Analysis Of Main Controlling Factors Of Hydrocarbon Accumulation In Chang 4+5₁ In Baibao-Nanliang Area Of ??Ordos Basin

With the deepening of petroleum exploration,some achievements have been obtained in Chang 4+51 reservoir of Baibao-Nanliang area,Ordos Basin.However,the hydrocarbon distribution remains unclear with big differences in two blocks,of which Nanliang areaowns relatively lower rate of water as 30%-40%,but that of Baibao area varies among wells with generally higher rate of water even up to 60%-70%in Well(s)Bai-157 while that of Well(s)Bai-191 is much lower which appears mainly as oil wells.Chang 4+51 in Baibao-Nanliang area is taken as the target to analyze the main controlling factors of hydrocarbon accumulation,guided by petroliferous basin analysis and petroleum geology,based on the core observation combining with well logging,thin sections,physical property and geochemical indicators,for researches on sand distributions,reservoir characters,migration systems and hydrocarbon accumulation characteristics.Both supplied by the northeastern sources,Chang 4+51 of Baibao area performs mainly as subaqueous distributary channels with great intermal connectivity while that of Nanliang area acts maiuly as estuary damsandstone which could be excellent reservoirs.Reservoir rocks are mainly composed of feldspar sandstone and lithicfeldspar sandstone come in second.And Chang 4+51 develops with various interstitial matters including chlorite membrane,ferrocalcite,hydromica and silicon,while chlorite membrane favorable for pores is dominant in Baibao area and ferrocalcite and hydromica unfavorable for pores are superior in Nanliang area.These reservoir sandstones possess bad roundness but good sorting and fine particle.And generally speaking,Chang 4+51 in Baibao area owns better physical properties than that in Nanliang area.Oil and gas of Chang 4+51 in Baibao-Nanliang area mainly originate from underlying Chang 7 high·quality source rocks with high maturity and great potential for hydrocarbon generation which is a little thicker in Nanliang area.The reservoirs,showing up as lithologic oil-gas reservoirs in studied area,mainly migrate in vertical or lateral through the pathway of good internalconnectivity sandstone and partly-developed fractures with the shelters of mudstone or updip pinchout sandstone.And the development of sandbody matches perfectly with the distribution of source rocks.Generally speaking,oil and gas accumulation of Chang 4+51 in Baibao-Nanliang area is under the control of sandbody type,low amplitude structure,sandbody structural type and physical property.After comprehensive comparisons on oil reservoirs from Baibao area andNanliang area as well as the different wells from Baibao area,it is clear that the physical property of subaqueous distributary channels performs better than that of estuary damsandstone,but the 0il saturation is much lower.And for the same subaqueous distributary channels,these sandbodies in the main part have much better physical property than the side part,both with high oil saturation.However,the oil saturation would be lower if there is no shelter for hydrocarbons in the channels resulting in less hydrocarbon accumulation.In a word,hydrocarbon accumulation of Chang 4+51 group in studied area is mainly affected by sandstone types and internal differences,low-amplitude structures and reservoir physical properties.Chang 4+51 in Nanliang area develops estuary damsandstone with favorable autogenetic traps,and owing to the bad internal connectivity,oil and gas could migrate and accumulate with sufficient oil-water differentiation,and are hard to be replaced by water.On the contrary,Chang 4+51 in Baibao area appears mainly as subaqueous distributary channels with great intermal connectivity.When there are no shelters,oil and gas would escape along the updip channels,and are easy to be replaced by water during the development process resulting in oil and water appear together with insufficient oil-water differentiation and fast rising rate of water,however,the case changes if there develop low-amplitude structures such as nose uplifts in Well(s)Bai-415 with relative lower rate of water than other wells in Baibao area.