Effects of Deep Fluids on Hydrocarbon Generation and Accumulation in Chinese Petroliferous Basins

Deep fluids in a petroliferous basin generally come from the deep crust or mantle beneath the basin basement, and they transport deep substances(gases and aqueous solutions) as well as heat to sedimentary strata through deep faults. These deep fluids not only lead to large-scale accumulations of CO_2, CH_4, H_2, He and other gases, but also significantly impact hydrocarbon generation and accumulation through organic-inorganic interactions. With the development of deep faults and magmatic-volcanic activities in different periods, most Chinese petroliferous basins have experienced strong impacts associated with deep fluid activity. In the Songliao, Bohai Bay, Northern Jiangsu, Sanshui, Yinggehai and Pearl Mouth Basins in China, a series of CO_2 reservoirs have been discovered. The CO_2 content is up to 99%, with δ~(13)C_(CO2) values ranging from-4.1‰ to-0.37‰ and ~3He/~4He ratios of up to 5.5 Ra. The abiogenic hydrocarbon gas reservoirs with commercial reserves, such as the Changde, Wanjinta, Zhaozhou, and Chaoyanggou reservoirs, are mainly distributed in the Xujiaweizi faulted depression of the Songliao Basin. The δ~(13)CCH4 values of the abiogenic alkane gases are generally -30‰ and exhibit an inverse carbon isotope sequence of δ~(13)C_(CH4)δ~(13)C_(C2H6)δ~(13)C_(C3H8)δ~(13)C_(C4H10). According to laboratory experiments, introducing external H_2 can improve the rate of hydrocarbon generation by up to 147% through the kerogen hydrogenation process. During the migration from deep to shallow depth, CO_2 can significantly alter reservoir rocks. In clastic reservoirs, feldspar is easily altered by CO_2-rich fluids, leading to the formation of dawsonite, a typical mineral in high CO_2 partial pressure environments, as well as the creation of secondary porosity. In carbonate reservoirs, CO_2-rich fluids predominately cause dissolution or precipitation of carbonate minerals. The minerals, e.g., calcite and dolomite, show some typical features, such as higher homogenization temperatures than the burial temperature, relatively high concentrations of Fe and Mn, positive Eu anomalies, depletion of 18 O and enrichment of radiogenic ~(87)Sr. Due to CO_2-rich fluids, the development of high-quality carbonate reservoirs is extended to deep strata. For example, the Well TS1 in the northern Tarim Basin revealed a high-quality Cambrian dolomite reservoir with a porosity of 9.1% at 8408 m, and the Well ZS1 C in the central Tarim Basin revealed a large petroleum reserve in a Cambrian dolomite reservoir at ~6900 m. During the upward migration from deep to shallow basin strata, large volumes of supercritical CO_2 may extract petroleum components from hydrocarbon source rocks or deep reservoirs and facilitate their migration to shallow reservoirs, where the petroleum accumulates with the CO_2. Many reservoirs containing both supercritical CO_2 and petroleum have been discovered in the Songliao, Bohaiwan, Northern Jiangsu, Pearl River Mouth and Yinggehai Basins. The components of the petroleum trapped with CO_2 are dominated by low molecular weight saturated hydrocarbons.

Effects of Deep Fluids on Hydrocarbon Generation and Accumulation in Chinese Petroliferous Basins

Deep fluids in a petroliferous basin generally come from the deep crust or mantle beneath the basin basement, and they transport deep substances (gases and aqueous solutions) as well as heat to sedimentary strata through deep faults. These deep fluids not only lead to large‐scale accumulations of CO₂, CH₄, H₂, He and other gases, but also significantly impact hydrocarbon generation and accumulation through organic‐inorganic interactions. With the development of deep faults and magmatic‐volcanic activities in different periods, most Chinese petroliferous basins have experienced strong impacts associated with deep fluid activity. In the Songliao, Bohai Bay, Northern Jiangsu, Sanshui, Yinggehai and Pearl Mouth Basins in China, a series of CO₂ reservoirs have been discovered. The CO₂ content is up to 99%, with δ¹³C_CO2 values ranging from –4.1‰ to –0.37‰ and ³He/⁴He ratios of up to 5.5 Ra. The abiogenic hydrocarbon gas reservoirs with commercial reserves, such as the Changde, Wanjinta, Zhaozhou, and Chaoyanggou reservoirs, are mainly distributed in the Xujiaweizi faulted depression of the Songliao Basin. The δ¹³C_CH4 values of the abiogenic alkane gases are generally >–30‰ and exhibit an inverse carbon isotope sequence of δ¹³C_CH4>δ¹³C_C2H6>δ¹³C_C3H8>δ¹³C_C4H10. According to laboratory experiments, introducing external H₂ can improve the rate of hydrocarbon generation by up to 147% through the kerogen hydrogenation process. During the migration from deep to shallow depth, CO₂ can significantly alter reservoir rocks. In clastic reservoirs, feldspar is easily altered by CO₂‐rich fluids, leading to the formation of dawsonite, a typical mineral in high CO₂ partial pressure environments, as well as the creation of secondary porosity. In carbonate reservoirs, CO₂‐rich fluids predominately cause dissolution or precipitation of carbonate minerals. The minerals, e.g., calcite and dolomite, show some typical features, such as higher homogenization temperatures than the burial temperature, relatively high concentrations of Fe and Mn, positive Eu anomalies, depletion of ¹⁸O and enrichment of radiogenic ⁸⁷Sr. Due to CO₂‐rich fluids, the development of high‐quality carbonate reservoirs is extended to deep strata. For example, the Well TS1 in the northern Tarim Basin revealed a high‐quality Cambrian dolomite reservoir with a porosity of 9.1% at 8408 m, and the Well ZS1C in the central Tarim Basin revealed a large petroleum reserve in a Cambrian dolomite reservoir at ～6900 m. During the upward migration from deep to shallow basin strata, large volumes of supercritical CO₂ may extract petroleum components from hydrocarbon source rocks or deep reservoirs and facilitate their migration to shallow reservoirs, where the petroleum accumulates with the CO₂. Many reservoirs containing both supercritical CO₂ and petroleum have been discovered in the Songliao, Bohaiwan, Northern Jiangsu, Pearl River Mouth and Yinggehai Basins. The components of the petroleum trapped with CO₂ are dominated by low molecular weight saturated hydrocarbons.