Identification of oil-water interface and evaluation of oil and gas distribution in Ordovician buried hill reservoir in Lungu：A case in Lungu-well block 7

Ordovician buried hill reservoir in Lunnan is located in the Lunnan ancient uplift in Tarim basin and the geological reserves of petroleum there are more than 6×108 t. Ordovician buried hill in Lunnan experienced Caledonian and other period multiphase tectonic movement, karst buried hill reservoir is mainly distributed between 0-200 m under the ancient weathering crust, solution pore, large water-eroded cave and fracture are major effective reservoir space. The 7 well block is located in the west Lungu, karst reservoir are mainly composed of micritic, sparry calcarenite in the middle and lower Yingshan group of the Ordovician system. The primary porosity in the limestone, only average in 1.2 %, is low. But karst structural seam hole is significant developed with the average porosity being 3.4 %. Fractures and solution pores communicate with each other forming seam-hole system, which is the main reservoir space of the carbonate reservoir. Identifying the location of oil-water interface in the buried hill is directly related to the exploitation effectiveness of oil. If the exploration well passes through the oil-water interface, large quantity of water yielding will take place; if the exploration well is far away from oil-water interface, oil and gas is likely to be missed. To find the suitable depth of exploration well, the position of oil-water interface should be fully understood at first. In order to provide scientific basis for exploration and exploitation of oil and gas, taking LG7 well block in Lungu buried hill as an example, the method to identify the position of oil-water interface is discussed in the paper.Through comparing the data of drilling, logging, testing and pre-production and analyzing the ancient karst landform, the paper conclude that the bottom of the karst valleys can indicate the oil-water interface position of the karst monadnock. Based on this, in combination with the production situation of completed wells, oil and gas reserves above the oil-water interface in karst monadnock are evaluated preliminarily. The results show that the well production capacity is closely related to the oil-water interface depth. ClassⅠarea (monadnock thickness >90 m) covers 13.86 km2, and oil and gas resource is 693×104t; classⅡarea (monadnock thickness :50~90 m) covers 27.13 km2, and oil and gas resource is 1 085×104 t; classⅢ area (monadnock thickness<50 m) covers 20.21 km2, and oil and gas resource is 404×104 t.