Sequence Stratigrahy And Favorable Reservoir Sandbodies Prediction In Xujiahe Foramation, Upper Triassic In The Center Of Sichuan Basin

Recently, the Upper Triassic Xujiahe formation which is potential strata has been an important area of natural gas exploration in the Sichuan basin. The new breakthrough of exploration of Xujiahe formation has been achieved in Guangan, Tongnan area. The huge thickness, extensive distribution, high proportionality of sandstone and mudstone are the characteristics of Xujiahe formation in the center of Sichuan basin. The study about the sedimentary facies model, the distribution of sedimentary facies and sedimentary microfacies is lacked.Guided by the principles and the methods of sequence stratigraphy , sedimentology and reservoir geology, using the integrated data of outcrops,cores, well log,and seismic, the paper carried out the sequence stratigraphy and sandbody distribution of Xujiahe formation in the center of Sichuan basin, deiscribed the favorable sedimentary microfacies distribution, predicated the distribution of the favorable sandbody, which provided abundant basis for the exploration to Xujiahe formation in the center of Sichuan basin. It is important and valuable when these research findings not only were used to the exploration successfully but also improved the study level about sedimentary sandbody in these area.The research findings indicated that:1) By using of the data of outcrops,cores, well log,and seismic, 3 sedimentary facies , 8 sedimentary subfacies and 17 sedimentary microfacies were distinguished in Xujiahe formation in the center of Sichuan basin. And the sedimentary model was established for Xujiahe formation .2) Based on the careful identification to the sequence stratigraphy, Xujiahe formation can be divided into four third-order sequences with twelve systems tracts. Lowstand systems tract which is the major deposition time for favorable sandbody is rich in delta depositional system, trangressive systems tract and highstand systems tract are rich in lake depositional system. On the base of the research, the distribution of favorable sedimentary facies were predicted in the paper. 3)Seven parasequence sets were distinguished for sequenceâ…¡, sequenceâ…¢and sequenceâ…£which are rich in reservoirs. The isochronal framework of parasequence sets stratigraphy for lowstand systems tract of Xujiahe formation were established to predicted the favorable sandbody.4) By means of the results of sedimentary microfacies, the multiple superposed channels and mouth bar models were designed for establishing its seismic faceis model using forward modeling in elastic wave equation . According to seismic facies parameters such as interior structure, exterior shape of seismic reflection units, reflected wave termination types, frequency, amplitude, continuity, bed velocity, six group and nine sub-group seismic facies are identified, in which the seismic facies characterised by lenticular-wave, unstable amplitude and medium continuous event is most favorable.5) Xujiahe formation in the center of Sichuan basin is the reservoir with low porosity, low permeability, in which space is intergranular dissolved pore, intragranular dissolved pore and remaining primary pore. The reservoir type is porous reservoir and fractured reservoir. The reservoir development has close tie with sedimentary microfacies and rock category. Underwater delta distributary channels is the favorable sedimentary microfacies. Mesograin and macrograin arkose arenite is the favorable rock.6) Based on the results, four favorable sandbodies of Xujiahe formation were divided in the center of Sichuan basin, and 3 exploration blocks,11 exploration targets were prior seeked using seismic inversion predict the distribution of reservoir in the key sandbody.