Sedimentary Evolution during Jurassic and Faces-controlling Reservoir Prediction in Sichuan Basin,China
Sichuan Basin is a focus region of oil-gas exploration in our country all the time,since it has better geological conditions of oil-gas reservoir forming and enormous resource potential,Sichuan Basin is not only the typical superimposed basin in large scale of our country,but also the earliest area to discover and exploit natural gas.Jurassic stratum are widely distributed in the basin with huge thickness and shallow depth,and it is one of the significant stratum of oil-gas exploration in Sichuang Basin where China Petrochemical Industry and PetroChina Company Limited.have got good results of exploration.This paper use many theory systems and technology methods refer to multi-subjects such as sedimentary geology,seismic stratigraphy,seismic sedimentology,geostatistics,structural geology and sedimentary geochemistry,and make some ideas on five steps: first is correlation and demarcation of stratigraphic units in all basin,second is mapping on lithofacies paleogeography,third is relative paleogeomorphy resuming in stratigraphic units,then is sedimentary evolution regulation and filling characters of basin,the last is faces-controlling reservoir prediction,the purpose of which is making search of tectonic background about three main orogenic belts of different time around sedimentary period in Jurassic,remolding basin shape,evolutionary regulation and sedimentary filling characters,looking for favorable exploration targets by targeting at 3D seismic region in advantageous phase region.The chief contents and obtained profits are as follows:(1)Based on understanding regional sedimentary patterns and summing up searching profits of predecessors,Jurassic stratum are integrate correlated and sedimentary systems are divided by using many data such as seismic,drilling cores,logging and field outcrop,and stratum are divided into nine class ranges that are Ziliujing formation(Baitianba formation),comprise of four sub-formations named Zhengzhuchong,Dongyuemiao,Ma Anshan and Daanzhai,Qianfoya of Middle Jurassic(Xintiangou formation,Lianggaoshan formation called by PetroChina Company Limited.),Lower and Upper Shaximiao Formation,Suining formation and Peng laizhen formation(Lianhuakou formation)of upper Jurassic.According to marks and characters of sedimentary system division,the Jurassic is defined as continental depositional system group which has five sedimentary systems.Lithofacies paleogeographic maps are made based on mapping units of nine stratum divided before.(2)According to practical grasp of data,an relative paleogeomorphy resuming method that adapt to continental stratum in Jurassic of Sichuan Basin is filterred to acknowledge ubiety of ancient depression,palaeoslope and palaeohigh.The method takes sedimentary character “filling up” of continent basin as the guideline,and makes use of “mirror” relationship between upper layers and paleogeomorphy to recover relative paleogeomorphy shape characters about nine stratigraphic units of Jurassic in Sichuan Basin which adopt impression method comprise of sedimentary thickness of upper stratum and ancient water depth parameters.This technique are mainly focused on seven aspects content such as remains stratigraphic thickness,apparent thickness correcting,hole deviation correction,denuded strata comeback,ancient water depth parameters,impression method to recover paleogeomorphy and paleogeomorphy examine which avoid two factors comprise of compaction and differences in tectonic subsidence.(3)Movement of three main tectonic belts around basin in nine sedimentary phase of Jurassic are deeply searched based on basin range transition,searching profits on macroscopy tectonic around basin of predecessors and make use of relative paleogeomorphy characters and spreading regularity of lithofacies paleogeography.Active stages of Longmen Mountain thrust belt of western basin are relative stabilization period of four sub-formations in Ziliujing formation,weak active stage in Qianfoya and lower Shaximiao formation,strong active stage in upper Shaximiao formation,weak active stage in Suining formation and strong active stage in Penglaizhen formation;Active stages of Trilobite biofacies-Dabashan structural belts of northern basin are stronger active stage in Zhenzhuchong formation,strong active stage in Dongyuemiao and Maanshan formation,relative stabilization period in Daanzhai formation,stronger active stage of Middle Jurassic,strong active stage in Suining formation and relative stabilization period in Penglaizhen formation;Active stages of seppox uplift of southeastern basin are stronger active stage in Zhenzhuchong formation,relative stabilization period in Dongyuemiao formation,stronger active stage in Maanshan formation,relative stabilization period in Daanzhai,strong active stage in Qianfoya formation,stronger active stage in lower Shaximiao formation,weak active stage in upper Shaximiao formation and strong active stage in Suining and Penglaizhen formation.(4)Based on movement analysis of orogenic belt around basin,the sedimentary evolution characters of basin get to be further acknowledged through migrate evolution rules of sedimentary center and sedimentary compensation in nine sedimentary phases,the basin nature of nine sedimentary phases are defined respectively as complex foreland basin of Dabashan-seppox uplift in Zhenzhuchong formation(with middleweak degree in sedimentary compensation),depression basin in Dongyuemiao,Maanshan and Daanzhai formation(with weak degree in sedimentary compensation),foreland basin of Trilobite biofacies-Dabashan in Middle Jurassic(with high degree in sedimentary compensation),complex foreland basin of Dabashan-seppox uplift in Suining formation(with middle-high degree in sedimentary compensation),complex foreland basin of Longmen Mountain-seppox uplift in Penglaizhen formation(with high degree in sedimentary compensation).(5)Discussed research technique on sedimentary facies based on seismic sedimentology,the technique solution is: First to divide the minimum isochronal stratigraphic unit in the study area,second to make search of relationship between lithofacies,sedimentary facies and seismic reflection characters,make quantitative characterization of seismic as basis that is to extract seismic attributes in comparative isochronal time window,the last is to work out sedimentary facies diagrams in minimum isochronal unit with description data about cores of single well and characters of sedimentary facies,all of which have embodied thought of combining point with line and surface,with use of which delicate sedimentary facies are researched and depicted in 3D seismic region in Zhenzhuchong and Daanzhai sub-formation of Ziliujing formation of lower Jurassic.(6)Glutenite reservoirs of fan-delta front are developed in Zhenzhuchong sub-formation,while coquina fractured reservoirs are developed in Daanzhai sub-formation of lower Jurassic through drilling data from Yuanba area in northeast of Sichuan.Take the two sub-formations as searching objects of reservoir prediction,and favorable exploration targets should be predicted under control of delicate sedimentary facies depiction by using 3D seismic data on high-resolution.The thickness of reservoir,oil-gas possibility,local structure and sealing conditions at top in Zhenzhuchong formation are got by wave impedance inversion,while with the same method the thickness of reservoir,oil-gas possibility,local structure,internal fractures development and sealing conditions at top are got in Daanzhai formation.At last,make use of searching profit above,the exploration target of a favorable structural-lithologic reservoir has been respectively predicted in the two formations.