Study On Conditions Of Hydrocarbon Accumulation, Sinian-Cambrian, Middle Of Sichuan Basin

The paper mainly discussed the conditions of hydrocarbon accumulation in An Pingdian-Gao Shiti structure,middle of Sichuan basin.Through the characteristics of biological maker in Sinian reservoirs bitumen,oil source could be identified.The diagenesis and its fective factors of Simian reservoirs have been studied and its porosity evolution has been drew. Using the apatite fisson track analysis and Easy%Ro chemical kinetics model, geothermal system of Sinian-Cambrian could be simulated. The palaeopressure could be recovered by the analysis of fluid inclusion when these inclusions were captured.Based on the relative order of fillers in hole,bore and fracture in the Simian-Cambrian reservoir,combining with characteristic of fluid inclusion and isotope of Sr,C and O,the sequence and stage of charging of fluid could be determinted. The dynamics relationships matched with each other in the time-space scales during the hydrocarbon generation,migration,accumulation ,dissipating and trap formation has been studied,and then build the model of muti-stage of hydrocarbon generation.So the hydrocarbon accumulation mechanism of Simian pools has been revealed.(1)Research on the biomarker of the reservoir bitumen and source rock ,it indicated that the oil and gas in Sinian reservoirs mainly comes from the black shale in Qiong Zhusi Formation of Cambrian.And it was oxygen free environment with some salinity when the source rock was deposited.And the living beings were mainly low aquatic life as homoneneae.As for bitumen of post-high maturity,using such biomarkers as Ts/(Ts+Tm) accorded with another biomarkers can reflect the relative maturity effectively of the specimens.(2)Using vitrinite reflectance and apatite fission track, palaeogeothermal could be recovered of the main formation in Well Anping 1. The Ro counted accorded with the Ro actual measured.The research indicated that the geothermal gradient of Sinian and Cambrian varied from decreasing to increasing,and to decreasing again.The burial history of strata and thermal evolution history of source rokcs was reconstructed.The pressure evolution history of the source rocks of Cambrian and the reservoir of Sinian in An Pingdian-Gao Shiti structural zone which varied from increasing to decreasing,and increasing again was recovered.The multidisciplinary analysis indicated that the main mechanism of the abnormal pressure generated in Sinian reservoir was the generation of hydrocarbon.(3) The sedimentary facies of Sinian-Cambrian reservoir in middle Sichuan basin were mainly restricted platform facies,and mingled with open platform limestone and shelf sandstone facies.Its porosity and permeability was low.The reservoir space contained corrosion pore,split and intercrystal pore.The Sinian reservoir was best in save ability,and second the Cambrian.(4)The diagenesises of Sinian reservoir contained 34 kinds such as recrystallization,dolomitizaton,silication,gypsification,denudation,packing action,and so on.Dolomitizaton impact on the reservoir quality of Denying Formation,but silication which happened at late period effected the reservoir quality more seriously.Denying Formation was reformed strongly by karst in the Tongwan event in the lata Sinian.Karstification could increased the porosity for 4%-5%.Tectonic karstification and organic acid could increased the porosity for 2%-5%,while recrystallization could resist part of cementation which can lead to decrease of porosity in diagenesis stage.That was the reason why pore space in Sinian reservoir could be maintained.(5)Through the study of isotope of Sr, C, O of mineral in core hole,bore and fissiur revealed that the saline water was not from adjacent rock but from allogenetic.Reservoir in Sinian presented multi-phase chargings,while the Cambrian probably appeared to be only one stage of fluid charging.Hydrocarbon and saline water in different layers coming from the same source,suggested that fault and fractures paly the part of the main conduction systems which fluid migrated vertically. (6)Sinian gas pools in An Pingdian structure evolved through some stages as follow: a.Palaeo-oil pools formed in late Silurian.b.Palaeo-oil pools were destroyed by the uplift and denudation in Caledonian event.c.From late Triassic to middle-late Jurassic,oil cracking to gas happened and water-soluble gas formatted.d.From late Cretaceous until now,natural gas exsolution took place and palaeo-gas pools were destroyed.Natural gas appeared to be lost,transferred and redistributed make the Sinian gas pools a remnant gas pools in An Pingdian-Gao Shiti tectonic zone.The fluid inclusion in Sinian reservoir recorded the homogenization temperature varied from 130-170℃,210-220℃,230-260℃.There was 3 stages fluid transferred into Sinian reservoir at least.From about 200-220℃,the bitumen could be seen in organic inclusion which showed the liquid hrdrocarbon had splited into dry gas in late Jurassic-early Cretaceous.