Effects Of Tight Reservoir Heterogeneity On Oil-Gas Reservoir Formation

Though several large-scale oil gas fields have been discovered after years of oil-gas surveys worldwide,the focus of surveys in some oil-gas-rich basins is not the large-scale uncompartmentalized oil gas reservoirs any more and turns to the relatively polyatomic and more complicated oil-gas exploitation.Following this trend,oilfield developers pay more attention to lithologic oil gas reservoirs.In lithologic oil gas reservoirs,the sandbodies are distributed randomly and dispersedly,and are mainly medium-and smallscaled,which result in severe reservoir heterogeneity and complicate our understanding about the oil-gas reservoir formation and distributive rules of lithologic traps.This work is targeted at the oil layer Chang 33 in Well 2 of Weibei Oilfield.We characterized the formation and distribution of oil gas reservoirs and determined the influence factors of different levels of heterogeneity on oil-gas distributions by making full use of rock core and logging data and referring to some physical simulations.The oil layer group Chang33 belongs to the delta front margin subfacies,especially underwater distributary channel microfacies and debouch bar microfacies.The major rock types in the reservoirs were quartz sandstone and feldspar sandstone.The major cements were siliceous and feldspathic,followed by iron calcite and hydromica.The major gap types in the reservoirs were intergranular dissolved pores and intragranular dissolved pores,and the oils and gases most aggregated in the intergranular dissolved pores.Mercury intrusion experiments showed that the replacement pressures of reservoirs were generally manifested as "relatively large median pressure and small median radius".For the reservoired rocks,however,their lithology was very tight,belonged to the type of "macropore-microscale throats",and the gaps were less connective.In particular,the intralayer and interlayer heterogeneities were very high longitudinally,and interlayers were very developed.The heterogeneity of reservoirs was controlled by sedimentary microfacies and late-phase diagenesis.Moreover,the effects of heterogeneity on oil-gas reservoir formation were analyzed from three aspects.Firstly,the major type of microcosmic gas of the Well2 in Weibei Oilfield was intergranular gaps,and the major storage spaces were large pores,which were both largely affected by heterogeneity.Secondly,the vertical rhythm largely affected the oil-bearing content and saturation of sandbodies,while the intralayer cross-beddings contributed to oil-gas aggregation.Also the fluid flow was affected by the presence of interlayers,but some thick homogeneous interlayers contributed to the oil-gas reservoir formation.Finally,the horizontal spread of oil-gas was mainly controlled by sedimentary microfacies,while the facies belts most contributive to oil-gas enrichment were the underwater distributary channel microfacies and debouch bar microfacies.