Study On Transgressive Delta Which Is Deposited In Shawan Formation Located In Western Junggar Basin

The Chunguang Block of Chepaizi Uplift is located in western Jungaer Basin, the integral direction is NW-SE, belong to a typical wage form of North thin-South thick depositional unit, and it is a long-term inheritance characteristics of the uplift. Oil and gas exploration in this block was began in 1950 s, Shengli Olifield reserchers drilled the Neogence Shawan Formation light oil reservoir through the well of Pai2 located in Chepaizi uplift west of Hong-Che fault zone, which opened a prelude to the exploration of light oil of Neogence. Early oil and gas exploration in this block was mainly depend on "highlight" mode, chun 2, chun 8 and pai 22 make breakthough in Shawan Formation, however, chun 3,chun 4,chun 5 and chun 7 come to nothing, the well of Chun 2 is not a "highlight" characteristic petroleum reservoir, and the oil strata is located in transgressive Braided River Delta deposited in Shawan Formation, the reservoir has better properties, the lithology is gravel bearing sandstone, and the diagenesis is low with high porosity and high permeability. Through the previous studies, the times of drill the transgressive delta is less, therefore the degree of study on transgressive delta is low. The oil-bearing strata is located in 2nd sandgroup set, Sha 1 member, the distribution characteristics of sandbody in the well region and the genetic mechanism of the transgressive delta sandbody lithologic trap are not clear, so it is necessary to study the sedimentary system and oil-gas geological conditions.Aiming at the above problems, this paper takes the 2nd sandgroup set of the Sha 1 formation(N1s1-2)in the Chepaizi Uplift as the research object, based on sequence straitigraphy and the related theory, combined with the drilling, logging and seismic data to establish the sequence stratigraphic framework, identify the transgressive and regressive system tract,coupled with velocity analysis and waveform analysis to establish the seismic response of the transgressive delta sandbody, to distinguish the sandbody deposited in transgressive period and finally to obtain the sandbody framework plane distribution map. Based on paleogeomorphic and seismic-sedimentary facies analysis, determining the deposited area of transgressive braided river delta, and through the analysis of reservoir, cap and barrier condition to figure out the genetic mechanism of the transgressive delta sandbody lithologic trap. The following conclusions are drawn from the study:1)On the basis of previous studies, through the mode of "T-R" to divide the Shawan Formation into Sha 1(s1), Sha 2(s2) and Sha 3(s3) three 3nd sequence. Within each sequence unit, there are a set of TST and RST, which is separated from the MFS.2)The sand-mud velocity analysis of Sha 1 member shows that sandstone is mainly I type sandstone, with a small percent of II sandstone. The waveform analysis carried out by synthetic seismogram, as though the sandbody set thickness is less than 1/4 wavelength, the sandbody corresponding waveform in seismic profile is right downdip skew-symmetric. In part II of the sandgroup set deposited sandstone must be combined with seismic-sedimentary facies to analyze and distinguish.3)Based on the waveform analysis of the block to distinguish the sandbody and finally to obtain the figure of frame sandbody plain distribution. Combined with the analysis of palaeogeomorphic map to figure out the frame sandbody in the northern gentle slope looks like a lobe, whereas located in northern slope is banded distribution and in center of southern sag is looks like fan frame appearance divergent bands.4)Though the typical seismic facies of the block, there are 5 kinds of seismic facies named high amplitude short shaft shaped reflection, high amplitude less continuous reflection, weak amplitude nearly blank reflection, middle-high amplitude discontinuous reflection and high amplitude moundly reflection are distinguished. According to the analysis of "seismic-sedimentary" facies and depositional paleogeomorphology comprehensive analysis, the 2nd sandgroup set developed braided river delta plain, braided river delta front, predelta of braided river, slope fan and basin fan group, the sandbody set is braided river delta-submarine fan depositional system.5)The genetic machism of lithologic trap of the well of Chun 2 is analyzed after the depositional range of the transgressive delta is determined. It is considered that the top of the well area sandbody is mouth bar sandbody, the sandbody is gravel bearing sandstone with good sorting performance and high porocity. The down-cutting stream is located west of mouth bar sandbody and is filled with mud at the MFS epoch, coupled with internal shelf is located at gentle terrain, the 3rd sandgroup set deposited sandbody away from the 2nd sandgroup set to be piled up and the sandbody can be pinchout at lateral direction owing to the lateral penetration of poor clay coating, thus forming the updip pinchout lithologic trap.