Sequence Stratigraphic Characteristics And The Selection Of Favorable Areas Of Jurassic In The Center Of Junggar Basin

Junggar basin has good oil and gas potential, however, the source rocks are deeply buried, with poorly developed positive elements. During the deposition of the Jurassic system, the major exploration-target interval, the basin was wide, shallow, and smooth, with multiple cycles of alternately deposited sand stone and clay. The sand stone intervals were scattered in the vertical direction extending horizontally in limited distance. Exploration results from a few wells show that, oil and gas in the center of the basin are controlled not only by the geological structures but also by the lithology. The non-structural traps discovered in the basin include upwarding check out trap, lithologic trap and stratigraphic overlap trap. Through the systematic study of the depositonal systems, reservoir characteristics, and the formation of oil and gas traps in the gently dipped Jurrasic intervals in the basin, this paper discusses the control of the stratigraphic sequences on the oil and gas resource gathering, and provides guidance to the selection and evaluation of the favorable exploration targets. The central part of Junggar basin has huge unexplored area, with few drilling wells and highly scattered seismic exploration. Based on the wells and seismic data constructed,the sequence stratigraphic theory and techniques are used to build the sequence stratigraphic frame of Jurassic in the center of the basin, which consists of 2 2nd and 7 3rd sequences. During the construction of the frame, the author fully used the well logs of peripheral area and salient of the basin as aid and reference. The research results show that all the 6 3rd sequences of Jurassic were formed in the gently dipped slope. However, due to the difference in dead forms and palaeoclimate,their deposition environments can be divided into three palaeogeographic groups:gentle dipped slope with a gentle break in a humid climate(Sq1,Sq2,Sq4); gentle dipped slope in a humid climate (Sq3); gentle dipped slope with a gentle break in an arid climate (Sq5,Sq6)). Accordingly, each group has different stratigraphic characteristics. Through the study of cores, logs, seismic crosses and provenances,this research has discovered that in the three different palaeogeographic groups,different deposition systems were formed from the verge to the center of the basin: meander river-delta plain-shallow lake; braided river-delta plain with braided rivers and delta front-shallow lake-deep lake;delta plain-shallow lake. In the sequence frame,micro-experimental techniques are used to do a detailed characteristic analysis of the reservoirs in the area where cores are available, for example, Zhuang1 in the 1st region and Dong1 in the 4th region.,and to discuss the geological factors that control the reservoir development and reservoir quality. It shows that the factors controlling the quality of the reservoir sandstone include sedimentary facies, the surrounding environment under low geothermal gradient and diagenesis. Through the study of source rocks, cap rocks, the combination of reservoir sandstone and cap rocks, migration, etc., the authors has constructed models for the formation of the oil and gas traps in the studied area. It is believed that three factors control the formation of Jurrasic oil and gas traps in the studied area: the existence of traps, the location of the traps in the oil and gas transfer path, and the reservoir quality. In summary, results from this research show that the sequence frame,in which different deposition systems are formed in the different deposition time and different palaeogeographic position, controls the formation of stratigraphic traps, the distribution and characteristics of the reservoirs,confining layers and migration passages,and, hence, controls the distribution of many obscure oil and gas reservoirs. The favorite areas are selected for all the 6 sequences in the studied area..The selection of favorite exploration target area is closely related to the three factors that controls the oil and gas reservoirs,i.e., the favorite area should have high quality reservoir rocks, there are available traps, and the traps exists in the migration passages.