Sequence architectures along the margins of rift basins are still poorly documented compared to passive continental margin settings. The Eocene Shahejie Formation on the rift margin of the Dongying Depression records a complex sequence stratigraphic distribution of conglomerate, sandy conglomerate, sandstone and mudstone. These facies have been mainly attributed to fan delta and marginal subaqueous fan depositional settings that developed during segmented fault activity along the rift margin. We utilize three-dimensional (3D) seismic data, conventional cores, and wireline log data to dissect the overall wedge-shaped upper sub-member strata of the Shahejie Formation's fourth member. The study interval is a third-order sequence formed between 45.4 and 42.5 Ma and contains a lowstand-transgressive systems tract (LST-TST) and a highstand systems tract-falling-stage systems tract (HST-FSST). We found that the LST-TST developed several huge amalgamated depocenters along the Chennan border fault; whereas, these depocenters gradually diminished or even disappeared during the HST-FSST period, illustrating less significant control by the border fault. Through calculations of strata growth rates, we confirmed that the segmented activity of the border fault influences the stratigraphic distribution and facies evolution during these two periods. Specifically, deep-water depositional systems, represented by marginal subaqueous fans were widely developed in the LST-TST period and were influenced by overall strong tectonic activity, including retrogradational (R) and aggradational (A) patterns. However, the aggradational to progradational (AP) deltas and progradational to degradational (PD) deltas dominated the generally muted tectonic activity setting during the HST-FSST. Additionally, influenced by localized segmented fault activity, these systems tracts exhibit incomplete vertical development, resulting in spatial variability in stratigraphic stacking patterns. Rift basins are interesting and complex, and this study examines stratigraphic architecture and facies evolution within a particular interval near a border fault with variable spatio-temporal activity.image

The study conducted physical simulation experiments on sandstone samples from the Junggar Basin to investigate how burial rates influence sandstone diagenesis and reservoir quality. Results show that the mechanical compaction under a negative burial rate (tectonic uplift) almost stops to destroy the sandstone reservoir space, the capacity of fluid seepage is the strongest and the sandstone tends to develop ‘weak compaction–strong dissolution’ diagenetic facies. For positive burial rates, sandstones with a low burial rate tend to develop ‘medium compaction–medium dissolution’ diagenetic facies; sandstones at a medium burial rate easily form ‘strong compaction–weak dissolution’ diagenetic facies, and sandstones at a high burial rate tend to develop ‘weak compaction–weak dissolution’ diagenetic facies. Experimental results indicate that the compaction strength and damage to sandstone reservoirs may not consistently rise with the burial rate. Faster burial rates do not always intensify compaction; the degree of compaction depends on fluid overpressure. If the increase in burial rate does not induce the fluid overpressure in sandstones, the burial rate is higher and the destruction degree of primary pores caused by mechanical compaction is greater; mechanical compaction also simultaneously causes the diagenetic system to be more closed and the dissolution to be weaker. If the increase in burial rate can induce the fluid overpressure in sandstones, the burial rate is higher, the inhibition of mechanical compaction by fluid overpressure is more pronounced. However, fluid overpressure also strengthens the closure of the diagenetic system, hindering the injection of external acidic fluids into the sandstone, which is not conducive to dissolution. Overall, low burial rates with normal pressure favour secondary pore development, high burial rates with overpressure preserve primary pores, while medium burial rates with normal pressure are unfavourable for primary and secondary pores.

Taking the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin as an example,the influences of the burial process in a foreland basin on the diagenesis and the development of high-quality reservoirs of deep and ultra-deep clastic rocks were investigated using thin section,scanning electron microscope,electron probe,stable isotopic composition and fluid inclusion data.The Qingshuihe Formation went through four burial stages of slow shallow burial,tectonic uplift,progressive deep burial and rapid deep burial successively.The stages of slow shallow burial and tectonic uplift not only can alleviate the mechanical compaction of grains,but also can maintain an open diagenetic system in the reservoirs for a long time,which promotes the dissolution of soluble components by meteoric freshwater and inhibits the precipitation of dissolution products in the reservoirs.The late rapid deep burial process contributed to the development of fluid overpressure,which effectively inhibits the destruction of primary pores by compaction and cementation.The fluid overpressure promotes the development of microfractures in the reservoir,which enhances the dissolution effect of organic acids.Based on the quantitative reconstruction of porosity evolution history,it is found that the long-term slow shallow burial and tectonic uplift processes make the greatest contribution to the development of deep-ultra-deep high-quality clastic rock reservoirs,followed by the late rapid deep burial process,and the progressive deep burial process has little contribution.

The Carnian Pluvial Episode (CPE) fingerprints global environmental perturbations and biological extinction on land and oceans and is potentially linked to the Wrangellia Large Igneous Province (LIP). However, the correlation between terrestrial environmental changes and Wrangellia volcanism in the Ordos Basin during the CPE remains poorly understood. Records of negative carbon isotopic excursions (NCIEs), mercury (Hg), Hg/TOC, and Hg enrichment factor (HgEF) from oil shales in a large-scale terrestrial Ordos Basin in the Eastern Tethys were correlated with marine and other terrestrial successions. The three significant NCIEs in the study section were consistently correlated with those in the CPE successions of Europe, the UK, and South and North China. The U-Pb geochronology indicates a Ladinian-Carnian age for the Chang 7 Member. A comprehensive overview of the geochronology, NCIE correlation, and previous bio- and chronostratigraphic frameworks shows that the Ladinian-Carnian boundary is located in the lower part of Chang 7 in the Yishicun section. HgEF may be a more reliable proxy for tracing volcanic eruptions than the Hg/TOC ratio because the accumulation rates of TOC content largely vary in terrestrial and marine successions. The records of Hg, Hg/TOC, HgEF, and NCIEs in the Ordos Basin aligned with Carnian successions worldwide and were marked by similar anomalies, indicating a global response to the Wrangellia LIP during the CPE. Anoxia, a warm-humid climate, enhancement of detrital input, and NCIEs are synchronous with the CPE interval in the Ordos Basin, which suggests that the CPE combined with the regional Qinling Orogeny should dominate the enhanced rate of terrigenous input and paleoenvironmental evolution in the Ordos Basin.

Predicting the reservoir spaces of paleokarst collapses is beneficial for enhancing the recovery of carbonate reservoirs and conducive to the sustainable development of oil fields. In general, geologists determine the sizes of paleokarst collapse reservoir spaces through a variety of geophysical methods. Based on the idea of ensemble learning, we proposed utilizing extreme gradient boosting (XGBoost) for the automated prediction of the reservoir spaces of paleokarst collapses. Compared with a multilayer perceptron (MLP), XGBoost increases the robustness of the associated model, significantly improves its parallel computation capability and is less affected by abnormal values. In addition, we used a conditional tabular generative adversarial network (CTGAN) to generate data to increase our dataset size. A practical application in eight blocks of the Tahe Oilfield showed that compared with those of other tree-based learning methods and support vector machines (SVMs), the prediction error of XGBoost was minimal, revealing the remarkable performance of XGBoost. Without CTGAN model fine-tuning, the generated fake dataset could still be used as training data, but its quality was worse than that of our original dataset. Finally, we randomly compared the prediction results of XGBoost and an MLP in collapse zones containing producing wells, and the results showed that the XGBoost method not only achieved high-accuracy predictions in most collapse zones but also performed better than the MLP. In this study, we introduce an innovative prediction approach leveraging ensemble learning and tabular data generation, crucial for accurately determining the reservoir dimensions within paleokarst collapses. This advantage enables us to more efficiently identify reservoirs with development potential, reducing exploration of ineffective or low-yield targets, thereby directly lessening environmental impact. By enhancing the accuracy of prediction models, we improve the economic benefits of oil and gas extraction and significantly reduce potential negative impacts on groundwater, surface, and ecosystems, promoting safer and more sustainable development practices.

Constrained by the geological burial history of Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin, the diagenetic physical simulation experiment was carried out with the low-mature sandstone samples taken from the outcrop area. Then, coupling with the regional geological data, the reformation of reservoirs with different diagenetic intensities by microfractures and the significance of microfractures for development of high-quality reservoirs were discussed. The results show that the large-scale microfractures were formed in the stage of late rapid deep burial, roughly equivalent to the period when organic acids were filled. The microfractures created good conditions for migration of oil and gas in deep and ultra-deep clastic rocks, and also enabled the transport of organic acids to the reservoirs for ensuing the late continuous dissolution of cements and particles. The existence of matrix pores and microfractures in the reservoirs before the rapid deep burial determined how the microfractures formed during rapid deep burial improved the reservoir quality. If matrix pores and microfractures were more developed and the cementation degree was lower before the rapid deep burial, the microfractures would be more developed and the dissolution degree would be higher during the late rapid deep burial, and so the reservoir quality would be improved more greatly, which can increase the reservoir permeability by up to 55%. If cementation was very strong, but matrix pores were not developed and microfractures existed locally before the rapid deep burial, the microfractures would also be more developed during the late rapid deep burial, which can increase the reservoir permeability by 43%. If cementation was strong, matrix pores were absent, and microfractures were not developed, limited microfractures would be formed during the late rapid deep burial, which can increase the reservoir permeability by only 16%. Formation of large-scale microfractures during late rapid deep burial and promotion of such microfractures to the dissolution of organic acids are considered as key diagenetic factors for the development of deep and ultra-deep high-quality reservoirs.

With the improvement of hydrocarbon exploration, turbidite reservoirs formed by deep-water gravity flow have become an important target for oil and gas exploration and development in faulted basins in eastern China. Due to the weak depositional differentiation and diagenesis damage, the overall quality of gravity flow sandstone reservoirs is poor. The prediction of high-quality reservoirs is becoming the key to restricting effective hydrocarbon exploration. In this paper, the authors study the distribution, lithofacies, and reservoir characteristics of gravity flow sandstone in the northeastern Nanpu Depression by cores, well-logging data, reservoir physical properties, and rock slices to explore the key control factor and development law of high-quality reservoirs. The result shows that the gravity flow sediments are mainly composed of eight lithofacies interpreted as slide-slump, sandy debris flow, muddy debris flow, and turbidity current. According to the statistical analyses of reservoir physical property parameters, it is proven that the gravity flow sandstone reservoir is with strong heterogeneity and that its quality depends on sandstone genesis, sand-mud structure, and dissolution intensity. The high-quality reservoirs are mainly from sandy debris flows and turbidity currents. Massive sandstones from sandy debris flows are usually high-quality reservoirs characterized by large single-bed thickness, strong calcareous dissolution of calcareous cement, and few muddy interbeds.In contrast, the sandstones from turbidity currents are of low quality due to small thickness, weak dissolution of calcareous cement, and many interbedded mudstones, and are formed in a closed diagenetic environment. This study provides an effective predictive idea for hydrocarbon exploration on deep-water gravity flow sandstone reservoir in a lacustrine basin based on analyses of sandstone genesis and sand-mud structure.

The Carnian Pluvial Episode (CPE) is a period during which the rainfall was extremely intense and greatly influenced the deposition in different sedimentary environments globally. However, the relationship between the CPE and turbidite deposition remains poorly known. Here we investigate a giant lacustrine turbidite system of the Triassic Ordos Basin in North China Plate, to determine how the CPE influenced turbidite deposition, by integrating geochronology, organic carbon isotope, major and trace elements, clay minerals and turbidite sedimentology. Results show that the lacustrine turbidite deposition in the Ordos Basin was strongly associated with the CPE. Although there is a temporal lag, the intensified turbidite deposition is considered as a response to the three humid periods during the CPE. Compared with marine environments, continental depositional settings tend to be more sensitive to the humid climate changes, and can record more detailed and long-term carbon fluctuations. This study highlights the potential for turbidite deposition affected by extremely humid events, and provides new insights into the prediction of deep-water turbidite systems. Furthermore, it enriches the theory of the CPE in terrestrial environments.

以准噶尔盆地南缘四棵树凹陷下白垩统清水河组辫状河三角洲碎屑岩为研究对象,采用岩心观察、岩石薄片观察、扫描电镜和压汞实验等方法分析成岩作用特征;根据岩心成岩相测井响应关系,进行测井成岩相识别和储层评价.结果表明:四棵树凹陷清水河组主要发育长石岩屑砂岩和岩屑砂岩,储集空间类型丰富,非均质性较强,既存在粗孔喉、连通性较好的粒间孔隙,为储层主要储集空间类型,也存在细微孔喉、低渗透率的粒内溶孔和粒间溶孔,属于特低孔—特低渗、超低孔—超低渗储层.研究区识别4类成岩相,其中强压实致密相多见蚀变凝灰质充填,发育于河道边缘薄层砂岩;钙质(铁泥质)致密胶结相集中发育于河道主体厚层砂岩的顶底部,孔喉结构较差;凝灰质致密充填相发育于分布最广的弱动力环境,粒间杂基多而物性差;欠压实溶蚀相主要发育于水下分流河道,排替压力较低、孔隙结构最好.自然伽马、声波时差、密度、中子测井及电阻率多元统计结果平均判识率达80.6％.该结果为取心井测井成岩相的研究和优质储层发育规律的认识、有利储层发育带的评价和优选提供支持.

The continuous breakthrough of deep and ultra-deep hydrocarbon exploration in China has attracted more and more attention. High-quality reservoir prediction is becoming one of the hotspots in deep and ultra-deep hydrocarbon exploration. Burial history, formation pressure and reservoir characteristics of the Qingshuihe Formation in the southern margin of Junggar Basin, China, were studied using thin section, scanning electron microscope, X-ray diffraction, stable isotopic testing of core samples, and homogenization temperature testing of fluid inclusion to explore the influence of burial history on the diagenetic process and reservoirs quality. Studies show that the Qingshuihe Formation went through four stages of the burial process after deposition, which successively consists of a long-term shallow burial stage, tectonic uplift stage, normal continuous deep burial stage and short-term rapid deep burial stage. The early long-term shallow burial and tectonic uplift stages not only can alleviate the strength of compaction, but also can maintain an open geochemical system in the reservoirs for a long time, which promotes the dissolution of intergranular volcanic ash by meteoric freshwater leaching and provide more secondary dissolution pores. The late short-term rapid deep burial process contributed to the development of overpressure, which effectively inhibits the destruction of primary pores resulted from compaction. Moreover, overpressure promotes the development of microfractures in the reservoirs, which can improve the permeability of the reservoirs and enhance the dissolution effect of organic acid. This study clarified the effect of burial style on the development of high-quality reservoirs by revealing the influence of burial process on diagenesis such as compaction, cementation and dissolution. It deepens the understanding of the development law of deep and ultra-deep high-quality clastic rock reservoirs, and also provides a new idea for the prediction of deep high-quality clastic rock reservoirs.