Title: Natural gas origin and gas accumulation model for deep volcanic rocks in Nanpu sag,Bohai Bay Basin
Abstract: A breakthrough has been made recently in natural gas exploration of deep volcanic rocks in Nanpu sag of Bohai Bay Basin,East China,indicating a very high potential in natural gas exploration of deep volcanic rocks in Nanpu sag.Therefore,it is quite important to study the origin and hydrocarbon accumulation model of natural gases for the future exploration.Natural gases accumulate mainly in deep volcanic rock reservoirs within the Paleogene Shahejie Formation.They are dominated by hydrocarbon gases in chemical composition,of which the methane content varies from 70% to 90%,ethane and heavy hydrocarbons are relatively high,ranging between 4%~27%,the dry coefficient varies from 0.71 to 0.96,indicating mainly a wet gas.Carbon isotopes of natural gases are relatively heavy,with δ13C1 mainly varying from-35‰ to-43 ‰ and δ13C2 mainly from-24‰ to-28 ‰.According to plots of C1/(C2+C3) versus δ13C1 and δ13C1 versus δD,the natural gas in deep volcanic rock reservoirs belongs to an oil-type condensate gas.The calculated thermal maturity of the gas ranges between 1.00%~1.50% by the δ13C-Ro method,it matches the thermal evolution of typeⅡ1 source rocks in the Es3 member.In addition,Pr/Ph ratios of associated condensates,light hydrocarbon parameters and hydrocarbon isotopes also show that the deep volcanic rock gas is an oil-type gas and it is dominated by a condensate gas.Carbon isotopes of individual hydrocarbons of associated condensates show that the gas in deep volcanic rocks is generated from source rocks of the Es3 and Es1 members,namely the natural gas below the Es1 stratum is mainly derived from source rocks of the Es3 member and composed of mainly a condensate gas,while the natural gas above the Es1 stratum is mainly derived from source rocks of the Es1 member,which generate chiefly oils.Thick mudstones of the Es1 member serve as not only a high-quality cap rock but also a good source rock.The high-maturity oil and gas generated from deep source rocks in the sag migrate laterally through inherited abysmal faults to and accumulate in deep volcanic rock traps,hydrocarbon accumulations are controlled both by volcanic individuals and by volcanic phases.
Publication Year: 2012
Publication Date: 2012-01-01
Language: en
Type: article
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Cited By Count: 6
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