The Main-M1 sandstone is becoming an important exploration target for structural-lithological plays in the Oriente Basin of Ecuador,in which accurate prediction of sand distribution and pinch-out belt is the key to successful trap definition. Oriente Basin is a retro-arc thermally subsiding basin in the Late Cretaceous and a foreland basin in the Cenozoic. It can be divided into a fold-thrust zone in the west,a fore-deep zone in the middle,and a slope zone in the east. In the fore-deep zone,there are a series of nearly N-S oriented elongate anticlines. In the anticline of the study area,a carbonate build-up is developed,which is characterized by mounded,low-amplitude but relatively continuous seismic reflections,and thick,blocky and low-amplitude GR well log motifs. This build-up may indicate that during the time of its deposition in Campanian,the anticline had begun to uplift and push its top close to the paleo-sea level. This brought about locally favorable conditions for the development of the carbonate build-up. Palaeogeomorphic reconstruction based on the back-stripping method shows that the anticline had started to uplift in the Early Campanian,ranging in magnitude from 7 to 10 m. As the uplift and carbonate build-up were developed prior to the deposition of the Main-M1 sand reservoir,the palaeomorphology may thus influence the emplacement of Main-M1. According to the statistics of Main-M1 sand thickness in dozens of wells on the anticline,the top of the anticline is overall Main-M1 sand prone,but the sand becomes very thin or absent on top of the carbonate build-up. In the southern extension of the anticline,the Main-M1 sand is absent on the structural top but becomes thicker at the downdip to the east. It is thus proposed that anticlinal paleo-morphology could influence Main-M1 sand deposition and force it to accumulate on the structural flap. This model opens up a new way of identifying structural-lithological traps through palaeomorphology reconstruction in the fore-deep of the Oriente Basin.
The Oriente Basin of Ecuador is one of the most important hydrocarbon-bearing basins in the Andean Foreland Basin Group of South America. The Main-M1 submember of the Upper Cretaceous Napo Formation,as one of the main exploration and developement formations,has been controversial in terms of its depositional environment,and its depositional system and evolutionary process remain unclear. Based on core,well logs and borehole cuttings,the main sedimentary microfacies types,depositional systems and evolutionary processes of Main-M1 submember were investigated in this study. The results show that the study area is located in a marine-continental transition environment with complex hydrodynamic conditions,and a tidal delta develops in the east,which gradually transitions into a shallow shelf environment in the west. In the study area,eight types of sedimentary microfaces including subaqueous distributary channel,subaqueous inter-channel,tidal channel,tidal sand bar,tidal flat,and tidal inter-channel,are recognized. According to the petrographic characteristics,the Main-M1 submember is further divided into three sub-layers. By analyzing the lithological assemblages of the three submembers,it is determined that during depositional of the Main-M1 submember,the Auca paleoarchelon gradually formed,and meanwhile the relative sea level of the study area rose due to the flexural effect. The three submembers overall show a gradual deepening upward trend and the tidal delta continuously retreated landward. Based on the reconstruction of the tidal delta-shelf depositional system during deposition of the Main-M1 submember,the subaqueous distributary channel and the leading sand bar in the eastern part of the study area are considered to have certain exploration potential for lithologic reservoirs.
Based on 3D seismic and borehold data,seven seismic sequence boundaries and seven drilling sequence boundaries are identified in the Carboniferous carbonate platform of North Troyes Oilfield. The KT-I reservoir group is divided into three and a half third-order sequences,and the KT-Ⅱ reservoir group is divided into three third-order sequences. The sedimentary environment evolved stratigraphically from open platform,through restricted platform,to evaporative platform. According to the palaeo-geomorphic restoration and sedimentary evolution within the sequence framework of the study area,the palaeogeomorphic and sedimentary evolution of the study area can be divided into three stages: the initial phase of differential platform uplift and depression(SQ2-SQ3 sequence),the finalizing phase of differential uplift and sedimentary differentiation fixing period(SQ4 sequence),and the inherited development phase(SQ5-SQ7 sequence). Further analysis shows that the uplift and depression pattern under the control of sequence framework controls the plan-view distribution of dolomite subclasses. The lower part of the sequences are dominated by micritic dolomite-gypsum and micritic dolomite-micritic limestone assemblages,which are characteristic of mainly lagoonal lacustrine deposits,while the higher part of the sequences are dominated by micritic dolomites,fine micritic dolomites and residual micritic dolomites.The results show that deposition of the KT-I oil formation in the study area results from inherited differential subsidence,rather than erosion and subsidence of the “western highland and eastern lowland”as previously thought. The overall palaeogeographical pattern is characterized by “Platform in the east,trough in the west,high in the north and low in the south”,which consistently controls the development and distribution of favorable facies and high-quality reservoirs. This finding is vitally important in screening for hydrocarbon exploration and production prospects in Pre-Caspian Basin.
Mahu sag,Shawan sag,and Pen-1 well west sag are the three major hydrocarbon rich sags in the west of Junggar Basin. Through the analysis of deep drilling wells,well logging,and deep reflection seismic profile data,these three sags were revealed to belong to the same depression in the early Permian,and the scope of the Western Depression of the Junggar Basin was determined accordingly. Three sets of regional unconformity surfaces were identified and three sets of structural layers were divided in the Permian to the upper Triassic within the Western Depression, i.e. ,the lower Permian structural layer,the middle Permian structural layer,and the upper Permian-Triassic structural layer. The fault related fold theory and balanced profile technique are used in the fine tectonic interpretation and tectonic evolution analysis. In combination with the previous analysis on thermal history and geochemical characteristics of volcanic rocks in the Junggar Basin,the tectonic evolution of the Western Depression in the Permian-Triassic period was mainly divided into three stages: the extensional rifting of the early Permian and the compression in the late Permian,the weak extensional depression of the Middle Permian,the compression uplift of the late Permian to Triassic period. The tectonic evolution of the western depression belt is spatiotemporally coupled with the anti-clockwise rotation of the western Junggar and Junggar Block. These results are of great significance for revealing the characteristics of the Junggar Basin during the Permian period and the evolution of intracontinental processes since the Permian period.
It is suggested that the spatio-temporal evolution of the Jehol Biota in northeastern North China is driven by the North China Craton destruction during the Early Cretaceous,due to the abrupt changes in paleogeographic environment. However,little quantitative work on the dynamic paleo-landscape evolution in North China has been done. In this study,we employed paleosoil weathering indices(PWI and CFXNa)and carbonate isotopes to reconstruct the paleo-elevation of North China around 145 ma. We then integrated factors such as tectonic movements,sedimentology,paleoclimate,and sea level changes using the Badlands software to model the Early Cretaceous paleo-landscape evolution of North China. Our findings reveal that the eastern North China experienced an abrupt geomorphological transition from the collapse of a paleo-plateau to the formation of the Bohai Bay Basin due to the subduction retreat of the paleo-Pacific Plate. The geomorphological transitions led to the formation of a series of eastward-migrating rifted basins,including several newly-formed isolated intermountain basins in the Yanshan area where the Jehol Biota first emerged. Frequent volcanic activity provides rich nutrients for the lakes,and the paleoclimate turns to warm and humid gradually,which provide favorable conditions for the prosperity of the Jehol Biota. The eastward migrating subsidence basin,eruption of volcanoes and suitable paleoclimate jointly controlled the eastward migration of the Jehol Biota.
The Yangtze River,Asia's largest river,represents a significant geomorphological event within the integrated tectonics-climate-landscape system of the Cenozoic era in China. A key point of debate in understanding its formation is the timing of the incision of the Three Gorges,situated between the Sichuan and Jianghan basins,which marked the emergence of the modern Yangtze River. Despite abundant geological data,there remains controversy over when exactly the Three Gorges were formed or incised. Previous studies usually focused on isolated factor affecting the river development,e.g., tectonic movements,sedimentology,paleo-climate and sea level changes,to resolve this key issue. In contrast,our study utilizes Badlands,a software for simulating paleo-landscape,to integrate these key factors quantitatively. Focusing on the area east of the “first bend”(Shigu Town in Yunnan Province)of the Yangtze River,we used Badlands to reconstruct the paleo-landscape and river system evolution process since the Late Cretaceous(80 Ma). We further validated our model results using seismic data from the Sichuan and Jianghan basins. The results revealed that the river flow direction in the Sichuan Basin was reversed to drain northwards due to the Late Eocene-Oligocene uplift in the eastern Tibet and the southwestern Upper Yangtze Plate. The Jianghan Basin maintained a consistently low base level during the early Paleogene,influenced by the continental rifting environment in eastern China. The reversal of the drainage direction in the Sichuan Basin and the long-lasting low base level in the Jianghan Basin eventually made the Three Gorges to be incised at the latest Oligocene. We propose that the reversal and subsequent capture of the Upper Yangtze River's flow by the middle Yangtze River played a crucial role in the incision mechanism of the Three Gorges.
