Ordos Basin, Laoshan - House Village Triassic Yanchang Formation Depositional Environments And Palaeogeography

Ordos Basin is a polycyclic superposition basin as well as the second-largest sedimentary basin on land in China which possesses abundant oil & gas resource and has bright prospect in oil & gas exploration. It has been becoming one of the important tactical area in oil & gas exploration and exploitation in China. The Yanchang Formation more than one kilometer in thickness is the main oil-bearing beds in the study areas.Its lithology characteristic indicates that the formation was deposited in fluvial-lacustrine environment.On the basis of the theories of sedimentary facies and petroleum geology, by means of geologic prospecting and research indoors ,the paper ,for the first time,study the stratum, sedimentary system and sedimentary facies of the Triassic Yanchang Formation in Laoshan and fucun areas ,and provide a solid base for the future geological development.The fluvial,delta,and lacustrine sedimentary system was identified according to the analysis of the fossils,sedimentary structure,sedimentary cycle,rhythmic bedding and other criteria in the Yanchang Formation.Opened out the rules of distribution of sedimentary facies and the rules of the evolution of palaeogeography in the research areas.Integrate with the district geological material,set up the stratum system of the areas.The Yanchang Formation of Laoshan and fucun areas oil field are divided into five lithologic segments and ten oil-bearing packages in terms of the features of lithology.And the time correlation in sequence stratigraphic unit was realized according to the temporal rock macker. The result indicates that the stratum thickness of the Yanchang Formation is increase from northeast to southwest, the depth of stratum isolines are basically paralleled.Research results indicate that sedimentation of the Yanchang Formation envolved from delta to fluvial sedimentary system.Changl and Chang3belong to fluvial sedimentary system.In detail,Changl is swamp and Chang2 is mainly of meandering river and braided channal partly. The channel microfacies,altered with the flood plain microfacies are distributed in the trend of NE-SW direction.The depositional patterns of the three sub-oil-bearing beds (Chang-21, 22and 23) are basically the same,except the location of the channels shifted slighted during different geological times when Chang-21, 22and 23were deposited.Chang4+5 and chang6 are featured by delta facies with shallow lacustrine facies in the southwestern part of the study areas.The Chang-64 is mainly delta-front facies,which is characterized by the connected distribution of subaqueous distributary channels.Whereas the chang63"61 are composed of delta plaint deposits with the distributary channels extended in the trend of NE-SW direction.The provenance might comes from northeast direction.