Fracture Characteristics Of Low Permeability Reservoirs And The Control Action Of The Accumulation Of Oil And Gas

Natural fractures occur commonly in the oil and gas reservoirs, especially in thearea with intense tectonic activities. For the sandstone reservoirs with low permeability,fracture systems are particularly important in hydrocarbon accumulation andsubsequent petroleum development. It has been proven by the exploration andexploitation of the oil pools in the member Chang8of southern Red River Oilfield inthe Ordos Basin studied in the paper for many years that fault and fracture systems arethe key to the hydrocarbon enrichment and large output in the development of thereservoirs. The characteristics, origination, stages, controlling factors of the fault andfracture systems and the relation with hydrocarbon enrichment in the Red River oilfieldare studied in depth, and then the distribution rule of the fault and fracture systems inthe reservoir of Member Chang8in the Red River Oilfield is figured out. Consideringhydrocarbon accumulation conditions and fracture controlling effects on theaccumulation, hydrocarbon accumulation models are constructed.Based on field investigation of cracks, description of core fractures and research offracture characteristics on imaging log, the basic characteristics of the cracks aresummarized that high-angle and vertical cracks mainly developed in the research areadominantly in the NE-SW group, secondly in the NW-SE group. The cracks cut throughthe different lithological layers vertically, extend a few meters to more than ten metershorizontally; shear ruptures and tensile ruptures occur together in the target layers in thestudy area among which shear ruptures developed in conjugated group, slickens andshear scratches in different directions or smooth fracture surfaces are visible, whereas tensile rupture surfaces are rough and uneven, bent-like, commonly with calcite fillingsand generally in parallel-group system. Fracture effectiveness is the best in NE-SW andSN groups is higher in the sandstone than that in the fractured shale. Based on theunderstanding of fracture characteristics, the paper analyzed logging responsecharacteristics of effective fractures, that is, low resistivity in various probing depth,increased acoustic value and slightly reduced density value. Therefore adoptingdiscriminant analysis, R/S analysis, probabilistic neural network analysis methodscombined with log interpretation parameters such as fracture porosity and permeability,the conventional logging standard for natural fracture identification in the reservoirs ofMember Chang8in Red River oil field is establish.It is believed through the research on control factors to the development of thenatural fractures in the reservoirs of Member Chang8in Red River oil field that thedevelopment of the natural fractures in the research area is related to faults, the ancienttectonic stress evolution, bed thickness, lithology and so on; it can be recognized on thebasis of the their characteristics and mechanical properties that regional tectonicfractures were superimposed by fault-associated (or fault-derived fractures). It isdetermined after rock acoustic emission experiment and stable isotope analysis offracture fillings that the natural fractures in the reservoirs of Member Chang8in RedRiver oil field developed mainly in the periods of Yanshanian and Himalayanmovements among which Yanshanian is critical period, the natural fractures developedin the two phases were evaluated in the paper.Paleostructure restoration in the paper simulated the ancient stress field in theperiods of Yanshanian and Himalayan movements, the distribution of tectonic fracturesin the periods is evaluated and predicted; in connection with fault-associated orfault-derived natural fractures, the control function to the development of differentscale faults is established, and the controlling distance of faults to the associatedfractures is determined. Finally the distribution pattern of the natural fractures in thereservoirs of Member Chang8was pointed out through superposition of the twogenetic types of cracks, and the development of the natural fractures is overall better inin the south part than in the north part, and in the fault zone than the non-fault zone, insandstone than in shale.Finally, the background and process of the hydrocarbon accumulation in thereservoirs of Member Chang8in Red River oil field are analyzed starting from thegeological conditions in the study area in the paper. After the matching of structures,faults, hydrocarbon source rocks and reservoirs is fully taken account of, five kinds hydrocarbon accumulation models for the reservoirs in the member Chang8are built,that is, cracks-divert accumulation model, fault-divert accumulation model,source-reservoir contacting accumulation model, the lateral-divert accumulation modelof stacked sand bodies and the combination-divert accumulation model; the controllingfactors to hydrocarbon-rich region obtained through well tests are complex, not onlythe fracture system but also the reservoir conditions, accumulation conditions and so on;the match of various accumulation factors and faults is the key to the oil and gasenrichment and to obtain high yield in the low porosity and low permeability reservoirsof Member Chang8in Red River oil field.