Sequence Stratigraphy and Prediction of Favorable Zones for Hydrocarbon Exploration of Badaowan Formation in Eastern Junggar Basin

Key words: 
• sequence stratigraphy /
• sedimentary facies /
• sequence filling /
• sedimentary evolution /
• Junggar Basin /
• Badaowan Formation

Abstract: Dishuiquan Area is located in slope area of Kalameili Mountain in the eastern Junggar Basin; at the south are Wucaiwan Depression and Dongdaohaizi Depression; at the west is Dishuiquan Depression.By using of the theory and method of sequence stratigraphy and sedimentology and the data of seismic, well drilling, well logging and core, the authors establish the sequence stratigraphic framework and divide sedimentary facies of Jurassic Badaowan Formation in the Dishuiquan Area. 
The sequence of Badaowan Formation is one and half, including 5 system tracts divided by two FFS, two MFS and the stable core inner J1b3. The whole experiences the process of retrogradation, progradation and then retrogradation again. Multilevel topographic breaks developed in the Dishuiquan Area in the early Jurassic. Superimposing patterns of system tracts are lowstand system tract, lacustrine transgressive system track and highstand system tract. As is mentioned above, JSB1 is the bottom boundary of the sequence and JSB2 as the top boundary. We divide the middle and lower part with the lower up part of Badaowan Formation into one sequence, from up to down, including lowstand system tract, lacustrine transgressive system track and highstand system tract. Its upper part of the lowstand system tract and lacustrine transgressive system track together with the lower member of Sangonghe Formation of highstand system tract make up another sequence.
The Badaowan Group is the first lake influx lake retreat cycle after the late Triassic tectonic uplift, which has the vertical sequence of bottomup development of coarse to fine to coarse complete cycle, and the sedimentary facies perform for the evolution of riverlakedelta. The tectonic activity of Badaowan depositional period is relatively stable, accompanied with regional sublacustrine concussion, and the climate is wet warm. Influenced by ancient water system in the eastern part of the Junggar Basin, the study area constitutes the sedimentary system with eastwest trending. The whole Badaowan Formation deposition in the time and space distribution has the inheritance and variability. 
The Badaowan Group in the study area mainly developed three kinds of deposition types, which are braided, braided river deltas and lakes, five kinds of sedimentary subfacies types and 10 kinds of sedimentary microfacies type. Braidriver deposits in the study area including the braided channel subfacies and floodplain subfacies, expressed as the peculiar asymmetry of the braided river "dual structure" in vertical. Braided river delta deposition of the study area included braided river delta plain subfacies, braided river delta front subfacies.
Viewing from the position of the sand body developing in the sequence, the reservoir sand bodies develop mainly in the low and high system tract, regional mudstone caprock mainly develop in the transgressive systems tract. The Badaowan formation in Dishuiquan area mainly develop two sets of large reservoircap combination.JSQ1 lowstand system tract formation of braided channel sand body has the role of filling the early valleys, also can be used as good reservoir. JSQ1 lake transgressive system tract formation of lacustrine facies mudstone can be used as underlying reservoir favorable cover; JSQ1 high system tract and JSQ2 lowstand system tract formation of braided river delta distributary channel sand body and JSQ2 lake transgressive system tract of lake deposition can be combined to form a reservoir-seal assemblage. Due to the JSQ lowstand system tract developing braided channel sand body, the lateral variation in lithology easy to form the oil and gas lateral occlusion. Therefore, the palaeogeomorphology of valley in study area of Badaowan Formation can be combined with widely distributed braided channel sand body of JSQ1 in the lowstand systems tract to form formation  lithologic trap which is the most favorable exploration targets in the study area.