Characteristics of Unconventional Reservoirs in Pingliang Formation,Southern Margin of Ordos Basin
Ordovician Pingliang Formation in the southern margin of the Ordos Basin develops two types of hydrocarbon source rocks:mud shale and carbonate rocks.In order to study the characteristics of the source-storage integrated unconventional oil and gas reservoirs in Pingliang Formation,the properties of hydrocarbon source rocks and reservoirs were simultaneously analyzed through field geological profile observations,core observations,and sample analysis tests.The results indicate that mud shale source rocks are mainly developed in the Longxian-Qishan region(i.e.the southwest part of the study area).The sedimentary environment is a plateau margin slope,and the lithology is mainly marl and micrite limestone.Carbonate source rocks are mainly found in the Xunyi-Tongchuan region(i.e.the central-eastern part of the study area).The sedimentary environment consists of platform edge reef shoals and open sea platforms,and with the main lithologies being mudstone,mud-crystal limestone.The kerogen type of source rocks in Pingliang Formation is mainly type Ⅰ,with a small amount of type Ⅱ1.The average organic carbon content is 0.41％,implying that they are moderately productive source rocks.The average vitrinite reflectance is 1.73％,and the average value of rock pyrolysis Tmax is 489 ℃,indicating a high mature stage.The Pingliang Formation source rocks exhibit high brittleness,with an average porosity of 1.38％,and an average permeability of 0.05 mD,and the physical properties of shale rocks are better than that of carbonate rocks.Methane adsorption capacity ranges from 0.64 cm3/g to 2.16 cm3/g,with an average of 1.71 cm3/g,and mud shale and marlstone show significantly higher methane adsorption than micrite limestone.Comprehensive analysis suggests that Pingliang Formation in the Longxian-Linyou region has well-developed mud shale,high content of siliceous brittle minerals,favorable physical and gas-bearing properties,indicating significant potential for further exploration of marine shale gas.In the central-eastern Xunyi-Tongchuan region,marl limestone source rocks demonstrate good hydrocarbon generation and methane adsorption capabilities,and provide basic conditions for the formation of self-sourced and self-stored marl limestone unconventional natural gas reservoirs.Micrite limestone source rocks in the same region can effectively generate hydrocarbons but exhibit poor storage and methane adsorption capabilities.It is recommended to conduct joint research on source-reservoir-caprock combination with reef-shoal facies reservoirs for carbonate rock natural gas exploration.
Ordos BasinPingliang Formationsource rockunconventional reservoirgeochemical characteristicsshale gas
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