Reconstruction of paleo river systems and distribution of sedimentary facies of Shanxi and lower Shihezi formations in southern Ordos basin

The study area is located in the south of Huanxian county,in Yan'an and Puxian counties and to the north of Xi'an.The Shanxi and lower Shihezi formations are important gas-bearing formations.Given our analysis of the direction of rivers,the contents of stable heavy minerals and of feldspar of palaeo river systems,the study area is divided into six palaeodrainage patterns corresponding to six feldspar regions and six sedimentary facies regions.On this basis,the distribution of sedimentary facies was also analyzed.During the Shanxi stage,a delta front was deposited in the Huanxian region and delta plains and fronts were deposited in the Pingliang,Chunhua-Yaoxian and Hancheng-Chengcheng regions.In the Yan'an-Daning region,only a delta front was developed.The distribution of sedimentary facies in the earlier Shihezi stage originated from the Shanxi stage.A delta front was developed in the Huanxian region while a delta plain and front developed in the Pingliang-Zhenyuan region during the same time.Lakes originated only in the Zhenyuan-Huanxian-Huachi-Zhengning and Daning-Jixian zones.This analytical method shows that different palaeodrainage patterns can be effectively distinguished in order to forecast sedimentary facies.

Sedimentary architecture of a sandy braided river with seasonal hydrodynamic variations:insights from the Permian Lower Shihezi Formation,Ordos Basin,China

A sandy,braided river is a typical type of river that exists in ancient and modern alluvial plains and is inherent with significant seasonal water discharge variations.The variations play an important role in the depositional process and the formation of the sedimentary architecture of braided rivers.In this paper,a braided river outcrop along the Yellow River in Fugu is used to describe the effects of seasonal hydrodynamic variations on braided river sedimentary architecture.The results show that the braided channel network exhibits two different patterns during flood period and normal period.During flood periods,the main braided channels surrounding channel bars and the secondary braided channels distributed on the top of the channel bars coexist,forming a highly braided channel network.Migration of the main braided channels control the formation of middle channel bars and side bars.The generation and evolution of the secondary braided channels reformed the upper part of preexisting channel bars and produced affiliated bars along their flow path.During the normal period,water levels decrease,causing the secondary river channels to be abandoned and forming abandoned channels,and only the main braided channels stay active.In the long term sedimentation process,strong water flow during the flood period continuously erodes pre-existing sediments and forms new sediments,while weak water flow during the normal period can only reform the main braided channels and their adjacent channel bar sediments.Based on differences in sedimentary processes and associated hydrodynamic conditions,braided river sediments are divided into two combinations.The strong hydrodynamic combination includes main braided channels,middle channel bar,and side bar,while the weak hydrodynamic combination includes secondary braided channels,abandoned channels,and affiliated bars.The proportion of strong hydrodynamic combinations is much larger than that of weak hydrodynamic combinations.Based on this,we construct a braided river sedimentary architecture model that is helpful for the fine characterization of subsurface oil and gas reservoirs.

Tracing of natural gas migration by light hydrocarbons:A case study of the Dongsheng gas field in the Ordos Basin,NW China

Based on the analysis of light hydrocarbon compositions of natural gas and regional comparison in combination with the chemical components and carbon isotopic compositions of methane,the indication of geochemical characteristics of light hydrocarbons on the migration features,dissolution and escape of natural gas from the Dongsheng gas field in the Ordos Basin is revealed,and the effect of migration on specific light hydrocarbon indexes is further discussed.The study indicates that,natural gas from the Lower Shihezi Formation(Pix)in the Dongsheng gas field displays higher iso-C5-7contents than n-C5-7contents,and the C6-7light hydrocarbons are composed of paraffins with extremely low aromatic contents(<0.4%),whereas the C7light hydrocarbons are dominated by methylcyclohexane,suggesting the characteristics of coal-derived gas with the influence by secondary alterations such as dissolution.The natural gas from the Dongsheng gas field has experienced free-phase migration from south to north and different degrees of dissolution after charging,and the gas in the Shiguhao area to the north of the Borjianghaizi fault has experienced apparent diffusion loss after accumulation.Long-distance migration in free phase results in the decrease of the relative contents of the methylcyclohexane in C7 light hydrocarbons and the toluene/n-heptane ratio,as well as the increase of the n-heptane/methylcyclohexane ratio and heptane values.The dissolution causes the increase of isoheptane values of the light hydrocarbons,whereas the diffusion loss of natural gas in the Shiguhao area results in the increase of n-C5-7contents compared to the iso-C5-7contents.