Heterogeneity And Hydrocarbon Control Of Salt Lake Source Rocks In Dongpu Sag

Dongpu sag is a typical salt lake rich oil and gas sag.For a long time,the evaluation of source rocks has been carried out using a large average method.The heterogeneity of source rocks and the control of hydrocarbons are weak.Based on previous studies,this paper explores the heterogeneity,main controlling factors of salt-water source rocks and its hydrocarbon control in the region by using geochemical techniques such as Rock-Eval,chromatography-mass spectrometry and carbon isotope of monomer hydrocarbons,combined with geological analysis.The source rocks of the Dongpu sag are heterogeneous: color and lithology are variable.The high-quality source rocks are mainly shale,oil shale and some dark gray mudstones;There are various combinations of source rocks and surrounding rocks,including three types: sand-shale interbedded type,thin interbedded layer of plaster and mudstone,and thick interbedded layer of thick layered mudstone.The quality of source rocks is closely related to gypsum rock.The abundance and type of source rocks in salt-bearing areas are better than those in unsalted areas,and the paste-soil layers are better than sand-shale layers.There are differences in the thermal evolution of the upper and lower source rocks of the gypsum rock.On the one hand,the gypsum rock can promote the thermal evolution of the source rocks on the salt,and on the other hand,it can inhibit the thermal evolution of the subsalt source rocks.The heterogeneity of source rocks is controlled by many factors.The differential tectonic movement and evolution control the spatial distribution of dark mud shale.The high frequency lake plane changes caused by climate control the lithology of source rocks and their The combined mode,paleoproductivity and paleo-oxygen phase control the organic matter abundance of source rocks.The heterogeneity of source rocks has a strong role in controlling oil and gas: The Dongpu oil and gas is mainly distributed between the two largest hydrocarbon-bearing subsag,the Qianliyuan subsag and the Haitongji subsag.Although the thickness of the dark mudstone is large in the southwest subsag,the quality of the source rocks is poor.Under the conditions of similar hydrocarbon expulsion and hydrocarbon reservoir properties,the ultra-thin and thin-layer reservoirs are more likely to be filled with oil and gas than the medium-thick and thick reservoirs,and the oil and gas enrichment layer is higher.Northern crude oil has the characteristics of low Pr/Ph,high gammacerane index and high homohopane index of typical salt lake phase crude oil,which is significantly different from southern freshwater crude oil.The shallow crude oil in Wenliu area has the three low characteristics of low diasteranes,low sterane isomerization and low 4-methyl sterane.The deep crude oil is high diasteranes,high sterane isomerization and high 4-methyl sterane.Significant differences in the nature of crude oil reflect the heterogeneity of oil sources.