Study On Accumulation Of Oil And Gas In Pinghu Slope Belt Of Xihu Depression

Xihu depression is situated in the northeast of the Donghai shelf basin, which isapproximately4.6×10~4km~2north-northeast trending strip of Cenozoic sedimentarydepression. The Pinghu slope zone is located in the the central segment of the westernslope. The main target layer is Eocene Pinghu group and Oligocene Huagang group,four oil and gas fields have been discovered, which is Pinghu，Bao Yunting, WuYunting, Kong Queting. It has frequent deposition of the transgressive and regressivein the Pinghu slopes, whose horizontal performance is the finger-like interactionbetween the deposition system or facies and on the vertical showing the phenomenonof thin interbeds of sandstone and mudstone. To find reserves of raw oil and gas, wemake the storage, transport, gather, scattered link into a unified whole, reevaluate theaccumulation power system of Pinghu slope, and find that in the thin interbeddedsedimentary area, lithology oil and gas accumulation condition is well.Forming conditions of the study area is good. First, source rocks are rich. darkmudstone, carbonaceous shale and coal in Pinghu Formation are the main sourcerocks, which is much better than Huagang group. Second, the reservoir performanceis good. Pinghu group developed tidal-flat sand, tidal channel sands and delta frontsands, which are all thin sandstone; the Huagang group development delta sand body,which are thick sandstone. Third, the regional seal and the local seal are rich. Themain cap rock is the gulf mudstone, silty mudstone, tidal flat mudstone and swampcoal and peat, and are always classed in I, II; in the upper part of Huagang, theabnormal fluid high pressure zone has seal role to the oil and gas in reservoir，whichform the pressure capping mechanism. The deposition of the pastry-like shape formsthree models of source-reservoir-cap assemblage and they are compositing, closingand lateral variating, which will help the formation of self storage lithologic traps.The source rocks of Pinghu Formation in Pinghu slopes mature at the end of theOligocene, in Miocene enter into the hydrocarbon generation peak，the trap, the sourceconfigurate well in the time. The Pinghu, Huagang deposited after the rifting period,the fault system formed in rifting period provide the vertical conduit of hydrocarbonaccumulation. The regional unconformity surface and local unconformities formed bythe tectonic movement can be lateral channels of hydrocarbon migration at differenttimes.Pinghu Formation in the Late Miocene (10Ma ago) reach the hydrocarbon expulsion threshold. most of the proved oil and gas reserves generated in the earlyPliocene (5Ma ago), and the oil and gas generated during the5Ma occur in thelithologic traps of Pinghu formation. The optimal zone is the the Yu Bi Ting structureand the Jing Xinting structure, they are in favorable directional zone of hydrecarbonmigration, and form hydrocarbon accumulation. Shi Lianting structure and KongQueting structure are also optimal, where there are rich rock source. In oil and gasmigration and aggregation in these two structures, always in the high parts, and oiland gas shows good in its surrounding.In summary, through the analysis of the accumulation and accumulationconditions, we know that the Yu Biting, the Jing Xinting, shi Lianting and KongQueting, can be considered as exploration targets of lithologic reservoir.