OUTCROP SPECTRAL GAMMA-RAY LOGGING OF DISTAL SLOPE CARBONATE DEPOSITS: DECONVOLUTION OF DETRITAL VS INSITU ORIGIN WITH IMPLICATIONS TO SUBSURFACE INTERPRETATION
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Date
2024-04-18
Authors
Umralin, Amir
Journal Title
Journal ISSN
Volume Title
Publisher
Nazarbayev University School of Mining and Geosciences
Abstract
Carboniferous carbonate platform and slope deposits outcropping in the Bolshoi Karatau, Kazakhstan have been used for decades as analogs for coeval giant oil and gas fields in the Pricaspian Basin, but their characterization by gamma-ray spectroscopy is lacking. This study aims to document the advantages of integrating spectral gamma-ray data with measured stratigraphic sections. An RS-230 gamma-ray spectrometer (assay 120 seconds) was used for high-resolution (0.4-0.5 m apart) measurements of potassium (K), uranium (U) and thorium (Th) content in carbonate slope deposits exposed in sections at Aktobe (103 m) and Akuiuk (25 m). Also, some specific areas were measured in Zhankurgan section. Below-detection-limit measurements (5 ppm for U and Th, and 1% for K) were removed from the results. Results show: (i) Predominance of CGR (computed gamma-ray; Th+K) in the lower part of the section due to the increased potassium and SGR (standard gamma-ray; U+Th+K) in the upper part, primarily due to increased uranium content in Aktobe section, respectively. (ii) Some fine- grained facies are characterized by increased potassium content, while others with increased uranium content. (iii) Isolated thorium spikes. (iv) Lack of correlation between upward- increasing thorium and potassium trend-cycle (up to 5 m thick) with facies observations (v) Increased thorium and potassium were determined in the clay facies in Zhankurgan section. Qualitative comparisons of gamma-ray data with facies/microfacies coupled with quantitative investigation of various uranium-thorium-potassium ratios suggest that: (1) Uranium readings can be increased due to the organic-rich facies, apatites from conodonts, carbonate fluorapatites within fractures and/or just carbonates that contain some uranium. (2) Potassium can be more abundant due to the clay rich facies or possbiel presence of glauconites. (3) Thorium spikes could be caused by apatite minerals or other heavy minerals that likely derived from volcanic ash-falls. It was identified that the most radioactive facies are wackestones and wacke- packstones. Interpretation of upward-increasing gamma-ray trends remains challenging. The results from Aktobe and Akuyuk sections were compared and some correlations were done with the subsurface data from the Tengiz field. In general, there is some similarities with the data from the Tengiz. These data and observations can be utilized to better interpret and understand subsurface log data in the Tengiz field.
Description
Keywords
Bolshoi Karatau, Outcrop gamma-ray logging, Carbonate slope deposits, Spectral gamma-ray logs, Type of access: Restricted
Citation
Umralin, A. (2024). Outcrop spectral gamma-ray logging of distal slope carbonate deposits: deconvolution of detrital vs insitu origin with implications to subsurface interpretation. Nazarbayev University School of Mining and Geosciences