The Deep Marine Hydrocarbon Accumulation Process Under Complex Tectonic Backround In The Northern Segment Of Western Sichuan Depression

Oil and gas exploration in the northern segment of the western Sichuan Depression?NSWSCD?is complex,long-term,and tortuous,and is one of the regions with the longest history of oil and gas exploration in the Sichuan Basin.Following the discovery of the Anyue Gas Field,the NSWSCD is the next important strategic oil and gas succession area for marine oil and gas exploration in the Sichuan Basin.Since the Late Triassic,the uplift of the Longmen Mountain and the formation of the western Sichuan Foreland Basin have caused deep marine formations with deeply-buried and strong-uplift characteristics.The deep background under the complex tectonic feature is the key geological characteristic of marine oil and gas accumulation in the NSWSCD.Based on the complete and comprehensive field geological data,seismic data and advanced geochemical methods,taking the tectonic evolution of Mianyang-Changning Intracratonic Sag?MY-CN IS?and Tianjingshan Paleo-uplift?TJS PU?as the starting point.This study has analyzed the geological characteristics of deep marine formations.Synthesizing the paleo-oil reservoir accumulation sequence of the foreland extension deformation zone to reveal the deep marine oil and gas accumulation process in the NSWSCD,this study finally establish deep marine hydrocarbon accumulation model and mechanism.Research has shown that:?1?Focusing on the evolution of the MY-CN IS and TJS PU,this study has established the early tectonic-sedimentary evolution process in the NSWSCD.The eastern boundary structural analysis at the northern section of MY-CN IS is completed.The NSWSCD is overlapping on the center of MY-CN IS.With a thickness of nearly 500m,which is also as the organic-rich shale sedimentary center of Lower Cambrian Maidiping-Qiongzhusi Fm.The tectonic movement at the Cambrian-Ordovician has a significant performance in the NSWSCD,reflecting as the formation of TJS PU.From tension to compression tectonic-sedimentary structure,the TJS PU is an important node in the Early Paleozoic;?2?After Late Triassic,the uplift of Longmen Mountain and sedimentation of Western Sichuan Depression show complex structural feature of banding strong-uplift and deeply-buried.The Majiaoba Fault is a boundary fault between the northern segment of the Longmenshan trust belt and the NSWSCD.The?1?fault divides the NSWSCD into a frontal-deformation zone on the northwest side and the western Zitong-Jiange Depression on the southeast side.Tectonic-buried history analysis shows that after the Late Triassic,the frontal-deformation zone has typical characteristics of medium buried-strong uplift-strong deformation;?3?According to the distribution characteristics of source rocks and the evaluation of organic geochemical indicators,the Lower Cambrian Maidiping-Qinzhusi Fm is the best source rock.The outcrop and well core analysis show that the Sinian-Permian reservoirs are dolomite-based.Due to the timely crude oil charging and relatively stable structure,the Zn₂dn²,Zn₂dn⁴,and P₂q² formations have become the best deep reservoirs.Seismic interpretation indicates that NSWSCD has a vertical-lateral composite bypassing system centered on the fault-unconformity.The deep and overpressure characteristics formed by oil cracking provide good initial static preservation conditions in the NSWSCD.Characterized by the Middle-Lower Triassic gypsum-salt rocks,the Lower Cambrian marine clastic rocks and thick Upper Triassic-Jurassic continental clastic rocks,which has shown multi-level capping features;?4?Based on the source rocks of the Lower Cambrian Maidiping-Qinzhusi Fm and Upper Permian Dalong Fm in the NSWSCD,bitumen in the Dengying Fm reservoir is used as a template to pass organic geochemical traces of paleo-oil reservoir system with different layers and different phases in the TJS PU area.The source of the entire paleo-oil reservoir is mainly from the Lower Cambrian Qiongzhusi Fm,while the Permian Dalong Fm may have a weak contribution only in the Guangyuan area;?5?Following the characteristics of Cambrian Qiongzhusi Fm as the main source of multi-source hydrocarbon supply,composite seal bypass system based on fault-unconformity and multi-level capping features dominated by the Trassic,the deep hydrocarbon accumulation process models in the NSWSCD can be divided into primary oil reservoir?primary gas reservoir and secondary oil reservoir?primary gas reservoir two types of reservoir-forming evolutionary sequences.The favorable distribution of deep-super deep marine hydrocarbon in the NSWSCD is controlled by typical combination of intracratonic sag-paleouplift and basin-mountain structure.With weakly late tectonic adjustment,either the Qixia Fm in deep Shuangyushi-Shejianhe latent tectonic belt,the structure-sedimentary characteristics of Zn₂dn-?₁ combination in the northern section of the MY-CN IS are similar with Gaoshiti-Moxi area in the middle Sichuan Basin.The high-quality source rocks and reservoirs have excellent reservoir-forming conditions for forming large-scale primary gas reservoirs.