The Developmental Characters of Sedimentary Environments and Lithofacies of Middle Ordovician Majiagou Formation in Ordos Basin

Key words: 
• Ordos basin /
• Middle Ordovician /
• Majagou formation /
• sedimentary model /
• lithofacies paleogeography map

Abstract: Ordos basin is located in the west of north china platform, middle Ordovician Majiagou formation is one of most important nature gas reservoirs. In Ordovician the overspread of the southern Helan rift and left strike slipping of the southern overthrust led to rift shoulders being warped and rise in the west and south of Ordos basin, and “L” sharp uplift or names of center uplift was formed. At the same time, as a result of equilibrium activity, in the east of the basin formed an unsymmetrical compensation basin (depression), which is named inner shelf basin. Two different sedimentary models are presented, one is sedimentary model of shelf and another is sedimentary model of distally steeped ramp. The sedimentary model of shelf is extend from east to west and pass through the center of inner shelf basin, which runs through main natural gas reservoirs area of Majiagou formation. The east part of three sub environments, that's to say, inter uplift lecuna and under-water uplift, inner shelf basin, open inner shelf, the latter most located out of the research area. In Majiagou 1,3,5 members depositional stage, the climate was dry and sea level was fall by the evaporation, the L-sharp uplift was close to the sea level or above it, or became the erodedarea, the inner shelf basin became semi-restricted or restricted environment. In the center of the basin deposited anhydrock, halilith and some dolostone compounds, in the basin marginal flat was deposited dolostone interbeded with anhydrock. In 2,4 stages, the climate was moist and sea level raised, the inner shelf basin became dolostone limestone-deposited limestone or dolo-limestone. In Majiagou 6 stage, because most of stratigraphy few wells, there's limestone shelf sedimentary environment. The west shelf lies on the L-sharp uplift, in all of the Majiagou stage was deposited limestone or dolostone. The steep slop, along Helan rift margin, developed carbonate gravity flow stones. However, in submarine through developed turbidites mixed with carbonate and silicon debris. The distally steepened ramp developed in all of Ordovician lies on the south of L-sharp uplift, that's to say, present Weibei uplift area. In 1,3,5 members depositional stage, there is dolstone distally steepened ramp. However, in members 2, 4, 6 present dolostone-limestone distally steepened ramp. According to the sedimentary model, in this paper present lithofaices paleography map from 1 to 6 member of Majiagou formation.