A Study On The Sequence Stratigraphy And Sedimentary Characteristic Of Middle-Lower Ordovician In Southwestern Area Of Tarim Basin

In this paper, using "sequence analysis technology" and "sedimentary systems analysis" as the two leading methods, I analyzed the sedimentary and sequence characteristics of the Middle-Lower Ordovician in Southwestern Tarim area. This paper, also folly used the outcrop, core, drilling, logging, seismic data and testing data, established the regional sequence stratigraphic framework and have a comprehensive study on the sedimentary facies of the southwest region in the Tarim Basin, finally propose the drilling and the seismic sequence division program, Analyze the spatial distribution and evolution of each sequence; On the basis of the sequence stratigraphic framework, I study the characteristics and models of the sedimentary facies of each sequence; According to the rocks characteristics, the combination of types and the control factors of the Ordovician carbonate reservoir, the paper reveals the distribution of favorable reservoir zone in the study area.This research made the following understandings:IThe deposition thickness of the Middle-Lower Ordovician in the Southwestern Tarim region is very huge,and have a strata distribution characteristics that thin to the south and to the west. The unconformities T80、T78 and T74 the subdivides the Middle-Lower Ordovician into two 2nd-order sequence, named Penglaiba formation and Yingshan formation. According to lithology and electrical characteristics of the 2nd-order sequence, Penglaiba formation can be subdivided into two 3nd-order sequence(the first to the second), Yingshan formation can be subdivided into four 3nd-order sequence(the first to the fourth). On the lithology, Penglai formation is characterized by interbed limestone and dolomite,, mainly for the dolomite; Lower Yingshan formation is given priority to with limestone, and the lower developed dolomite. Penglaiba formation is slightly thicker than the Yingshan formation. Except the first Yingshan formation had been denuded in the He4 well region, other sequence have a regional extension and are well comparaed. The paper established Middle-Lower Ordovician’s regional sequence stratigraphic framework, and each sequence is described in detail by many methods.2 Carbonate platform sedimentary system as the main body of early ordovician sedimentary system in the study area is the weakly rimmed type of platform marginal, where the depositional environment is diversity. Several kinds of facies, such as restricted platform facies, open platform facies, platform marginal facies, slope facies and shelf facies are identified on the basis of seismic data and drilling data. These facies developed from north to south. In this paper, sedimentology mode patterns are compiled in the 2nd sequence and the show layout of flat sedimentary facies are compiled in the 3nd sequence. The sedimentary environment of the Penglaiba formation mostly are restricted platform, and with the rising of sea level in the sedimentary period of Yingshan formation, the restricted platform facies gradually transformed into a large area of open platform, have the evolution characteristics of inheritance development.3 Based on the basic characteristics of the reservoir, the type of the reservoir room in the study area include pore-fracture-vug reservoir, pore-fracture reservoir and fracture-vug reservoir. Through research and analysis, concluded that the junction point of the the Bachu uplift with the Magaiti Slope and the east of the southern margin in the Magaiti Slope are the most prospected regions for oil and gas exploration and development in southwestern Tarim basin.