The Analysis Of Sequence Stratigraphy And Depositional System For Dongying Formation In The Southern Liaozhong Depression

This paper applys the new theory and methods of sequence stratigraphy?structural stratigraphy?sedimentary geology and basin modeling to analyze structural sequence and depositional system of Dongying Formation in the southern Liaozhong Depression with using data of three-dimensionnal seismic?LWD and core synthetically . The sequence stratigraphic series and the fine isochronous stratigraphic framework have been set up, the evolution discipline of the depositional system have been clarified, and how the deferences of the tectonic subsidence rate and the changing of the relative lake-level to control sequence—depositional system have been revealed in this paper.From the open question of the paper to now, a lot of work have been done and many conclusions have been gotten. There are four major ones as follows.There were three third-order sequences and six fourth-order sequences during Dongying Formation in sourthern Liaozhong Depression by analysis. Espacially, the first section and the up-second section of Dongying Formation formed SQD1, the low-second section of Don- gying Formation was SQD2 which could be divided two fourth-order sequences: SQd2x1 and SQd2x2. And the third section of Donying Formation was SQD3 which could be divided three fourth-order sequences:SQd3-1?SQd3-2 and SQd3-3. Sequence stratigraphic framework have been built by comparing the stratigraphy sequence series of some key wells and seismic profiles.There were fan or braided river deltas?distal deltas?lake floor fans or turbidite fans?shallow lake beach bars?deep lakes or semi-deep lakes and so on in sourthern Liaozhong depression.There were three stages during the evoluting process of these depositonal systems. The first stage is SQD1 depositional period during which the water is usually very deep and fan deltas or braided river deltas grow up in southern Liaozhong depression. The second stage is SQD2 period. During SQd2x2 period semi-deep lake or deep lake developped with some turbidite fans sometimes. Delta depsitional systems developped along with water and there were usually some turbidite fans in the front of them during SQd2x1 period. The third stage is SQD1 period during which Deltas from east and west sides convergenced to the center.Tectonic subsidence rate have been analyzed in the studying area by the method of multi-screen views back to the stripping. They were very different among different area and they controlled the development of high-level sequences fundamentally?the evolution of the palaeogemorphology and the shape and range of the basin. Tectonic subsidence rare was the trigger power of sediment and was also the important factors of sedimentary type.In addition, the change of relative lake-level during Oligocene in southern Liaozhong depression have been analyzed by the method of super point of sea shore and the curve of relative lake-level change and the curve of shoreline trajectory have been made. It was thought cycles of large-scale changes in lake level controlled the formation of third-order sequences and high-frequency cycles of lake-level change controlled the formation of fourth-order sequences.