where I log denotes the total natural gamma ray/potassium/thorium reading in the zone of interest, A minute shows the average natural gamma ray/potassium/thorium response of the clean (clay free) zone, A maximum indicates the average natural gamma ray/potassium/thorium response of the pure clay zone .
There are studies where in actuality the Th/You ratio was utilized to determine the depositional environment [39, 40]
However, some variations of the above-mentioned parameters can induce large effects on the log responses, so that the linear equation, called gamma ray index (IA in Table 1 when A is the gamma ray log readings), gives an overestimate of the clay contents in the reservoirs. Thus, the linear gamma ray index should be modified using one of the empirically derived non-linear calibration equations such as those introduced by Larinov , Clavier , Steiber , Dewan , or gleeden Bhuyan and Passey (Table 1). These transforms are all in the non-linear form except for that of Bhuyan and Passey , which is developed based on the assumption that the estimation of weight percent clay can be modified by multiplying the gamma ray/potassium/thorium index by an empirical correction factor, i.e., C. This factor is basically determined from the weight percent clay content of average shale adjacent to the zone of interest , and commonly ranges from 50 to 70 [10, 13, 14]. Accordingly, the rock can be differentiated as clean, if the calculated clay content is less than 10%; shaly sand if it ranged from 10 to 33% and if it is more than 33%, it is considered to be shale .
The newest gamma ray activity regarding development isn’t exclusively relevant on the clay minerals. Ergo, whilst the sheer gamma ray logs are often the best evidence of clay posts, they should be used with caution [ten, 11]. New problems in the estimation of clay content regarding wireline logs shall be smaller, if they’re calibrated resistant to the mineralogy investigation obtained from cores. Numerous degree lead an excellent correlations within laboratory-derived clay content material in addition to beliefs determined on altered pure gamma beam diary and its spectral areas data since others did not assistance this particular fact. For example, Heslop debated the altered absolute gamma beam log render a good correlations with the X-beam diffraction studies. Johnson and you will Linke , according to the cation change strength analysis within the Mackensie delta town, figured the fresh new changed natural gamma beam approach really does supply the reputable clay articles. Instead of, Causey discovered that the altered pure gamma ray diary try by far the most real clay sign within his investigation, although it performed overestimate the brand new clay contents in comparison with the fresh new lab research. Jurado affirmed that altered pure gamma beam and its spectral components investigation are useful and you can energetic devices throughout the character/quantification of clay minerals, because of their higher and you will lowest readings regarding the shale and you may brush zones, correspondingly. Almost every other some research has already been achieved by using the integration away from new research, together with journal-derived clay nutrient viewpoints from the subsurface formations [18, 29,30,32,33,34].
The latest thorium in order to uranium (Th/U) ratio may differ to the sedimentary processes and you can affairs together with towards depositional ecosystem; thus, it can be used to distinguish the environment and processes
The potassium and thorium concentrations are not only linked to the clay content, but also to the changes in the weathering regime of sediments in the source lands [35,36,37]. The Th/U ratios in the sedimentary rocks range from less than 0.02 to more than 21 . This ratio in the continental, oxidizing environment, weathered soils are above 7, while the most marine deposits, gray and green shales, and graywackes have ratios that are > 2 and < 7. However, in marine black shales and phosphates, it reaches below 2 . Schlumberger has developed several cross-plots to estimate the clay mineralogy in the subsurface formations in the absence of laboratory data. Many authors used them to recognize different clay minerals types [18, 20, 42, 43].
