| Ordos Basin is one of the most potential basin for natural gas exploration inChina, with a long tectonic and sedimentary evolution history. The forming andevolution of central paleouplift played extremely important role in the tectonicevolution of whole basin, and is connected with natural gas accumulation closely.With application of geophysics, sedimentary, sequence stratigraphy, petroleumgeology and basin analysis, based on quantity drilling data, outcrop and logging,focused on early Paleozoic era of central paleouplift and its surrounding area, thispaper have studied geodynamics, forming evolutionary mechanism of paleouplift,sedimentary characteristic, sequence stratigraphy framework, sequencepaleogeographic, accumulation effect and sedimentary filling, and revealedaccumulation and distribution regularity of different sediments in forming process ofcentral paleouplift and its surrounding area. The main achievements are as follows.First, based on predecessors’achievements, to check lower Paleozoic stratumunit of Ordos Basin, to divide Cambrian and Ordovician stratum of lower Paleozoic,and establish stratigraphy division plan and comparison framework.Second, based on comprehensive analysis on data of outcrop, drilling core andlogging, by study on sedimentary facies mark, the lower Paleozoic sedimentarysystem is divided in central paleouplift and its surrounding area of Ordos Basinduring the forming and evolution process of central uplift. Six sedimentary systemsand several sedimentary units were distinguished, which include shore, shelf,carbonate ramp, carbonate platform, continental slope and deep sea basin.Third, based on study on sequence boundary characteristic, genetic type ofsequence boundary and sequence division mark, sequence stratigraphy have beendivided in central paleouplift and its surrounding area, it can be divided into5super sequences and27third-order sequences, which contain2super sequences and12third-order sequences in Cambrian and3super sequences and15third-ordersequences in Ordovician. Then, the characteristic of each super sequence is studied,and sequence is compared in each different period.Fourth, the morphology change of central paleouplift is described fine, withapplication of latest data of drilling, logging and seismic. By analysis onpaleogeographic evolution period and its difference of Ordos Basin, it presented thatthe base formed in middle to late Proterozoic, paleouplift begun to appear rudimentin early to middle Cambrian, formed in late Cambrian, developed in earlyOrdovician, uplifted to erosion in middle to late Ordovician and withered away afterCarboniferous.Fifth, based on study of sedimentary facis characteristic and establishment ofsequence stratigraphy framework, taking third-order sequence as mapping unit, havestudied sequence paleogeographic characteristic of each stage during the evolutionof paleouplift. The results show, it was mixed sedimentary environments ofcarbonate and clastic rocks of ramp in stage of∈SQ1to∈SQ3. Transgression waslasting and4old lands developed in stage of∈SQ4to∈SQ8, sedimentaryenvironment transited to epicontinental sea, and the central paleouplift emerged therudiment. Huge cratonic basin formed in stage of∈SQ9to∈SQ12. Sedimentsdistributed around the paleouplift, and it was carbonate ramp around uplift in stageof OSQ1to OSQ3. Due to blocking by paleouplift, west side of paleouplift wascarbonate ramp, southeast side was epicontinental sea in stage of OSQ4to OSQ5, itdeveloped gypsic saline lake. Part of center of paleouplift drowned by water anddeveloped carbonate tide flat in stage of OSQ6. Water covered most of basin andgypsic saline lake vanished in stage of OSQ7, One set of saline lake developed againin stage of OSQ8. Denuded zone formed in stage of OSQ9, one set of carbonateplatform and basin facies developed along edge of uplift denuded zone. Most ofbasin were uplifted and weathered to erosion in stage of OSQ10to OSQ11. Westedge was deep water shelf to continent slope, and south edge was carbonate platformto basin with development of reef. Margin reef and shoal developed in platform edgein stage of OSQ13to OSQ14.Sixth, accumulation effect controlled by central paleouplift reflected in manyaspects. The main type of Cambrian source rock was dolomite which distributed likeshape of L surrounding the central paleouplift, the thickness became thinner fromeast to west and south to north. Ordovician source rock was shale and carbonate of Majiagou Formation and shale and marlstone of Pingliang Formation, the thicknessof source rock and distribution of abundance of organic matter were all controlled bycentral paleouplift obviously, it was thinner near the paleouplift and thicker far awayfrom it. The forming and evolution of central paleouplift controlled distribution ofsedimentary facies, as well as regional distribution of different reservoir. Dolomitereservoir mainly distributed in Emeng uplift, central paleouplift, saddle ofSanmenxia-Funiu uplift. Oolitic shoal reservoir of Zhangxia Formation distributed insouth of central paleouplift on some scale. Reservoir of weathering crust mainlydistributed in slope of paleouplift. According to continuity of cap rock, it can bedivided into areal one and local one. Areal cap rock mainly developed in upperPaleozoic, with feature of wildly distribution, high thickness and good lateralstability. Local cap rock developed in lower Paleozoic and distributed in small area.Seventh, by study on sediments distribution pattern of central paleouplift and itssurrounding area, it reveals sediments filling process in each evolution stage of centralpaleouplift and the different within each stage. The characteristic is as follows, centralpaleouplift emerged rudiment in early Cambrian, sedimentary environment transitedto mixed sedimentary environments of clastic rocks and carbonate from clastic rocksof continent margin. Transgression was speeding up and4old lands developed, thesedimentary was still epicontinental sea, oolitic shoal appeared in Zhangxia stage. Itwas forming period for central paleouplift and huge cratonic basin formed in lateCambrian. Central paleouplift began the period of adjusting and developing in periodof early Ordovician. Paleouplift almost covered the whole study area in early periodof early Ordovician, the nature of continental margin transformed from passive toactive in Majiagou stage, due to the developing central paleouplift, which wasseparated into the east and west side of paleoocean, west side of paleouplift wascarbonate ramp, the east was carbonate epicontinental sea environment. The lastsediments filling process of early Paleozoic began from Pingliang stage of middleOrdovician. It was uplifting period of paleouplift. The different sediment pattern waslike this, west side was deep water shelf to continent slope, south side was carbonateplatform to basin, with reef developing at edge of platform. It was open platformalong the erosion area and margin reef shoal along the other edge in late Ordovician.Water retreated and lower Paleozoic ends sedimentation since the period... |
PDF Full Text Request