The Features And Forming Condition Analyses Of Tight Oil Accumulations Of Chang 7 Member Of Yanchang Formation,Ning 89 Well Area Of Ordos Basin

The porosity and permeability of Chang 7 reservoirs in Ning 89 well area is very low,and does not satisfy the basic demands of forming conventional oil accumulation.Yet,lots of tight oil accumulations have been found in those reservoirs,there must be some conditions making formation of tight oil accumulations in large scale possible.Based on data of well logs,loggings,rock cores,test of formation and a series of experiments,studies on tight oil accumulation features have been conducted.Chang 7 reservoirs are intermediate-to-oil wet;the quality of Chang 7 source rocks is very good,with tens to one hundred meters thickness;the source-reservoir assemblage type is "selfgeneration and self-accumulation";pressure accumulated in good quality source rocks can be tens of MPa,which can only be slightly alleviated by organic pores,source rocks will be cracked many times,hydrocarbons will be expelled episodically.Based on former studies on tight oil accumulation features,analyses of accumulation forming conditions can be conducted.Firstly,good quality source rocks can provide enough oil resources to tight oil accumulations,and give oil strong migration force to charge reservoirs episodically;secondly,the intermediate-to-oil wet property of reservoirs can compensate the ineligibilities of reservoirs;finally,the "self-generation and self-accumulation" assemblage can decrease migration distance and lower demand for migration force,and provide preservation conditions for accumulations.Those studies on accumulation features and forming conditions will provide theoretical guidance for the exploration of tight oil in Ordos basin and other areas.