Study On Depositional System Of 2^nd Member Of The Shawan Formation In Chunguang Block Of Junggar Basin

Junggar Basin is the second largest basin in China, which is rich in oil or gas. And the Chepaizi area is located at the northwest margin of the Junggar Basin. At present, the oil or gas exploration of this area has stepped into a deep study phase. Because the study area belongs to a nose structure belt, it is lack in structural trap and the lithologic trap is the main goal. The considerable industry oil production has been found in the low level Shawan formation whose age is neogene. So we suppose that this area has oil exploration potential. And it has not carried out the sequence stratigraphy and sedimentary system research which is assisted to find the lithologic trap.For solving the problem above, we take advantage of the seismic, log and drilling data integrated the geologic background as well as instructed by seismic stratigraphy, sequence stratigraphy and sedimentology theory to divide the sequence stratigraphy and compare them. And then we build the sequence stratigraphy framework of the Shawan formation. At last, we study the sedimentary system systematically and have some new knowledge as followed:1)Instructed by the sequence stratigraphy calibrating theory raised by O. Catuneanu(2009), we divide the Shawan formation into three Third-order sequences: SQ1,SQ2,SQ3, which correspond the three sections of the Shawan formation: N₁s¹,N₁s²,N₁s³ approximately. And each section can be subdivided into four tracts including: Lowstand System Tract (LST), Trangressive System Tract (TST), High System Tract (HST) and Force Regressive System Tract (FRST).2)We select one typical profile which goes through the whole study area and is Source Direction strike. And then we carry on curve tracing with it as well as original basins restored. At the same time, we determine internal structure, deposition trend, accumulation mode, contacting relationship and the distribution of the source direction of the SQ2.3)By extracting the values of the velocity from large number of sandy mudstone sections and interpreting them, we find three types of sand in our study area, which include:Ⅰtype, the velocity of sand is slower than the mud stone;Ⅱtype, the velocity of sand is about the same as mud stone;Ⅲtype, the velocity of sand is faster than the mud stone. And the second section of the Shawan formation has theⅠtype sand. By comparison and analysis of the well and seismic profile, the trough indicates the top of the sand.4) Based on the study above, we scan the sand body by application of the seismic scanning and interpreting method and get the distribution of the sand framework in the plane. Using the individual well and seismic face analysis, we think it develops shallow water braided river delta plain, delta front, slop and floor fan facies. Besides, the plain includes distributary channel and interdistributary estuary sand mainly; the delta front develops subaqueous distributary channel and mouth bar sand; the slop and floor fans develop a few of subaqueous distributary channel sand and the latter has much of sand body overlaying.5) Integrated the source direction and tectonic background, we think that the third sand bed in the second section of the Shawan formation develops shallow water braided river delta plain- shallow water braided river delta front-slop fan-floor fan in the west and central part of the study area, in the east area there is only shallow water braided river delta plain.6) Overall, we build the sedimentary facies model of the third sand set in the second section of the Shawan formation, point out the location where the lithologic traps develop.