Sequence Stratigraphy And Sedimentary Study On The Ordovician To Devonian System In Northwest Tarim Basin, Xinjiang, China

Tarim Basin is the largest inland basin in China, covering an area of 560,000 km2, and also the last domestic large sedimentary basin which is insufficiently studied in terms of oil and gas exploration. Thus this basin is the focus of the national energy exploration. However, the exploration is impeded by arduous natural condition and the multiple tectonic changes which the basin has experienced.The lithoogy, lithofacies and phase sequence in the Keping area in northern margin of Tarim has been studied in this paper, in order to understand the characteristics and the stacking pattern of meter-scale cycles. The study focus is the meter-scale cycles under the theory of outcrop sequence stratigraphy, cyclic stratigraphy and sedimentology. Furthermore, the different cycles and hierarchic rank of the development of sequences are analyzed; the Ordovician to Devonian sequence framework and sedimentary facies in particular localites, along expedition routes as well as large areas in Keping area. The following conclusions have been reached:Three sedimentary facies could be recognized in Ordovician to Devonian strata in studied area: shore facies, shelf and carbonate platform deposits.The Ordovician in Keping could be divided into three 3^rd sequences under one 2^nd sequence. These 3^rd sequences mainly comprise TST and HST, lacking **** and ***. In certain sequences, HST could be subdivided into LHST an LHST. CS is only observed in sequence 3. The six Ordovician sequences are well developed in Sishichang area. Eastwards to Gusibulong and Subashi area, they decrease to three sequences. The framework of sequences suggests that, in the sea-level fluctuation, the middle and upper Ordovician transgressed from east to west.In the middle and lower part of the Silurian and Devonian in Keping, seven 3^rd sequences could be established. The sequence 1-5 are Silurian and the remaining is early to middle Devonian.In Akesu area, measurement of magnetic susceptibility of middle to late Ordovician carbonates reveals that the sedimentary environment of carbonates is related to their magnetic susceptibility. In general, deep water environment resulted in low magnetic susceptibility. The magnetic susceptibility fluctuated and show the trend of increase as the water become shallow. The shallow water, such as shore, produced exceptionally high magnetic susceptibility. Meanwhile, magnetic susceptibility of carbonates is also related to the sequence boundary, parasequence and sequence. Compared with the strata under the submerged sequence boundary, strata above the boundary has obviously lower magnetic susceptibility. In parasequence formation, the lower part generally has low magnetic susceptibility and straight curve, but the converse is true for the upper part.