Analysis Of Main Controlling Factors Of Hydrocarbon Accumulation In Chang 8₁ In Heshui Area Of ??Ordos Basin

With the increase of petroleum exploration intensity,some achievements have been obtained in Chang 81 reservoir group of Heshui area,Ordos Basin.However,hydrocarbon enrichment remains unclear and oil wells are concentrated distribution in the northern part of Heshui area,while oil wells are distributed dispersedly in the southern part.Chang 81 in Heshui area is taken as the target to analyze the main control factors of hydrocarbon accumulation guided by petroliferous basin analysis and petroleum geology and based on the core observation combining with well logging interpretations,casting thin sections,petrophysical data and geochemical parameters,for researches on sandstone distributions,reservoir characters,source rock distributions,migration systems and hydrocarbon accumulation dynamics.Delta distributary channel sandstones are regularly developed in Chang 81 of Heshui area,and are distributed differently on the flat surface.Sandstones with good internal connectivity are thicker in northern part than in southern part of Heshui area.Reservoir rocks are mainly composed of lithic feldspar sandstone and feldspar lithic sandstone comes in second.Interstitial material types vary in the studied area including chlorite membrane,ferrocalcite,hydromica and silicon.It possesses more chlorite membrane in the northern part than the southern part of Heshui area,while the contents of calcite and hydromica are opposite.Reservoir sandstones possess bad roundness and fine particle but good sorting in Chang 81 of Heshui area.The average porosity and permeability is respectively 7.36%and 0.29mD in the north part of the studied area while they are lower in the south part which is 6.37%and 0.13mD respectively.Oil and gas of Chang 81 in Heshui area mainly originate from overlying Chang 7 high-quality source rocks.It can be concluded from the analyses that source rocks of Chang 7 in Yanchang formation have tremendous hydrocarbon generation potential,but are distributed with some differences in the studied area.Mudstone distributes more widely in northern part comparing to the southern part of Heshui area.The reservoir types are mainly belonged to lithologic oil and gas reservoirs in Heshui area.Sandstones with good internal connectivity and partly-developed fractures are the major migration systems.Abnormal overpressure caused by hydrocarbon generating pressurization and undercompaction is the main reservoir-forming dynamic.In a word,hydrocarbon accumulation of Chang 81 formation in Heshui area is mainly affected by source rock distributions,sandstone scales,reservoir physical properties and fracture distributions.The northern part of Heshui area has concentrated reservoir distributions and good exploration effects because of widely-distributed Chang 7 source rocks,well-developed delta distributary channel sandstones with good internal connectivity and physical properties and partly-developed fractures.However,conditions of southern part are completely opposite with poor internal connectivity and physical properties and no fracture development,which makes hydrocarbon difficult to migrate into a large scale and leading to a bad exploration result.