Identification And Prediction For The Subtle Oil Reservoir Developed At Yongjin Area In Junggar Basin

The Yongjin Oilfield had been explored to be a large oilfield with in-place volume of hundreds millions ton in Junggar Basin. It had been proved by drilling data that its oil reservoirs were mainly the basal conglomerate reservoir entrapping in Cretaceous, sandstone reservoir below the coal bed and stratigraphic-lithologic reservoirs above the coal bed entrapping in Jurassic Xishanyao Group. It was difficult to predict and evaluate the reservoirs due to deep buried target, thin sandstone developed and poor reservoir physical property, which made the hydrocarbon exploration difficult at present.The factors controlling lithologic or stratigraphic-lithologic reservoirs and its corresponding matching relationships had been studied through using high accuracy sequence stratigraphy, reservoir seismic prediction techniques and strata unconformity analysis basing on regional geologic background and subtle oil reservoir characters analysis. Finally the subtle traps had been analyzed and evaluated through analyzing reservoir physical property, porocity and prestack property oilness prediction, comprehensive unconformity and stratigraphic-lithologic traps analysis, and then the benefit reservoir development zone and two benefit targets had been predicted for drilling.The sequence stratigraphic time and space framework for Jurassic and Cretaceous bottom had been built according to individual well sequence analysis and seismic data lateral tracing combining with geologic data, geophysical exploration data and logging data, as a result eight third order sequence interfaces depositing at Jurassic and Cretaceous bottom had been classified. Four types deposit facies(braided stream, meandering stream, delta and lakes) had been determined through studying depositional system and depositional facies finely. The conditions for subtle traps configuration and their controlling factors had been studied to build corresponding sequence stratigraphic model for hydrocarbon entrapment.The seismic identification and description methods applied to analyzing subtle traps had been studied systematically in order to determine structure geometry and fractures distribution and predict reservoirs distribution in space, which were consisted by trap identification technique, reservoir prediction and hydrocarbon prediction techniques. The individual sandstone distribution in plain had been identified through seismic inversion with Gr curve for the benefit depositional facies zone. The reservoir physical property had been analyzed and classified through properties analysis and spectrum decomposition method. The variable velocity mapping was used to resolve questions of velocity great change in lateral and time and depth transformation under conditions of deep buried dept. it could be used to identify low amplitude structure. The coherence analysis, dip and azimuth scanning and frequency division amplitude were all applied to identify strata pinchout boundary and describing correctly faults distribution. The inversion technique used could differ sandstone and mudstone each other when they could not separated from each other using conventional method. The property analysis basing on constrain inversion was applied to predict reservoirs physical property and their development. The hydrocarbon could help predicting subtle traps bearing hydrocarbon. The subtle traps had been evaluated comprehensively to optimize the targets for drilling and provide reliable evidence after the reservoirs developed at Cretaceous-Jurassic had been predicted and described through analyzing unconformities structure and their physical property, porosity and pre-stacked extracted property, identifying strata onlapping and denudation zone As a result of study three new techniques had been developed. They are as following:①The developed technique for identifying correctly thin sandstones developed in deep formation in Yong1 well areaThe variable velocity mapping was used to resolve questions of velocity great change in lateral and time and depth transformation under conditions of deep buried dept. it could be used to identify low amplitude structure. The coherence analysis, dip and azimuth scanning and amplitude spectrum obtained by frequency division were all applied to identify strata pinchout boundary and describing correctly faults distribution.②The developed inversion technique with less wells drilled for thin sandstone characters prediction in Yong1 well area.It was difficult to use only the impedance to predict correctly reservoir distribution due to impedance overlapping in the area of benefit reservoir and non reservoir. So it was first developed that combining logging data to reconstruct characters property reflecting the change of reservoir in space at well drilled, and then applying neural network technique to inversing the characters cube, by with finally the reservoir distribution in space could be predicted correctly.③The developed technique for evaluating reservoirs in Yong1 well areaThe reservoir developed characters had been predicted combining variable prediction and evaluation techniques from different respects. Firstly it used inversion technique constrained with pseudo acoustic impedance and Gr curves to predict sandstones. Secondly the reservoirs physical property had been calculated in quantity. Finally aφ-AC model had been built to predict porosity, the oil saturation had been predicted with sequential gaussian simulation modeling, the reservoir oilness had been predicted using spectrum decomposition and prestack AVO analysis, as a result an effective prediction and evaluation for reservoir had been developed.