| Qiongdongnan basin is a typical basin of high temperature and high pressure. Using the overpressure as the study point to deepen the cognition of hydrocarbon migration and accumulation can provide criterion for further exploration of the basin. Now the hydrocarbon migration direction in the study area is not very clear, which restricts to understand the hydrocarbon migration and accumulation and the favorable exploration zones. Under the guidance of hydrocarbon accumulation dynamics, sedimentology and geophysics, this paper is mainly to study the special hydrocarbon migration dynamic mechanism of the basin based on the data of drilling, lithology, well logging, velocity spectrum and seismic data. The processes include building overpressure model using velocity spectrum, seismic data and drilling data, identifying and sorting the fluid activities information in the seismic profile, simulating the evolution process of the paleopressure and hydrocarbon migration and finally forecasting the favorable zone in the study area.The research results and conclusions include the following fields:(1) Current overpressure characteristicsQiongdongnan basin exists overpressure commonly. There are two overpressure types, double overpressure structure which only exists in Yc21-1 and Yc 19-2 structure near the Yinggehai basin and single overpressure structure which exists other areas in Qiongdongnan basin.In the north uplift zone of the study area, there exists overpressure partially. If it is farther from the central depression area, the pressure coefficient will be lower, while the pressure coefficient becomes lower from west to east in the same burial depth. In the central depression area, the overpressure is common while the pressure coefficient is very high, the deeper the higher, unless in some lower uplifts and the area near the faults. The southern uplift pressure is the smallest where generally don't develop overpressure or only develop weak overpressure, except in some deep small fault depression,(2) The flow activities information's classification, of characteristics and the significance to hydrocarbon migration.Various fluid activities can be identified on the seismic profiles in Qiongdongnan basin which include bright spot, mud-fluid diapir, seabed pockmark, gas leakage path, magma diapir and disorderly reflection zone etc. the bright spots mainly occur in neogene shallow layer, mainly distribute in north area controlled by No.2 fault while in the central depression area and the south uplift area the bright spots are less which are controlled by diapir or overpressure. In the west, the bright spots are dispersive while in the east they are mainly concentrated in the east of Songtao uplift, Baodao sag and Changchang sag. Generally, on the seismic profile, the fluid activities appear not separately but in the form of combination. There are three types of combination, diapir+diapiric cracks+bright spots, the gas leakage path+bright spots and diapir+ diapiric cracks. The former two combinations can be effective hydrocarbon migration channels and positive indicator to hydrocarbon migration and accumulation, while the last combination generally is negative indicator because it lacks of bright spots.(3) The evolution characters of overpressureThe simulation results of the single well show that the overpressure formed from the Meishan formation depositional stage and massively generate in Huangliu and Yinggehai formation stage. The main overpressure genesis mechanism is under compaction. In the pressure evolution graph, Ledong sag and Lingshui sag maintain strong pressure all the time, Ya'nan sag has not overpressure, Songnan and Baodao sag also develop overpressure but not strong as Ledong and Lingshui sag, Changchang sag had high pressure in the early time but low in the late time. Before 5.5Ma, the overpressure centers of the west and east were separate, Longdong-Lingshui sag in west while Changchang sag in east; after 5.5Ma, the center transferred to the west, forming the pressure distribution patter by LeDong sag as overpressure center.(4) The characteristics of hydrocarbon migrationIn the section of 2D simulation, we can see that the basin have strong ability to generate hydrocarbon. Before 15.5 Ma, the oil formation was small, and then oil formation increased, especially after 5.5 Ma, the intensity of hydrocarbon migration increased. The hydrocarbon migration is mainly vertical migration while the interfaces of hydrocarbon migration arrived in the both sides of faults are shallower than the around area which implies that fault zone is the main research of hydrocarbon migration channels. According to the simulation results, we can see that the hydrocarbon migration have the following main characteristic:oil transport is conducted the faults, especially the main fault which controls the basin's sedimentary; the lateral hydrocarbon migration is not evident; hydrocarbon mainly generates in the central depression and migrates to the relatively high hypsography traps in the north and south.(5) Forecasting of the favorable exploration zoneWith the pressure distribution and hydrocarbon migration simulation results, we know that from the perspective of oil and gas migration, the favorable hydrocarbon accumulation zones are the areas near the faults, with low pressure and low fluid potential such as Yacheng area, Badao-Songtao area and the uplifts in south. However, the favorable hydrocarbon accumulation zones not always are the favorable exploration zone, the sealing condition must be considered. After the analysis to the mudstone thickness and fluid activities information in the study area, we know that only the north has the good sealing condition, other areas are all poor. So at last, the areas according Ya'nan sag and north of Baodao-Songtao area are forecasted as the favorable exploration zones.The major innovations are as follows:(1) Using geophysical methods, this paper identified, interpreted and classified the fluid activities (such as bright spot, mud-fluid diapir, gas leakage path, diapir crack, polygon fault and so on) on the seismic profile and then analyzed the hydrocarbon migration characters based on all the information.(2) This paper built the overpressure model of Qiongdongnan basin (deep water area included) using the velocity spectrum data, simulated the overpressure evolution characters and hydrocarbon migration process in the entire basin. The job is very significant for understanding the overpressure distribution and hydrocarbon migration characters of the study area especially the deep water area.
|
PDF Full Text Request