| The Ordos Basin, which developed in the Middle-Late Triassic, is a petroliferousbasin where the Meso-cenozoic strata deposit upon the Palaeozoic the North Chinacraton basin. The North Jiyuan Area, located in the northwest Ordos Basin, is situatedbetween the obduction zone in the western margin and the slope in the north of Shanxi.And multiple-period tectonic movements have happened since the deposition.This study, based on the data of drilling, log and stratigraphic division, uses thechange rate of the reference layer thickness to estimate the erosion thickness of theYanchang Formation, builds the buried evolution history of individual wells, andanalyzes the buried evolution history of the Yanchang Formation. By core observationand with the imaging log data, the fractures are identified and the occurrence of thefavorable ones is analyzed. The response features of the fractures as shown in the logcurve are analyzed. The factors relating to the fracture development are analyzed, soare the forming periods of fractures. Based on the existing test and analysis data, theauthor studies the migration features and the types of the oil reservoirs in the Chang-8and Chang-6formations, and evaluates the reservoir-forming characteristics of theresearch area.1. According to the data of drilling, log and stratigraphic division, drew the topstructure map of the Chang-8and Chang-6formation. It is found that the northwestand the eastsouth parts are higher, and a NE-SW low-lying area develops in themiddle. The recovery of the eroded thickness of the Yanchang Formation suggests thatthe erosion degree is small (stronger in the west and weaker in the east) since theeroded thickness is between60m and180m.2. Core fracture observation and the imaging logging fractures indicate that thenatural fractures develop in the Chang-8and Chang-6formations, and85%arehigh-angle oblique-crossing fractures. The vertical fractures are the next. And thereare no low-angle oblique-crossing fractures and horizontal fractures. The fracturestrike is mainly NEE-SWW, and the direction of dip is SSE. The fractures in Chang-8and Chang-6formations are mainly open fractures, filled with calcite.3. The factors relating to the fracture development are lithology, formation thickness, faults, tectonic stress, with tectonic stress being the major forces. By thesimulation analysis of the stress field, it is found that the stress during the time whenthe fractures are formed are along the direction of NWW-SEE, identical with thestress field direction during the Yanshan Period of the basin. The study of the fractureforming periods suggests that the fractures of Chang-8and Chang-6are mainlyformed in the Yanshan Period.4. The study of the oil reservoir characteristics suggests that the rock types ofChang-8and Chang-6formations are mainly feldspatite debris and lithic feldsparsandstone. The pores of the reservoir are mainly residual primary intergranular pores.The reservoir porosity and permeability increase from Chang-63to Chang-61, theaverage porosity and permeability increase, and the physical properties get better. Theporosity and permeability shares the same trend of change from Chang-81to Chang-82.The change of the correlation between the median pressure and the poroperm is moreobvious than the displacement pressure, indicating the complexity of the porestructure.5. The oil reservoirs in this area are mainly lithologic and structural-lithologicreservoirs. The force behind petroleum migration is mainly thehydrocarbon-generating bulging force. There are two paths of migration. One is themigration along the reservoir sand bodies under the hydrocarbon-generating bulgingforce. The other may relates to the fracture system or minor faults in this area, i.e. themigration network of sand body-fracture (fault), which serves as the transient path ofmigration or later adjustment. Petroleum generation began in late Jurassic epoch, andthe early Cretaceous epoch is the major hydrocarbon-generating period. |
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