Sequence Stratigraphy And Lithofacies Paleogeography And Carbonate Reservoirs Evaluation Of The Lower-Middle Ordovician In Tarim Basin

Based on the integrative investigation of outcrops, cores, and thin sections and drillings, and seismic data, third order sequence stratigraphic framework and its characteristics of the Lower-Middle Ordovician in Tarim basin are analyzed systematically under the guidance of sedimentology and sequence stratigraphy and basin analysis and other theories, and thereby the lithofacies paleogeographic maps on the third order sequence stratigraphy as a unit are established with the combination of former works and previous researches. Further, comprehensive evaluation and prediction of carbonate reservoirs is carried out in this thesis with incorporation of main reservoir controlling actors. Thus, it is of great significance for the implementation of new oil and gas exploration breakthrough in Tarim basin. Main conclusions of the thesis are as follow.(1) The transformation from regional extensional to compressive geodynamics of Tarim plate started in the latest Cambrian, and the most intensive activity occurred in the middle-late Middle Ordovician. Contemporarily, the Proto-Tethys ocean in West Kunlun region in the Cambrian began subducting to the bottom of the Tarim plate, and in the Early-Middle Ordovician the subduction is strong, and the Proto-Tethys ocean basin has probably been closed in the late Ordovician. On the contrary, the south Tianshan region during the Cambrian-Ordovician was in the heyday of the ancient Asian ocean expansion. And also it is found that the basin configuration, carbonate platform internal palaeo-geomorphology and the change of sedimentary framework have been affected by this regional dynamic conversion.(2) Five sequence boundaries have been identified in the Lower-Middle Ordovician of the Tarim Basin, which is divided into four third order sequences. In which Penglaiba Formation is divided into one third-order sequence, and Yingshan Formation into two third-order sequences, and Yijianfang Formation into one third-order sequence. On the other hand, the sequence structure of the Cambrian and the Lower-Middle Ordovician and the interior of Lower-Middle Ordovician in the southwestern part of the today basin presents a retrogradational style, whereas Gucheng and other areas in the basin with a prograde style. And it is inferred that the dissimilarity in sequence structure style may be closely related to the differential tectonic subsidence activities in different basin regions, which were caused by the middle Caledonian tectonism during the Early and Middle Ordovician. (3) Through compiling the lithofacies paleogeographic maps on the third order sequence stratigraphy as a unit, each lithofacies palaeogeographic framework of the Lower-Middle Ordovician is comprehensively studied. As a result, it is revealed that the evolution of palaeogeographic framework of SQ1and SQ2and SQ3was successive, while SQ4was the significant transformation period of palaeogeography. In the late Early Ordovician, Tarim Basin entered the tectonic-sedimentary transformation period from extension to compression, forming micro-palaeogeomorphology of shoal alternated with depressions, providing favorable conditions for the development of intnerplatform shoals of Yingshan Formation. In the middle and later Middle Ordovician, Hetian, Tazhong and Tabei palaeohigh began to take shape and formed early karst landform due to local exposure and erosion of early strata in these paleohigh regions.(4) The carbonate reservoir distribution of the Lower-Middle Ordovician in Tarim Basin was mainly controlled by sedimentary facies belts, diagenesis and T74uncomformity. High-quality carbonate reservoir rocks of Penglaiba Formation and lower Yingshan Formation were mostly distributed in intraplatform lagoon-tidal flat facies. While that of upper Yingshan Formation mainly distributed in intraplatform middle and high energy shallow shoal facies, secondly at platform marginal high energy reef and shoal facies. Most of Bamai area, Tabei and Tazhong areas were located within toplap and truncation areas of T74uncomformity, which were the most favorable carbonate reservoir developed in middle and lower Ordovician in Tarim Basin.