Depositional System And Trap Development Model Of The Paleogene,Gentle Slope Of The "Island Chain" Liaoxi Uplift

The segement nature of the Liaoxi uplift has a clear response in the physical and deposits characteristics in the gentle slope zone,which in turn affects the type and distribution of the sand.Despite the progression of research on the"island chain"features,a dearth of systematic investigation on their evolutionary trajectory has limited our understanding of this phenomenon.Furthermore,a lack of comprehensive analysis of the types and spatial distribution patterns of the depositional system and trap under the control of this feature has impeded the exploration of lithologic-stratigraphic traps in the region.According to the principles of sequence stratigraphy,the 3rd member of Shahejie Formation-2rd member of Dongying Formation are divided into five third-order sequences（SQs₃,SQs₂,SQs₁,SQd₃,and SQd₂）,while the SQd₃and SQd₂are divided into three system tracts,each consisting of five and four quasi-sequence sets,respectively.On this basis,it was explained that during the process of"island chain",the stratigraphy of the Shahejie Formation was well developed on the uplifts but poorly developed in the middle and southern slopes,and thin in the northern segment.The Dongying Formation had well-developed stratigraphy on both the uplifts and slopes,with steeply dipping layers in the middle and southern segments and gently dipping layers in the northern segment.Furthermore,a study was conducted on the stratigraphy of the Dongying Formation in the central and southern parts of the slope zone,where the northern part is filled with mud.Six slope-fold systems and sequence patterns were established,consisting of two tectonic slope-folds（"island chain"and boundary faults）and three slope faults.Based on the characteristics of core,logging,and seismic facies,four types of sedimentary systems,including（near-and far-source）braided river deltas,carbonate platforms,and sublacstrine fans,were identified in the gentle slope zone.On this basis,it was clarified that the depositional system in the gentle slope evolved from the near-source braided river delta,carbonate platform in the northern segment to the sublacstrine fan,far-source braided river delta in the middle and southern segments,as the Liaoxi Uplift changed from an island chain in the middle and southern segments and an uplift in the northern segment during the Shahejie Formation period,to an island chain in the middle and southern segments and disappearance of the island chain in the northern segment during the Dongying Formation period.Meanwhile,the controlling effects of slope and grain size on the morphology of the sublacustrine fan were revealed,with steep slope and rich sand forming a tongue-shaped fan,gentle slope and rich sand forming a lobe fan,gentle slope and rich mud forming a channelized fan,and gentle slope and mixed sand-mud forming a leaf-shaped fan.Furthermore,the control mechanism of sand enrichment of the sublacstrine fan by coupled transport systerm（multi-stage fault,sedimentary slope folding,and single-stage fault）during the high lake level period is elucidated for the source channels under the control of"island chain".Using seismic inversion technology,four types of deposits sand were identified and characterized,and it was clarified that sand bodies,unconformities,and faults are the three elements for delineating reservoir traps.Expanding on this,a comprehensive classification of traps in the gentle slope zone has been established.The identified trap types include unconformity traps,sandstone upturned nose-out lithology traps,sandstone lens-shaped lithology traps,single fault structural traps,block traps,structural-stratigraphic traps,and structural-lithologic traps,resulting in a total of seven distinctive trap categories.Combining with the evolution characteristics of the uplift,the coupling mechanism and distribution patterns of four types of trap were revealed.These included the"island chain source supply"unconformity trap,formed by near-source braided river delta sand;the"island chain ingress"unconformity trap,formed by clastic shoal sand;the"island chain channel"lithology trap,formed by sublacstrine fans sand;and the"island chain disappearance"structural traps,formed by far-source braided river delta sand.