Sequence Stratigraphy Analysis And High Quality Source Rock Identification In Typical Permian Sections, Yangtze Area

Marine deposits are very important hydrocarbon source beds and reservoirs in the world. There are 61%available reserve come from marine deposits.40%depositional area is coverd by marine sediments in China,but a few hydrocarbon pool is explored in marine sediments compared with worldwide.Hereby,marine deposits have significant hydrocarbon exploration potential in China.The previous research shows that Permian system is one of the most important marine resource rocks in Lower Paleozoic in south China.In this paper 3 typical Permian profiles in Yangtze region have been studied.Taking the sequence stratigraphy of carbonate rock outcrops as theory guidance,geochemical testing of trace elements as well as carbon and oxygen isotope is used to systematically study the sedimentary facies and the sequence stratigraphy of the typical Permian profiles,and to discuss the basic characteristics of sequence achitectures,with the integration of field work and the microscope observation of thin sections.On the basis of sequence stratigraphy analysis and the previous studies involved with the habitat types in this area,and with the integration of the results from residual organic carbon testing,organic matter enrichment of the Permian in Yangzi region is discussed,in turn the intervals with high quality source rocks developed on the typical profiles have been identified.A preliminary study on the identification for marine source rocks is made by Geo-biology methods in this paper.Finally,the development model for high quality source rocks is concluded in the study area.The main results and conclusions are as follows:1.According to the observation of fieldwork and thin sections,depositional facies in 3 typical Permian profiles are divided,and the geochemical characteristics in each depositional facies are summarized.The average contents of Co,Ba,V,Rb showing distinct marks for depositional facies become higher toward deep marine environments,but Sr/Ba ratio decrease from platform facies to basin facies.In the Luodian profile,total REE(∑REE) increases gradually from fore-platform,outershelf,slope to basin facies,and obvious Ce anomaly in REE Patterns of every faces has been observed. Carbonate sedimentary models in Guangyuan,Huayingshan and Luodian areas have also been summarized.Carbonate platform deposits are dominated without reef in the Guangyuan and Huayingshan sections,where a few continental slope deposits between outershelf and basin facies.While carbonate slope facies are developed well in the Luodian area.2.According to the field description and the test results of carbon and oxygen isotope as well as trace elements,the Permian deposits in Guangyuan,Huayingshan and Luodian were divided into 9 third-order sequences,6 third-order sequences and 7 third sequences,respectively. The achitecture characteristics of each sequence and system tract are discussed individually in detail:Based on lithologic association,8 basic subsequence achitecture patterns were summarized;and the characteristics of the sequence architecture as well as the distributions of geochemical elements in the system tract were described in the sedimentary backgrounds of carbonate platform,slope deposit and basin floor respectively.Rare earth elements(REE) and total REE(∑REE) gradually reduce from the transgressive systems tract to the highstand systems tract;the La/Yb ratio in the transgressive systems tract is higher than that in the highstand systems tract,which indicates a higher enrichment degree in the former one than the latter.Then,the stratigraphic sequence correlation of the Permian systerm were discussed,and the isochronous stratigraphic framework of the lower Permian was established;the main controlling factors for sequence development,in our research area,were also concluded as sea-level change,tectonic subsidence,climate conditions,sediment supply,and so on,of them sea-level change and climate condition are taken as the main controlling factors.3.Through the analysis of those typical profiles,the results indicate that(1) the organic matter enrichment is closely related to the lithology:the organic matter is most enriched in thin layer siliceous rocks interbedded with siliceous shale,or laminated limestone interbedded with shale,and the organic matter decreases apparently with the increasing thickness of beddings in limestone,the laminated limestone and the thin layer limestone has the highest TOC;(2) the distribution of residual organic matter in the Permian shows a close relationship with the habitat types,and high quality source rocks are supposed to be most likely developed in habitat types ofⅣ2 andⅤ2,then the habitat type ofⅢ;(3) the sequence development has certain influences on the organic matter enrichment,for example,the organic matter enrichment degree in TST is obviously higher than that in HST.In the Shangsi profile in Guangyuan,three intervals of high quality source rocks are mainly identified as the lower Qixia group,the top Maokou group and the middle Dalong group;In the Huayingshan profile in Chongqin,two intervals of high quality source rocks are mainly identified as the middle Maokou group and the lower Xixia group;but in Luodian profile in Guizhou,high quality source rocks are apparently absent.4.The following relationship between known source rock and the Geo-biological indexes has been discussed adopting the discriminant value with the less than 0.3%TOC in non-hydrocarbon source rock and more than 1.5%in high quality source rocks:(1) the average contents of Cr,Ni,Cu,Zn,Ba,Mo,U,as proxies for pre-productivity,and the ratios of Ni/Co, V/Cr,as proxy for sedimentary environment,reveal a regular change from non-hydrocarbon source rocks to high quality source rocks.Significant increasing trends are shown with these contents as well as ratios(the ratio of V/(V+Ni) is an exception,whose change is not so obvious as others);(2)Contrasting levels of the above elements and ratios in different lithology(limestone, siliceous rocks and mud),in high quality source rocks,the contents of Ni,Cu,Cr,Ba,Mo,U in mudstone is much higher than those in limestone and siliceous rocks,however,the ratios of Ni/Co and V/Cr among them are similar;the contents of Ni,Cu,Cr,Ba between limestone and siliceous rocks are close,but the contents of Mo,U is much less than these of siliceous rocks.In non-hydrocarbon source rock and common source rock,most of element contents in siliceous rocks are similar to these in limestone,but the content of Cr is obviously more than in limestone.5.On the base of organic matter distribution in the typical profiles,the development model of high quality source rock is summarized.Occurrence of hydrocarbon-generating organisms and its abundance are essential conditions for the formation of high quality source rocks,and preservation condition is also another important factor.Deep fluid and upwelling activities are throught to be main reasons to result in the formation of high ancient productivity for high quality source rocks in northwestern Sichuan.