The Main Controlling Factors Of Tight Oil Enrichment And Sweet Spot Prediction In The Qingxi Sag Of Jiuquan Basin

In recent years,great progress has been made in the field of lacustrine pelitic dolostone reservoirs in Qingxi Sag.And it has recently been an important exploration field of tight oil in Jiuquan Basin.This paper bases on analysis of thin sections,drilled cores,scanning electron microscopy,logging data and test data,and studied characteristics of tight oil source rocks,reservoirs,source-reservoir combinations,preservation conditions of the Xiagou formation in Qingxi sag.The major controlling factors of tight oil enrichment are identified.The thickness of source rocks in Xiagou Formation is huge.The source rocks have an average TOC of 1.6% and are dominated by Type II kereogen.Ro values range from 0.6% to 1.0%.The main reservoir spaces of the Xiagou formation in Qingxi sag are micro-fracture,dissolution pore and intracrystalline pore.The results of magnetic resonance imaging and high-pressure mercury injection indicate that pore throats of reservoirs are a large amount of scale from nanometer to micrometer.The result of oil-source correlation shows tight oil distributes widely around and within source rocks in the Xiagou Formation.The types of source-reservoir combinations contain self-souce and self-reservoir,lower-source and upper-reservoir,upper-source and lower-reservoir,sandwich and thin-layer interbedded.Statistic results of oil testing yield,magnetic resonance imaging test and physical modeling experiment for hydrocarbon accumulation of tight oil indicate that the self-source and self-reservoir combination type is the best for the enrichment of tight oil,the sandwich is the second,the upper-source and lower-reservoir is the third,lower-source and upper-reservoir is the fourth while the thin-layer interbedded one is the worst.The analysis of the kind and salinity of formation water,the relationship between the development degree of fault and fissures and oil production and formation pressure coefficient shows that the preservation conditions of tight oil in the Xiagou formation in Qingxi sag is good.With analysis of typical profiles,Main Controlling Factors of tight oil enrichment in the Xiagou formation,Qingxi sag are high quality source rocks wide distributed,top quality source-reservoir combinations types and the development of cracks.Combined the distribution eara of tight oil with the main controlling factors of tight oil enrichment,Sweet spot of tight oil is predicted which is mainly located in the superposition of multiple factors.