The Relationship Between Fault Characteristics And Petroleum Accumulation Of Carboniferous Volcanic In Karamay Oilfield

The Carboniferous volcanic oil reservoirs in Karamay Oilfield were located in the hanging wall of Ke-bai Fault Belt, and faults played an important role in petroleum accumulation. Based on the analysis of typical volcanic oil reservoirs, the fault sealing of main faults was evaluated quantitatively, and the relationship between faults and volcanic petroleum accumulation was also discussed. The research results could be summarized as follow.The Carboniferous volcanic oil reservoirs in study area were controlled by faults, their scales became larger as they got closer to the primary faults. The oil reservoirs were rich in petroleum with continuous sheets in plane and multilayers in vertical. It was proved that fault block reservoirs and fault anticline reservoirs with weathered crusts mainly grew in Huwan Area, while fault block reservoirs and fault bulge reservoirs with weathered crusts mainly grew in 6th-7th-9th Area.Through the discussion of relationship between fault filler and surrounding rock density, a new parameter called “Density Difference between Fault Surfaces” had been built, which solved the problem of fault sealing assessment in complex lithologic stratum. The method not only showed the lithologic variation between two walls of a fault, but also showed the attitude configuration between faults and strata. The bigger the parameter was, the better the fault sealing would be.According to fault sealing mechanism in volcanic rocks, fault sealing patterns of diagenetic compaction and lithologic juxtaposion were built, and the synthetical assessment of current fault sealing was finished with parameters including normal stress on fault surface, tightness index, density difference between fault surfaces, fluid index. The results showed that the current fault sealing got better as the fault level became higher, making the oil reservoir scale get larger. The sealing of deep-formed faults was better than that of shallow-formed faults, so the fault block reservoirs with bigger scale grew close to deep-formed faults, while the fault block or fault bulge reservoirs with smaller scale grew close to shallow-formed faults.Through paleo-drop analysis, the fault activity was studied, and the main active stages were also determined. Then, combined with paleo-stress calculated by tectonic history and tectonic stress history, the fault sealing history got restored. The results showed that the paleo-migrating of faults in T2~T3 was better than that in J, while the paleo-migrating of primary faults was better than that of subordinate faults. In detail, the paleo-migrating in active stage, the paleo-sealing in later intermittent stage and static stage of deep-formed faults were all better than that of shallow-formed faults, so deep-formed faults were more favorable to petroleum migration and preservation.Based on the summary of Carboniferous volcanic petroleum accumulation conditions, combined with fluid inclusion measurement, hydrocarbon history from generation to expulsion and burial-thermal evolution history of strata, 3 accumulation stages were divided, such as T2, the end of T~J1 and J2~J3. After discussion on the relationship between faults and those factors including hydrocarbon source rock, reservoir, petroleum miagration and accumulation, it was found that the fault effects on volcanic petroleum accumulation consisted of source-rock-controlling effect, reservoir-reforming effect, petroleum-migrating effect and petroleum-sealing effect.Finally, combined with tectonic history, hydrocarbon history from generation to expulsion, fault sealing history and trap growing history, the petroleum accumulation evolution patterns of Carboniferous volcanic rocks in Karamay Oilfield were built, they were the protogenous accumulation in T2, the early-mixed accumulation from the end of T to J1 and the late-mixed accumulation from J2 to J3.