1 Introduction
The distribution of radioactive nuclear elements in rock records, such as thorium (Th), uranium (U), and potassium (K), is linked to lithology, mineral composition, diagenesis, and groundwater activity (
Guo et al., 1996;
Chi, 2003;
Adamu et al., 2020;
Ghosal et al., 2020). Generally, sedimentary rock’s Th content decreases with grain size (
Zhang et al., 2006;
Ghosal et al., 2020). The main factors controlling Th distribution in sedimentary rocks are clay mineral adsorption on Th and the presence of Th in stable minerals. Furthermore, Th has good chemical stability and a low migration capacity. As a result, there is a linear relationship between Th content and argillaceous deposit content in sedimentary rocks (
Lima et al., 2005;
Ghosal et al., 2020). Groundwater activity is closely related to increased U content in the permeable stratum. Because of its active chemical properties and solubility in water, U is easily migrated to the deep by groundwater along main faults or fracture zones formed in the karst or tectonically active zone. Then, it is reduced to tetravalent U and precipitated under reducing conditions. As a result, the U content has a strong relationship with hydrodynamics. Furthermore, there is a linear relationship between K concentration and argillaceous deposit concentration in sedimentary rocks. In fluvial deposits, K has a higher value than Th and U. The natural gamma-ray spectrum content of fluvial deposits is generally higher than that of carbonate deposits (
Davies and Elliott, 1996;
Lima et al., 2005). Furthermore, the high Th and K contents indicate a stable and low-energy humid sedimentary environment. In contrast, a high U and K contents indicate a turbulent and high-energy sedimentary environment with a dry climate (
Ruffell and Worden, 2000;
Wang and Zhu, 2002;
Chen and Zha, 2004). According to empirical statistics, Th/U > 7 characterizes continental mudstone and bauxite, which primarily reflect continental sedimentation with complete weathering, oxidation, and leaching. When the Th/U value is between 2 and 7, it generally represents a marine sedimentary environment with a small amount of terrigenous supply. When Th/U < 2, the environment is generally marine sedimentary, with relatively deep-water and little terrigenous supply (
Liu et al., 2000;
Yang et al., 2003;
Wang, 2004). It can be concluded that the natural gamma spectrum has beneficial effects on determining the ancient sedimentary environment.
Natural gamma-ray content, sporopollenin content, and clay mineral content are all consistent (
Ehrenberg and Svana, 2001;
Koptíková et al., 2010;
Feng et al., 2016). Sporopollen content and clay mineral content, for example, can reveal climate and sea-level changes (
Chen et al., 2001;
Liu et al., 2009;
Zhang, 2017). As a result, the natural gamma-ray spectrum can serve as an indicator of paleoclimate (
Yang et al., 2003;
Ghasemi-Nejad et al., 2010). The U, Th, K, and KTH in the natural gamma-ray spectrum can reflect energy changes in the sedimentary environment and can be used to determine the ancient sedimentary environment by combining lithology associations, conventional logs, and seismic reflections (
Liu and Zhou, 2007;
Yang et al., 2010;
Wu et al., 2011).
Over the last few decades, research on the Ordovician sedimentary environment in the eastern Tarim Basin is still in its early stages (
Lin et al., 2012,
2013;
He et al., 2017). Furthermore, determining the sedimentary environment is difficult due to limitations in industry data sets and the influence of special and complicated settings in the eastern Tarim Basin. As a result, only a few investigations of the sedimentary environment were conducted. At the time, the energy of the sedimentary environment was unofficially proposed. According to previous research, there is a misunderstanding between energy and the sedimentary environment (
Lin et al., 2012,
2013;
Gao et al., 2016;
He et al., 2017). The determination of energy has always been based on grain size analysis (
Wang and Zhu, 2002;
Zhao et al., 2010;
Zhang et al., 2020). Owing to data limitations, conducting grain size analysis in the eastern Tarim Basin is challenging. However, previous studies using natural GRS logs to reveal changes in paleoclimate and sedimentary environment yielded fruitful results (
Ruffell and Worden, 2000;
Chen and Zha, 2004;
Ghasemi-Nejad et al., 2010). A corresponding investigation is conducted based on the natural GRS log to better address these issues. The goal is to systematically determine the ancient sedimentary environment and associated energy in deep-buried marine carbonates. The findings of this study will guide future hydrocarbon exploration in similar sedimentary environments. Most importantly, this research aims to investigate the ratio of Th, U, and K can be used as a proxy for the ancient sedimentary environment and associated energy in deep-buried carbonate reservoirs for the first time in borehole wireline logs.
2 Geological settings
The Tarim Basin, a superimposed or multicycle basin located in north-west China, has undergone long-term tectonic evolution and has hosted numerous petroleum reserves (Fig.1(a)) (
He et al., 2005;
Lin et al., 2012, 2013;
Laborde et al., 2019). The Tarim Basin is 56 × 10
4 km
2 in size (
Wu et al., 2002;
He et al., 2005;
Lin et al., 2013). The eastern Tarim Basin is bounded to the east by the Luobopo Low Rise, to the west by the Awati-Manjiaer Transition Zone and the Gucheng Low Rise, to the south by the South-eastern Depression, and to the north by the Tabei Uplift (Fig.1(b) and 1(c
)) (
Han et al., 2009;
Dong et al., 2014;
Wu et al., 2018).The eastern Tarim Basin formed due to the interaction of subduction and collision between the Northern Tianshan Mountain, the Yining-Junggar Plate, the Eurasian Plate, the Southern Arkin, the Lhasa Block, the Qiangtang Block, and the India Plate. Therefore, the tectonic framework of the eastern Tarim Basin is complicated (
Tang, et al., 2003;
Han, et al., 2009). The research area, Gucheng Area, is in the south-west of the eastern Tarim Basin (Fig.1(c) and 1(d)). It is bounded to the south by the Cheerchen Fault, to the north-east by the Manjiaer Depression, and to the north-west by the Awati-Manjiaer Transition Zone (Fig.1(d)) (
Wu et al., 2015, 2018). The coverage area is roughly 6100 km
2 (
Wang et al., 2011;
Wu et al., 2015). It has a broad and gentle nose-like structure with an NW strike. The southern part is heavily influenced by the Cheerchen Fault, which has resulted in a series of complex thrusting structures. In contrast, the northern part is affected only marginally by the Cheerchen Fault, with gentle structures and locally faulted anticlines (
Wu et al., 2015;
He et al., 2017).
During the Cambrian-Middle Ordovician period, the Gucheng Area was in an extensional setting (
He et al., 2017). Platform facies (mostly restricted platform, semi-restricted platform, and open platform) and platform marginal facies (mostly foreslope) predominated (
Wang et al., 2014a;
He et al., 2016). The lithology included dolomite, limy dolomite, and limestone. The influence of the first episode of tectonic movement in the middle Caledonian eroded the top of the Middle Ordovician. Sea level rose rapidly in the Early Ordovician, resulting in deep-water basin facies (primarily neritic shelf and shelf slope) (
Wang et al., 2011;
Wang et al., 2014b). The Gucheng Area was influenced by the third episode of tectonic movement in the middle Caledonian, which resulted in a high in the south and a low in the north. Lower Ordovician Penglaiba Formation, Lower-Middle Ordovician Yingshan Formation, Middle Ordovician Yijianfang Formation, Upper Ordovician Tumuxiuke Formation, and Upper Ordovician Queerqueke Formation are the main Ordovician strata preserved in the Gucheng Area, eastern Tarim Basin (Fig.2) (
Feng et al., 2007;
Lin et al., 2012, 2013;
Wu et al., 2015). Dolomitic limestone, granular dolomite, and limy dolomite dominate the Penglaiba Formation, with thicknesses ranging from 120 m to 300 m (
Wu et al., 2015). The Yingshan Formation is divided into two sections with thicknesses ranging from 370 to 990 m (Fig.2) (
Lin et al., 2012;
Zhang et al., 2021). The lower section comprises granular dolomite, limy dolomite, and dolomitic limestone, while the upper section comprises calcarenite, micrite, and dolomitic limestone (Fig.2) (
Zhang et al., 2021). The Yijianfang Formation is composed primarily of calcarenite and granular limestone with thicknesses ranging from 70 to 190 (Fig.2) (
Wu et al., 2015;
Zhang et al., 2021). The Tumuxiuke Formation is primarily composed of argillaceous limestone with a maximum thickness of 50 m (Fig.2). The Queerqueke Formation contains the thickest sandstone and mudstone, ranging in thickness from 1200 to 2400 m (Fig.2) (
Wu et al., 2015;
Zhang et al., 2021).
3 Database and methodology
3.1 Database
The data sets are obtained from the Exploration and Development Research Institute, Daqing Oilfield Company, PetroChina. The data sets predominantly include wireline logs, 2D seismic lines, sieve residue logs, core photos and thin sections. The wireline logs (from 12 wells) primarily consist of DT, GR, and PE logs with a sampling rate of 0.125 m. The DT log is used to determine the lithology and porosity. The GR log is used to detect radioactivity in lithology associations. The purity of lithology and its associations are determined using the PE log. Furthermore, natural GRS logs are collected from boreholes using a portable device. A single pristine NaI crystal is commonly used in the device, surrounded by oscillation detectors. The crystal᾽s design allows for continuous documentation of the gamma-ray hit number on the crystal. Furthermore, the device enables analysts to differentiate the intensity of radioactive element sources that emit light pulses. The hitting energy is then converted in a photomultiplier tube by splitting the intensity of the light pulse into the corresponding voltage pulse of the original gamma-ray energy. Borehole gamma-ray spectrometers are intended for use in natural sedimentary environments to analyze the most common radioactive elements (Th, U, and K). As a result, the spectroscope᾽s numerical value is equivalent, but not identical, to other common methods of gamma-ray radiation or elemental abundance. Alternatively, the devices for measuring radioactive intensity have been in use for a long time. The numerical value has been calibrated in terms of errors and deviations from the API standard. The study area is traversed by 553 km of 2D seismic lines with a dominant frequency of 30 Hz. The sieve residue logs (from 12 wells) were sampled every 5 m. Every 100 m, the lag time is measured. The core images are from wells scattered throughout the Gucheng Low Rise, primarily wells GC 4, 6, 7, and 8. The total length of these well cores are approximately 35.1 m. The thin sections are performed at the Tarim Oilfield Company᾽s test center using a Germany Zeiss microscope with different digital zoom scales.
3.2 Methodology
3.2.1 The theory for determining ancient sedimentary environment and associated energy
The integrated superposition method seeks to conduct integrated and quantitative analysis by using multiple typical parameters derived from natural GRS logs. The superimposition process begins with the analysis of typical individual parameters, followed by the superimposition and combination of the results of each parameter into the final results. Unlike the previous method, in which each typical parameter is first superimposed and then analyzed, the integrated superimposition method can fully exploit the role of each parameter during the analysis process, while the final parameters are well integrated, making the determination results more objective and accurate. With the help of cores and thin sections, the integrated superimposition method can best use every typical parameter to determine and finally quantify the energy of the sedimentary environment in ancient deep-buried carbonate reservoirs.
3.2.2 The optimization for parameters in determining the ancient sedimentary environment and associated energy
In natural GRS, the ratio of numerical values (Th, U, and K) is calculated as Th/U, U/(U + K + Th), Th/(U + K + Th), Th/KTh, U/(Th + K), Th/K, K/(U + Th), U-K, U/(Th + K)-Th/KTh, U/(Th + K)-Th/KTh + U-K. The calculated ratios and the original data are then optimized to calibrate the energy based on the integrated analysis of cores and thin sections. Finally, the parameters that correspond well with changes in the sedimentary environment and associated energy are chosen for characterization. The final optimized parameters primarily include U-K, U/(Th + K)-Th/KTh (referred to as Parameter 1/P1), U/(Th + K)-Th/KTh + U-K (referred to as Parameter 2/P2), Th/U, PE.
4 Results and interpretations
4.1 Determination of energy
The cores and thin sections aid in determining multiple energies in the sedimentary environment, as well as the calibration of typical wireline parameters in various energy sections. Then, in conjunction with changes in parameters and borehole conditions, a standard for determining energy resulting from wireline parameters can be established (Tab.1).
Different boreholes may have slightly different determination standards. However, the values in the determination standard can be slightly adjusted to changes in parameters and borehole conditions. This method is found to be ineffective in determining the energy of dolomite. Therefore, the dolomite is identified before the limestone energy determination. The specifics are as follows. To begin, the PE values are used to differentiate between dolomite and limestone. The energy of the limestone is then calculated. The GRS parameters (including U-K, parameter 1/P1, parameter 2/P2, and Th/U) are used to determine low, medium, and high energy. Finally, all parameters are determined, and the results are displayed (Tab.2). The table shows the determination results in some depth sections of well GC7. It is concluded that the results are highly comprehensive, avoiding discrimination of individual parameters and quantitatively determining energy changes.
The determination results are plotted as a curve to produce continuous changes in energy from top to bottom. This method is used to calculate the energy of wells GC4, GC6, GC7, and GC8. The results of energy determination in some depths of well GC7 can be seen. However, this method cannot accurately and reasonably determine the energy of dolomite. To distinguish the energy of dolomites from that of other lithology associations, the energy of dolomites is assigned the lowest value (Fig.3(a)). The energy change curve reveals that the Upper Ordovician has massive energy (Yijianfang Formation and first member of Yingshan Formation). In contrast, low energy dominates the upper part of the second member of the Yingshan Formation. Instead, the lower part of the second member of the Yingshan Formation and the upper part of the third member of the Yingshan Formation is characterized by an interaction of high, medium, and low energy, with the high-energy dominating. The lower part of the third member of the Yingshan Formation is made up of high and low energy. In the fourth member of the Yingshan Formation, the upper part is dominated by low energy, while the lower part is dominated by high energy. Unlike the energy changes in the Yingshan and Yijianfang formations, the energy in the Penglaiba Formation is primarily composed of low energy, with a small portion of high energy in the middle and lower parts (Fig.3(a)). The energy changes are well correlated with lithology associations, which adds to the validity of the determination results.
The statistics and analysis of thin sections in the Ordovician of the eastern Tarim Basin show that the dolomites are primarily composed of medium-crystalline and fine-crystalline dolomite. Therefore, the conclusions are that the energy of the sedimentary environment where the dolomites are deposited is medium-low energy.
Groundwater activity is closely related to increased U content in the permeable stratum. Because of its active chemical properties and solubility in water, U is easily migrated to the deep by groundwater along faults or fracture zones formed in the karst or tectonically active zone. Then, it is reduced to tetravalent U and precipitated under reducing conditions. As a result, the U content has a strong relationship with hydrodynamics. Furthermore, the high Th and K contents indicate a stable and low-energy humid sedimentary environment. Furthermore, the high U and K contents indicate a turbulent and high-energy sedimentary environment with a dry climate (
Ruffell and Worden, 2000). As a result, in natural GRS, the U, Th, and K contents can reflect energy changes in the sedimentary environment. The analysis reveals that determining dolomite energy is challenging. Therefore, the dolomite is first determined. Following that, the energy of limestone is determined. U, Th/U, parameter1, and parameter2 are the most commonly used parameters for determining energy. Both parameters have already been mentioned. Based on the values from the determination parameters and parameters calibrated by cores or thin sections, the energy in the sedimentary environment where carbonates are deposited is divided into three categories: high, medium, and low energy. The precise parameter ranges are then displayed (Tab.3). Through energy changes, the final transition of parameters can reflect the sedimentary environment.
The identification of lithology associations and determination of energy in the Ordovician of well GC4 depicts energy changes resulting from wireline logs in general, which not only reflects overall energy changes but also shows energy changes in detail (Fig.3(b), 4(a), 4(b), 4(c), 4(d), 4(e), 5(b), 5(d), and 5(g)).
The dolomite content is highest in well GC8. As a result, identifying dolomites takes precedence. The determination of energy in limestone and the identification of lithology associations reveal alternate transitions of high and low energy (Fig.3(c), 4(f), Fig.6(g), Fig.4(h), and Fig.4(i)). The energy determination in well GC4 reveals that high energy dominates the upper part. In contrast, the middle section generally denotes low energy, followed by a small portion of an alternation transition between high and medium energy. The characteristics from middle part to upper part reveal increasing energy from upper Yingshan Formation to the Yijianfang Formation. The lower part is dominated by a series of thick dolomites, the energy of which is difficult to determine. As a result, energy determination focuses on limestones (Fig.3(b), Fig.6(a), Fig.6(b), Fig.6(c), Fig.6(d), Fig.6(e), Fig.4(d), and Fig.4(j)).
4.2 The determination of ancient sedimentary environment
The distribution of radioactive elements derived from natural GRS, such as U, Th, and K, not only aids in energy determination but also in revealing ancient sedimentary environments using thin sections, cores, and seismic data sets. In general, the Th and K content in sedimentary rocks decrease with grain size. Th and mudstone content are linearly related. The high Th and K content indicates a stable and humid sedimentary environment with low energy. In contrast, the high U content indicates a dry sedimentary environment. Furthermore, U content and hydrodynamics are linearly related. The carbonates were deposited in the eastern Tarim Basin during the Middle-Lower Ordovician. It is dominated by limestone (primarily calcarenite) in the upper Yingshan Formation and the Yijianfang Formation (Fig.2, Fig.4(a), Fig.4(d), Fig.4(e), Fig.4(k), and Fig.4(l)). However, dolomite (primarily granular and limy dolomite) predominates in the lower Yingshan Formation and Penglaiba Formation (Fig.2, Fig.4(c), Fig.4(f), Fig.4(i), and Fig.4(j)). The interpretation of seismic sections aid in determining that it is a typical carbonate platform (Fig.5, Fig.6(h), Fig.6(i), Fig.4(i), and Fig.4(l)).
The study area᾽s facies are mostly made up of platform facies and platform margin facies. The energy curve from well GC7 shows that it has high energy from the upper part of the third member of the Yingshan Formation to the Yijianfang Formation. The concentration of the single radioactive element Th is exceptionally high. The values of the single radioactive elements U and K vary considerably. The composite parameter values (parameters 1 and 2) are relatively high, while the composite parameter values Th/U are relatively low. These characteristics reveal a dry sedimentary environment with turbulent shallow water. Calcarenite dominates the lithology, deposited in a psammitic shoal in the platform margin facies. It is distinguished by the interaction of high, medium, and low energy from the lower part of the third member of the Yingshan Formation to the Penglaiba Formation. U values vary significantly, followed by the composite parameter (Th/U). It generally denotes a dry, active sedimentary environment with various lithology associations. Dolomitic limestone, micrite, granular dolomite, and limy dolomite are typical lithologies in platform margin facies, reflecting psammitic shoal and interbank sea (Fig.3(a)). The wireline properties of well GC8 are similar to those of well GC7 (Fig.3(c)). The energy curve from well GC6 demonstrates the interaction of high and low energy from the upper part of the second member of the Yingshan Formation to the Yijianfang Formation. In contrast, it is distinguished by low energy from the second to third members of the Yingshan Formation. The values of Th, U, and K vary significantly, indicating an unstable sedimentary environment with transition characteristics from high to low energy. The lithology comprises micrite and fine-crystalline limestone deposited between the platform margin facies and the platform facies (Fig.7).
The Ordovician well correlation shows that the gentle carbonate slope gradually incorporates into the carbonate platform margin with obvious slope during the Early Ordovician in the study area (Fig.8). On the platform margin’s high-energy belts, the platform marginal reef-shoal is well-developed. The platform margin’s outer side develops a neritic a shelf and shelf slope. However, the platform margin is usually quite visible, evolving into a steep platform margin during the middle-late Ordovician (Fig.8). The energy of the sedimentary environment in the first member of the Penglaiba Formation is low, with low Th and K values. Instead, it is distinguished only by high energy in well GC8 and high parameters 1 and 2 values, indicating a relatively deep-water depth. The energy of the sedimentary environment decreases toward the platform in the second member of the Penglaiba Formation. In comparison to the first member of the Penglaiba Formation, the study area evolves into a locally drowned platform in the second member. It demonstrates low energy in the fourth member of the Yingshan Formation with low values of Th/U and Th, implying that it was a platform facies during transgression. The energy in the third, second, and first members of the Yingshan Formation is generally high, with low Th and K values and high values of parameters 1 and 2, indicating an open platform (platform facies)-foreslope (platform margin facies). The Yijianfang Formation᾽s energy is a mix of high and low energy, indicating an evolutionary transition from a foreslope in platform margin facies to a locally drowned platform in platform facies. The integrated analysis of energy and sedimentary environment produced by natural GRS shows that sea level rises in general during the Early Ordovician (Penglaiba Formation-fourth member of the Yingshan Formation) and falls during the middle-late Ordovician (third member of Yingshan Formation-Yijianfang Formation).
5 Discussion
The novel approach of natural GRS log is employed to investigate the Ordovician deep-buried carbonates of the eastern Tarim Basin to reveal genetic relationships between radioactive elements such as Th, U, K, and sedimentary environment and associated energy. The natural GRS log is produced by artificially measuring changes in radioactive element concentration and gamma-ray intensity in rock records during nuclear decay. The GRS log has become widely available and used in the energy industry (
Davies and Elliott, 1996;
Ruffell and Worden, 2000;
Lima et al., 2005;
Ghosal et al., 2020). This is because it is capable of reconstructing sedimentary environments and paleoclimates in sedimentary basins. Furthermore, it can reveal the hydrodynamics, water depth, and energy of sedimentary environments (
Chen and Zha, 2004;
Lima et al., 2005;
Ghosal et al., 2020).
Because argillaceous content and radioactive element concentration are linearly related, the natural GRS log is used to effectively determine argillaceous content in rock records based on changes in the radioactive element concentration (Th, U, and K). Furthermore, the ratio of thorium/uranium content (Th/U) and thorium/potassium content (Th/K) can be used to calculate changes in the sedimentary environment and relative ancient water depth (
Davies and Elliott, 1996;
Yang et al., 2003;
Zhang et al., 2006).
Modern statistical studies on the interpretation of natural GRS logs show that U chemical properties are relatively active (
Davies and Elliott, 1996;
Chen and Zha, 2004). The reduction and adsorption of U by organic matter during the diagenetic process is the primary mechanism of U enrichment in sedimentary rocks. As a result, U enrichment in sedimentary rocks reflects a deteriorating environment. Th has a more stable chemical property than U (
Liu et al., 2000;
Lima et al., 2005;
Zhang et al., 2006). As a result, three sedimentary environments can be distinguished based on the change in Th/U value: when Th/U > 7, it generally indicates shallow water under oxidation conditions or in an exposed environment. When 2 < Th/U < 7 is present, it indicates an interactive neritic shelf sedimentary environment with transitional reduction-oxidation conditions. Th/U < 2 generally indicates deep-water under strong reduction conditions, with sediments characterized by gray or green shale (
Wang and Zhu, 2002;
Wang, 2004;
Lima et al., 2005;
Ghosal et al., 2020). Changes in the Th/K value of sedimentary rocks can also indicate changes in the sedimentary environment and water depth. The high Th/K value indicates that the stratum has been exposed to weathering. The Th/K value of long-exposed large-scale weathering crust is frequently greater than 7; low Th/K values indicate deep-water environments with low-energy reduction conditions (
Ruffell and Worden, 2000;
Zhang et al., 2006;
Ghosal et al., 2020).
6 Conclusions
The integrated superposition method produced effective and fruitful results in determining the ancient sedimentary environment and associated energy in deep-buried marine carbonates based on natural GRS logs from the Ordovician in the eastern Tarim Basin.
1) Analyzing the energy and radioactive element distribution can objectively reflect the sedimentary environment. Furthermore, lithology associations and seismic reflections can be used to determine the Ordovician sedimentary environment in the eastern Tarim Basin.
2) During the Ordovician, the carbonate platform dominated the sedimentary facies in the eastern Tarim Basin. From the Penglaiba Formation to the Yijianfang Formation, the sedimentary cycle transitions from transgression to regression. The platform margin facies are deposited in wells GC4, GC7, and GC8. Instead, the well GC6 is in the transition zone between the foreslope in the platform margin facies and the open platform in the platform facies.
3) Based on well correlation from wells GC4, GC8, GC7, GC6, and GL1, the facies generally change from foreslope in platform margin facies to open platform and restricted platform in platform facies.
4) In summary, using lithology associations and seismic reflections, the natural GRS log can be used to determine ancient sedimentary environments and associated energy. The workflows described in the manuscript can provide solid insights and useful guidance for determining the sedimentary environment and associated energy in deep-buried carbonate reservoirs. Because the petroleum exploration in deep-buried carbonate reservoirs has important scientific implications in today᾽s oil industry.
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