| Yitong Basin is a Cenozoic graben basin located in the north part of the Tanlu Fault Zone,and Luxiang Depression is one of its sub-tectonic units with good conditions of hydrocarbonaccumulation and rich hydrocarbon resources. Based on predecessor's experiences, we study onthe basis conditions of hydrocarbon accumulation using logging data, seismic data, and sampletesting data, and then analyze the types of reservoirs, petroleum distribution and its influencingfactors, and last do deeper researches on the accumulation mechanisms of petroleum reservoirs ofShuangyang Formation in Luxiang Depression using methods of basin modeling and fluidinclusions.First, we study on the basic hydrocarbon conditions such as source rocks, reservoirs and sealrocks. The results show that the source conditions are good: the dark mudstone is thick, and theorganic is medium abundance and in the mature phase. Specifically, the third member ofShuangyang Formation has the thickest dark mudstones and then the first member; the organicabundance and maturity of the first and third members of Shuangyang Formation are better thanthe second member. So, the best petroleum source bed is the first member of ShuangyangFormation, and then the second member; Danan Sag is the best petroleum source area, thesouth-west and south of Luxiang Depression is the second best ones, and the east of the depressionis not as good as the other places. The reservoirs conditions are good, too. The reservoirs aremainly made up of fan-delta sand body and underwater fan sand body, and the rock types are mainlymedium-fine grain arkoses and lithic arkosics; the pore types are mainly primary pores such asintergranular pores and micropores and the secondary pores such as dissolved pores and fracturepores, and the pore structure grade are ⅡB-ⅡA; the porosity grades are medium, low andparticularly low, while the permeability grades are low and particularly low. So, the secondmember of Shuangyang Formation has the best reservoirs conditions, and the third member hasthe second best ones; the edge of Luxiang Depression has the best reservoirs conditions and thecentral part has relatively bad reservoirs conditions. The seal of petroleum in ShuangyangFormation in Luxiang Depression is mudstone, mainly developing in the first and third members,and the seal rocks distribute widely and have strong seal ability.Then, we analyze the types of reservoirs, petroleum distribution and its influencing factors inLuxiang Depression of Yitong Basin. There are five types of petroleum reservoirs: complex faultblock reservoirs, basin-edge ligulate lithologic reservoirs, deep-lake lenticular lithologic reservoirs,fault screened-lithologic combinational reservoirs and palaeo buried hill reservoirs. Overall, thesecond member of Shuangyang Formation has the richest petroleum, and the first member has thesecond richest while the third member has the poorest; petroleum mainly develops most in thenorth part of the depression and then the central and south part. Additional, the structure, sedimentation, diagenesis, source-reservoir-seal assemblages and petroleum migrationtransportation systems are all important controlling factors to petroleum accumulation. To bespecific, the structure characteristics influenced the migration, preservation, accumulation anddistribution of the petroleum reservoirs in the Shuangyang Formation; the sedimentary conditionsand environments influenced the grades and distribution of source rocks, reservoir rocks, seal rockand types of source-reservoir-seal assemblages, and then influenced the distribution and scales ofpetroleum reservoirs; the compaction and cementation made the physical properties of reservoirsbecome bad while the dissolution improved the physical properties of reservoirs because it couldmake many secondary pores; the source-reservoir-seal assemblages were mainly self-generate andself-storage assemblage and new-generate and old-storage assemblage, which is good forpetroleum accumulation; the petroleum migration transportation systems were made up ofconnected sand bodies, faults and unconformities, which provides good conditions for secondarymigration of petroleum.Further, we modeling bury history, thermal history, maturity history, hydrocarbon-generationhistory and hydrocarbon-expulsion history using the basin modeling technology: as for buryhistory, the average eroded strata thicknesses are96.83m,201.33m,220.85m and192.3mrespectively; as for thermal history, geothermal gradient fell from5.02-4.87℃/100m inShuangyang period to3.64℃/100m now; as for maturity history, the source rocks in the firstmember went into early-maturity phase in32-38Ma, while the source rocks in the second and thirdmembers went into early-maturity phase in26-32Ma and21-16Ma respectively, and Xing-3in thenorth part of Danan Sag, Xing-27in the central and Liu-2in the south part were more mature thanothers; as for hydrocarbon-generation history, the source rocks in the first member of ShuangyangFormation began to generate oil in36-37Ma, and ones in the second members and third memberbegan to generate oil in31-35Ma and25-28Ma respectively; as for hydrocarbon-expulsion history,the source rocks in the first member of Shuangyang Formation began to expulse oil in32-38Ma,and ones in the second members and third member began to expulse oil in31-35Ma and18-26Marespectively. Then, we analyze the petroleum migration times by homogenization temperatures offluid inclusions and the results of basin modeling, showing that there were two times (fourepisodes) of petroleum injecting. The first and second episodes were the first time, which wasabout37.6Ma-30.2Ma, while the third and fourth episodes were the second time, which was about22.2Ma-15Ma.In conclusion, we analyzed and summarized the petroleum accumulation mechanisms ofShuangyang Formation in Luxiang Depression of Yitong Basin: in Shuangyang period, YitongBasin began to rift, and the stratums of Shuangyang Formation deposited on the weathered crustof basement, among them, the first member and third member had very thick mudstones whichwas benefit for the source rocks and seal rocks while the second member had very thicksandstones which was good for the reservoir rocks; in late Wanchang period, the strong tectonicactivities in the north-west and the north-east part of the depression formed many faults, makingmany tectonic traps and petroleum migration channels, and source rocks in first member ofShuangyang Formation began to expulse hydrocarbon, bringing the first time of petroleum migration in large scales; in late Qijia period, the basin was compressed and reversed strongly,with north-west part of the depression lifting, and the source rocks of three members ofShuangyang Formation were expulsing hydrocarbon, making the second time of petroleummigration in large scales. As a whole, petroleum which expulsed from Danan Sag migrated in thepetroleum migration transportation systems such as connected sand bodies, faults andunconformities to the edges of Luxiang Depression, especially the north of Wuxing tectonic zoneand the south of Danan Sag, forming many different types of petroleum reservoirs, such ascomplex fault block reservoirs, deep-lake lenticular lithologic reservoirs, and palaeo buried hillreservoirs. |
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