Lithologic Prediction Of Paleogene In Lenghu No.7 Tectonic Structure,the Western Part Of Northern Qaidam Basin

The Lenghu tectonic belt is an important hydrocarbon accumulation area in the western part of the northern Qaidam Basin.The Lenghu No.7 area is currently at a low level of exploration.It is of great significance for further exploration to predict the distribution of clastic reservoirs in the target zone.Based on comprehensive analysis of geological data(such as cores,logging,and cast thin sections)and seismic data,attribute extraction and colored inversion techniques were performed on seismic data to predict the distribution of the clastic reservoirs of Paleogene in Lenghu No.7.The results show that the Paleogene reservoirs in Lenghu No.7 are mainly composed of siltstones,fine sandstones and conglomeratic sandstones,which deposited in the front subfacies and plain subfacies of braided river delta respectively,and transit southwest to siltstones and conglomeratic sandstones deposited in the shore-shallow lake subfacies.The target segments,S1,S2 and S6 sand groups in N₁ Formation,X5 and X6 sand groups in E₃²Formation and X1 and X3 sand groups in E₃¹Formation,were confirmed by the analysis of lithological characteristics and the oil and gas display.According to the analysis of results of the attribute extraction,the RMS amplitude matches the lithology best.By extracting attributes of the target sand groups,the result shows that the sand bodies in N₁Formation are scattered,and the continuity of sand bodies at the bottom of E₃² and E₃¹Formation are more continuous.Results of colored inversion indicate that wave impedance has a good correlation with the lithology.The most part of sand bodies of N₁Formation are mudstones except those rocks developed at the bottom and the distribution of sand bodies on the plane is discontinuous.The stratum at the bottom of E₃² is filled with great number of sandstones with thin layers of mudstone where the continuities of the sand bodies of X5 and X6 sand groups are the best.A large sets of mudstones are developed in the middle and top of the E₃²Formation as a regional cap.The sand bodies of the X1 and X3 sand groups in E₃¹ mainly concentrated on the southeast side of the anticline and small part of sand bodies of X1 sand group distribute along the tectonic trend in the north of the anticline.The pore spaces are more developed in the E₃²Formation compared to others with the analysis of pore characteristic of the Paleogene.According to the calculation results of the percentage of sandstone in the key sand layer groups and analysis of geological conditions for the accumulation of oil and gas,the sand bodies of the X5 and X6 sand groups in the south of the Lenghu No.7 anticline are the most favorable exploration areas in the study area,followed by the sand bodies of the X3sand groups in the south and east of the anticline.