The Lithofacies Paleogeogra Phy And Depositional System Of The Cambrian In The Southwestern Margin Of Ordos Basin

Marine carbonate strata is widely distributed with great depositional thickness and great prospect of exploration and development,which has superior oil and gas accumulation condition in the early Palaeozoic era in Ordos basin.In recent years.The researches are mainly concentrated on the Ordovician strata.However,Study on the Cambrian strata is relatively rare.Therefore,The Lower Paleozoic Cambrian carbonate rocks are studied in the southwest margin of Ordos Basin.Combined with previous research achievements,observed outcrop features,measured field profiles,drilling results and logging curves.Based on the theory of sequence stratigraphy,sedimentology,lithofacies palaeogeography.The research will focus on the distribution of Cambrian strata,sequence stratigraphic characteristics,depositional system types and lithofacies paleogeographic characteristics.The understandings and conclusions obtained are as follows:The study area can be divided into western margin and southern margin depositional subareas.The Cambrian stratum system in every depositional subarea is described in detail.The shape of the Lower Cambrian stratum presents a "L" type characteristics,The Middle and Upper Cambrian stratum is widely distributed in western margin and southern margin depositional subareas.both has thinning tendency when it extends to the interior of the basin.Two two-level sequences and ten three-level sequences can be recognized in the Cambrian strata.The first two-level sequence is composed of the Xinji period to the Xuzhuang period.The second two-level sequence is composed of the Zhangxia period to the Fengshan period.During the whole Cambrian epoch,The western margin and southern margin both have experienced twice cycles of sea-level change.The ten three-level sequences are not fully developed.Different three-level sequences are lost in different regions.The sequence stratigraphic characteristics is mainly controlled by relative sea level changes of Qinling-Qilian sea in the southwest margin of Ordos Basin.According to the signs of sedimentary facies and tectonic evolution background analysis.Five depositional systems of Cambrian strata can be distinguished in the southwestern margin of Ordos Basin.Including shoreland,platform,gentle slope,slope and deep trough.The shoreland sediments are mainly found in the Lower Cambrian and the platform sediments are mainly found in the Middle Cambrian to Upper Cambrian.The gentle slope which belongs to even gentle slope is developed in the whole Cambrian.The slope and deep water trough sediments are found on the lateral surface of gentle slope sediments.The depositional evolution model is established on the basis of depositional system features description.The single factor analysis and multifactor comprehensive analysis research technique is applied to describe the lithofacies paleogeography distribution.Firstly.According to the sequence stratum division results.Seven mapping units are confirmed.Secondly.Four factors which include the stratum thickness.the grain content in carbonate rocks.the limestone content and the deep-water carbonate rocks sediments content are chosed to analyze and the corresponding contour maps are drawed.Combined with the single factor comprehensive analysis and depositional evolution process.The lithofacies paleogeography maps of the Cambrian period are presented in the southwest margin of Ordos Basin.In the Early Cambrian,The southwest margin starts to accept transgression.The shore sediments are developed along the continental.The lithology is also gradual transform from the phosphorous clastic rocks to the mixture of clastic rocks and carbonate rocks.In the Maozhuang period and Xuzhuang period of the Middle Cambrian,The transgression still continues,The sea is gradually covering the southwest margin and extending to the whole basin.The central ancient land has been divided.Meanwhile the restricted platform and gentle slope sedimentary facies are widely developed.Since the Zhangxia period,The open platform and gentle slope sedimentary facies are widely distributed.The lithology has been entirely composed of marine carbonate rocks,which is not mingled with the terrestrial debris.When reaching the Late Cambrian period.the restricted platform and gentle slope sedimentary facies are mainly distributed.The central ancient land reappears and dolomite develops,which marks the ending of transgression and the beginning of the regression.