Well Logging Evaluation On The Reservoirs Of Braided-River Delta Facies

Yuejin II oilfield, which is in the southern district of the western part of Qaidam Basin, with the sedimentation of mainly braided-river delta, have the characters such as numerous layers, thin sands, well-developed faults, narrow facies belts, dramatic changes of facies and strong reservoir heterogeneity. Sedimentary facies are of great influence to the reservoir property, as a result, it is very important to the remaining oil exploitation and production adjustment program that carrying out logging reservoir evaluation with the control of sedimentary facies.With N₂¹ and E₃¹ reservoir of Yuejin II oilfield as study object, combining many types of information including geology, logging, testing and production dynamic data, basing on the logging facies research, the reservoir parameters interpretation models are established, and the work of logging interpretation is completed by layer systems and sedimentary microfacies, oil and water layers are divided, water flood layers are evaluated, the spatial variation of reservoir parameters is described, and a comprehensive evaluation of the reservoir is carried out. Knowledge and results obtained are as follows:Relying on conventional logging curves, selecting six non-associated primary components which can reflect sedimentary characters such as lithology and size grading of reservoir, using mathematic methods such as cluster and discretion, logging facies models in the study area are established by layer systems and sedimentary microfacies, logging facies are divided and discriminated quantitatively with correct rates over 80%.The calculation models of reservoir parameters such as the porosity, permeability are established according to different facies and different layers with good effect that the degree of fitting of these formula are over 0.8. The interpretation results are verified by core analysis, well test, production profile of liquid, and production data, a good explanation effect is achieved. The type character of water flooded layer of the study area is that the SP curve baseline shifts intensely, and the resistance rate changes with "U-type" at different development time. With entropy weight method, the quantitative assessment of flooded layer is carried out and water flooded layers in the study area are divided into 6 levels such as non-flooded layer, weak flooded layer, et. al. The water flooded layers of Yuejin II oilfield are mainly moderate flooded and strong flooded. The sands in mouth bar and underwater distributary channel facies in the main district of the study area are relatively with abundant oil accumulation , the water-flooded degree of which is high; the district along downflow well, active aquifer and the zone of the main fault are also easier to be flooded.According to physical properties of Yuejin II oilfield, the reservoirs in the study area can be divided into 4 types, of which N₂¹ reservoir is mainly IIa type, and E₃¹ reservoir is mainly IIIb type. Underwater fluvial channel of N₂¹ formation is I type, while mouth bar and sheet sand are IIa type. Distributary channel of E₃¹ formations are mainly IIIb type and IIb type, while natural levee is not very developed, with less number of sands.The distribution of oil&gas in study area is mainly controlled by factors such as physical property, depth, and sedimentary environment. As to oil&gas abundance extent, the mouth bar facies are the richest, underwater distributary channel secondly, sheet sands are the least in N21 formation. Natural levee in E₃¹ formation are nearly non-oil reservoir, almost all oil and gas is within the body of the distributary channel. The remaining oil is abundant in the edge of different facies, the district near the fault and the zone with improper well pattern.By layer systems and sedimentary microfacies, the logging interpretation models are established and logging manipulation and interpretation is completed with the improved interpretation accuracy.