Community paleoecology formed by bioturbation and its control over reservoir development in Ma 4 Member of Ordovician Majiagou Formation in eastern Ordos Basin

The Ma 4 Member of the Ordovician Majiagou Formation in the Ordos Basin was formed during the period of maximum marine transgression sedimentation, belonging to a normal marine sedimentary environment, and having extremely abundant marine organisms. Strong bioturbation structures have formed in carbonate sediments, especially in lime mud sediments. Based on a comprehensive study of outcrop geological profiles in the eastern margin of the basin, and a large number of core data of exploration well drilled within the basin in recent years, this article analyzed systematically the characteristics of bioturbation structures（trace fossils）and possible paleoecological characteristics of the community in the Ma 4 Member of the eastern basin, and explored the control effect of biological disturbance on the development of rock reservoirs. The main understandings had been formed as follows:①The biological disturbance structure in Ma 4 Member has the characteristic of universal development, which is mainly manifested in the continuous development of biological disturbances in most layers vertically and their widespread distribution horizontally. This indicates that benthic communities had abnormal activity throughout the sedimentary development process, presenting as a basically similar living, life and ecology scene that continues endlessly without interruption throughout a long period of geological history;②There are various types of biological disturbance structures in the Ma 4 Member of this area. Based on the identification of fossil fragment categories and fossil burial analysis, combined with the research on bioturbation structures（trace fossils）, it is believed that the strong bioturbation structures in the Ma 4 section of this area are mainly caused by creeping worms, gastropods, trilobites, and ostracods, etc. Through the combination of further research on rock microfacies, the Ma 4 Member of this area can be divided into five typical types of paleontological communities（community paleoecology）. The evolutionary sequence also represents the main stages of ancient ecological environment（sedimentary environment）evolution from early open sea to late algae mats of supratidal dolomite flat.③The biological disturbance extensively developed in Ma 4 Member of this area provided the basic conditions for selective dolomitization, further more, provided a basic guarantee for the formation and effective maintenance of reservoir pores. Therefore, the biological disturbance indirectly controlled the development of effective reservoirs.