Ordos Basin Post-transformation And Its Impact On Oil And Gas Accumulation

Ordos basin developed from Mid-late Triassic to Early Cretaceous, then, underwent reformation. As liquid, in response to the changes of environment hydrocarbon tends to flow, changes its distribution and place, so not only the structural framework during and before basin-forming, but also any tectonic movements or any kind of reformation that occur after basin-forming affect its occurance . Late Jurassic-early Cretaceous is the rush hour of the hydrocarbon's generation and accumulation in Ordos, then the basin experience multiple tectonic disturbances of many kinds, absolutely, they would have a significant effect, such as result in the destruction, adjustment, redistribution, leakage and the modify of energy field.In this paper, according to the geological events, evolution of dynamic setting, stratigraphic contact and change of depositional environment et al, we divide the stages of the late stage reformation after Late Crataceous in Ordos and its adjacent area, systematically discuss the main manifestation of reformation and spatio-temporal distribution feature of its intensity. Based on this, starting with many geological-geochemical evidences resulted from oil and gas leakage in reformed process, taking the depressurizing in Neopaleozoic due to erosion as clue , analyzing the typical and representative examples, primarily discussing the influnce reformatin to hydrocarbon.Since Late Crataceous, evolution of Ordos basin and its adjacent area can be divided into 5 phases : ①Late Crataceous-Palaeocene;②Eocene-Oligocene;③Early-Middle Miocene;④Late Miocene;⑤ Quaternary . Basin base has experienced differential, detailedly say, east strong and west weak, having a gradually increasing trend eastward. Uplift and erosion till about 8Ma. At about 8Ma ago, erosion action ended and aeolian deposits occurred in eastern block, the framewok that uplift east and subside west since Late Jurassic changes, Ordos block reversed . From then on, block still continuous differential uplift by this time. The effect of tectonic movements discussed above on hydrocarbon have two sides, its destruction are:(l) as erosion proceeds, the thickness of the overlying formation becomes smaller, or even it results in the objective interval coming out to the day in edge , consequently changes subsurface environment and increases velocity of hydrocarbon leakage;(2) upturning of beds causes readjustment of gas in Ordovician top weathering crust gas reservoir, because of good seal, the gas loss is very little. Its construction are (1): moderate erosion induces release of overpressured fluid in compartment, and forms secondary gas reservoir;(2): natural gas release from groundwater and crude oil, coal-bed gas desorption occurs, these gases can be a new additional gas source for accumulation.