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

Abstract

By comparing nonhydrocarbon compositions,carbon isotope ratios of natural gases and reservoir inclusions in Weiyuan Area(W) and Ziyang Area(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 orogeny and the latter Indosinian Orogeny,but Z structure disappeared in Yanshanian Orogeny. So,natural gases for W are made up of oil and kerogen cracked gases,but the latter is important. However,natural gases in Z are mainly made from oil cracked gases. C 1/C + 1 ratios of natural gases in W and Z are more than 99.62%,therefore,natural gases are 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 Area is higher than 6% ,and less than 1.22% in high yield wells and higher than 4.17% in low yield well in Z Area. C 1/C 1+ ,contents of hydrocarbon gases and nitrogen hold out that natural gases in W are high and over matured,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 kerogen 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 interfused by kerogen cracked gases contain high content of helium and nitrogen gases. Methane carbon isotope ratios are nearly 5‰ higher in W than in Z . R o(%) ,based on δ 13 C 1(‰)? 玆 o(%) formula, is 1% higher in W than in Z. Furthermore,natural gases in W are kerogen cracked and those in Z are oil cracked. Homogeneous temperature(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 W is in the range of 200℃ and 240℃. Homogeneous temperature of hydrocarbon inclusions in Z is in the range of 80℃ and 248℃,and having liquid,gaseous,gaseous liquid,liquid gaseous,mainly liquid hydrocarbon inclusions. HT of liquid hydrocarbon inclusions is in the range of 80℃ and 160℃,and the rest are in the range of 150℃ and 248℃. Accumulated oil is small and biodegraded,and oil cracked gases in W are lack.Accumulated oil is large and oil cracked gases in Z are abundant. Differences of nitrogen and helium gas,methane carbon isotope ratios,HT and phase of hydrocarbon inclusions prove that natural gases originating from lower Cambrian black shale in W are kerogen cracked,and those in Z are oil cracked.