The Sequence Pattern And Sandbody Distribution Principle In The Compressed Shallow Basin

Turpan Sag is a shallow lake basin under the compressed structural background, characterized with rapid subsidence, frequently-changing lake level, evident sequence stratum boundary, nearby and multi-material source and multi-sedimentary center. Under the instruction of the theory of high-accuracy sequence stratum, the type, distribution and the evolving law of the sedimentary facies in the framework of sequence stratum were studied, the composing pattern of the sequence in middle and lower Jurassic of Turpan sag was summarized and combined with the genetic type and the controlling mechanism of the slope break for the sand in the basin, and the favorable distribution of the sandbody in the sequence was analyzed and forecasted. In the paper, the work was done in the following 5 aspects:(1)Based on the study of the outcrop section, bore log and seismic data, the framework of the high-accuracy sequence stratum in middle and lower Jurassic of Turpan sag was established, 5 triple sequence cycle(JSQ1—JSQ5) and 10 quadruple sequence cycle(SC1—SC10) were identified and correlated in the region, and the vertical superposing pattern and the horizontal evolving law of the sequence stratum in the middle and lower Jurassic were summarized.(2)The types of the main sedimentary facies in the middle and lower Jurassic of Turpan sag were collated and stipulated, and it was pointed out that the river and delta facies widely developed in middle and lower Jurassic and the shore lake subfacies and shallow lake subfacies were the main lake facies. Controlled by the structural background and evolution of the basin, there were several stages in vertical evolution of the sedimentary facies, that is, the shallow sedimentary background of the gentle slope at the stage of SC1-SC5 cycles and the sedimentary background with evident slope break at the stage of SC6-SC10 cycles.(3)By analyzing the vertical cycling characters of the datum level change, the composing patterns of the sequence under the different background of structural belts of accommodation space were discussed; Under the condition of low accommodation space and high accommodation space, the composing patterns of the sequence of the sag at theshoreland area of the margin of the gentle slope, the shallow area of the edge of the lake, the shallow lake area of the front of the steep coast and the shallow lake area at the center of the lake were summarized; The sequence pattern of the shallow lake basin of middle and lower Jurassic under the background of gentle slope and slope break were summarized and it was pointed out that the sequence under the background of shallow lake basin was characterized with the evident dual structure.(4)3 genetic type slope breaks in the middle and lower Jurassic of Turpan sag were recognized, that is, the sedimentary-geomorphy slope break, the structural bending slope break and the syndepositional fault slope break. By the study of the distribution and space-time evolving law of different slope breaks, the controlling of the paleogeomorphology slope break on the sedimentary facies belt and the sandbody of the middle and lower Jurassic in different periods was summarized.(5)It was concluded that there were several sandbody types, including the low-stand fan, the sandbar of shore lake, the branch-river sandbody of the delta and the delta front sandbody. The forming condition, the distribution law and main controlling factors of different reservoir sandbody in the sequence stratum framework were indicated and the time-space distribution law of the favorable sandbody in the main sequence was summarized.