By Azelin Mohamed Noor and Lim Chia Wei,
Department of Management & Humanities, Universiti Teknologi PETRONAS
Geology is the study of solid rock, the understanding of rock composition and various processes that involve rocks. For non-geologists, identification of lithology is challenging especially when rocks and its structures are not fully understood. Much of the identification is done by guessing from observing the earth's crust. Anything that lies below remains a mystery. For engineering geologists, guessing is never an option. Calculated risks are measured at every step of oil and gas exploration to locate and identify reservoir and non-reservoir zones. Determining the structure and lithology of formations are crucial before drilling can be done. Methods to identify lithology such as sonic logs, spontaneous logs and neutron logs can be used to reveal buried resources. In this article, the gamma ray logs or GR logs is explained because of its widespread use.
Identification of Lithology
Subsurface lithology is traditionally determined by GR logs. It is used to measure the natural radioactivity of formations by emitting gamma rays to rocks. Thorium, uranium and potassium are the main radioactive isotopes emitting radiation. Isotopes concentrated in clay such as shale, contains many complex minerals which are more radioactive as compared to sand. Sand contains quartz. As shale content increases, the GR log response increases because of the concentration of the radioactive materials in shale. Shale emits high gamma rays which are approximately 100gAPI (American Petroleum Institute). However, radioactive elements can barely be developed in quartz. As a result, quartz is referred to as clean sand where the readings of gamma rays are low in formations. The readings of gamma rays of clean sand are approximately 12gAPI. Typically, shale with high gamma rays indicates non-reservoir rocks. Sedimentary rocks (non-shale) such as coal, dolomite and sandstones with low gamma rays indicate reservoir rocks.
Estimation of Shale Content
GR log is one of the most useful indicators of shaliness of sand in porous reservoirs. Once the main lithologies are distinguished, gamma ray values obtained can be used to estimate the amount of shale volume of the rock. Minimum level of radiation concentration value of the formation is essential for geologists to help draw a distinction between reservoir and non-reservoir zones. The volume of shale is calculated as:
Figure 1: Formula for calculating shale content
Increased volume of shale in a formation decreases effective reservoir capacity. Non-reservoir and reservoir zones of wells may be interpreted after their shale volumes are determined. Figure 2 is an example of a well-log data obtained from using gamma rays.
Figure 2: Well-log Data obtained from Gamma Ray Log
One common method used for well evaluation or reservoir characterization is depth matching. Depth matching for well-logging is the process of correlating several types of logging data that were aligned at different depths for the purpose of well evaluation or reservoir characterization. The high reliability and high vertical resolution of gamma ray logs result in their extensive use to perform borehole drills for depth matching. The ultimate ability of GR logs to accurately match the depth from each run makes it a powerful logging tool in well-logging and it is being utilized as part of almost every tool combination. Normally, a number of logs are applied on a wireline for a run. For instance, sonic log, resistivity log and gamma ray logs are combined in one run. Every log will be assigned at different depths at different times and gamma ray logs are used as reference points for each of these logs. The recordings of individual logs are usually different at any particular depth of formations. The sonic and resistivity responses are matched against the GR logs responses. Depth matching is extremely functional in fine rocks correlation of stratigraphic unit and estimation of shale content.
Factors affecting GR tool response
Despite the simplicity and consistency of GR logs, a few factors need to be considered before running the GR logs to avoid inaccuracy. A change in mud weight, hole size, casing in the hole, cement thickness positions of devices in the hole will affect the gamma ray counts. GR logs can be badly affected if the borehole suffers from caving. Consequently, correction charts are available to compensate the effective downhole for the variables mentioned. Time constant to logging the data and logging speed are modified to reduce fluctuations of the logging data in order to provide precise outcomes on well properties and achieve optimal production.
GR logs are extremely functional for well-to-well and stratigraphic log correlation, quantifying of shale content and depth matching. For this reason, it is essential to build an understanding of the physical properties of the earth's interiors through well logging. Appropriately documented and developed results from well logging are important, without these results, validity and accuracy of well evaluation or reservoir characterization can be questioned.
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