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Lower Triassic Tight Sand Reservoirs Prediction Technology Combined of Post- and Pre-stack Seismic Inversion in MX Area, Junggar Basin


Geological background: The main payzone, T1b in Manase Lack Depression (MLD) of JUNGGAR basin, deposited in a rapidly transgressive lacustrine environment, consists of 3 bottom-up retrograding members vertically. The sand-prone subfacies of these members are merely overlapped with each other and almost all sand bodies are isolated with each other within a single member. Petrophysics and HC accumulation conditions: Underlain 50-100 meters mudstone on top of Middle Permian Urhe Groud (P2w) and widely developed T1b3 lacustrine mud are the stable floor and roof for those isolated T1b1 and T1b2 fandelta front sandstones to forming lithological traps. A series strike slide faults with small vertical displacement and high dip played a crucial role of pathway for oil migrating from Permian source rocks to the T1b reservoirs. So the key element for T1b sands trapping oil/gas is the porosity. Reservoirs Prediction: Based on well log data analysis, the P-wave impedance (IP) of sandstone ranges from 10,000 to 13,000 g/cm3*m/s, while that of mudstone ranges from 9,000 to 12,500 g/cm3*m/s. The peak impedance of sandstone (about 10,850 g/cm3*m/s) is higher than that of mudstone (about 11,850 g/cm3*m/s). That makes IP, which can be easily derived through well-restrained seismic inversion, a useful means for roughly lithology identification. But because of the negative correlation between the IP and porosity of sandstone, the effective reservoirs of relatively high porosity have a relatively low IP which falls in the main peak range of IP distribution of mudstone. Therefore the thin sands and the upper/lower boundary of the thick sands would have higher predicted porosity than that of well-developed thick sands inner parts. The relatively high predicted porosity of those thin sands and thick sands boundaries is related to the relative low IP based on poststack seismic inversion. The relatively low IP of the thin sands and the thick sands boundaries is mainly caused by the influence of low speed surrounding mudstones rather than high porosity. The ratio of P-ware velocity to S-ware velocity (VP/VS) provide not only a more precise alternative for lithology identification but also an effective approach for reservoirs prediction combined with P-wave impedance. 1.75 can be set as ceiling VP/VS value for pure sandstones. The lower the IP, the higher the porosity in condition of below this VP/VS threshold.