| The object of this study is carbonate shoals that are extensively developed in the Ordovician of the Tarim Basin,China,and the main objective is to explore the formation mechanisms and the development regularitys.This study employs the theories of Sedimentology,Sequence Stratigraphy,Paleontology,Geochemistry and Geophysics.The following conclusions are drawn through the multiple means,including field geological investigation,core observation,thin sections identification,and the analyses of well logging data,scanning electron microscope,cathodoluminescence,grading test,and geochemical analyses.The quantitative classification of shoals in the Ordovician Tarim basin is established.It is point out that four types of shoals are developed,namely,intraclastic shoal,boiclastic shoal,oolitic shoal,and algal shoal.The shoal identification methods based on the conventional logging and FMI logging are clarified.The vertical development characteristics of four types of shoals are established,and it is demonstrated that intraclastic shoals,oolitic shoals and algae shoals show progradation sequences,and the bioclastic shoals are in the sequence of progradation or retrogradation.Twelve shoal combination patterns are built,and they are four types of reef-shoal combinations(reef-bioclastic shoal,reef-algae shoal,reef intraclastic shoal and reef-oolitic shoal),six types of shoal-shoal combinations(algae shoal-intraclastic shoal,algae shoal-oolitic shoal,boiclastic shoal-intraclastic shoal,bioclastic shoal-ooiltic shoal,intraclastic shoal-oolitic shoal,and intraclastic shoal-rudaceous shoal),and two types of restricted shoals(lagoon-intraclastic shoal and lagoon-algae shoal).Each combination pattern shows the sedimentary cycle of upward coarsening that indicates the process of sea-level drop.It is founded that different sedimentary combination patterns are developed in different sedimentary environments: platform margin mainly develops the reef-shoal combination followed by the shoal-shoal combination;Open platform mainly develops shoal-shoal combination and then reef-shoal combination;Restricted platform develops restricted shoal combinations.Through summarizing the difference among these shoal types,the shoal sedimentary abundance distribution model is established for the Ordovician of the Tarim Basin.Detailed analyses show that shoals are well-developed in the platform margin with large thickness and limited horizontal extent,and the main shoal type is bioclastic shoal that is developed in the backreef area.The second shoal type is algae shoal that is vertically interbedded with reef,and their areal distribution are sill-like;Shoals developed in the open platform facies are thin but with large horizontal extent,and the main shoal type is intraclastic shoal,which shows gently moundy or sill-like spatial characteristics.The followings are bioclastic shoals and oolitic shoals developed in the highland near the platform margin.Rare shoals are developed in the restricted platform,and are mainly intraclastic shoals and algae shoals,which distributed in striped shape that parallel the shoreline.It is believed that all four shoal types are directly controlled by hydrodynamic and are driven by sea level changes.Intraclastic shoals are also controlled by the micropaleogeomorphology that is driven by tectonic movement;Bioclastic shoals are also controlled by the development of the biological reef that is driven by platform structure and palaeoclimate;Oolitic shoals are affected by the paleosalinity and Ph;Algae shoals are also controlled by the development of phytoplankton.In addition,different types of shoals are interbeded and formed the overall spatial pattern of shoals.This study suggests that the carbonate shoal facies is more conducive to the reservoir development than the non-shoal facies.The controlling factors for the development of reservoir that related to shoal faices include material basis,sequence interfaces,dolomization and tectonic movement. |
PDF Full Text Request