Marine Geophysical Research最新文献

Empirical methods often fail to accurately depict in-situ gas hydrate saturation distributions, despite their relationships with petrophysical and elastic properties remaining partially unclear. We proposed a data-driven approach to estimate gas hydrate saturation employing several machine learning techniques, including radial basis function neural network (RBFNN), random forest (RF), extreme gradient boosting (XGBoost), Adaptive Boosting (AdaBoost), support vector machines (SVM), and k-nearest neighbors (kNN). This study involved pre-processing data from laterolog deep resistivity and p-wave velocity logs, defining their increments as differences from the lowest values in gas hydrate zones. We identified data-driven patterns between pairs of laterolog deep resistivity and p-wave velocity increments, as well as core information corroborated with the traditionally predicted gas hydrate saturations, by adopting machine learning (ML) approaches. The approach tested on four wells in the Krishna-Godavari (KG) offshore basin (India) is extremely feasible. During the training and test phases, the minimum correlation coefficient between the true and predicted responses exceeds 0.94 and 0.88, respectively. The model accuracy hierarchy was RBFNN > AdaBoost > RF > XGBoost > KNN > SVM during training, and AdaBoost > XGBoost > RF > RBFNN > KNN > SVM during testing. This approach allows interpreters to select the most accurate ML model based on training phase performance. The proposed ML-based method is efficient, synergising p-wave and resistivity data increment, significantly improving gas hydrate saturation predictions, and avoiding the complexities of traditional calculations. The study indicates that gas hydrate saturation in the Krishna-Godavari region ranges from 0.17 to 86.84%.

Lake Fuxian is a tectonic lake located on the Yunnan–Guizhou Plateau in southwest China. It is the deepest freshwater tectonic lake in the Yunnan Plateau. The present study focused on examining the structural changes, faulting patterns, and their influence on fault subsidence in the Lake Fuxian basin. Seismic interpretation showed uplift in the SSW area and subsidence in the NNE region. Subsidence is more pronounced on the northern survey lines, where the sedimentary strata had a maximum sedimentation of 1200 m. The seismic interpretation findings showed a horst block in the southern basin and a graben block in the northern half of the basin. L-14 demonstrated the steeper with maximum throw and parallel character of normal faults and provided the evidence of crustal extensional regime. Thirteen main faults were identified by fault modeling in the lake basin. The analysis of fault characteristics revealed that faults in the northern basin are characterized by greater depth, steeper angles, maximum displacement, and are actively moving owing to low resistance and negative asperity values, and poor edge detection values. Faults in the southern basin have an opposite character to those in the northern basin. Major faults in the northern lake basin have a stronger influence of fault subsidence compared to faults in the center and southern lake basins. Overall, the lake Fuxian basin showed horst-graben structure with parallel normal faulting with a crustal extensional regime.

Particle breakage at coral sands–structure interface is common in marine geological environments and is critical to the stability of geotechnical structures. However, due to the knowledge gap regarding the relationship between structure roughness and coral sand particle breakage, previous studies have not provided a clear understanding of this phenomenon. To address this gap, we conducted ring shear tests to investigate the evolution of the fractal dimension of particle breakage and its dependence on structure surface roughness and vertical stress based on fractal theory. The results show that both the roughness of the steel plate simulating the structure surface in the ring shear test and the particle size of coral sand have significant impacts on the evolutions of particle breakage and morphology at the contact interface. In detail, the particle size distributions (PSDs) of coral sands after shearing have obvious self-similarity and converge to the limit distribution state, especially when the sands contain more large particles. When the steel plate is smooth, the fractal dimension of the broken coral sand is relatively low, which indicates an early stage of fractal dimension development. Moreover, the relationship between the fractal dimension and vertical stress exerted on a rough steel plate can be approximately described using a second-order polynomial function. Moreover, there exists a critical vertical stress that corresponds to the maximum fractal dimension for each kind of coral sand in our tests. The particle breakage rates of coral sand samples on smooth steel plates are substantially lower than those on rough steel plates.

Subducted seamounts are recognized as structures that influence seismicity in subduction zones. Understanding the detailed structures of seamounts, including before and during subduction, is essential for a comprehensive grasp of their influence. Of particular importance is their competency and deformation history during subduction. To better understand seamount subduction and related processes, we analyzed seismic reflection profiles around the Daiichi-Kashima Seamount, the Katori Seamount, and a knoll situated on the oceanward slope. These three features are located at different distances from the trench axis but all fall within the hinge-line of the subducting plate. The Daiichi-Kashima Seamount is currently subducting at the junction between the Japan and northern Izu-Ogasawara trenches, while the Katori Seamount and the knoll have not yet reached the trench axis. A thick limestone layer capping the Daiichi-Kashima Seamount indicates that the core of the Seamount is at least partially intact. On the other hand, our work reveals a large number of trench-parallel or sub-parallel faults within each of the edifices. The seismic profiles also reveal sediments draping the flanks of the edifices, interpreted as turbidite and/or debris flow deposits originating from slope failures. Consequently, our findings show that seamounts and other topographic features begin brittle deformation and failure due to flexural bending of the incoming oceanic plate long before actual subduction, with implications for the strength and competency of seamounts during subduction.

This research project aims to conduct a comprehensive geophysical evaluation of the oil potential within the pre-rift Matulla Formation in the Rabeh and Edfu-Saqara fields, situated in the Gulf of Suez Basin. The investigation relies on the analysis of wireline logs from twelve wells, with eight drilled in Rabeh field (Nageh-1, South Malak-1, Tawoos-1, RE-8, RE-22, RE-25, RE-2 & RE-4) in the onshore south-western Gulf of Suez Basin, and four wells (GS323-1 A, GS323-4 A, Edfu A-3 & Edfu A-5 A) drilled in Edfu-Saqara field in the offshore central Gulf of Suez Basin. Additionally, the interpretation of twenty seismic sections covering the Rabeh field reveals the prevalence of NW–SE normal faults, supplemented by minor E–W faults. These faults play a crucial role in creating structural traps conducive to retaining oil and gas. The analysis of well logging data indicates encouraging petrophysical indicators for the Matulla sandstones, suggesting their potential as reservoirs in the studied fields. These reservoirs exhibit a moderate net pay thickness ranging from 25 to 400 feet, a fair to good net/gross ratio between 0.07 and 0.68, low shale content ranging from 0.03 to 0.20, excellent effective porosity ranging from 0.10 to 0.20, minimal water saturation ranging from 0.10 to 0.35, and high hydrocarbon saturation ranging from 0.65 to 0.90. The obtained results revealed that the Matulla Formation is being considered as a prospective hydrocarbon reservoir in addition to the widely recognized Nubia sandstones and Miocene reservoirs in both Rabeh field and Edfu-Saqara field. Moreover, the constructed iso-parametric maps for the calculated petrophysical parameters in the Rabeh field recommend a strategic focus on the eastern side of the Rabeh field for potential drilling locations, given the presence of high-quality Matulla sandstone reservoirs in that area.

The reef limestone specimens selected in this experiment can be divided into four types according to their morphologies: strongly-cemented compact-type (BM-type), weakly-cemented compact-type (M-type), weakly-cemented loose-type (BS-type), and strongly-cemented loose-type (S-type). Based on the split Hopkinson pressure bar (SHPB) test, the aims of this study were to investigate the dynamic mechanical response and energy dissipation characteristics of reef limestone under impact loads and discuss the relationships of the dynamic fragmentation fractal characteristics with the strain rate and energy dissipation of reef limestone. The results indicated that the length of the compaction section for compact-type reef limestone compared with that of the loose section, which is more significant in the case of decreasing strain rate. The fractal dimension is linearly positively correlated with the strain rate; the fractal dimension of compact-type reef limestone is lower than that of loose-type reef limestone; meanwhile, the dynamic fractal dimension of compact-type reef limestone is more sensitive to the strain rate. The fragmentation fractal dimension of reef limestone under impact loads shows exponential growth with the increase in dynamic strength. The fragmentation fractal dimension of reef limestone is linearly, and positively, correlated with energy dissipation density.

One of the main drilling challenges in the offshore deep-water Nile Delta is the overpressured Tertiary shales, which causes formation fluid influxes, kicks, and very narrow drilling window, thus contributes to non-productive times and enhanced drilling risk. Accurate understanding of pore pressure distribution is crucial for casing design, mud optimization and safe and successful drilling. This study presents first ever in-depth analysis of the pore pressure distribution within the 4500 m thick Oligocene-Pleistocene stratigraphy from the West Delta Deep Marine block in deep-water Nile Delta. Direct formation pressure measurements indicated around 0.06–0.1 PSI/ft (1.36–2.26 MPa/km) gas gradient in the Pliocene El Wastani and Kafr El Sheikh sandstone reservoirs, while the Miocene Qantara sandstones are water-bearing with a 0.42 PSI/ft (9.5 MPa/km) pressure gradient. Shale porosity distribution exhibited additional porosity retention within the montmorillonite and mixed clay-dominated Late Pliocene and deeper sediments and marks the onset of overpressure at the top Kafr El Sheikh Formation. Based on the loading trends and acoustic slowness-density relationship, we inferred compaction disequilibrium as the primary overpressure generating mechanism resulted from high sedimentation rate. Shale pore pressure was interpreted by utilizing wireline logs by utilizing compaction trendline-based approach and calibrated with drilling events and mudlog data. Qantara and Tineh formations are characterized by 0.75–0.77 PSI/ft (16.96–17.41 MPa/km) pore pressure gradient leaving a narrow drilling mud window of 1.7–2 PPG. Based on vertical effective stresses, two significant overpressure compartments were identified in the Late Pliocene and Early Miocene-Late Oligocene, which were separated by the Middle Miocene Sidi Salem Formation acting as a pressure seal.

Abstract

The northern region of the offshore southern Tanzania contains structurally controlled hydrocarbon discoveries. The Permo-Triassic, Jurassic, Cretaceous and Cenozoic source rocks are reported to have possibly charged the existing hydrocarbon reservoirs in the region. However, there is no a detailed record of possible heat sources that may have matured the reported source rocks. Additionally, focused academic research on the possibility of different petroleum traps in the area has also not been done before. This study has used conventional 2D seismic interpretation to reveal presence of several igneous intrusions in the study area. The revealed igneous intrusions include igneous dykes and sills present in the Cretaceous and Cenozoic parts of the studied stratigraphy. Together with the existing burial depth, the associated magmatic heat is interpreted to have facilitated maturation of the Cretaceous-Cenozoic source rocks in the region. This suggests that the offshore southern Tanzanian Basin contains an atypical petroleum system where source rock maturation did not entirely rely on burial temperature. The igneous intrusions have also created hydrocarbon migration pathways from different stratigraphic levels. This is reflected by fracturing of the immediate reservoir intervals and seal rocks above the mapped igneous dykes. The used seismic datasets have also allowed mapping of structural, stratigraphic and combination traps in the study area.