Differences between Kerogen-and Oil-cracked Gases in Sinian Reservoirs of Weiyuan and Ziyang Area

YIN Chang he; WANG Ting dong; WANG Shun yu; LIN Feng

Bv comparing nonhvdrocarhon compositions carbon isotope ratios of natural gases and reservoir inclusions in Wcivuan Areas( W ) and Zivang Areas( Z ) and lower Cambrian black shale is the main sources of natural gas.Time of structure formation is different between W and Z.The former is during Yanshanian orogcny and the latter Indosinian Orogcny, but Z structurc disappeared in Yanshanian Orogcny.So, natural gases for W arc made up of oil and kcrogcn-cracked gases, but the latto is important.However natural gases in Z are mainly made from oil-cracked gases. C₁ / C₁+ ratios of natural gases in W and Z are more than 99.62 %,therefore natural gases arc high and over-matured gases, and hydrocarbon gases of natural gases in Z are more abundant in high-yield wells than in low-yield ones.Contents of nitrogen gas in W Arca is higher than 6%,and less than 1.22% in high-yield wells and higher than 4.17 % in low-yield well in Z Arca. C₁/C₁₊,contents of hydrocarbon gases and nitrogen hold out that natural gases in W are high and over-ma- turgid, and nitrogen gas originate from high and over-matured lower Cambrian black shale.Natural gases with high content of nitrogen and helium gases in W are mainly kcrogcn-cracked.Natural gases in high-yield wells in Z are oil-cracked.Because content of radioactive element and nitrogen in oil are much lower than in source rock oil-cracked gases must contain low content of helium and nitrogen gases.Natural gases in low-yield wells intcrfuscd by kcrogcn-cracked gases contain high con- tent of helium and nitrogen gases.Methane carbon isotope ratios are nearly 5‰ higher in W than in Z.R_a(%),based on δ¹³C₁(‰)-R_a(%) formula, is 1% higher in W than in Z.Furthermore natural gases in W arc kcrogcn-cracked and those in Z are oil-cracked. Homogeneous temperatures( HT) of liquid hydrocarbon inclusions in W is in the range of 80℃ and 160℃,and amount of liquid hydrocarbon inclusions is small.Homogeneous temperature of abundant gaseous hydrocarbon inclusions in Wis in the range of 200℃ and 240℃.Homogeneous temperature of hydrocarbon inclusions in Z is in the range of 80℃ and 248℃,and having licquid, gaseous gaseous- liquid liquid-gaseous mainly liquid hydrocarbon inclusions.HT of liquid hvdrocarbon inclusions is in the range of 80℃ and 160℃,and the rest are in the range of 1500C and 2480C.Accumulated oil is small and hiodegraded, and oil-cracked gases in W are lack.Accumulated oil is large and oil-cracked gases in Z are ahundant. Differences of nitrogen and helium gas, methane carbon isotope ratios, HT and phase of hvdrocarhon inclusions prove that natural gases originating from lower Cambrian black shale in W are kerogen-cracked and those in Z are oil-cracked.

Institute of Southwest Petroleum,Nanchong,Sichuan 637001

Received Date: 1999-06-22

Rev Recd Date: 1999-10-25

Key words:

Abstract: Bv comparing nonhvdrocarhon compositions carbon isotope ratios of natural gases and reservoir inclusions in Wcivuan Areas( W ) and Zivang Areas( Z ) and lower Cambrian black shale is the main sources of natural gas.Time of structure formation is different between W and Z.The former is during Yanshanian orogcny and the latter Indosinian Orogcny, but Z structurc disappeared in Yanshanian Orogcny.So, natural gases for W arc made up of oil and kcrogcn-cracked gases, but the latto is important.However natural gases in Z are mainly made from oil-cracked gases. C₁ / C₁+ ratios of natural gases in W and Z are more than 99.62 %,therefore natural gases arc high and over-matured gases, and hydrocarbon gases of natural gases in Z are more abundant in high-yield wells than in low-yield ones.Contents of nitrogen gas in W Arca is higher than 6%,and less than 1.22% in high-yield wells and higher than 4.17 % in low-yield well in Z Arca. C₁/C₁₊,contents of hydrocarbon gases and nitrogen hold out that natural gases in W are high and over-ma- turgid, and nitrogen gas originate from high and over-matured lower Cambrian black shale.Natural gases with high content of nitrogen and helium gases in W are mainly kcrogcn-cracked.Natural gases in high-yield wells in Z are oil-cracked.Because content of radioactive element and nitrogen in oil are much lower than in source rock oil-cracked gases must contain low content of helium and nitrogen gases.Natural gases in low-yield wells intcrfuscd by kcrogcn-cracked gases contain high con- tent of helium and nitrogen gases.Methane carbon isotope ratios are nearly 5‰ higher in W than in Z.R_a(%),based on δ¹³C₁(‰)-R_a(%) formula, is 1% higher in W than in Z.Furthermore natural gases in W arc kcrogcn-cracked and those in Z are oil-cracked. Homogeneous temperatures( HT) of liquid hydrocarbon inclusions in W is in the range of 80℃ and 160℃,and amount of liquid hydrocarbon inclusions is small.Homogeneous temperature of abundant gaseous hydrocarbon inclusions in Wis in the range of 200℃ and 240℃.Homogeneous temperature of hydrocarbon inclusions in Z is in the range of 80℃ and 248℃,and having licquid, gaseous gaseous- liquid liquid-gaseous mainly liquid hydrocarbon inclusions.HT of liquid hvdrocarbon inclusions is in the range of 80℃ and 160℃,and the rest are in the range of 1500C and 2480C.Accumulated oil is small and hiodegraded, and oil-cracked gases in W are lack.Accumulated oil is large and oil-cracked gases in Z are ahundant. Differences of nitrogen and helium gas, methane carbon isotope ratios, HT and phase of hvdrocarhon inclusions prove that natural gases originating from lower Cambrian black shale in W are kerogen-cracked and those in Z are oil-cracked.

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Differences between Kerogen-and Oil-cracked Gases in Sinian Reservoirs of Weiyuan and Ziyang Area

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