Characteristics and geological significance of middle Caledonian palaeodrainage pattern in the northwest of Tahe oilfield

The Tahe oilfield is located in the main part of Akekulei nose arch, which is situated on the Shaya uplift, northern Tarim basin, Xinjiang. The main target reservoirs are developed within paleokarst weathering crusts at the tops of middle and lower Ordovician successions, which underwent paleokarstification from the middle Caledonian to early Hercynian periods, and formed large-scale fractured-cavern reservoirs. Large amounts of previous studies on the paleokarstification in the Tahe oilfield mainly focused on the axial zone of Akekulei nose arch, which is the major part of production and displayed intensive early Hercynian paleokarstification below the overlapping lower Carboniferous shale. While the northwest part of Tahe oilfield was supposed to locate on the low-relief northwest slope of Akekulei nose arch, and preserve a large number of production of the middle Caledonian paleokarstification, which has not attracted the attention of many scholars and has been poorly studied. Starting from the factors controlling the hydrological conditions, the authors analyzed the distribution of overlying strata,determined the atmospheric freshwater confluence characteristics of each stage of paleokarstification, and explored the paleokarstification model in the northwest of Tahe oilfield. On one hand,by comparing the upper and lower Ordovician strata with several drilling data, it is revealed that the successions overlying on the weathering crusts are correlated. Shale interval in the upper Ordovician Sangtamu formation is found locally to overlie directly on the middle-lower Ordovician carbonate successions and the successions counterpart to the Qiaerbake and Lianglitage formations are absent. This context of successions in the slope zone of Akekulei nose arch is distinct from that in the axial zone and suggests the interference of the episode II of middle Caledonian. On the other hand, 3D seismic data and related techniques were applied to identify and characterize the drainage distribution patterns of episodes of middle Caledonian paleokarstification. Based on further research of the multi-episode sedimentary and tectonic evolutions and sequences of the middle Caledonian in the northwest of Tahe oilfield, it is pointed that there are two independent drainage systems on the top of Lianglitage and Sangtamu formations of the upper Ordovician successions. This research has contributed to the identification and discrimination of drainage systems during the episode II and III of the middle Caledonian paleokarstification. It is proposed for the first time that the episode II of the middle Caledonian movement at the end of Lianglitage formation has favorable conditions for the development of paleokarstification. The model of middle Caledonian paleokarstification is established provisionally. It does provides new and favorable evidence for further studying the characteristics and evolution model of the middle Caledonian paleokarstification in the Ordovician strata of the whole Tahe area. And this research provides an important clue to revealing the development of the Ordovician fractured-cavern reservoirs in Tahe oilfield, and to predicting the exploration of high-quality reservoirs in Tarim basin.