Abstract:
In previous achievements, depositional system, paleogeomorphology, geological evolutionary and sedimentary characteristics of Ordos basin has been quite clearly recognized. However, the research precision and depth in local area still has certain limitation. As geological data's supplement and corroborative evidence of Ordos basin, the research on precise depiction of geological features, sedimentary characteristics as well as their matching relationship with practical production in local area, undoubtedly, has its necessity and inevitability.
Most of the previous researches on petroleum geology in the eastern Gansu were focused on Triassic, and the research on Jurassic is few, scatter, and lacking of integration and systematicness. The petroleum exploration history on Jurassic went through three stages: most of the achievements are mainly on regional geological surveying, researches on Jurassic are few(before 1970s); Several oil fields of Jurassic, such as Maling, Yuancheng, were successively founded in study area, and Fuxian Formation, Yan′an Formation of Jurassic become the major explorative horizon(form 1970 to 2000); New oil reservoirs of Y10,Y9 and Y8 member of Yan′an Formation are founded continuously in local regional (from 2000 to now). In the past few years, exploration practice indicated that Y10 and Y9 members are the main oilbearing layer of Yan′an Formation, owning great resource potential. Paleogeomorphic form is the basic factor of Y8Y10 oil reservoir.
In view of petroleum accumulation of Jurassic controlled by paleogeomorphic forms, this paper will rebuild the paleogeomorphic formwork of preJurassic, and first make up the 3D simulation restored maps, which are based on geological analysis(such as sedimentary background, tectonic setting), changing tendency of sand layer thickness and formation thickness of each members(especially the formation thickness of Y10 Formation), calculating key parameters(such as ancient landform evaluation difference, slope gradient, paleocurrent direction), combining with seismic data and so on, adopting remnant thicknesscompensation cast and synthesis method of sedimentary geology.
First, according to the stratum development features, five paleogeomorphic units are divided: erosion valley, paleoupland, paleoslope, paleoterrace, interchannel mound. There are three paleouplands, four ancient rivers, six interchannel mounds and one platform in eastern Gansu area. Between paleouplands and ancient rivers, there are slopes and terraces from high to low in order, which are strictly controlled sedimentary facies and compactiondrape tectonic of Fuxian formation and lower Yan'an formation.
Second, according to spatial distribution of oil reservoir already founded and combined with analysis of paleogeomorphic features and reservoir forming conditions, six paleogeomorphic reservoir models are summed up in the study area: edge of deep dissection valleydelta front, edge of deep dissection valleydelta plain, ancient valleydelta plain, paleoterracedelta plain, paleouplanddelta plain, and interchannel mounddelta plain combination models. Exploration practices shows that formation conditions are very complicated, which is controlled by many factors, and paleogeomorphic features is only one of the important factors.
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