Research Of High-resolution Sequence Stratigraphy And Evolution Models In The Tertiary In Qaidan Basin

The Qaidam Basin is 121000km~2. The Tertiary whose thickness is generally 5000-6000m and sometimes can reach to 8000m extensively distributes. At present, there are thousands of wells that penetrate the tertiary. The basin is covered by the intensive two-dimensional seismic grid. Moreover, the 3D seismics are still carried on in the partial region. The petroleum exploration for more than 50 years indicates that the tertiary is the most important hydrocarbon formation in the Qaidam Basin. The tertiary discovered reserves account for more than 90% of the total discovered reserves. There is a tertiary gas field among the five discovered gas fields. Three resource evaluations indicate that just in the west Qaidam Basin (the area is 30000km~2, accounting for 1/4 of the basin), the total resources is 21.93×10~8t. Among them, the petroleum is 15.35×10~8t and the nature gas is 5972.5×10~8m~3. Therefore, the tertiary resources are abundant and have great prospecting potence. Currently, the basin's backup reserves are shortage, so it is difficult to increase the production. At the same time, the sharp structural traps have almost been found and the backup targets are shortage. So the favorable place for prospecting must be nailed down.The tertiary is divided into seven second-older sequences and these second-older sequences contain twelve third-older sequences, by outcrop observation, well drilling, seismic, paleontology, geochemistry, and so on. The main sedimentary facies are identified, which include alluvial fan, alluvial plain, delta facies, fan delta, braid delta, meare, sublacustrine fan, semi-deep and deep lacustrine facies, salt lake and beach bar.This research that studies the precise lithofacies pleogeography in the tertiary in Qaidam Basin is on the basis of the sequence stratigraphy and sedimentology and take"the single factor analysis and multifactor comprehensive mapping method"as the methodology, System tracts or sequences are taked as graphical unit and the quantitative lithofacies palaeogeographic maps of the whole basin are mapped. Accompanying with the evolution of the basin, the deposition center of the tertiary continuously migrated and has the following trend of migration: from the west to the east, from the south to the north. The sequence stratigraphic model is established, which includes abrupt slope model and gentle slope model. The abrupt slope model generally appeared in the actic region when the basin continuously subsided. The sedimentary facies types of the abrupt slope model include alluvial fan, fan delta, and so on. Moreover, the facies changed rapidly and the sorting of the rock is bad. The gentle slope model generally appeared in the gentle region when the basin degenerated. The sedimentiary facies types of the gentle slope model mainly include fluvial plain, delta facies, braid delta, meare. The facies belt changed slowly. The grain size become finer and the sorting become better.