The Evolution Of Pressure System And Hydrocarbon Accumulation In Shizigou Tectonic Belt, Qaidam Basin

Shizigou oilfield is located in the western Qaidam basin, under co-effect of Kunlun Mt. thrust faults and left-lateral strik-slip Altun Tagh faults, the geological conditions in study area are complex and there have various types of pressure system, which bring some difficulties to oilfield exploration and development combing with destruction and adjustment of hydrocarbon reservoirs caused by late-stage tectonic movements. In view of above problems, typical reservoirs have been finely analyzed and also to various geological factors and its influence on formation pressure through taking the process of hydrocarbon reservoirs formation and evolution as main line, then on this basis genetic mechanism of different pressure system are found out, hydrocarbon accumulation mechanisms and its models are summarized. The results and recognitions are as following:From shallow to deep, there have three pressure systems in study area including low pressure, normal pressure and overpressure, and the relationship between the test pressure data and depth is becoming more obvious with the deep increasing. In addition, pressure data in different producing formation appear oil layer>oil-water layer>water layer, which indicate that oil injection intensity control the reservoir pressure in some extent.On the basis of pressure compartment research, multi-level fault in the area divide the integrated compartment into more compartments that are not connected to each other, every sub-compartment have an independent pressure system, so that many compartments overlap vertically and juxtapose laterally.Analysis suggests that the main causes of abnormal overpressure are disequilibrium compaction, hydrocarbon generation and tectonic compression. With uplift and denudation of strata in later period, even light hydrocarbon diffusion where preservation condition is poor. Consequently, the overpressure in reservoir because of fluid rapid injection from deep formation gradually disappeared, and the fluid pressure in trap dropped gradually to the extent of negative pressure lower than hydrostatic pressure.It is determined that there are two accumulation stages by combining with fluid-inclusion petrography and thermodynamic study. Deep hydrocarbon reservoir, belonging to self-generation and self-store type, mostly has the characteristic of early charging, multi-stage charging and long-term charging. By the restriction of its own source rock and early carrier system, oil and gas in shallow strata come mainly from mature hydrocarbon of several deeply buried source rocks, which is the typical process of episodic petroleum accumulation under fault-overpressure control, the formation of hydrocarbon reservoir have the characteristic of short duration, rapid and high efficiency, and the main reservoir type is lower generation and upper store.From the research of typical reservoir accumulation mechanism, the comprehensive model of hydrocarbon accumulation in Shizigou tectonic belt are systematically summarized: ①initial accumulation stage of E3 reservoir between E₃¹ and late N₂¹;②enrichment stage of E3 reservoir between N₂² and middle N₂³;③E3 reservoir destruction and N1-N₂¹ reservoir formation between middle N₂³ and late N₂³;④adjustment stage of reservoir in shallow and deep between late N₂³ and Q.