This study applies seismic facies analysis to the Eocene and Oligocene sedimentary successions in the northern Jeju Basin of the Korean South Sea, using 3D and 2D seismic reflection data integrated with lithological and paleontological information from exploration wells. Within a recently established 3rd-order sequence stratigraphic framework, six seismic facies types were identified and mapped based on their reflection characteristics. Interpretation of these facies indicates that during the Eocene, deposition occurred mainly in structurally segmented rift sub-basins, where an alluvial setting was well developed, including alluvial fan, channel, and localized lacustrine environments. From the Late Eocene, a gradual environmental shift from fluvial to coastal settings occurred as a consequence of relative sea-level rise. During the Oligocene, basin infilling promoted the transition of the depositional environments from the upper coastal plain to the lower coastal plain and shallow marine settings. Our study results have revealed that the Jeju Basin has gradually experienced an environmental change from non-marine in the early rifting stage to marine settings, and provided important constraints on the timing and spatial distribution of depositional environments.
Korean South Sea, encompassing Korea-Japan Joint Development Zone, Block 4, Block 5, and Block 6-2, has been estimated to have high potential for petroleum resources. Nevertheless, the stratigraphy of this sea was not in consensus. This paper introduces not only theoretical and technical issues, encountered during the industry-academia collaboration to establish integrated stratigraphic framework, but also provides the solutions for these challenges. The key theoretical and technical issues are summarized as follows: 1) previous stratigraphic frameworks with lack of consideration of the tectonic diachroneity in the rift basin system, 2) absence of description and classification criteria for the Mesozoic and Cenozoic index microfossil of the Korean seas, 3) the stratigraphic discrepancy between the Korean South Sea and East China Sea, 4) quality differences among the seismic reflection data caused by different acquisition parameters and processing methods, 5) resolution contrasts in the maps resulting from different density of the seismic reflection data, 6) disharmony between existing time-depth relationship and newly obtained seismic reflection data, and 7) absence of official area and nomenclature for the sedimentary basins in the Korean South Sea. These theoretical and technical challenges are the issues that the researchers will inevitably encounter again when we establish basic stratigraphy for assessing hydrocarbon resources and Carbon Capture & Storage (CCS) reservoir in other Korean seas. Thus, this paper, which introduces the solution of those issues, is expected to serve as a guideline for the future research around the Korean Peninsula.
Pebbles derived from shallow-marine carbonate rocks have not previously been reported from the Nakdong Formation, the lowermost unit of the Gyeongsang Supergroup. This study reports, for the first time, chertified carbonate pebbles formed in shallow-marine environments from conglomerates and pebbly sandstones in the middle part of the Nakdong Formation in the Waegwan area, southeastern Korea. These chert pebbles consist mainly of skeletal packstone to grainstone and ooid grainstone. Petrographic observations reveal abundant skeletal grains, including bryozoans, benthic foraminifera, coral fragments, and fusulinids, indicating deposition in a shallow-marine carbonate environment associated with reefal settings. When considered together with previous studies, including paleocurrent data, the occurrence of these chertified shallow-marine carbonate pebbles suggests that a pre-Cretaceous accretionary complex was present in the western source area during the early development of the Gyeongsang Basin.
To analyze rock properties and sedimentological characteristics to interpret subsurface structures and accurately model petroleum systems, improving the quality of acquired data is essential. This process serves as a foundation for petroleum system modeling. In this research, we develop an AI-based 3D volume expansion method for 2D seismic data and a 3D lithofacies classification method using the expansion method. The 3D volume expansion method was developed based on a generative artificial neural network. The 3D volume transformation method, based on an artificial neural network utilizing random noise vectors, demonstrated high performance in interpolating gaps in widely spaced 2D data. Furthermore, the lithofacies classification method was developed based on an artificial neural network model based on U-Net. In this reteach, the network was trained using data with completed lithofacies inversion from the acquired data, thereby reflecting the correlation between seismic data and lithofacies distribution. The 3D seismic data expanded using the volume expansion method were then used as supplementary data when generating a 3D lithofacies distribution using geostatistical methods. Through this, a 3D lithofacies model within the target area reflecting the characteristics of the elastic wave data was finally obtained.
Late Triassic granitoids are widely distributed in the Deokjeok-Soya-Ijak islands along the western coast of the Korean Peninsula. This study presents zircon U-Pb ages and whole-rock geochemical characteristics of granites and quartz monzonite from the area. Together with previously published data, our results indicate that these plutons were emplaced during approximately 223-217 Ma. Geochemically, the rocks show high-K calc-alkaline to shoshonitic affinities, and their rare earth and trace element patterns closely resemble those of other Late Triassic granitoids in the central Korean Peninsula. These results suggest that the granitoids formed as products of post-collisional magmatism associated with crustal extension following continental collision. Our findings extend the duration of Late Triassic post-collisional magmatism in the central Korean Peninsula to around 217 Ma and provides new insights into the tectonic evolution of the continental crust during this period.
This study conducts a 3D petroleum system modeling of the Jeju Basin to evaluate whether the proven petroleum systems of the adjacent Xihu Basin can also be effective in the Jeju Basin. To enhance the reliability of the study, the latest 3D seismic data and geochemical datasets analyzed by advanced and high-resolution geochemical analytical techniques were integrated. In particular, geochemical re-analyses based on detailed lithofacies classfication of source rocks indicate that high-quality shale and coal-bearing strata were deposited during the Eocene and Oligocene, providing critical evidence for petroleum potential in the study area. Modeling results suggest that Eocene and Oligocene petroleum systems are working in the Jeju Basin: The Eocene petroleum system is mainly effective in the western slope belt, whereas the Oligocene petroleum system is evaluated to be effective in the central anticline inversion belt. Despite significant tectonic movements during the Late Miocene, hydrocarbon generation and expulsion are interpreted to have continued after major structural deformation events. This study have academic significance in that it visualize the timing of hydrocarbon generation, migration, and accumulation in 3-dimensions and evaluates regionally effective petroleum systems. Overall, the petroleum system proven in the Xihu Basin is considered applicable to the Jeju Basin. The results of this study are expected to contribute to establishment of future exploration strategies and to the reduction of exploration risk in the Jeju Basin.
Earthquakes are generated by the dynamic propagation of ruptures along faults, and the resulting deformation structures can be preserved within fault cores and damage zones. This study investigates the formation of seismic slip indicators and the development and evolution of pulverized rocks in the Quaternary strike-slip fault zone at two localities in Geumsan-ri, Yangsan, Korea. At Site 1, a fault with an orientation of N26 degrees E/83 degrees NW juxtaposes Quaternary sediments in the southeast against Cretaceous granite in the northwest. Clay-rich gouge is injected from the slip zone into the sediments, with a width-to-length ratio (aspect ratio) of approximately 0.3. Within the granite on the northwestern side of the fault, healed fractures and open fractures occur at high density, together with deformation bands and folded deformation bands. At Site 2, a fault with an orientation of N60 degrees E/87 degrees SE is developed within Cretaceous granite. Here, pulverized rocks are concentrated in the northwestern side, containing both healed and open microfractures, whereas only healed fractures are developed in the southeastern side. The gouge injection structure at Site 1, with its high aspect ratio, is interpreted to have formed not only by fluid overpressure induced by frictional heating but also through the contribution of dynamic tensile stresses. Pulverized rocks at both sites evolved through three pathways during repeated seismic slip: (1) pulverization-successive pulverization, (2) pulverization-fracture healing, and (3) pulverization-fracture healing-pulverization. Fracture healing during interseismic periods served as a critical factor in determining the occurrence and location of subsequent pulverization. The asymmetric distribution of pulverized rocks is attributed to bimaterial interface effects and lithological property contrasts, but it can also occur in homogeneous rocks depending on rupture directivity and stress field distribution. This study suggests that fracture healing within damage zones can control subsequent pulverization and location, thereby contributing to the development of long-term fault zone evolution models.
In this study, a machine learning-based model was developed to estimate total organic carbon (TOC) from well logging data for enhanced source rock evaluation. The model enables estimation of a continuous TOC curve along well logs, even in core-limited intervals. The dataset consisted of well logging data and TOC analyses of sidewall core samples obtained from the O, J3, and J5 wells in the Jeju Basin. After pairing well logging data with corresponding TOC values, input features were selected considering the number of available data pairs. Subsequently, a well-to-well standard normalization was performed to account for inter-well variability of the gamma-ray log responses. A total of 118 datasets were divided into 113 for train and 5 for test data. A performance comparison between random forest (RF) and extreme gradient boosting (XGBoost) models revealed that XGBoost demonstrated superior performance. Specifically, on the test dataset, XGBoost achieved a coefficient of determination (R-2) of 0.84 and a mean absolute error (MAE) of 0.09 wt.%, significantly outperforming RF (R-2 = 0.11, MAE = 0.19 wt.%). Application of the developed model to intervals within the O well where core data were unavailable revealed an underestimation in sections with TOC exceeding 1 wt.%. Future improvements within high-TOC intervals can be achieved through data augmentation or training with TOC experimental data using cutting samples.
Mercury intrusion capillary pressure (MICP) analyses were conducted on caprock core chips obtained from seven wells across four exploration blocks in the southern offshore Korean continental shelf. Caprock intervals were selected based on well logs and lithofacies information, and representative-depth samples were analyzed to determine pore-size distribution, porosity, permeability, pore-entry pressure, threshold pressure, and breakthrough pressure. Contact angles and interfacial tensions for CO2-brine and oil-brine systems were applied to convert capillary pressures into allowable CO2 and oil column heights, and caprock sealing capacities were compared using the relationship between free-water level and water saturation. Although pores smaller than 1 mu m dominated the total pore volume in all samples, the distribution of pore sizes and the relative proportions of nano and micro scale pores varied significantly among wells, indicating strong heterogeneity in pore structures. Pore-entry and threshold pressures ranged from 0.2 to 36 MPa, while breakthrough pressures varied widely from approximately 75 to 283 MPa. Samples G-1 and J5-4 exhibited the highest breakthrough pressure (283 MPa) and maximum allowable CO2 column height (6,012 m), indicating excellent sealing capacity; in contrast, sample J1-1 showed the lowest breakthrough pressure (75 MPa) and minimum CO2 column height (1,586 m), suggesting apotential weakness in capillary sealing. The maximum allowable CO2 column heights for the CO2-brine system ranged from 1,586 to 6,012 m, whereas allowable oil column heights based on breakthrough pressure for the oil-brine system were higher, at 2,157 to 8,175 m. Overall, the caprocks of the southern Korean continental shelf exhibit favorable sealing capacities for geological CO2 storage; however, substantial heterogeneity in pore structures and capillary properties among wells highlights the need for spatially refined caprock assessments in storage-site design.
In this techinical report, with a view to developing a zircon OSL dating technique, we investigated the physical properties of OSL signals in zircon using LM-OSL (linearly modulated optically stimulated luminescence) measurements. By applying a general-order kinetic model, the zircon LM-OSL signals could be deconvoluted into four constituent signal components (fast, medium, slow1, and slow2), the photoionization cross sections of which were in the range of 10(-17) cm(2) and 10(-21) cm(2). We then examined the signal resetting efficiency of each LM-OSL component in response to optical stimulation. In this experiment, the fast signal component, which has the largest photoionization cross-section, was observed to decrease linearly and to be fully reset within similar to 6 s of blue-light stimulation. On the other hand, the medium and slow1 signal components exhibited exponential decay, reaching similar to 10% and similar to 20% of their initial count rates, respectively, after 50 s of optical stimulation. The slow2 signal component showed no distinguishable decrease in intensity over 180 s of blue-light exposure. As the absorbed dose increased, the intensities of the fast, medium, and slow1 signal components showed clear dose dependency, which was well described with a single saturating exponential function. The fast signal component reached dose saturation at similar to 170 Gy, whereas both the medium and slow1 signal components became dose-saturated at similar to 350 Gy.
Rift basins commonly host multiple coexisting and interacting sediment sources; however, most previous studies have focused on systems with a single upstream source, limiting our understanding of geomorphic and stratigraphic evolution in multi-sediment-source settings. To address this limitation-the inability of single-source based models to explain boundary migration, stratigraphic development in multi-source rift basin systems-we conducted three-dimensional tank experiments simulating two sediment sources and examined the evolution of a coupled alluvial fan-delta system under three different rates of sea-level change. The experimental results showthat the boundary between the two depositional systems oscillates in response to relative supply dominance and autogenic channel migration, with the magnitude of boundary migration increasing downstream. In addition, systematic variations in topset slope, boundary migration amplitude, and oscillation periodicity were observed as a function of sea-level change rate. These findings provide experimental constraints on the mechanisms controlling the stratigraphic development of interaction zones arising from multiple sediment sources and sea-level fluctuations, commonly observed in rift-basin depositional systems.
Water stable isotopes (delta & sup1;O-8, delta & sup2;H) are key tracers used to interpret hydrological cycles and climate change because they respond sensitively to phase change processes such as evaporation and condensation. In particular, quantitatively identifying the trend of increasing water stable isotope values during the evaporation process provides crucial information for understanding the hydrological cycle and reconstructing past climate and environmental changes. In this study, distilled water was evaporated under open-system conditions to track changes in the residual ratio (f) and water stable isotope composition, and the control factors influencing these factors were analyzed. When the results of the evaporation experiment were applied to the Rayleigh model, the effective fractionation factor (alpha eff) was calculated to be 1.0171 for delta & sup1;O-8 and 1.0792 for delta & sup2;H. Subsequently, the Craig-Gordon model was additionally applied to distinguish between equilibrium (alpha eq) and kinetic (alpha k) fractions during the evaporation process, and the results were evaluated as alpha(eq) = 1.0113 and alpha(k) = 1.0180 for delta O-18, and alpha(eq) = 1.0830 and alpha(k) = 1.0064 for delta & sup2;H. This is interpreted as delta O-18 being more significantly affected by kinetic fractionation, which is sensitive to changes in relative humidity, and delta & sup2;H being more significantly affected by temperature-dependent equilibrium fractionation. These results provide an experimental basis for interpreting the relationship between water evaporation intensity and climatic factors (temperature, relative humidity, etc.) by quantitatively evaluating the characteristics of stable isotope fractionation during the evaporation process, and are expected to improve the precision of evaporation and humidity reconstruction in natural water bodies.
Jeju Basin is a Cenozoic rift-related sedimentary basin located in the southern offshore region of the Korean Peninsula, where terrestrial to marginal-marine environments prevailed during the Eocene-Oligocene rifting stage, resulting in the repeated occurrence of shale and coal layers. In this study, HAWK-2 pyrolysis data and vitrinite reflectance (VRo) measurements from borehole J4 in the southern Jeju Basin were integrated to reassess the organic matter characteristics, thermal maturity gradient, and petroleum generation potential of Oligocene source rocks. HAWK-2 results show that shales are widely distributed over abroad depth interval with TOC values ranging from 0.3 to 7.0 wt% (average similar to 1.6 wt%). In particular, some intervals within the Late Oligocene shales exhibit high HI and S-2 values and occur within the oil window, indicating that these strata represent the most effective source rock intervals in the southern Jeju Basin. In contrast, Early Oligocene shales occur in deeper, highly mature zones where HI and S-2 values are significantly reduced. Coal layers display very high TOC values (10 similar to 70 wt%) but are stratigraphically restricted and show gas-prone characteristics in deeper intervals. VRo and T-max increase systematically with depth, defining a typical thermal maturity gradient with the oil window occurring at approximately 2.6 similar to 3.4 km, followed by transition to the gas generation zone at greater depths. Regression analysis of Tmax versus VRo reveals different slopes for shale and coal, indicating that lithology-specific correction is required when interpreting source rock maturity. The present generative organic carbon (GOC) ratios derived from HAWK-2 data are generally below 20% and decrease with increasing depth. However, by combining depth-dependent HI variation with thermal maturity-based transformation ratios (TR) and applying the method of Jarvie et al. (2012), the initial generative organic carbon (GOC(0)) was conservatively reconstructed. The estimated GOC(0) reaches similar to 20% in immature intervals and exceeds 58% in deeply overmature intervals. These results indicate that, although the current residual generation potential is limited, the source rocks originally contained a substantially higher fraction of generative organic carbon and were capable of significant hydrocarbon generation in the past. Overall, the southern Jeju Basin source rock system is interpreted as a composite system composed of mixed-origin (Type II/III) shales and organic-rich coals. Among them, the Late Oligocene shales are identified as the most effective petroleum source rock intervals. The results of this study provide an improved understanding of source rock characteristics in the Jeju Basin and offer fundamental constraints for future petroleum system modeling and gas-focused exploration strategies.
This study investigated the thermal maturity of organic matter (Tmax and vitrinite reflectance, Ro) in mudstone and coal fragments from the J5 well in the Jeju Basin. Tmax and Ro ranged from 430 to 535 degrees C and 0.5-1.8%, respectively. Considering the thermal maturity of organic matter, the oil window is estimated to begin at a depth of approximately 2,500 m, while the gas window is reached at around 3,400 m. Based on Tmax and Ro measurements, a Jeju Basin optimized correlation equation (Ro = 0.0123 & times; Tmax - 4.5769) was established. Using this equation, the reconstructed paleogeotemperatures indicate values predominantly above 170 degrees C in the early Oligocene interval, mostly within 120-170 degrees C in the late Oligocene interval, and gradually decreasing to 108-120 degrees C in the Miocene interval. Geothermal gradients derived from these paleogeotemperatures show a decreasing trend from the Oligocene to the Miocene, which is consistent with the progressive evolution of the rift basin. Discrepancies among previously published Tmax-Ro correlations were attributed to the lack of overmature samples. By incorporating overmature samples from the Jeju Basin with previous data, this study proposes an improved, internationally applicable Tmax-Ro correlation (Ro = 0.01 & times; Tmax - 3.5854).
Source rocks play a critical role in controlling both the quantity and type of hydrocarbons accumulated in reservoir rocks. Nevertheless, evaluations based on drill cuttings can introduce significant uncertainties in assessing hydrocarbon generation potential due to the admixture of lithic fragments. To minimize these limitations, this study analyzed the distribution of mudstone and coal fragments in drill cuttings from G well and independently evaluated the organic matter characteristics of each fragment to precisely assess the source rock properties in the northern Jeju Basin. The late Eocene mudstone-dominated source rocks (GM1, GM2, GM3) exhibit Fair-Good hydrocarbon generation potential and are capable of generating both oil and gas. The early Oligocene mudstone-dominated source rock (GM4) shows Good-Very good hydrocarbon generation potential, favorable for oil generation. The late Oligocene coal-bearing source rock (GC1), despite containing less than 20% coal fragments, exhibits higher hydrocarbon generation potential than the mudstone-dominated rocks, and considering the maximum HI (HImax) of coal fragments, it can generate both gas and oil. Compared to the late Eocene source rocks, the Oligocene source rocks are thinner but have higher hydrocarbon generation potential per unit area, potentially resulting in greater total hydrocarbon yield. However, due to relatively shallowburial depth, they may not reach sufficient generation temperatures, which could limit the actual hydrocarbon generation amount.
Apatite fission track (AFT) analysis, track length distribution measurements, and time-temperature modeling were performed on Oligocene-Miocene cutting samples from wells J3 and O to reconstruct the geothermal history of the Jeju Basin in the South Sea Shelf. The central ages of J3 samples are 28.4 and 19.2 Ma, while O1 samples show ages of 59.0 to 48.0 Ma, which are older thantheir depositional ages, indicating preservation of the cooling history of source rocks. Time-temperature modeling reveals that J3 experienced cooling of approximately 30-40 degrees C during the middle to late Miocene (13-7 Ma), corresponding to erosion of approximately 1.2-1.6 km thickness, whereas well O shows no distinct cooling event. This spatial variation indicates that the Longjing tectonic movement of the East China Sea Shelf Basin (ECSSB) affected the Jeju Basin heterogeneously,with the central part of the Jeju Basin experiencing tectonic influence from the middle Miocene while the northern part was minimally affected. These results suggest that the tectonic history of the sedimentary basins in the South Sea Shelf, including the Jeju Basin, may differ from the regional ECSSB framework, highlighting the necessity for basin-specific independent tectonic studies.
