rigorous methodology in the studied field of North Oman for the Middle Cretaceous Natih-A carbonate reservoir to understand the reservoir heterogeneity and properties and their impact on the water saturation profile variation with the help of borehole images, NMR (Nuclear Magnetic Resonance) and core measurements. This work aims at developing an initial field-wide understanding. The addition of new wells and acquisition of more data will help to validate the concepts evolved in this study in an integrated framework.

The borehole micro-resistivity images were processed and interpreted to identify facies based primarily on variations on image textures in conjunction with conventional open hole logs and cores acquired. The conductivity data from the images were transformed into porosity distribution maps and histograms to capture the subtle porosity variations due to the heterogeneity in the formation, which were not able to be depicted by conventional open hole logs because of the resolution and azimuthal coverage. The images derived porosities and textural quantitative indicators among the three studied wells illustrated the heterogeneity variations from well-to-well and revealed a trend for diagenetic and bioturbation changes in terms of prospective flow units that were collaborated with core data.

Afterwards, the need to understand the fluids distribution (saturation profile) and their production potential led to the integration of NMR data with the image derived results. The pore size distribution, bound fluid volume, and permeability measurement from NMR helped to understand the fluid saturation profile on studied wells.

The conventional open hole logs were suggestive of variable hydrocarbon saturation profile not in an obvious relation to porosity variation and the advanced integrated analyses with borehole images and NMR helped in gaining a better control on understanding its nature. The analysis results suggested two prospective flow units in terms of bound fluid volume, pore size distribution and the textural variations. A few sub-units were also identified to address the minor heterogeneities which influence fluid flow through them.

The methodology developed here helped in understanding the hydrocarbon saturation profile behavior on field scale and provided an opportunity to be utilized for optimal exploitation of the reservoirs. To date, the well B & C have been completed with laterals crossing low saturation flow units and produces negligible volume of water.

It is recommended to refine the current study results by integrating the MICP (Mercury Injection Capillary Pressure) data when available and attempt to establish an empirical relationship with conventional logs enabling the prediction of reservoir heterogeneity on field wide scale.