Development Environment And Shale Gas Resources Assessment Of The Wufeng-Longmaxi Formation

Black shale of Wufeng Formation and Longmaxi Formation,main target of China's shale gas development currently,is widely deposited on the Yangtze Platform from late Ordovican-early Silurian.Along with global climate changes during this period,and with effects of global and regional Guangxi tectonic movement,the Yangtze Platform had experienced paleogeographic patterns transform.To have a better understanding of these two shale beds,especially the formation mechanism and reservoir characteristics of Wufeng Formation,it is very important to study the relationship betweent tectonic event and water environment evolution and deposition of black shales.We select two sections(Datianba section in Xiushan and Shuanghe section in Changning)along the advancing direction of Guangxi tectonic movement on the Yangtze Platform,analyse C-Fe-S relations of samples,and compare the results with that of the Wangjiawan Riverside section of Yichang city which located in the open sea sedimentary environment.It is revealed that Guangxi tectonic movement has impact on the sedimentation of Wufeng Formation and Longmaxi Formation.After that,we explored characteristics and main control factors of pores structures of shale.The main achievements of our study are as follows:(1)Different paleogeographic background is responsible for vary water environment evolution models.Wangjiawan Riverside section in open sea environment showed anoxic and ferruginous characteristics before and after the ice age,and oxic during glaciation;Whereas,Datianba section of Xiushan region in restricted area of the Yangtze Platform was not influenced by the ice age,and displayed anoxic feature throughout the whole late Ordovican-early Silurian period and euxinic before and after the ice age,ferruginous during the i ce age;Shuanghe section of Changning region in the inner part of the Yangtze Platform was not influenced by the ice age as well,and also revealed anoxic characteristic in the turn of the Ordovican-Silurian and ferruginous before and after the ice age,e uxinic during the ice age.(2)Water evolutions of Wangjiawan Riverside section,Datianba section of Xiushan region and Shuanghe section of Changning region were influenced by different fators.Wangjiawan Riverside section was effected mostly by the ice age,showing clear relations between the oxidation-reduction alternation of water column and sea-level fluctuation.However,Datianba section and Shuanghe section were affected dominantly by Guangxi tectonic movement.More specifically,Datianba section of Xiushan region was located near the advancing direction of the Guangxi tectonic movement,and due to the uplift by the Guangxi movement in Southeast initially,it received terrestrial weathering input from Southeastern margin and became constrained.As a result,Datianba section formed the euxinic wedge earlier.Afterwards,with the continuing advance movement from Southeast to Northwest and sea level drop in the ice age,the euxinic wedge grew towards the Northeast,which leads to the removement of the Datianba section to the euxinic wedge area,indicating ferruginous environment.During this process,Shuanghe section of Changning rigion depressed and enter into the euxinic wedge area,showing euxinic environment.After the ice age,sea level rise,uplift was still continuing,and the euxinic wedge area was back into the Datianba section,revealing euxinic environment,causing an outlet of the euxinic wedge in Shuanghe section and regain ferruginous environment.(3)The formation of black shale of the Wufeng Formation was closely related with the Guangxi tectonic movement,which was correspond with the tectonic timing and environment in the late stage of the Ordovician.This tectonic movement occurred in the Sambit,early stage of the late Ordovician,and brought about the end of the long term tensile-type tectonic background of the South China,which converted to the extrusion and convergence phase of the Yangtze Black and the Cathaysia Block.In detail,it changed the basement geographic pattern of ?high west low east? from the Neoproterozoic to the early Paleozoic into ?three sides constrained,high east low west?.This transformation is manifested as black shale rimmed carbonate platform-carbonate platform decay-the Wufeng Formation deposited in restricted sea.Besides principal tectonic movement control,low oxvgen content,high nutrient input,and circulation of nitrogen,phosphor could promote the formation of black shale.(4)The organic matter in samples from Wangjiawan Riverside section,Datianba section at Xiushan and Shuanghe section at Changning are type?.The TOC contents in samples derived from three sections are all high in Wufeng and Longmachi Formation,but had diverse TOC distribute mode respectively.The TOC content of Datianba section during the Wufeng Formation was higher than that of the Longmaxi Formation,and Shuanghe section had a higher TOC content in the Longmaxi Formation over the Wufeng Formation,and Wangjiawan Riverside section had high TOC content all over these two formations.These distinctions not only were a result of tectonic movement and sea level changes,more importantly,it can be better to help us in searching the favorable area of shale gas.All samples at three sections contain brittle minerals(e.g.quartz,feldspar,cal cite and dolomite)and clay mineral(e.g.illite and chlorite),among which the quartz contents are the highest.The quartz contents in samples from Shuanghe section at Changning have some correlation with the TOC contents,indicating some quartz are biogenic origin.The reversed relationships are observed at three sections.The results of calculated brittleness index indicate the rock have well fracturing performance at three sections,thus are better for the later development of shale gas.(5)Shale samples derived from the Wangjiawan Riverside section,the Datianba section and the Shuanghe section revealed great development of pore structures,which have good relationships with the TOC contents.While the different correlations between the pore in same scale and the TOC contents among different sections and between the pore in different scales and TOC contents in the same section show the complexity of pore development in shales.The poor relationships between the pore structure parameters and the TOC contents in samples from Riverside section at Wangjiawan display the influence of co-existing between organic matter and the minerals.All samples show the micropore provides the largest surface area and mesopore occupies the largest proportion of total pore volume.The most pore in samples are focus on the scales of 0.3~0.6nm,1.2~1.4nm and 2~10nm.From the bottom to the top,there are no obvious changes of pore size distribution in samples from Datianba and Shuanghe sections,while samples from Wangjiawan Riverside section show some change,this may be due to the relationship between organic matter and the mineral in these samples.(6)The fraction research using samples from Datianba and Shuanghe sections shows there are double fraction characteristics in pore.The high fraction dimensions D1 and D2 demonstrate the complexity of pore surface and structure.The poor relationships between fraction dimensions D1 and D2 and the TOC and the mineral contents reveal the influencing factors of the fraction dimensions are not quantity.Using the general negative relationships between the pore size and the fraction dimensions,the research results show micropore can promote the fraction dimensions but the mesopore can reduce the fraction dimensions.The fraction dimensions of shales are determined by these adverse two factors.(7)The processes of removing minerals have no influences on the contributions of micropore in providing surface area and the mesopore in providing the pore volume,respectively.The volume of mesopore and macropore are keeping increase during the processes of removing the minerals,while the reducing of the volume of micropore illustrate there are some micropore in carbonate,which can be related to the corrosion holes.The effects of removing mi nerals are different on the pore size distribution of micropore and mesopore.For micropore,the pore becomes more concentrated after removing the carbonate,the pore size distribution are not affected by the silicate,while there are some micropore appear after removing the pyrite.For mesopore,pore in any size are emerge after removing the carbonate,while there are no influences of removing silicate and pyrite on the pore size distribution except change the relative proportion of pore.All these phenome nons illustrate that the characteristics of pore in organic matter will be affected by the co-existing relationships of organic matter and the minerals,however,the rock sample still can be used to characterize the features of pore in organic matter,which is the most important controlling factor in the development of nanopore in shale.