| Source-reservoir-cap assemblage is well-developed in fluvial to lacustrine facies transition,which is an important target for oil and gas exploration.Shallow-water sedimentary systems are extensively developed in shallow water non-marine basins.However,there have been inadequate discussions in the literature on the characteristics and variations of river-lake interaction sedimentary systems.In particular,little attention has been paid to the study of systematic river-lake interaction depositional model and formation mechanism in such systems.The water conditions of the river-lake interaction area change frequently,and the hydrodynamic conditions of the two systems are significantly different in temporal and spatial variations.Thus,the depositional characteristics,depositional models,and hydrocarbon enrichment in the river-lake interaction system are also obviously different from the traditional fluvial and deltaic depositional systems.Traditional deep-water delta is characterized by constantly deep-water and stable facies zones.In contrast,shallow-water lacustrine basins are characterized by flat topography,wide width,shallow-water,frequent water oscillations,large water lateral migration,broad facies zones,and facies vertical reciprocal superposition.River-lake interaction sedimentary systems reciprocal overlapped in-unit geological scale,which made it difficult to be characterized by a single facies zone.In this study,we first innovatively proposed the fluvial to lacustrine interaction sedimentary system,which emphasized the dynamic deposition process of river-lake interaction,to analyze the frequently modified deposition area of river and lake as a dynamic interaction area and independent unit,and to explore and construct the sedimentary model of river-lake interaction deposition in the shallow-water lake basin.The results have important theoretical and practical significance.In this paper,we took the modern shallow-water fluvial to lacustrine deposition of Poyang Lake as the starting point and took the ancient shallow-water river-lake interaction sedimentation of the Neogene Guantao Formation in the eastern Bohai Bay Basin as the main object to conduct the research on the river-lake interaction sedimentary system of Guantao Formation in the eastern Bohai Sea area.An integrated3D seismic data,drilling cores,logging data,major and trace elements,paleontology,and zircon U-Pb dating data were employed to comprehensively analyze by the guidance of stratigraphic stratigraphy,seismic sedimentology,comparative sedimentology,and source to sink system theory.Overall,we have built the high-frequency sequence stratigraphic framework during the shrinking period of the lake basin based on the"qualitative-quantitative"and"well-seismic"combination,restored the three-level sequence-scale source supply process and the temporal and spatial evolution of the multiple source intersection area,clarified the method to divide the sedimentary units of fluvial-lacustrine,established typical sedimentary facies sequence and recognized criteria,systematically dissected the sedimentary characteristics of the interaction of rivers and lakes,summarized development rules and main controlling factors of fluvial to lacustrine deposition,and finally,established the sedimentary model of fluvial to lacustrine interaction system,reconstructed the sedimentary process,explored the hydrocarbon reservoirs enrichment mechanism and predicted the favorable reservoir zones.The following results can be drawn:(1)A sequence stratigraphic framework during the shrinking period of the lake basin was constructed.From bottom to top,the Guantao Formation of the Neogene in Bodong area can be divided into three third-order(SQ1,SQ2,SQ3)and six fourth-order(SQ1-1/SQ1-2/SQ2-1/SQ2-2/SQ3-1/SQ3-2)high-resolution sequences and paleo-morphological framework.The interior of the sequence changes episodic and the ancient landforms transform from inherited separated landforms to gently connected landforms correspondingly.Based on basin tectonics,paleo-geomorphic features,and fault styles,three primary structural-sedimentary units,A(extensional fault control zone),B(strike-slip clamping zone),and C(stable depression zone),were identified,which is corresponding to the river-dominated area,river-lake interaction area,and lake-dominated area.(2)The paleogeographic background of shallow water deposits in Guantao Formation was systematically analyzed.During the deposition of the Guantao Formation,the Bohai bay basin belongs to a subtropical monsoon climate that is warm and humid.For the early stage of the Guantao Foramtion(SQ1),both temperature and humidity show a gradually decreased trend;In the middle depositional period of Guantao Formation(SQ2),the temperature and the humidity increased;In the late depositional period of the Guantao Formation(SQ3),the temperature and humidity decreased again.The topography is gentle with the slope less than 1°,the lacustrine basin is broad and shallow with the average water depth of 12.4m,which show a typical characteristics of a shallow continental lake basin.(3)The spatial and temporal provenance interaction of multiple sediment sources in continental down-warped lacustrine basins was reconstructed.The Triassic detrital zircon ages together with 170-180 Ma seem to be unique from the Liaodong Uplift,which can be used as effective provenance footprint for differentiating potential source areas.The northern part of the Bodong area generally exhibits relatively complex age spectrums and is characterized by multimodal patterns,which indicate that the sediments were mainly derived from the Liaodong Uplift.In contrast,the sediments in the southern part of the Bodong area commonly exhibit markedly different age populations with the absence of 170-180 Ma and Triassic age populations.We interpret this to suggest that the detritus was sourced from the Jiaodong Uplift.The provenance intersection area migrated from north to south corresponding to the sedimentation of the Guantao Formation from N_g~L to N_g~U.The provenance interaction area was thus constrained in the PL19 structure.Spatial analysis of the results suggests that sediment contributions from the Jiaodong Uplift gradually reduced whereas that from the Liaodong Uplift continued to enhance.(4)The method to divide the sedimentary units of fluvial-lacustrine was proposed.The characteristics of different sedimentary units in the shallow water river-lake interaction in Poyang Lake were analyzed.Taking on the highest flooding lake shoreline and the lowest dry season lake shoreline as the boundary,we effectively-identified three sedimentary units,namely the river-dominated area(A),the river-lake interaction area(B),and the lake-dominated area(C).The river-dominated area(A)always above the highest flooding lake shoreline is characterized by without influence of the lake,stable skeleton drainage system and channel sand body,positive rhythmic cycle,occasionally isolated point water body;The river-lake interaction area(B)between the highest flooding lake shoreline and the lowest dry season lake shoreline is a dynamic interaction area which characterized by complex sand bodies with positive and negative rhythm superposition,strong water segmentation,and multiple banded water units;The lake-dominated area(C)below the lowest dry season lake shoreline is characterized by a multi-center(different terrain depth)sheet-like water distribution pattern,lacustrine mudstone with thin laminated sand.(5)The sedimentary model of fluvial to lacustrine alternating sedimentary system was constructed.For the early deposition stage of the Guantao Formation,braided river delta(SQ1)and shallow water delta(SQ2)sedimentary models were developed in the Bodong Area with sedimentation derived from intrabasinal near-source and local denudation sources of basin margin;In the late period,with the shrinking of the lake basin and filling up and the decreased difference of geomorphic pattern,the meandering river,and shallow water delta depositional model was developed,which was dominated by relatively distant sources.(6)The favorable reservoir zones in key areas were predicted based on seismic sedimentology analysis.We proposed a new multi-method interactive comprehensive method to predict the favorable reservoir belt in the PL19-20 structure by plane partition(block)and vertical segmentation.For the SQ1 and SQ2,the wave impedance of sand and mudstone is similar,which makes it difficult to differentiate the sand and mudstone using the seismic data.Thus,we proposed a reservoir prediction method that is dominated by lithologic assemblage combined with seismic-based analysis,including the lithologic assemblage,seismic facies analysis,frequency division inversion;For the SQ3,it is easy to differentiate the sand and mudstone using the seismic data.Therefore,we proposed a reservoir prediction method that is dominated by seismic-based analysis combined with lithologic assemblage,including RGB frequency division fusion,multi-attribute fusion,frequency division inversion,drilling calibration.The sedimentary characteristics of the PL20 sandstone derived from two sediment routing systems(PL20-2 and PL20-3)are dominated by frequent lamellar and dendriform interaction and superposition.The dominant sand body is developed in the main channel zone.In the early deposition stage of PL19,braided river delta deposits supplied by Jiaodong were developed,and the provenance was converted in the late stage,then the relatively long-source meandering river transport deposits were dominant.In the early stage,the favorable reservoir sand body of the PL19 were deltaic front lobes developed in the southwest of PL19.However,for the late stage,the favorable reservoir sand body was concentrated in the meandering river channel in the northeast of PL19. |
PDF Full Text Request