Sequence Stratigraphy Research And Stratigraphic Traps Recognition In Large-scale Depression Of Lacustrine Basin

Junggar Basin is one of large-scale petroliferous sedimentary basins in western china. The sequence stratigraphy and deposition facies developed a typical feature of lacustrine basins filling in the depression, that is the basin area was very big and water was quite shallow since Jurassic period. Thus, the attitude of stratum was gentle; the scale of the reservoir of the research interval was large; the facies belt was wide, and the sandbodies was difficult to pinch out laterally, etc. It is very important in academic and practical study for stratigraphic traps exploration in Luxi-Dabasong area and some others like this area, through researching the features of sequence, deposition and stratigraphic hydrocarbon reservoirs forming in this kind of lacustrine basins.On the base of observation of core, depending on the data of geo-logging, well logging, drilling and seismic, combining the analysis of the regional geologic setting, the seismic strata interpertation was proceeded using the technique of lithofacies analysis and electrofacies analysis and applying the principle and methodology of high resolution sequence stratigraphy and sedimentology. Then, the isochronous stratigraphic framework of studied area was established using lots of andvanced techniques such as straitigraphic correlation integrating well and seismic data, sand ratio calculation matching well and seismic data, seismic attribute analysis, frequency division processing, sand body carving, seismic strata flattening, seismic time slice, multiwell constrained seismic reservoir inversion, etc. Following the research path of point(well)-line(profile)-tract(temporal evolution and spacial distribution)-point(trap), the depositional system's planar distribution and vertical evolution were studied in the isochronous stratigraphic framework of studied area, and then the sequence stratigraphy filled model of lacustrine basin of this depression and the depositional facies distribution features were studied. Finally, the beneficial explortion targets were found out and stratigraphic traps were recognizated.The research strata were divided into three second order sequences and subdevided into four third order sequences. Meanwhile, three kinds of depositional facies, which were fluvial, delta and lacustrine facies were made certain. Based on those work, the reservoir forming features and favorable exploration areas were evaluated, and three new stratigraphic traps wererecognizated and ascertained, in the stratigraphic framework of studied area.The research suggested that the sequence of lacustrine basin in depression should be made up by lowstand system tract, transgressive system tract and highstand system tract, which were abbreviated as LST, TST and HST. The LST mainly developed in the stage of lacustrine basin oscillating and reconstructing; therefore, it mainly developed progradational wedge and deposited large-scale conglomerates and sandstones. The TST mainly developed the transgressive deltal and lacustrine mere deposition; thus, the sandstones and mudstones deposited alternatively, and the gravity flow could form under some inducement mechanism in some breakover belts of lacustrine basin. The HST deposited large-scale mudstones and mingled with a few of lenticular finestones in the influence of the lacustrine basin filled and leveled up and swamping; thereby, good reservoirs of HST developed a few.Sangonghe Formation was lacustrine depositional environment on the whole and had typical features of lacustrine filling, and the second Mumber of Sangonghe Formation (J_1S2, for shot) was deposited when the lacustrine was unquiet. In the early stage of J_1S2, the water body was very unquiet, and the fluvial system, large-scale lowstand braided delta which deposited many conglomerates were developed very well, so the structural traps were our main targets. In the later stage of J_1S2, the TST was developed and the maximum flooding surface (MFS, for shot) in Jurassic period occured in this Mumber, and the sandstones and mudstones deposited alternatively; therefore, the stratigraphic traps were our main targets.Xishangyao Formation was deposited in the backgroud of the lacustrine basin was filled and swamping and detals were abandoned, so the depositional topography was very gentle. The normal detal plain, meandering stream and distal sandy braided stream were main depositioinal facies types, and the structural-stragraphic traps were our main targets.Toutunhe Formation was remain second order sequence deposition, which developed meandering stream, transgressive detal and lacustrine mere, sandstones and mudstones deposited alternatively; thereby, the stratigraphic traps were our main targets.The first Member of Qinshuihe Formation was the initial deposition of the Cretaceous Period and it had features of the lacustrine basin reconstructed and the gully and valley filled. In the early stage, the deposition mainly occured in the remained accommodation (in gully andvalley), and it was bypassed on the high; however, in the sag, large-scale conglomerates were developed, so the structural traps were our main targets. In the later stage, there developed lacustrine transgressive deposition on the high; meanwhile, some gravity flows could deposited in the sag which was located at the lower part of breakover belt (probably, formed by some faults); therefore, and the stratigraphic traps were our main targets.