Paleogeographic Pattern of the Carbonate Platform in the Middle-Upper Yangtze Area During the Deposition of the Ediacaran Dengying Formation and Distribution Pattern of the Reservoir Facies
[Objective]The dispute in the paleogeographic pattern of the carbonate platform and distribution of reser-voir facies during the deposition of the Dengying Formation in the middle-upper area,hinders the expansion of the ex-ploration domain from the Mianyang-Changning intracratonic sag to other areas.[Methods]Detailed lithofacies and facies analysis was conducted on more than 30 sections(wells),[Results]further confirming that the mound-shoal is marked by massive peloidal/ooidal dolograinstone and columnar,domal dolostromatolite,which were deposited around the platform(or inner ramp)margin of the Middle-Upper Yangtze Platform gradually shifting outward into tempestite of the middle-outer ramp or slump dolobreccia,argillaceous dolomite and chert of slope-basin facies.Ow-ing to the mound-shoal barrier,its back was dominated by tidal flat and lagoon.Of these,the tidal flat facies were marked by microbial dololaminite,domal dolostromatolite,dolothrombolite with minor peloidal dolograinstone,and dolomudstone,which are commonly arranged into meter-centimeter scale cycles.The lagoon facies were composed mainly of dolomudstone and peloidal dolowackestone.In general,the mound-shoal facies is present in the lower Dengying Formation along the platform to slope transition(or inner ramp to middle ramp transition).In the Upper Yangtze Platform,it occurs in the Deng 1(e.g.,wells Pengtan 1,Ziyang 1,and sections Qingping,Beidoushan)or Deng 2 members at the Baijiaba,Yulin,and Meiziwan sections,or in both the Deng 1 and Deng 2 members at the Songlin-Yankong areas.On Middle Yangtze Platform,it occurs in the Hamajing(e.g.,sections Sancha,Bailuya,and Tianping)or Baimatuo members of the upper Dengying Formation at the Miaohe section.Specifically,the mound-shoal facies shows repeated transgressions and regressions and occurs in different stratigraphic position of the Dengying Formation at many sections(Baiguoping,Xiaofenghe,Dengyingxia,Wushan,Xuejiadian,and Dengjiaya).In contrast,the tidal flat and lagoon facies are widely present on the whole Middle-Upper Yangtze Platform,commonly associated with the grape-like dolomite at the lower-middle part of the Deng 2 member(or correlated stratigraphy).The spatiotemporal distributions of these facies show a progradational trend of the Dengying Formation on the Middle-Upper Yangtze Platform,which consists of 2.5 depositional sequences with three progradations(shallowing)and two retrogradations(deepening).The regressive system tract of Sequence 1 is formed by the lower Dengying Formation(the Deng 1 to middle Deng 2 or Hamajing members),characterized by the upward evolution of lagoon or mound-shoal facies into tidal flat facies.On the Upper Yangtze Platform,Sequence 2 occurs in the upper Deng 2 member,marked by the regression of mound-shoal facies and the presence of more lagoonal dolomudstone,followed by the reoccurrence of tidal flat facies.On the Middle Yangtze Platform,Sequence 2 is present in the Shibantan member,represented by the deepening into middle ramp limestone or lagoonal dolomudstone and then shallowing into mound-shoal or tidal flat facies.Sequence 3 on the Middle Yangtze Platform shows a similar deepening(transgression)and shallowing(regression)cycle.On the Upper Yangtze Platform,the transgressive systems tract of Sequence 3 is located in the siliciclastic rock of the Deng 3 member and the following transgressive systems tract is marked by the transition of lagoonal dolomudstone to tidal flat dolomicrobialite.[Conclusions]Owing to the denudation of the Dengying Formation at the platform margin-slope area and Upper Yangtze Platform beyond the current plate boundary,the plat-form marginal mound-shoal facies are locally present in the Deng 4 member.In this case,certain areas(e.g.,the Songlin-Yankong area and periphery of the Middle Yangtze Platform)with platform marginal mound-shoal facies in the lower-middle Dengying Formation(Deng 1 to 2 or Hamajing to Shibantan members)are important exploration targets.Moreover,the lower Deng 2 and upper Deng 4 members(or coeval strata)formed during progradations composed of microbial dolomite throughout the Middle-Upper Yangtze Platform(or inner ramp)interior are also important exploration targets.Because the microbial dolomite was widely developed throughout the Middle-Upper Yangtze Platform interior,decoding the formation mechanism and distribution pattern of high-quality reservoir is the key to finding new hydrocarbon provinces.
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