The Accumulation Mechanism Of Tight Oil Of Lucaogou Formation In Jimsar Depression,Junggar Basin

Tight oil reservoirs are important for China and even global petroleum exploration in the future,thus research on the accumulation mechanism of tight oil reservoirs is significant for tight oil exploration and predicting favorable target areas.Lucaogou formation in Jimsar depression located in Junggar Basin was taken as research object,based on geological,well logging,oil testing data and in combined with geochemical analysis and other testing method,the source rock,tight oil reservoirs,source-reservoir configuration and migration dynamic conditions were systematically studied,the main factors controlling oil accumulation were expounded and finally the model of oil accumulation was established.The analysis of geological features of tight oil accumulation showed that dark mudstone of Lucaogou formation sedimented in semi-deep lake to deep lake environment,and the wide spread source rocks were characterized by high organic content,superior organic type and in low mature-mature stage.The source rocks entered the primary time of hydrocarbon generation and expelling from Cretaceous till now,which provided sufficient oil sources for oil accumulation;Rock types of tight oil reservoir in study area are mainly transitional rocks between dolomite and sandstone with rich developed micro-nanometer pores.Besides,secondary dissolution pore is the main pore type,and the sweet spots with relatively high porosity and permeability are favorable for oil enrichment;Lucaogou reservoir is of typical source-reservoir structure,which is favorable for hydrocarbon charge,and the lower reservoir and formation thickness ratio is,the better for oil enrichment.Analysis on the dynamic conditions of tight oil accumulation revealed that overpressure was widespread in Lucaogou source rocks and reservoirs with lateral stability and connectivity,which suggested the feature of strata-controlled;With longitudinal differences in hydrocarbon-generated ability in different strata,resulting in uneven distribution of the overpressure.Strong overpressure difference between layers provided the main force for the tight oil charge,and the distribution belt which has big difference of overpressure between layers might be the favorable area for tight oil enrichment.Based on the above studies,summarized the main factors controlling oil accumulation,and finally the models of oil accumulation in tight oil reservoir were established as follows: “source rock and tight reservoir joint controlling,hydrocarbon adjusting accumulation” in macro,“oil charge controlled by sustained and strong overpressure gradient,and tight oil preferentially enriches in sweet spots” in micro.