Identification of Low Resistivity Pay Through Mud Gas and LWD NMR – A Case Study in the Guantao Formation, Bohai Basin
Authors: Shi Xinlei , Zheng Yang , Gao Qiangyong , Yang Wangwang , Li Huanran , Saikat Das , Wei Gang , Holger Thern , Rex Sy 
Published: February 2024

Currently, the proven reserve of low resistivity pays (LRP) exceed 100 million cubic meters in the Guantao Formation, Bohai Basin. These LRP reservoirs are a crucial component in increasing both reserves and production in this oilfield. In general, LRP can be defined as reservoirs with a resistivity ratio of less than 2 between oil-bearing and adjacent water-bearing intervals. Production and testing in the Guantao Formation have confirmed that the resistivity of the oil-bearing intervals is even lower than that of the neighboring water-bearing intervals. As a result, identification of LRP zones can be quite challenging, particularly in development wells where spontaneous potential and micro-resistivity logs are not available, because operators depend solely on logging while drilling (LWD) for achieving complex well trajectories. Furthermore, due to the prevalence of highly inclined and horizontal wells in offshore oilfields, the use of radioactive sources in logging, namely density and neutron data acquisition, poses a significant risk and hence is not used. As a result, special workflows were developed at different phases to identify LRP reservoirs in the Bohai Basin without density and neutron data.

In the first phase, prior to 2015, the LWD services for development wells primarily relied on gamma ray (GR) and resistivity curves. But this limited dataset was unable to reliably and accurately identify LRP. As a result, mud gas analysis was used to support the interpretation. The traditional Pixler Plot and Triangle Plate methods were used in this area with some success. However, this technique required a large number of sample wells, the interpretation process was cumbersome and could only achieve a qualitative identification of LRP. Additionally, the success rate of identifying LRP reservoirs with this method was low. To address this issue, we proposed a new mud gas analysis method in this paper for identifying LRP based on the Hydrocarbon Index and Hydrocarbon Abundance. This approach is based on the differences between light and heavy hydrocarbon components in oil and water reservoirs and is capable of evaluating LRP reservoirs quantitatively. Applying this methodology in the example wells, we have found that the Hydrocarbon Abundance method is relatively more effective, with an identification compliance rate of 89%.

In the second phase, after 2015, LWD nuclear magnetic resonance (NMR) logging was introduced in the Bohai Basin. Initially, the main purpose was to address the issue of measuring porosity when density-neutron logging was not available because of avoiding radioactive sources. However, upon analyzing the T2 spectra of the LWD NMR, it was discovered that light hydrocarbon components exhibited a trailing effect (i.e., a late T2 feature) in the spectrum. This noticeable difference in the response between oil and water helped to successfully identify the LRP zones. This phenomenon was also confirmed through core NMR experiments.

The combination of the Hydrocarbon Abundance method of the mud gas analysis with LWD NMR logging has successfully addressed the challenge of identifying LRP in these oilfields. Successful application of this technique has resulted in the discovery of an additional 12 million cubic meters of oil reserves in the Guantao Formation of the Bohai Basin. The interpretation method of LRPs in the development phase drilling campaign has achieved a compliance rate of over 95%, in more than 44 high-yield horizontal wells drilled in the Guantao formation, resulting in a cumulative oil production of 750,000 metric cubic meters. This has ensured the smooth implementation of the Oilfield Development Program (ODP) program.