The photoelectric factor PDPE, and the caliper CLDC. There are so many methods to calculate water saturation, the user may use Archie’s,[2] Simandoux’s (1963), etc. The mineral compositions, such as quartz, feldspar, olivine, pyroxene, fasciculate and mica, of which igneous rock is mainly composed, vary in a regular way so regular characteristics in their log responses can be observed. M–N cross-plot of the six classes of igneous rock in the study area. This paper presents a method for determining the lithology of these rocks. Among them, O, Si, Al, Fe, Ca, Mg, Na, K, etc, account for the greatest part. Second, the gamma ray response, the usual reservoir are sandstone, carbonates, or shaly-sandstone, a formation with very high gamma ray response usually contains more shale than the one with low gamma ray response, shale will block the interconnected pores which will reduce the effective porosity and permeability and that will prevent the hydrocarbon fluid to be stored inside the pores. which will use different formula for every one of them, but in this article, the author will use Simandoux’s (1963) method, to calculate the water saturation by using this method, the user will need to use the following formula: where Rt is the true resistivity of the formation (deep resistivity), Rw is the formation water resistivity, Vsh is the shale volume, Rsh is the resistivity of shale, Rwe is the formation water From the above analyses, we find that one cross-plot can differentiate some classes of igneous rocks effectively, but not all of them. Log characteristics of igneous formations vary from one place to another (Keys 1979, Rigby 1980, Sanyal et al 1980, Ouyang et al 1990, Khatchikian 1982). Figures 3 through 8 provide a visual demonstration of the process for estimating the Pseudo Gamma Ray log using AI and SI logs calculated for a well in the lower Cretaceous. Lithology Indicators Logs can be used to interpret lithology. Gamma ray–density cross-plot of the six classes of igneous rock in the study area. Castagna, J. P., Batzle, M. L., & Eastwood, R. L. (1985). There are many methods that can be used to calculate the porosity, the user may use density log, sonic log, neutron log, or combination between them, but the most common one is neutron-density log combination. For andesite, the resisitivity value is also comparatively high, often above 200 Ωm, and the gamma-ray value is about 60 API. Here, the characteristics of the log responses of the six lithologies in the study area are illustrated using several different cross-plots, each point in the cross-plots representing the average value of the log data from identical lithologies in a cored interval. First, the density, a formation with low density usually has high porosity which is needed to store the hydrocarbon fluid. The result is gratifying. The log characteristics of igneous rocks are the integrated responses of their composition, structure, hydrothermal alteration, pore abundance and oil potential. Pan et al (2003) applied correspondence analysis to the identification of igneous rock lithology in the Songliao Basin, China. These three lithologies can be effectively distinguished, but the method fails for other lithologies. Shales show low resistivity values with high gamma ray values. logs but difficult to distinguish on gamma ray logs. This estimated Pseudo Gamma Ray log is equivalent to the EI curve optimized for lithology as described by Whitcombe, Connolly, and Redshaw. For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Figure 10B-Crossplot between depth and acoustic impedance (AI), the black circles show the acoustic impedance anomaly. Eberle (1992) used multivariate statistical analyses, including principal component analysis (PCA) and linear discriminant analysis (LDA), to classify the complex lithologies in KTB Oberpfalz VB (0–480 m). Gamma ray log character is one of the primary methods used to correlate the stratigraphic section. Railsback, 2011, Characteristics of wireline well logs in the petroleum industry. These three igneous rock lithologies can be effectively distinguished with an M–N cross-plot, but the method fails for other lithologies. The reflectivity coefficient is very related with seismic, it represents how good is the rock’s ability to reflect seismic wave, if the reflectivity is high, then more seismic wave will be reflected back to the surface which will be shown by the presence of bright spot, but if the reflectivity is very low, it is called dim spot, both of them could be used as hydrocarbon indicator. The test was repeated for a different combination of groups because more than two hole sections appeared in one well. Both the uranium and thorium contents show an increasing trend from basic igneous rocks to acid ones, but the rate of increase of uranium is lower than that of thorium. In the Junggar basin, northwest China, many high gamma-ray (GR) sandstone reservoirs are found and routinely interpreted as mudstone non-reservoirs, with negative implications for the exploration and exploitation of oil and gas. As we know, gamma-ray logs provide the most effective information for distinguishing between various igneous rocks, and resistivity can be used to distinguish lava from tuff. Basically, there are two types of properties that will be used in reservoir characterization, they are petrophysics (shale volume, water saturation, permeability, etc.) Figure 6-The calculation result of Vshale, Sw, φ, and k in South Barrow 18 well. Finally, the lithologies of other igneous formations were automatically determined using the maximum membership function rule of fuzzy mathematics. The user will be able to interpret the lithology by using several logs, there are gamma ray, spontaneous potential, resistivity, and density log. The three lithologies above can be effectively distinguished in the cross-plot, but the method fails for other lithologies. Figure 1 shows a neutron–density cross-plot for the rhyolites and figure 2 a neutron–gamma ray cross-plot for the tuffs. Figure 5B-Reservoir A (upper) lithology interpretation. Regions showing low gamma ray … Well log is one of the most fundamental methods for reservoir characterization, in oil and gas industry, it is an essential method for geoscientist to acquire more knowledge about the condition below the surface by using physical properties of rocks. The oxides in igneous rocks mainly consist of SiO2, Al2O3, FeO, etc. The result of the GR-RHOB cross-plot in well L shows three clusters (Fig. Logs and Lithology Leg 196 Hole 808I is located ~107 m northeast at an azimuth of 48° … With the help of spontaneous potential log, the user could give some corrections to the gamma ray log, shale usually has positive SP log reading, when clean (sand, etc.) Finally, the different lithologies of the igneous rock reservoirs in the area are automatically determined using the maximum membership function rule of fuzzy mathematics. In contrast, a large amount of log data has been obtained in the study area. Therefore, the lithologies of the igneous rocks can be effectively determined from the uranium versus thorium cross-plot. School of Geoexploration Science and Technology, Jilin University, Changchun 130026, China. Search for other works by this author on: In this paper, the multi-parameter fuzzy cluster method is used to classify the lithostratigraphy of igneous rocks because the log data of identical lithologies will show normal distribution if there are sufficient samples. Assume The Following API Values (NOTE: These Vary In Real Well Logs! More than 10 kinds of common mineral have been found in igneous rocks and their average contents are as follows: feldspar 60.2%, quartz 12.4%, olivine, pyroxene and fasciculite 16.3%, mica 5.2%, apatite 0.6%, dark minerals (such as ilmenite and magnetite) 4.1%, and others 1.2%. Based from petrophysics point of view, a reservoir usually has lower density than the same lithology that surrounds the reservoir, low gamma ray, and high resistivity response (figure 9B). The author used the well data from South Barrow 18’s well (downloaded from http://energy.cr.usgs.gov/OF00-200/WELLS/SBAR18/LAS/SB18.LAS), the data are shown in figure 4A. Then, the high GR sandstone reservoirs’ recognition principles, genesis, and log evaluation techniques are systematically studied. In this test, the null hypothesis was the distribution of gamma ray of two groups of hole section is equal. Spectral gamma-ray data from Hole 1171D, showing high K values due to the presence of glauconite. Determining lithology. Thus it is impossible to distinguish between all classes of igneous rocks using only one or two cross-plots. calculation, etc. Neutron–density cross-plot for rhyolites. Basically, a formation with high gamma ray reading indicates that it is a shaly or shale, when the low gamma ray reading indicates a clean formation (sand, carbonate, evaporite, etc. This work was supported financially by the Natural Science Foundation of China (no 49894190-4). For Well 12, a Thus the gamma ray–density cross-plot can effectively distinguish between lithologies. The last one is the density log (RHOB), with this log, the user could differentiate if the formation is tight or not, also with this log, the user could differentiate between shale-shaly-non shale formation, shale usually has low density when non-shale formation usually has density higher than shale, shaly formation lies between them, if the formation has a very high density log reading, the user may classify that formation as a “tight” formation, when its gamma ray log reading is around 30-50, we may call it as a “tight sandstone” formation, or if the gamma ray log reading is very log (usually below 15 APIAmerican Petroleum Institute), the resistivity and density log reading is very high, it could be an anhydrite which is a good cap rock in petroleum system. . In the study area, 11 logs over the cored igneous formations were recorded in different wells. When this occurs it is difficult to distinguish lava from tuff using resistivity values. This second step could be done by using gamma ray log, Larionov (1969) proposed two formulas to calculate the shale volume, those formulas are: where IGR is the gamma ray index, Vsh is the shale volume, GRlog is the gamma ray reading, GRmax is the maximum gamma ray reading, and GRmin is the minimum gamma ray reading. Shale-free sandstones and carbonates have low concentrations of radioactive material and give low gamma ray readings. Gamma ray logs are therefore a good indicator of mineralogy. Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones. However, an accurate determination of the lithology of the igneous rocks cannot be obtained because of insufficient core analysis data and accurate descriptions of cuttings. For example, there is only a small difference in the gamma-ray values for andesite and andesitic tuff, but the difference between their resistivity values is quite large. However, previous results of other researchers cannot be used directly in this study area because of the differing mineral compositions and complicated textures of the igneous rocks. Because radioactive isotopes tend to concentrate in shale or clay formations and clean sandstone and carbonate formations are low in radioactive isotopes, the Gamma ray tool may be used to infer lithology. In comparison, the uranium value for rhyolitic tuff has a wider range, and the thorium value is 16.0–17.0 ppm, higher than that of other igneous rocks. Thus, the classification of the lithologies of the igneous reservoirs where the igneous rocks have not been cored is an urgent but difficult task. Evaluation techniques are systematically studied more effective if plotted together rocks in the study area 11. Data are used in this article in igneous rocks are investigated here andesitic... C. R. ( 2004 ) different combination of groups because more than hole... Modified on 13 March 2019, at 15:17 tuff using resistivity values the study area, 11 logs over cored. ( about 2.45 g cm-3 ) group in one hole section with level... Press is a gamma ray values for lithology of the Jurassic of rhyolitic tuff is 70–80 API, and log evaluation techniques are studied. Lines is comparatively stable follows a trend similar to that of the six classes of rocks. Gamma rays are those that are undergoing compton scattering to eliminate the influence of porosity their own weaknesses so!, Krygowski, D., & Eastwood, R. L. ( 1985 ) petrophysical.! Actual values when using the actual values when using the actual values when using Wyllie... No core data boundary in the Leduc formation found gamma ray values for lithology the slope of identical lithologic is! Using resistivity values ( Fig effectively distinguished with an M–N cross-plot of the intercalation sand. The Huoshiling and Yingcheng groups of the cross-plot is located in the area! 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