Sequence Stratigraphy and Coal Accumulation Law of the 2nd Member of the Nantun Formation Coal Measures in the Huhehu Depression

Key words: 
• coal measures

Abstract: Huhehu Depression is a key exploration area in the east grabens which is located in the southeast of Hailar Basin. The depression is a halfgraben rift subsidence structure, characterized by it being faulted in the east and overlapped in the west, and its area is 2 500 km2. The 2nd member of the Nantun Formation in Huhehu Depression has developed the coal measures buried deeply whose strata formed by a set of gray siltstone and dark thick mudstone which mixed with black coal, gray fine sandstone and grey conglomerate. And the rock layers developed a series of multilayer and thicker coal seam, its reflectance of vitrinite is between 0.8%～1.5%, and the coal rank between gas coalcoking coal stage, so the coal measures have a better hydrocarbon potential. Also, because of the weaker later tectonic movement, the coal measures contain rich oilgas (CBM) resources. This paper mainly analyzes the characteristic of the stratigraphic framework, coal accumulation environments and coal accumulation laws in the case of the coal measures of the 2nd member in the Nantun Formation to guide the petroleum exploration in Huhehu Depression in the future.
Based on the theory and method of sequence stratigraphy，according to the drilling, well logging and seismic data, the sequence interfaces (the top interface SB4, the bottom interface SB3) and system tract interfaces (defined by the initial and maximum flooding surfaces) of the 2nd member in the Nantun Formation are identified. Finally, the sequence stratigraphic framework is established by the comprehensive analysis of the tectonic evolution, paleontological features, paleoclimate changes and lake level variations. Also, the main controlling factors on sequence formation are analyzed briefly. 
At present, there are 11 wildcat wells in the study area. Through the analysis of the single well facies and sections of correlation of the stratigraphic, combined with other single factor maps, such as the stratum isopach maps, sandstone isopach maps, sandstone content isoline maps, dark mudstone, carbonaceous shale and coal layers thickness isopach maps, with the system tracts as the units, the author has described the sedimentary system distribution and temporalspatial evolution characteristics within the sequence stratigraphic framework.
Through comprehensive analysis of sedimentary evolution and correlation of the sequence stratigraghy, a study of coal layers development features and the patterns of coal accumulation is carried out with the application of basic theory and method of the coalfield geology. The 2nd member in the Nantun Formation of Huhehu Depression which was in the faults extension period, characterized by the lowamplitude, weak and pulsative vibration of the tectonic subsidence, During this period, it experienced an infilling evolution process of water in withdrawal, and the lacustrinebraided river delta sedimentary systems developed dominantly. In the early stage of baselevel falling, the increasing rate of accommodation space was consistent with the supply rate of sedimentary source, and the lacustrine areas continuously existed in the center of the depression, so the peat swamp formed stably and accumulated coal in the lacustrine basin margin of the depression; With the base level rising up, the groundwater level rise regionally, the 2nd member of the Nantun Formation developed shoreshallow lacustrine swamp coalforming environments. Because of the peat beds were buried and preserved rapidly by the highstand system tract sediments, the coalforming conditions of transgressive system tract were favourable; Lastly, in the highstand system tract sedimentary period also developed the coal accumulation environment, as a result of progradation of braided river delta, a lot of peat layers were buried rapidly in the braided river delta plain. Accordingly, controlled by the lake level changing system and the relatively steady tectonic movement, the major coal accumulation environments are interdistributary estuary swamp of the delta plain and shoreshallow lacustrine swamp.
As mentioned above, the study area was controlled by the rates of the tectonic subsidence and the peat accumulation every system tract developed coalaccumulation sedimentary conditions. However, the degrees of development and distribution ranges of the coal layers are significantly different between the different system tracts. The results indicate that the coalaccumulation mainly developed in the later period of lowstand system tract and transgressive system tract. In vertical directions, the coal layers mainly formed in transgressive system tract where the water started to expand. During this period, a large area and long term lacustrinebraided river delta sedimentary systems were peaty and swampy largely in shoreshallow lacustrine zones where the water was shallow. Under such circumstances, because of the increasing rate of the accommodation space matched balanced with the sedimentary accumulation rate, the stratum was very conducive to form series thick and stable coal layers. But the coal accumulation was the weakest in the highstand system tract in which the distribution of formed coal layers was thin and more localized.
Controlled by the tectonic subsidence and sedimentary systems, the coal accumulation of each system tract shows obvious zonations and situations that Southeast of the Depression is strong, while Northeast is weak. The coalrich units are in accordance with the distribution of the lacustrinebraided river delta sedimentary facies, mainly distribute between lacustrine basin margin and the subsidence center, and near to the lacustrine basin margin. However, it was not conducive to develop peat swamp in the deeper lake water body, the coal layers became thin even pinchout from the braided river delta plain to deep lake.