Fracture-cave Reservoirs Characteristics And Controlling Factors Of Ordovician In Area 7 Of Tahe Oilfield

Tahe Oilfield has the largest reserves of carbonate fracture-cave reservoirs in China. Accompanied with further studies on this kind of reservoir, there are still “triple-low problems” in oil extraction, which refer to low developed reserves ratio, low oil recovery rate and low recovery ratio. Based on the observation of cores in the study area and Ordovician palaeokarst outcrop in the north of Tarim Basin, also make full use of date as well logging, seism and exploitation, this article carried out research on the growth characteristics of karst fracture-cave reservoirs in District 7 of Tahe Oilfield, probe into distribution regularities and controlling factors of reservoir development.On the basis of previous achievements, take full advantage of core drilling data of several wells in the study area, combining with detailed observation of core and thin section, the article summarized the petrologic features and characteristics of diagenetic of Ordovician carbonate in Tahe Oilfield, also established diagenetic sequence.Through above analysis, the author considers that the reservoir spaces of the study area can be divided into three types: fracture, cavern and pore; then the reservoirs can be divided into karstic reservoir, fractured reservoir and fracture-cave reservoir. The filling types and sequences in karstic fractures and caves are also analyzed.According to systematic summary of characteristics of core, well logging and seismic response of all kinds of reservoir, classify reservoir type of single well and take well-tie section correlation, then study the reservoir distribution regularities. Reservoirs of the study area can be divided into three belts vertically, separated by compact layer in the middle. And there are more highly filled caves on the shallow unconformity plane. Horizontally, same reservoir belt developed different types of reservoirs, their distribution regularities are related to filling, local structures and faults.By means of analyzing the formation mechanism and main factors of fracture-cave reservoir distribution, the author considers that the crux of factors controlling the development of fracture-cave reservoir includes fault, karst palaeogeomorphology, ancestral rivers, unconformity plane, lithology and paleoclimate.