THE CHARACTERISTICS AND CONTROLLING FACTORS OF THE SURFACE OIL AND GAS GEOCHEMICAL ANOMALY IN THE ORDOS BASIN

Based on the abundant actual data and the joint mapping of these data,the authors systematically studied the characteristics of the geochemical background of the Ordos Basin,and compared the geochemical background values of this basin with those of the main petroliferous basins in China.The results show that the Ordos Basin is characterized by middle to high geochemical background values,and relatively even geochemical field among the main petroliferous basins in China.Comprehensive research result shows that the formation of the background and the anomaly within the basin is the results of the combined reactions of various underground geological conditions,various near-surface sedimentary features and various landscapes.Of witch,oil and gas source is the material basis for the formation of the changes in the near-surface concentration of the geochemical indicators,and acts as the main controlling factor for the formation of the near-surface oil and gas geochemical anomaly.Oil and gas from different sources have different compositional characteristics and can cause different near-surface anomaly combinations.The study also shows that the tectonic movement is the main drive force for oil and gas to migrate vertically and laterally from the source rocks to the reservoirs,the difference in trap types plays a controlling role in the near-surface anomaly combination form or anomaly pattern,while the near-surface sedimentary characteristics and the landscape conditions play an important role in affecting the anomaly distribution regularity and the application effects in near-surface oil and gas geochemical exploration.