Research On Sedimentary Characteristics Of Huadian Formation Of Paleogene In Huadian Basin

With energy demand growing, oil shale, as a kind of significant non-conventional resource, is acting as an important amount of supplementary oil resource in the furture. Exploration and development of oil shale have been in national energy strategies with highly importance. The oil shale resource in Jilin Province is the richest in China. However, because of less exploration and the expansion of oil shale exploiting areas, the identifying amount of resources can not meet the current needs. Searching large-scale and high-quality oil shale deposits become the primary target for exploration. Presently, only a small amount of drillings and outcrop have been used to explore oil shale. It is lacking for detailed description of oil-shale bearing sedimentary layers and for the study on horizental and vertical distribution of oil shale-which all are hindering further exploration for oil shale. To meet those chanllenge, the author takes the Huadian Basin, which is small but rich in oil shale, as a example. A comprehensive study with a large number of cores and testing data will be taken to ilustrate the sedimentary characteristics of oil shale-bearing strata in Huadian basin. This research will help reveal the major sedimentary facies and tectonic-sedimentary evolution in this area, which also can determine the sedimentary environment when oil shale forming. By establishing sequence stratigraphic framework in Huadian basin, we can indentify the location of oil shale in this framework. Thus, this study will provide hypothesis in sedimentary geology about oil shale forming mechanism.Located on the Dunhua-Mishan fault zone, Huadian basin belongs to the northern branch of Tanlu fault. The primary strata is Paleogene Huadian formation, which can be divided into three members-the lower (pyrite-bearing), the middle(oil shale-bearing) and the upper (carbon-mudstone-bearing).Cores are the first-hand materials to study the sedimentary characteristics of Huadian basin. Core observation shows that there are mudstone, oil shale, sandstone and thin marl and coal deposited in this area while some glutenite appear at the bottom of Huadian Formation. The sandstone of Huadian basin is feldspathic lithic sandstone while tuff lithics and rhyolite lithics are the major lithics. Calcium nodules are widely appearing around sandy particles. The filling-ups in the spaces between particles can be divided into two groups-one is matrix filling and the other is calcareous cement (some of cement is intercalary crystallized calcite, which represents the strong hydrodynamics and the supplying lack of terrigenous clastic. The sandstones of Huadian Formation are classified into net sandstone and graywacke. The former is supported by the particles and shows better sorted, and the latter is supported by the matrix and shows poorly sorted.SEM and X-ray are used to analyze the mineral composition of oil shale. The major clay minerals are montmorillonites, then are the kaolinites and illites. Illite-smectite mixing layers are also occasionally identified in the oil shale. The average content of montmorillonite is 30.7%, and its shapes are tablet-like, flocculent and connecting under SEM. The average content of kaolinite is 16.1%, which appears like sheet. The average content of illite is 12.6%. They are like wire-lea and scals. Pyrites and Epidotes are also identified in oil shale and sandstones.Strata in different sedimentary facies presents various rock combinations, beddings, logging data, grain size characteristics and bioturbation. Based on data got from Huadian basin, sedimentary facies models in single wells are constructed.There are four sedimentary facies in Huadian basin-alluvial fan, Fan-delta, lacustrine and subaqueous fan. They are divided into eleven sub-facies and eighteen micro-facies. Fan-delta and lacustrine facies are the most common. Alluvial fan facies appears at the bottom of Huadian Formation in the western part of Hudian basin. Grain size analysis, thin section and cores observations are used to study hydrodynamics in different sedimentary facies, the beach dam at the lake shore appears the features of tractive current-deposition and shows the characters of bidirectional flow. In semi-deep and deep lake, there were suspended particles deposited. Rhythmic deposition is commonly seen in lacustrine mudstone. They are the interbeds of clay and terrigenous clastic or calcareous thin layer, which are generated in different sedimentary enviroments. Fan delta is mainly traction-flow regim, while in the fan delta plain there develop some gravity flow deposits and in the Fan delta there are liquefied slumped deformation. Alluvial fan and subaqueous fan are mainly built by gravity flow deposits. However, in the middle fan tracting flow deposits can be found. Through thin section observation, semi-deep and deep lake deposition, especifically oil shale, contain lots of carbonized plant and flocculus deposits, which have been mostly bituminizated.The types of sedimentary facies and their scals are controlled by the base level. The sequence characteristics of single well, key bed among wells, the vertical changes of oil yield and sporo-pollen assemblages, vertical changes of trace elements are used to establish sequence stratigraphic framework of Huadian basin, which is beneficial to reveal the sedimentary facies. There is one second-order sequence and four third-order sequence in this area.Oil shale layers occur in one third-order sequence,which is divided into four system tracts-low stand system tract(LST), transgressive system tract(TST), high stand system tract(HST) and regressive system tract(RST). Oil shale was deposited in TST and HST. In the LST and RST there deposited dark mudstone and coal. Based on the vertical changes of oil yeild and types of organic matter and the ones in TST are lean ore and in HST are rich ore. The relationship between tectonic subsidence rates and deposition supplying ratse determines the changing of accommodation space, which may reflect oil shale location in the sequence stratigraphic framework and its features on horizental distributions. The horizental distributing areas of oil shale become larger and larger in the TST which indicates retrograding deposition. In the highstand system tarct, covering area of oil shale reaches to its largest which distributs stable and shows aggradation deposition.The thickness of strata and that of sandstone layers, the percentage of sandstone thickness in total strata are the basis for identifying the centre of deposition and the direction of sediment source. Based on these information of each mumber of Huadian Formation, the sedimentary centre of lower pyrite member is the northeast part of Huadian basin and the dirction of major sediment source is northeast. And the northwest and west part of Huadian basin was not covered by lake water and exsited two secondary sediment sources. In the middle of Huadian Formation (oil shale bearing), the deposition centre was also in the northeast part of Huadain basin, and the sediment sources which were in west and northwest were disappeared. The one from northeast remained. In this mumber, the influence of sediment source was negligeable. In the upper carbonaceous mudstone member, the northeast sediment source becomes smaller and smaller. The deposition centre and sediment source was changing to the side of major fault The thickness and the area of sandstone layers became larger and lager.On the basis of sedimentary source and sedimentary facies analysis, three evolution stages in Huadian basin have been identified. The initial subsiding stage started at the lower pyrite member when the area of Huadian Lake was small, and in the northeast part of Huadian basin was developing delta deposits. In the middle was shore-shallow lacustrine, and in the side of basin-controlling fault there was sub-deep lake deposits. The western part of this basin wasn't covered by lake water, and there were mainly depositing alluvial fan and flood plain. The maximum settlement stage was at oil shale member. In that period, the fan-delta were depositing in the northeast part of this basin and the eastern side of basin was developing samll areas of subaqueous fan. In the gentle slope of this basin there was shore-shallow lacustrine In the middle of Huadian basin and in the side of basin-controlling fault was sub-deep and deep lake. That was the period when oli shale deposited. The atrophic stage of Huadian basin can be identified in upper carbonaceous mudstone member In the eastern and western basin-controlling fault there was developing fan delta, and in the northeastern part of this basin was beach bar In the western part there were depositing small scal of fan delta. In the gentle slope area there was developing lacustrine bog. Towards to the basin-controlling fault there was shore-shallow lacustrine. And in the middle of this basin there was sub-deep and deep lake.