Evaluation On Efficient Source Rock Of Cambrian-Ordovician In Tarim Basin, China

It is displayed the large resource potential of marine strate in Tarim Basin for many years oil and gas exploration, and Cambrian-Ordovician source rock had established the position of chief source rocks in Tarim Basin, but there are many controversial issues about Cambrian-Ordovician source rock, such as the lower limit of organic carbon,the scope of distribution of the efficient source rock, etc. According to the deficiency the author will study in this paper.On the basis of the work of predecessors, and considering the real geochemical characteristics of Cambrian-Ordovician source rocks, the author consider that the lower limit of organic carbon of old marine carbonate being efficient source rock should not be too low, and consider that 0.4% is appropriate.On the basis of single well geochemical profile detailed analysis, we adopt multiple sedimentological and seismic research methods to study the distribution of efficient source rock in vertical profiles, and the horizontal distribution. We consider that the lower-middle Cambrian is the aim efficient source rock in Cambrian formation, and the good source rock are developed at the basin facies in east and lagoon facies in west, in the middle of the Tarim Basin, the efficient source rock is poor. There are two sides of new knowledge about Ordovician source rock. First, we consider that the heituao formation has the similar sedimentary characteristics and the quality of source rock with the lower-middle Cambrian source rock, it is the best source rock of this period, second, although the lim-mud reef of lianglitage formation of the Ordovician is widely distributed in the central Tarim area, the heterogeneity of source rocks is relatively strong and the accumulation thickness of efficient source rocks are relatively thinness, so we consider that the lianglitage formation is only the moderate quality source rocks in partial well area.Through outcrops, drillings and seismic sections sequence division and correlation, we consider that condensed sections of Cambrian are mainly distributed in the Early-Middle Cambrian(corresponding to super sequence SSq1-SSq2), in the Early-Middle Ordovician(corresponding to super sequence SSq4-SSq5), Manjiaer depression and its east, in the middle-late Ordovician(corresponding to super sequence SSq7-SSq8), the condensed sections are mainly distributed in the west of basin, such as wushi, keping area. The efficient source rocks are generally distributed in low-stand system tract of super sequence and transgressive system tract or the condensed sections, part of them extend to the early period of high-stand system tract while most of them are distributed transgressive system tract.On the basis of the work of predecessors and the analysis of typical drilling Ro profiles which distribute in the northern Tarim uplift and the central uplift in Tarim basin, we consider that the efficient source rocks of lower-middle Cambrian are the main oil source rocks in the Caledonian reservoir formation event, at terminal of Permian, the hydrocarbon-generation of lower-middle Cambrian source rocks is weak or stopped. The three times of oil generation of middle - lower Ordovician source rocks happened in late Caledonian, Yanshanian and late Hercynian. The hydrocarbon-generation of upper Ordovician source rocks firstly happened in late Hercynian, and the peak of oil generation in most of areas happened in Himalayan period.