Structural Feature Of Southeast Uplift In Songliao Basin And Its Control On Oil Pool Formation

Southeast uplift is one of key controlling factors for oil and gas poolaccumulation in Songliao basin. To carry out deep study of relationship betweenstructure and oil pool genesis is of great significance. This paper, based oninterpretation of geophysical data, drilling data and other geological data, and newtheory involved with structural interpretation, oil pools formation and reservoirgeochemistry, analyzed structural evolution, fault development, structural type andtheir combination type, and classified effect of structural accommodation belt on itscontrol of depositional system. For typical reservoirs, its formation period andprocess in accordance with laboratory test of fluid enclave and reservoir geochemicaltest are determined in combination with hydrocarbon generation and structuralhistory. The major controlling factor for oil pool formation is analyzed, andstructural pattern is established. Based on these study, oil pool formation conditionand its distribution rule are outlined and some suggestion on exploration is putforward.The area of interest is regarded as the result of six evolution periods, includinguplifting period, rift period, sag period, depression period, atrophic upfold period,weakly extending concave period, among which, Sifangtai and Mingshui formationreveal atrophic upfolding depositional structure, formed by squeezing during the lateNenjiang –Mingshui period.Structure configurations in study area are mainly extending structure andreversed structure. Extending structure is mainly developing in the rift-subsidencelayer, with feather type structure, stair type structure, "y" shaped structure,hemihorst-hemigraben as its main combination structure type. Whereas, reversedstructure are formed mainly in the depression layer, of which upper "reversedstructure"-lower "normal" structure, upper and lower both "reversed structure",lower "rift structure"-upper "upfold structure" and upper and lower both "normalstructure" during depression period take predominance. Diversity of structure typeand combination type provide sound condition for formation of structural trap andoil migration pathway.The types of the rifts in study area are mainly accordant, opposed and backward.The basic unit of the structure is an expanding rift which is divided into the slope,depression center and gentle slope. The steep slope is bounded by a largedepression-controlled fault. The gentle slope usually develops the secondary positiveantithetic fault. The sediment is unconformably overlying on the slope. The oil & gasusually migrates from the depression center to the gentle slope, and then migratesvertically and accumulates in the reversal structure.The study area can be divided into two typical primary structural adjusting beltsand eight secondary structural adjusting belts. The primary structural adjusting beltextends nearly in east-west direction, which divides up the rift with the north-eastextension and results in the block distribution of the south and north in the study area.The secondary structural adjusting belt lies between the marginal lateraldepression-controlled positive faults, which is the main entrance of the sedimentarysource of the rift.The reservoir in study area shows two periods of migration and accumulation. Theearly period is at the end of Yingcheng Formation, and the late period is at the end ofNenjiang Formation. The secondary reservoir shows the late primary migration andaccumulation, with its migration and accumulation period being at the end ofNenjiang Formation.The study area is characterized by developed source rocks, varied trap types,developed reservoir-seal assemblages and superior migration conditions. Thestructural movement is the key factor for generation, accumulation and preservationof oil & gas. The reservoirs which accumulate in Shahezi Formation and YingchengFormation in the early period are seriously damaged by the tectonic movement at theend of the deposition of Yingcheng Formation. The deep source rock experiences thesecondary hydrocarbon-generation period during Qingshankou-Nenjiang period,which is the important period for formation of reservoirs. Most of the primaryreservoirs are further adjusted due to the tectonic movement at the end of thedeposition of Nenjiang Formation, which forms the shallow secondary reservoirs.The uplift of the formation at the end of the deposition of Nenjiang Formationresults in the large loss of the early oil & gas and the stagnation of hydrocarbongeneration of the deep source rock, which makes the oil & gas abundance in thesoutheast uplift decrease a lot, and the oil & gas are in a disadvantage state that thelosing volume is lager than accumulation volume. The large oil & gas reservoirs arehard to form in medium-shallow formation due to the influence of late tectonicmovement, but there is still an exploration prospect for oil & gas reservoirs whichare better preserved in deep formation.