Sedimentary Facies Of The Fourth Member Of The Ordovician Majiagou Formation In The Southern Ordos Basin

In recent years, Ordovician Majiagou Formation multiple hundreds million tuns gas field discovered in the Ordos basin,showing the carbonate reservoir rich oil and gas resource potential, but mainly concentrated in Member5of Majiagou Formation.In this paper, to deal with the deficiency of sedimentary facies studies on Ma4Member of Majiagou Formation, Ordovician in the south of Ordos Basin, guided by the theories and methods of multiple disciplines such as sedimentology, petrology, tectonic geology and geochemistry, combined with current status of studies and exploration in the area, the expected targets of the selected subjects are completed and the fundamental basis of depositional model established through in-depth analysis and studies taking the sedimentary facies of Member4of Majiagou Formation of Ordovician in the south of the basin as the main content of study using a variety of technical means combined with field cross sections, downhole cores as well as logging and laboratory analysis authentication data, obtain the following main conclusions. Through integrated studies on surface outcrops, downhole cores and laboratory analysis data, facies indicators are identified. Member4of Majiagou Formation in the study area is generally classified as a depositional environment of carbonate platform and trough only at the west and south in the basin. The platform facies is specifically divided into6subfacies and11corresponding microfacies including platform, depression, shoal, tidal flat, slope, platform edge; trough is divided trough subfacies and2microfacies.On the other side,the microfacies sequence as well as the sedimentary environmental models of the middle-east and the west are established respectively based on longitudinal progressive changes and planar distribution of microfacies. On this basis, through facies cross-section analysis and depositional thickness analysis, the lithofacies paleo-geographic features of Member4of Majiagou Formation in the study area is reproduced.