Research On The Hydrocarbon Accumulation Condition And Enrichment Mechanism Of Tight Oil Of Lower Cretaceous In The Huahai Depression Of Jiuquan Basin

Tight oil exploration is in the initial stage in Huahai depression,accumulation conditions and enrichment mechanism of tight oil are lack of in-depth research.This paper analyzes the condition of tight oil accumulation in the study area by means of geological statistics,experimental test and observation of thin section under microscope etc.Studies show that the quality of hydrocarbon source rock is good in Huahai depression,high quality and effective hydrocarbon source rocks are mainly distributed in Xiagou formation and lower Zhonggou formation of the middle and south part of the depression.The tight reservoir and source rock are adjacent to the symbiotic,and wide range distribution.The sandwich type high quality source and reservoir combination developed in the depression,and the preservation condition is good.Through basin simulation and the analysis of tectonic evolution,study on the process of tight oil accumulation in the depression,this paper clarifies that the accumulation of tight oil in the study area mainly through the accumulation process of early Cretaceous time and two transformation process of Himalaya period.The late early Cretaceous to early late Cretaceous is the main accumulation stage of tight oil in Huahai depression,there is no secondary hydrocarbon generation.The tight oil accumulation is mainly controlled by hydrocarbon generating pressurization and capillary pressure,mainly with the overpressure filling.Source reservoir pressure difference is the main driving force for tight oil filling.Nuclear magnetic resonance experiments indicate that Lower Cretaceous strata of Huahai depression generate a limited tight oil.the vast majority of oil is adsorbed on the surface of micro-nano scale pores though the form of oil film(adsorption state).A few occur in the pore throat by the form of oil droplet.Irreducible oil saturation is 1.76%~18.21%,and most of samples is not more than 10%.Movable oil saturation is between 0.23%~2.96%.Through the tight oil filling physical simulation experiment,it is found that seepage flow migration of tight oil in the tight reservoir has obvious stages and jumping.The size of the driving pressure determines the lower limit of the pore throat of the tight reservoir,and the lower limit of the pore throat of the tight reservoir calculated in the depression is 65 nm.Through the tight oil enrichment of main controlling factors and favorable zone optimization,forecast the dessert areas mainly distribute in the well zones of Huatan7-Huashen1-Huatan10-Huatan9.The well areas of Huatan7-Huashen1 have good reservoir conditions and oil and gas shows,which can be used as the favourable regions of tight oil exploration in the future.