Reservoir Depiction Of Ma5₁₊₂ Formation In Shan262Well Area, JingBian Gas Field

JingBian gas field is located in central of east slope of Ordos basin, which is gentle structure with dip less than one degree, and the fractures and traps are not very developed in this field. With rich hydrocarbon resources, it has great potential for oil and gas exploration. In our country, it has became one of the most important strategic areas, of which oil and gas resources abundance and proven resource directly influences the development of Chinese oil industry.According to previous research results we can know that JingBian gas field with weathering crust is paleo karst reservoir, under which is Lower Ordovician Majiagou formation Ma5carbonate, and on top of which is Ma51+2ancient landform or lithology gas resevoir related to paleo karst or unconformity. There is a close relationship between Majiagou gas reservoir development of Ordovician top of Ordos basin and eroded palaeogeomorphology especially the ancient buried platforms and ditches. In the area of ancient buried platforms, Majiagou formation preserves well, but in the ancient buried ditches, the main reservoir of Majiagou formation was eroded by massive denudation which resulted the remaining thickness is small. Besides, in platforms major reservoir develops solution pores which are good for hydrocarbon accumulation, and the situation is opposite in ditches.Majiagou reservoir is primarily dominated by karst landscape especially the corrosion shape of Ordovician top surface influences reservoir physical property greatly. For depiction, It is necessary to research Ordovician in lower Paleozoic weathering paleogeomorphology and summarize objectively the laws of karst distribution. In this paper, the seismic inversion methods will be used to roughly delineates the distribution of ancient buried platforms and ditches. Then, paleogeomorphic restoring method is used to depict Ma51+2reservoir precisely. After this research, some points are outlined as follows:First, in order to interpretate the horizons accurately, we should first do a research about seismic reflections influenced by ancient buried platforms and ditches. Because there exists a big velocity difference between Carboniferous coal formation and Ordovician tight dolomite, it will form strong wave impedence interface. The eroded layer on top of Ordovician in buried ditches makes BenXi formation(Tc) and Ma51+2member(O_m52) seismic waves reflect lag behind, therefore the seismic reflection will be sinked, twisted, widened or amplitude increased. According to geologic and seismic waveform characteristics, three different seismic wave characteristic models are summarized:normal type, phase widened type and multi-phase type.Second, seismic signals depend on petrophysics which are comprehensive reflection of the characteristics of strata. Spatial variation of strata would inevitably lead to changes in seismic wave characteristics, thereby affecting the changes in seismic attributes. These characteristics point the way for inferring buried ditches through seismic attribute inversion and depicting the ancient weathering crust reservoir.Third, in order to identify the buried ditches, the prestack simultaneous inversion and post-stack instantaneous information are combined. Compared to post-stack inversion, the prestack simultaneous inversion, taking full advantage of nonzero offset information to obtain a reliable petrophysical characteristics, is much easier to explain the erosion surface accuratly. In prestack simultaneous inversion, firstly S-wave impedance profile and slice because of lithological difference is used to recognize ancient landform, then using the P-wave impedance and S-wave impedance computes the λρ and μρ attributes volume, what are more sensitive to lithology and gas. Apart from λρ and μρ, S-wave impedance data volume is helpful to recognize palaeogeomorphology. In post-stack inversion, through extracting the instantaneous information, the buried ditches can be interpretated fine.Fourth, based on the overlay relations and stratigraphic development characteristics between the top of Ordovician Majiagou formation dolomites and the overlying Carboniferous strata, two methods are adopted to recover the Ordovician top palaeogeomorphology of Shan262well area in JingBian gas field. The first method is to draw the outline of Majiagou formation and BenXi formation top structure. Second, we used residual thickness map of Majiagou formation top structure and thickness map of BenXi formation to explain the Ordovician weathering crust. In addition, together with the information inferred from prestack simultaneous inversion and instantaneous information seismic inversion, the technology for ancient karst landform recovery turned out well.In view of special geological structure and gas accumulation background of Majiagou gas resevior, integrating prestack simultaneous inversion, instantaneous information and ancient geomorphological recovery techniques, erosion surface is mapped accurately and Ma5gas reservoir is interpratated precisely.