Characteristics And Potential Of Collapsed Structure Of Ordovician Reservoirs In Tahe Oilfield

The reservoir type of the fracture and vug reservoir of carbonate rocks in the Ordovician of Tahe Oilfield,mainly belongs to karst cast.When many stages of cave systems are buried in the ground,the surroundings top rock of the opening channel form collapse.Because of its low producing degree and reservoir description accuracy,the structural characteristic,recognition method,genetic model and the petroleum potential of the collapse still require further studies.Through analyzing the cores,conventional logging,imaging logging and the seismic characteristic,combined with the constructional curvature method,this paper carries out research in the aspects of the distribution,controlling factors,types and the development potential of the karst collapse zones in Tahe Oilfield.The main results of the paper include the following aspects:(1)Determine the characteristics of the karst collapse structure through cores,conventional logging,imaging logging and the seismic characteristic.The lithology of the void fillings of the collapse breccia mainly consists of cave-collapse breccia,the gray sandy conglomerate of the underground river deposition,lime sandstone,lime siltstone,lime mudstone and so on.The FMI data clearly reflect the crack of the karst cast near sidewall,the karst cast and the syntagmatic relation of each other.The forward results show the collapse on the seismic section mainly manifest as the sag in the upper strata,for which the morphology similar to the interface fluctuation of T74.The bottom of the karst exists relatively significant bottom interface.The main development of the collapse are strong amplitude and blank reflection.The boundary of the both sides of the collapse(collapse surrounding rock breccia)are disorder reflection(string bead).(2)Through analyzing the seismic horizon,constructional curvature method has a better advantage in determining the boundaries of the collapse of the Tahe Oilfield.their identification features in seismic section can be distinguished by using the difference in the genesis,and three kinds of negative structure can be divided:gully,syncline and collapse.From logging interpretation of single well,the shallow cave in the collapse band is more developed,and the degree of filling gradually decreases from shallow to deep.(3)The main control of the distribution of the collapse structure are tectonic factors,karst and the effects of gravity load.Combine the cause with the mechanism of the collapse structure,the collapse structure can be divided into three types,of which the collapse of fracture is 71,load collapse is 32 and the collapse of karst is 19.(4)Through statistical analyzing of different types of drilling data in the collapse,the karst collapse is the favorable zone,which has the highest abundance of oil and gas and the highest proportion of high-yielding wells.Regarding the yield abundance and the venting dropout as two important criteria for classification,combining with the fault density,rate of amplitude change and the seismic reflection characteristics,the development potential of karst collapse structure is divided into I,II and III class.