Multi-stage Hydrocarbon Accumulation Mechanism In The Silurian Of Shuntuoguole Uplift,Tazhong Northern Slope,Tarim Basin

The Silurian bituminous sandstone is widespread in the Shuntuoguole area of Tazhong northern slope,accompanied with heavy oil,normal oil and natural gas.It is characterized by multi-period hydrocarbon accumulation and multi-period hydrocarbon adjustment,which accounts for extremely complicated hydrocarbon accumulation mechanism.In this study,the geochemical characteristics of crude oil and extracts,micro FT-IR analysis of bitumen and petrography observation,both in macroscopic and microscopic scale,and fluid inclusion analysis were carried out for hydrocarbon accumulation analysis.Hydrocarbon charging differences in upper(S₁k^3-3 and S₁k^3-1)and lower?S₁k¹?Kepingtage Formation in well Shun 10 and Shun 9 well block have been summarized.Kepingtage Formation of Tazhong northern slope has been in late diagenesis stage,including mechanical compaction,chemical cementation,dissolution and metasomatism.According to petrography and fluorescence characteristics,the diagenesis in Shun 9well block is oil trapped in internal crack of the quartz,in quartz overgrowth and crack which across the quartz but not across quartz overgrowth,in later calcite cements and crack which across both the quartz and quartz overgrowth in chronological order,while in well Shun 10 it was in the chronological order of internal crack of the quartz,in quartz overgrowth and crack which across the quartz but not across quartz overgrowth.In addition,micro FT-IR characteristics of bitumen,individual hydrocarbon inclusion and oil were analyzed comprehensively to explore hydrocarbon accumulation mechanism in Shuntuoguole area.However,the spectra were visibly overlapped because of the sophisticated structure of oil inclusion and solid bitumen.By comparison of Gaussian,Lorentz and Gaussian–Lorentz curve fitting in different noise levels,Gaussian–Lorentz curve fitting with 1–10 noise level is confirmed to be a better choice for individual peak calculation to quantify bitumen and oil inclusion concentration with a fitting accuracy of 99.5%.Most maturity parameters for Tarim oils are limited result from the complicated geological settings.According to alkylbiphenyls'thermal stability of ortho,meta and para substituents,several new parameters are tentatively proposed for maturity assessment.Theyare4–/2–MBP,4–/2–EBP,?2,4+2,4'?–/2,2'–DMBP,4,4'–/?2,4+2,4'?–DMBP,3,3'–/?2,4+2,4'?–DMBP,3–MBP/3–EBP,4–MBP/4–EBP,3,3'–DMBP/3–EBP,4,4'–DMBP/4–EBP,3–MBP/3–MDPM and 4–MBP/4–MDPM.In combination with petrography,micro FT-IR analysis,petroleum geochemistry and paleo-fluid study,three hydrocarbon charging models are proposed in Shun 9 well block,i.e.earlier charged hydrocarbon was biodegradated into bitumen without later charging;earlier and later charged hydrocarbon were mixed;only later hydrocarbon was charged.However,hydrocarbon suffering from oxydative degradation and heating stress alteration,turned into bedding-following bitumen without later charging.Oil inclusions in S₁k^3-3 were characterized by turquoise fluorescence in internal crack of the quartz and cross quartz crack.Normal alkanes of oil sand extract in S₁k^3-3was dominated by nC_14-20 with various UCM humps.In microcosmic scale,bitumen was in fractures and abundant in particle pores while in macroscopic scale,it was mainly in block patterns.Hence,hydrocarbon in S₁k^3-3 was mainly charged in earlier stage with high maturity.Bitumen was the product of thermal cracking,biodegradation and water-washing.Oil inclusions in S₁k^3-1 were characterized by turquoise and yellow-green fluorescence.Some of normal alkanes in S₁k^3-3 were similar to those in S₁k³-3-3 while there were some with full series n-alkanes and obvious UCM hump.In macroscopic scale,bitumen was in spot–like patterns and bedding–following distribution while in microcosmic scale,it was mainly on the particles'fringe or in bedding–following distribution.In combination with micro FT-IR analysis,hydrocarbon in S₁k^3-1 was oxidated and biodegraded into bitumen,and then mixed with later charged hydrocarbon.Paleo-fluid in S₁k¹ had some similarities with that in S₁k^3-1.Oil inclusions with orange and yellow-green fluorescence accounted for a larger proportion indicating that low maturity hydrocarbon was dominated in S₁k^3-1.From the perspective of micro FT-IR,earlier charged hydrocarbon suffered from severe biodegradation while the later was influenced by oxidation alteration.In combination with bitumen occurrence,biomarkers and paleo-fluid analysis,hydrocarbon in S₁k^3-3 of Shun 9 well block was mainly from Cambrian–Lower Ordovician source rocks while that in S₁k^3-1 and S₁k¹ was a mixture of Cambrian–Lower Ordovician and Middle–upper Ordovician source rocks.In Shun 10 well,hydrocarbon was mainly from Cambrian–Lower Ordovician source rock.According to bitumen and fluid inclusion analysis,there were three charging periods in Shun 9 well block,Late Caledonian?419.6–398.1 Ma?,Late Hercynian?271.5–224.0 Ma?and the Himalaya?11.4–2.2 Ma?as well as gas charging in 11.1–1.1 Ma.However,there were only two charging periods in well Shun 10 in Late Caledonian?419.6–408.4 Ma?,Late Hercynian?271.6–236.8 Ma?.According to 3D seismic profile and seismic interpretation,hydrocarbon in the Silurian migrated from the source rock below T₇⁴ by NE strike–slip fault and laterally accumulated into reservoirs speeded up by pull-apart and stress-releasing.In Late Caledonian,Cambrian–Lower Ordovician source rock was mature and generated hydrocarbon in a large scale.Then,it was accumulated into the reservoir along NE strike–slip fault.However,due to a series of secondary alteration resulting from the tectonic uplift in early Hercynian,e.g.water–washing,oxidative degradation and biodegradation,early charged hydrocarbon was destroyed with widespred bituminous sandstone in the Silurian.What's more,late magmatic intrusions in early Permian also contributed to the earlier reservoir destruction.Fortunately,it also provided a migration pathway for later hydrocarbon accumulation.In the Himalaya,hydrocarbon from Middle-Upper Ordovician source rock was charged in S₁k¹ in Shun 9 well block but no hydrocarbon was charged in well Shun 10 resulting from a long distance from NE strike-slip fault.Consequently,hydrocarbon accumulation model of Kepingtage Formation in Shuntuoguole area is multi-period hydrocarbon accumulation from mixed source rocks,earlier charging reservoirs destroyed but charged with later hydrocarbon.The favourate target for hydrocarbon exploration in Shuntuoguole area is the lower Kepingtage Formation in Shun 9 well block.