On Vertical Sequence and Reservoir Characteristics of Alluvial Debris Flow Sedimentary Unit in Fault Basins,Eastern China ——Based on a case study of Yong an alluvial fan, Dongying Basin

Key words: 
• fault basin /
• debris flow /
• alluvial fan /
• sand -cong lomerate body /
• sediment ary unit /
• sedimentary pattern /
• reservoir /
• reservoir characteristics

Abstract: Debris flow sedimentary strata consisted of mixed deposites of sands and conglomerates are the most significant oil and gas reservoir in alluvial fans developed along the steep-slope in Mesozoic-Cenozoic Fault Basins, Eastern China. Based on a case study of cores through Yong-an alluvial fan, Dongying Basin, the lithological characteristics of deberis flow deposits were firstly generalized,which are (1)conglomerate with a complex component,a poor degree of sorting and a very poor roundness;(2)a typical matrix support texture with the gravel axis directed randomly;(3)the common gravity structures as the massive,the fiamme and the deformation bedding ect.,the common bottom surface consisted of well developed erosion surface. Three types of vertical sequence of debris flow sedimentary unit were recognized and modelized,and they are(1) the fining upward sequence mainly developed in the upper fan area,with the fining upward either gradually or in a multicycle way;(2)the nongraded sequence typically formed in the main channels in the upper fan facies and (3)the composite graded sequences combined with a lower corsing upward interval and an upper fing upward interval and,this type of sequence is hardly well developed due to the strong erosion in the alluvial fan area. Bsed on the studies above,a complete sequence model of alluvial fan unit was suggested.This complete unit was composed of three intervals,that is (1)the lower coarsing upward sand fine gravel interval which was deposited in the first debris forming stage,(2)the thick middel upper fining upward interval that was formed in the typical debris developing stage and (3)the upper sandy interval seposited in the water channels during the past flood time.Futhermore,the lower interval can be recognized into two subintervals named ?sandy part formed in normal tractive current channels before flood and the fine gravel part deposited in the low dense debris at the first stage of flood;the middle upper interval can also be devided into two subintervals,that is the thick middle lower coarse conglomorate as a result of normal debris;?the fine conglomerate corresponding to the late stage of a flood;the upper interval is right the subinterval ?Because of the strong erossion process,the complete sequence of ,were not common in strata record,contrastly,the sequences like, ect.were very common.Understanding the sequuence model would be of significance to sedimentological and reservoir studies of alluvial fan in lacustrine basins.In order to understand the permeability structure of a commplete debris unit in profile,a corresponding reservoir characteristic model was finally established based on a number of core permeability test in lab,and the testsshowed that the thin subinterals ?and ?be well permeable,the was relative poor,and the ?? have a great range.So,the high remindering oil should tend to be formed in a debris reservoir unit.