Study On Abnormal Formation Pressure Evolution And Petroleum Accumulation Mechanism In Junggar Basin

Abnormal pressure is discovered in many petroliferous basins. It is closely related to petroleum generation, migration and accumulation. As an important component in basin analysis, the research on pressure evolution history benefit to find out petroleum geological process, and it plays more and more important role in petroleum resources exploration and prospective assessment. Up to now, although many methods can be used to predict formation pressure, basin numerical simulation, a method and means has original advantage to study formation pressure. Many software systems with different characteristics can be found in the market. The BasinMod software, developed by the American Platte River Company (PRA), is used to predict fluid pressure in this study.Junggar basin, as a large-scale superimposed petroliferous basin, is developed from the fold basement at the late of Hercynian. The pressure generate mechanism, distribution and relationship with petroleum accumulation are complicated. The central, western and southern parts of Junggar Basin are study areas in my dissertation. The central part includes four exploration blocks of SinoPec in Junggar. The western part is made up of Chepaizi uplift. Based on collecting all the basic geologic, geophysical, geochemical and well test data, taking the quantitative modeling technique in petroliferous basin as the major means, the generate mechanisms, distribution and evolution of pressure of some typical wells and sections in the studied area are simulated and the contribution of different generate mechanisms to pressure are also analyzed. Burial history, thermal history, maturation history, hydrocarbon generation expulsion and migration histories are also studied. So the relationship of formation pressure with petroleum generation, migration and accumulation can be discussed and the beneficial exploration aim can be confirmed. Some conclusions in this study are got as follows.1. Seperated by Jurassic, the pressure can be divided into early and late periods. There are two deep and shallow pressure systems in today geology section. Overpressure in P1j main distribute in Mahu depression with relative low excess pressure and Changji depression with high excess pressure. Three independent overpressure centers in P2w locate in Mahu depression, Changji depression and Mobei uplift, respectively, and the biggest value is in the western of Changji depression. In Jurassic, there is only one generate overpressure center which locates in Mid—West part of Changji depression.2. Undercompaction is the generate pressure mechanism of early pressure system. The late pressure system was caused by undercompaction, hydrocarbon generation and quartz cementation in Changji depression, but undercompaction is the most important However, hydrocarbon generation and quartz cementation play the important role as the pressure generate mechanism in Penyejingxi and Dongdaohaizibei depressions. The pressure in the southern part is related to tectonic stress. Different folded zone has different pressure.3. The burial, sedimentation and tectonic subsidence histories manifest that sedimentation rate varies in different tectonic evolutionary stage. Early tectonic subsidence and sedimentation rates are great at the rifting stage of Junggar Basin in Permian period. However, it decreased in depression basin from Triassic to Jurassic. The sedimentation proceeds at the late Cretaceous. From Neocene to Quaternary, tectonic subsidence and sedimentation rates in Changji and Chepaizi area become rapid again because of Xishan Movement which also result Tianshan Mountains uplifting and cause strata erosion in well Dal.4. The thermal and maturation histories also indicate that the temperature rate is related to sedimentation rate. The heat flow and maturity is different as the different matrix thermal conductivity. Moreover the pressure enclosure can prevent the heat dissipation which makes the maturation retardation.5. Different source rock has different maturation and hydrocarbon expulsion process. Oil-source rock correlation confirms that petroleum main comes from Permian and Jurassic strata. Hydrocarbon migration is affected by tectonic movement especially Yanshan and Xishan movements. The distribution of petroleum is not the same in different reservoirs. Petroleum found in P₂x strata main distribute in Hongche fault zone, Maqiao and Baijiahai uplift. T₁₊₂ reservoirs distribute in Baijiahai uplift, east of Chepaizo fault zone and south of Changji depression. As to J₁b reservoir, the benefit area is Hongche fault zone, south of Chepaizo fault zone, northwest and east of J unggar basin.6. The main factors affecting the hydrocarbon accumulation are fault, unconformity, Chemo palaeohigh, Xishan movement and overpressure. These factors have different affection to different areas which make different plays.