Tertiary Sedimentary Characteristic And Lithologic Reservoir Prediction In Thrust Belt, North Qaidam Basin

This study area, north of Qaidam basin, which is a first structure unit, is one of oil-gas-rich structure of Qaidam basin. It includes the North Kunlun fault-step belt, Dahonggou uplift, Yuka-Hongshan fault-depression and Delingha fault-depression. There are four fault-depression stages of Meso-cenozoic regional tectonic evolution in north Qaidam Basin:fault-depression sedimentary stage, local uplift and erosion stage, mass settlement depression stage and strong thrust and fold stage. Tertiary system is main composed of clastic sediments and the hydrocarbon rocks are mostly formed by a series of Jurassic carbonaceous mudstone. This formation forms a assemblage of lower grneration-upper accumulation.Take Tertiary system of north Qaidam basin as the main target stratum, the palaeogeographic environment and sedimentary evolution were systemically studied and the framework of sequence stratigraphy was established. The study shows that the medium cycle controlled the formation of lithologic traps. After the study of lithologic reservoir types, key controlled factors and favorable accumulation condition in salt lake basin, the favorable reservoir zones, the prospecting exploration domains and zones are totally predicted. The above studies systemically guided the lithologic reservoirs exploration in salt lake basin.The sequence stratigraphy of Tertiary system in north Qaidam basin are divided into3super-long-period datum level cycles,9long-period datum level cycles, the LSC1, LSC2and LSC3formed in the early developmental phase, LSC4and LSC5, in the expand phase and LSC8, LSC9in the extinct phase of lake basin. The Sequence assemblages controlled the distribution of lithologic traps. The favorable reservoir mainly situated in the following medium cycles, which are the asymmetry cyclic arrangement deepen upward in facies and inadequacy symmetry cyclic arrangement where the thickness of the upper hemicycle is far great than the down hemicycle.The thrust belt sedimentation of north Qaidam Basin has a characteristic of multi-source and near-source. And vertically, it has the one of inheritance and rebirth. There are three different sedimentary sources:Altun source, Saishiteng source, and Yuka-Lvliangshan source. Controlled by the three sources, the thrust belt develops three depositional systems in Tertiary.The formation of lithologic reservoirs in Tertiary of salt lake basin of north Qaidam basin are controlled by the distribution of source rock, the distribution of depositional system, structural background and migration pathway. Reservoirs of north Qaidam Basin are mainly distributed in Lenghu-Nanbaxian and Mahai-Pingdingshan-Yuka structural zones. They can be divided into two types: primary type and secondary type which mainly contain structural reservoirs of anticline, faulted anticline, broken nose structure and block. Simultaneously, there also develop non-structural hydrocarbon bearing traps, such as disconformity trap and lithologic trap.Fine calibration of microfacies and reservoir lateral prediction are two key technologies for lithologic reservoir exploration. Using this approach, the exploration targets of clastic reservoir and algal limestone are predicted effectively.The innovation of this study is firstly using integrated data to demonstrate the characteristics of stratigraphic sequence and deposition, and pointing out that the exploration be worked around the secondary depression of western Lenghu and the depression of northern Yikeyawuru. We should take near-source exploration as the purpose of exploration, and carry out "lower slope and lower depression near-source exploration" for searching deep reservoirs.