| The shale oil reservoir,which has a broad shale oil resource prospect,are characterized by high organic carbon content from stara Q1 to Q9 in the Cretaceous Qingshankou Formation,Gulong Sag,Songliao Basin.The characteristics of lithology,laminae structure,reservoir space,occurrence state of oil and gas,and paleoenvironment of the study area were studied using core,thin section,fluorescence thin section,CT,major and trace elements,and XRD experiments.The results show that lithology is divided into four types,including clay shale,felsic shale,shell limestone,and dolomite.According to the scale,the multi-scale laminae structure can be classified into millimeterscale laminated rocks,centimeter-scale layered rocks,and decimeter-scale massive rocks.The thickness of individual lamina is less than 0.01 m in the laminated rocks,and the layered rocks have bundles of genetically related laminae thickness ranging between 0.01m-0.1 m.In massive rocks there is no visible layering.According to the content of mineral compositions,the laminae were divided into four types: felsic laminae,clay laminae,carbonate laminae,and mixed laminae.According to mineral type and superposition and assemblages from thin section observation,laminae assemblage can be divided into five types: I clay laminae interbedded with felsic laminae,II clay laminae combined with felsic laminae,in which the thickness of clay laminae is larger than that of felsic laminae,III clay laminae intercalated with carbonate laminae,in which the thickness of clay laminae is larger than that of carbonate laminae,IV clay laminae intercalated with mixed laminae,in which the thickness of clay laminae is larger than that of mixed laminae,V clay laminae interbedded with mixed laminae and felsic laminae.The reservoir space of shale oil reservoir is diverse,mainly composed of clay minerals intercrystalline pores and interlayer fractures,secondary dissolved pores,and organic pores.The development of lamellation fractures greatly improves the reservoir property and enhances the connectivity between the pore and throat.The dominant occurrence state of oil is free or adsorbed state,and the oil film is the most common,mostly distributed along the edge of pores.The shale oil reservoir of the Cretaceous Qingshankou Formation is deposited in the lacustrine depositional environment,which is characterized by freshwater with humid climate and strong reduction.The water body reaches the deepest in the Q7 strata and then gradually becomes shallower.The sedimentary facies transition from deep lake to semi-deep lake to terrestrial delta.According to high resolution image log,especially the slab and button conductivity curve derived from the image logs,multi-scale laminae structure can be recognized.Conventional petrophysical log suites are used to recognize lamellation fractures.The effective porosity was selected as the main parameter of reservoir physical property evaluation.Oil saturation and total organic matter content were selected as the parameters of oil-bearing property evaluation.Nuclear magnetic resonance(NMR)logs were used to calculate the effective porosity and oil saturation.The exchanged T2 cutoff value model was the best method to calculate effective porosity.The variable baseline Delta Log R acoustic wave method is the best to calculate the total organic matter content.The calculated results of each model are in good correspondence with the experimental data and can be used for the evaluation of related parameters.Two-dimensional NMR(2D NMR)experiments are used to identify the reservoir fluid types.Results show that the oil-bearing shale intervals are dominated by laminated rocks and layered rocks,accounting for 90% of the studied Qingshankou Formation.The lamellation fractures result in a positive deviation from deep and shallow resistivity logs,and they are associated with low gamma ray values.In addition,three porosity logs are not sensitive to lamellation fractures in shale oil reservoirs.Effective porosity ranges from 3% to 15%,with an average value of 7%.Strata Q1,Q4,Q8 and Q9 have the highest effective porosity,which is higher than the average value.The oil saturation value varies greatly in different strat,and the average value is 50%,among which the oil saturation of Q2,Q4,Q5,Q8 and Q9 layers is higher,above 50%.The Total Organic Carbon(TOC)content ranges from 0.05% to 4.06%,with an average value of 1.94%.The organic carbon content of Q1,Q2,Q6 and Q9 layers is higher than the average value,and the abundance of organic matter in Q1 and Q2 layers has a large variation range and a high abundance.The average oil saturation(mainly irreducible oil)calculated by two-dimensional nuclear magnetic log is 46%,and the highest oil saturation in Q9 layer is 50%,followed by Q1 layer and Q4 layer is slightly higher than the average.The results also show that the fluid types are divided into asphaltene,irreducible oil,movable oil,clay mineral bound water,irreducible water,and movable water.According to the changes of 2D NMR maps under three different conditions of original state,drying at 105℃ and saturated kerosene,the classification criteria of T2 and T1/T2 for different fluid types were established,and the main controlling factors of fluid type content were the abundance of TOC,free hydrocarbon(S1)and pyrolytic hydrocarbon(S2).The distribution of the calculated saturation of different fluid types in a single well is completed by two-dimensional NMR logs.The relationship between multi-scale laminae structure types and reservoir property,oil-bearing property parameters and mobility were unraveled.the layered rocks play an important role in improving reservoir quality and the laminated rocks present moderate reservoir quality.The massive rocks show poor reservoir quality.Moreover,an increased proportion of felsic mineral content favors increased oil potential in oil shale reservoirs.Combined with the changes in sedimentary environment and sequence,the distribution characteristics and formation mechanism of multi-scale laminae structure and laminar types are clarified,and the enrichment mechanism of shale oil and gas is improved,which provides insight into the prediction of high-quality sweet spots of unconventional oil and gas reservoirs. |
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