Characterestics of Sequence Sedimentary Organic Facies and Distribution of Source Rocks of Majiagou Formation,Lower Ordovician, Ordos Basin

Key words: 
• Ordos basin /
• Majiagou Formation /
• sequence stratigraphy /
• sedimentary organic facies

Abstract: A sedimentary organic facies is defined as a stratigraphic unit, in which there are the similar characteristics of depositional environment, biological assemblage, diagenetic environment, reduction-oxidation condition, and the genetic type of organic matter. As the distribution of sedimentary organic facies is controlled by sequence stratigraphic framework in sedimentary basin, sequence sedimentary organic facies analysis can become a very useful tool to determine and predict the distribution of source rocks. Based on the analysis of sequence stratigraphy and sedimentary organic facies, Majiagou Formation of Lower Ordovician in Ordos basin is divided into three sequences and the distribution of sedimentary organic facies in each sequence is discussed in this paper. The results indicate that the distribution of sedimentary organic facies in sequence stratigraphic framework of Majiagou Formation present a certain regularity. In the western and southern part of the basin, the sedimentary organic facies of deeper- water sequence are mainly composed of open platform C facies from bottom to top, in which the content of organic carbon is 0.08% ～ 0.18 %. In the middle of the basin, the sedimentary organic facies of transgressive system tract of shallow- water sequence are mainly composed of open platform C facies and restricted platform D facies, in which the content of organic carbon is 0.12% ～ 0.25 %, whereas the sedimentary organic facies of highstand system tract are composed of restricted platform D facies and evaporation platform E facies, in which the content of organic carbon is 0.17% ～ 0.49 %.The best source rocks are mostly distributed over highstand system tract of shallow- water sequence in the middle of the basin