Deposited Sequences And Reservoir Genesis Of Penglaiba Formation In Maigaiti Slope And Surrounding Areas

The Middle Lower Ordovician carbonate reservoir in the Maigaiti area has great exploration potential.To clarify the sedimentary facies distribution and reservoir genesis of the carbonate rocks in the Penglaiba Formation,We based on the drilling,core,and thin section data of the Penglaiba Formation drilled in the surrounding area mentioned by Maigai,this paper conducts research on the sedimentary environment,sequence framework,and reservoir genesis.The research results are of great significance for clarifying the sedimentary model and multiphase dolomitization of the Penglaiba Formation in the Maigaiti area.Based on the division and establishment of lithofacies and lithofacies association of Penglaiba Formation,the sedimentary model of carbonate rocks is proposed and the main controlling factors of sedimentary facies development are defined.There are eight types of lithofacies in the Maigeti area,including granular cloud rock and grain cloud rock.The sedimentary facies are mainly confined platform and open platform,and are further divided into granular beach,interbeach sea,cloud flat and other microfacies.During the whole period of Penglaiba Formation,the sedimentary pattern in the study area gradually changed from limited platform facies to open platform facies.On the basis of microfacies analysis,the high frequency sequence stratigraphic framework of Penglaiba Formation was established,and the sedimentary microfacies evolution in the sequence framework was identified.The Penglaiba Formation as a whole can be divided into a third order sequence,and three fourth order sequences are identified within the Penglaiba Formation.The micro relative ratio of sequence lattice in Lianjing profile shows that confined platform cloud flat and high energy platform grain beach are mainly developed in SQ1 period,while open platform interbeach sea and medium-low energy grain beach are mainly developed in SQ2--SQ3 period.In the plane,the open platform is characterized by a large area of interbeach sea and granular beach facies.The restricted platform is characterized by grain beach and cloud flat microfacies.Based on the study of sequence and sedimentary facies,according to the characteristics of reservoir type and reservoir space,it is clear that the reservoir in Maigeiti area is mainly dolomite reservoir,and the reservoir space mainly develops intergranular pores and intergranular dissolved pores.The classification of reservoir evaluation is mainly class Ⅱ and Class Ⅲ.According to the distribution of reservoir in the high frequency sequence framework,the sedimentary microfacies such as grain beach and cloud flat are the basis of reservoir development.By using laser ablation inductively coupled plasma mass spectroscopy(LA-ICP-MS)to test major,trace and rare earth elements in different structures of the main lithofacies in the study area,it is found that dolomitization fluids can be divided into two classes.The class,characterized by low values of Fe and Mn and high values of Sr and Ba,was the main fluid of the perissyngenic to early diagenetic period.The other class of fluids is characterized by high Fe,Mn,low Sr,and Ba,and is the dominant fluid in the sustained burial phase.The micrite dolomitization was formed by micrite in the quasi-syngenetic period,and the granular dolomitization was formed by the return infiltration dolomitization of granular limestone in the early diagenetic and shallow burial stage.Influenced by dolomitization in early diagenetic period,these two types of dolomites are usually better able to preserve matrix pores.During the burial period,some granular and silt-fine crystalline dolomitization formed mesocrystalline dolomitization,and the continuous growth of dolomite resulted in the destruction of the original pore structure of crystal.The comprehensive analysis shows that the best reservoir is the granular dolomitization which preserved the early matrix pores under the high energy granular beach environment in the early diagenetic stage.