Composition And Pore Characteristics Of Shales From Ediacaran Lantian Formation In Southern Anhui Province

The Ediacaran period was a critical transitional time in the history of the Earth,during which pronounced changes occurred in the biosphere and ocean and atmosphere.The discovery of the Lantian biota,which was preserved largely in situ as carbonaceous compressions in black shales of the Ediacaran Lantian Formation in southern Anhui Province,offers a new window to elucidate the complex but crucial role of ocean oxidation in the Ediacaran and the emergence of complex multicellular life.Furthermore,marine shales rich in organic matter deposited in various environments during this period are well-developed and preserved in the Yangtze Block,South China,and have recently been related to shale gas exploration inspired by the great success of exploration and production of unconventional shale gas in North America.Marine shales are mainly in the Ediacaran Doushantuo Formation,the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in the Yangtze Block,which are some of the most promising strata for shale gas exploration in China.To date,unlike the Lower Cambrian and Silurian shales,detailed characterisation of the Ediacaran shales is not available.Based on the samples collected from a shallow drill core and a roadcut section in the Ediacaran Lantian Formation in southern Anhui Province,this thesis sets out a detailed investigation of the geochemical composition of the Lantian shales.Sequential extraction and catalytic hydropyrolysis(HyPy)were utilized to investigate the free,adsorb and bound fractions of biomarkers and carbon isotopic composition.The characteristics of OM enrichment for shales from various sedimentary facies of the Yangtze Block were compared and primarily summarized.Otherwise,an integrated approach combining several methods sets out a detailed investigation of the composition and pore characteristics of Lantian shales.The effects of the composition of the shale on its porosity and pore structure are discussed.The main results are as following:1.Shales containing highly abundant OM were mainly developed in Member II and Member IV of Ediacaran Lantian Formation,and the thicknesses of black shales were about 90 m and 10 m,respectively.Based on the overall variation in the trend of ?13Corg values,the Member II was subdivided into two shale intervals: the ?13Corg values of Shale Interval I show a increasing trend from –34‰ to –29‰,Shale Interval II was characterised by a stable ?13Corg values of –29‰ that vary little with TOC content and depth.TOC content of most shale in Member II is higher than 2%(up to 12%),and more than 40 m-thick shale had a TOC content higher than 4%,which suggests that Member II was a typical OM enrichment shale strata.The compiled data showed that the Member II of the Lantian Formation(equivalent to the Doushantuo Formation)deposited in the slope-basin setting from Southern Anhui Province is the best among three major layers with respect to OM enrichment during the Ediacaran period(Doushantuo Member II and Member IV,and Dengying Formation).This result suggests that sedimentary face controlled the organic matter enrichment,and shale rich in OM was favoured by the deep-water environment.2.EVRo values calculated from the measured solid bitumen reflectances and the Raman spectra analysis were ranging from 4.14% to 4.32%(average 4.22%),and 3.98% to 4.12%(4.04%),respectively,and which indicate that the Lantian Formation evolved into the highly over-mature stage over its geological history.3.As to the mineralogy,the Lantian shales are dominated by quartz(33–72%),and generally have low clay mineral or feldspar content(2–20%),and a highly varied carbonate content up to 60%.The Lantian shales are notable for their high pyrite content,up to 19%.The relationship between the TS and TOC content suggested that the Lantian shales were deposited in generally anoxic,frequent variations between Euxinic and suboxic/oxic and sulphate rich bottom water body.The relationships between the TS and TOC content and between the TOC and carbonate content suggested that depositional environments of Shale Interval I and II were very different: Interval I has less sulfate ion or clastic iron mineral content,and favoured for OM-enriched shales.Interval II has more terrigenous supply,and an anoxic water body rich in sulfates.4.The bulk porosities of the investigated shales ranged from 1.0% to 7.9%(average 4.1%),and showed a good positive correlation with TOC content.SEM images support the crucial importance of OM content on pore development: OM uniformly fills the intergranular spaces of a variety of minerals,pyrite framboids and clay aggregates,and intraparticle pores smaller than 100 nm are quite common in the OM.Interparticle pores are mainly developed along the boundaries between OM and minerals,and also between minerals,but are much larger than the pores in organic material.Fractures were rarely and almost only developed along the boundaries between the OM and clay aggregates.Certain relationships were indicated between mineral content and bulk porosity,but such relationships are also related,at least to some degree,to the correlation between OM and mineral content.5.Pore-size distribution plots illustrate several modal characteristics for pores in the Lantian shales.Pore volume shows an excellent positive correlation with TOC content for micropores and a good correlation for mesopores,but a poor correlation for macropores.Two samples with TOC contents higher than 7% were found to possess a very low macropore volume and a medium mesopore volume.These results suggest(i)a close relationship between OM with relatively small pores,and(ii)a greater compaction effect on large pores than on small pores in highly OM-enriched shales.6.Study on the composition and pore characteristics of outcrops and cores from the same fossils lithostratigraphic horizon have showed that the mineral components and chemical elements of shales will change when they lift up to the Earth's surface and affected by weathering,even the fresh samples.The He-porosities of the investigated outcrops ranged from 10.4% to 17.3%,and much bigger than cores' porosity values,which were ranging from 1.4% to 3.8%.SEM images showed that outcrops have more mineral matrix pores,and the pores' sizes were much larger than cores.The largest difference between them was outcrops have more fractures,and not only developed along the boundaries between the OM and clay aggregates,but also the boundaries between minerals,and even inside the mineral crystals.Pore-size distribution plots illustrate that outcrops developed abundant pores with sizes of 20–500 nm and several to tens of micrometers,but a poor connectivity between the pores.While,the cores have a higher proportion of micropores and mesopores,and a good pore system connectivity.