Concave-uplift Combination Structure Of Upper Paleozoic Source Rocks In Ordos Basin And Its Relationship With Faults, Sand Bodies And Gas Reservoirs

The upper Paleozoic gas reservoirs in Ordos Basin are mainly coal formed gas,tight sandstone gas and lithologic gas reservoirs.Predecessors have summarized the main control factors and characteristics of different types of gas reservoirs,but there are great differences in the classification standards and basis of gas reservoir types,and there are great differences in the classification standards of some of the same types of gas reservoirs.Based on the data of sag and uplift assemblage structure,basin sedimentary facies,two-dimensional earthquake and rock slice,this paper analyzes the plane association relationship between sag and uplift assemblage structure,sand body and fault of Upper Paleozoic source rocks in Ordos Basin,and divides four sand body types and twelve association relationships.Based on the characteristics of Upper Paleozoic lithologic assemblage,fault strata assemblage,sag uplift assemblage structure and the relationship with discovered gas reservoirs in Ordos Basin,a new division method of Upper Paleozoic gas reservoirs in Ordos Basin is explored,and the distribution characteristics and main controlling factors of gas reservoirs are analyzed and discussed.The main understandings are as follows:(1)There is a strong correlation between the distribution of gas reservoirs and the development of ridge lines in sag uplift assemblages.The sand bodies in the sag uplift assemblage are divided into four types: lenticular sand body,cross slope sand body,reverse slope sand body and along slope sand body.Lens sand body and reverse slope sand body are the most favorable for natural gas enrichment,followed by cross slope sand body.Better trap conditions are required for natural gas enrichment in along slope sand body.The sand bodies of Taiyuan Formation and Benxi Formation are mainly lenticular sand bodies,the sand bodies of Shanxi Formation and upper Shihezi Formation are mainly slope sand bodies,and the sand bodies of Lower Shihezi Formation are mainly cross slope sand bodies and slope sand bodies.(2)The discovered gas reservoirs in Benxi Formation are mainly distributed in lenticular sand bodies.The bottom of Taiyuan formation is in the depression and uplift at the end of Early Cretaceous.The discovered gas reservoirs in Taiyuan formation,Shanxi formation,Lower Shihezi Formation and upper Shihezi Formation are mainly distributed in the sand bodies along the slope.The bottom of Taiyuan formation is in the depression and uplift at the end of Late Cretaceous and now,and is mainly distributed in the sand bodies on the cross slope.There are different types of sag uplift assemblage structures,sand bodies and faults in different strata.(3)According to the exploration of new gas reservoir classification scheme,the upper Paleozoic gas reservoirs in the basin are divided into three categories: source rock gas reservoirs,which are mainly characterized by enrichment in the source rock series and self generation and self storage;The sandstone gas reservoir adjacent to the source rock is the gas reservoir in direct or indirect contact with the source rock,including the sandstone gas reservoir in the source rock series and the overlying sandstone mudstone gas reservoir in the adjacent source rock series;The distribution and formation of fault-rock combination structural gas reservoirs are greatly affected by fault activities.(4)The distribution of source rocks and dark mudstones in coal measures controls the distribution of gas reservoirs in source strata.Sandstone mudstone combination hydrocarbon bearing is an important gas bearing type in sandstone adjacent to source rock,accounting for the highest proportion(43%).Shiqianfeng Formation develops typical fault-strata structural gas reservoirs.Fault strata structural gas reservoirs are distributed in Shihezi Formation and Shanxi formation.