Flowing Mechanism Of Fault Fluid In Foreland Thrust Belts And Its Significance In Petroleum Geology

Faults formed during tectonic movement play an important part on fluid flowing and hydrocarbon accumulation. Nowadays, the research of issues include the part and capability of vertical transporting fluid of faults during the activity stage, the active faults influence on hydrodynamic field, and their relationship with hydrocarbon accumulation is weak and the further study is necessary. Thus, the studying of fluid flowing mechamism in forland basin and its influence on hydrocarbon migration and accumulation has not only great theoretical significance but also contributing to improving the exploration level.Combining observation of outcrop in Longmen Mountain thrust belt of Northwest in Sichuan province, Kelasu thrust belt under the typical tectonic compressive stress invironment in Kuqa forland basin is the main study area. Focused on the vertical transporting fluid parts of fault and their distribution prediction in the activity stage, the hydrodynamic field after faulting, the transient flow field during faulting, the estimate of vertical transportation capability of faults in the activity stage, and their relationship with hydrocarbon accumulation, the flowing mechanism of fault fluid and its significance in petroleum geology are tried to state.Based on outcrop observation and the samples tests, fluid flowing related with faulting is identified and the flowing regulation of fault fluid is analyzed. Finally, the favorable vertical transportation parts of compressional faults are summarized. Finite-element method based on geomechanics is used to restoring regional stress field during faulting by ANASYS10.0. Then, combining the force mechanism analysis, the distribution of favorable vertical transportation parts of faults is predicted. The hydrodynamic after faulting is restored by numerical simulation of overpressure stemming from undercompaction, the model of increasing pressure stemming from the maximum horizontal principal compressive stress of tectonic, the calculation of overpressure transferred by faults. The transient flow field during faulting is characterized by the software of Processing Modflow. During the modeling, hydraulic conductivity, the key parameter of faults in activity stage is extracted. Then, the flow coefficient of faults, which characterized the transportation capability of faults and considered the permeability of faults and hydrodynamic, is calculated. Finally, by using above research results and combining geochronology of hydrocarbon accumulation, hydrocarbon distribution and difference of oil and gas reserves, etc., the relationship between hydrocarbon accumulation and opening faults during activity stage is analyzed. The main research results are as follows:1) The damage zones of faults are the favorable pathway of fluid vertical migration, which usually develops in the zone of stress perturbation. The zones of plane stress perturbation in Kelasu thrust belt mainly distribute in the fault transformation belts, fault segment intersection and fault termination, which also are the favorable part of fault damage zone development and vertical fluid migration.2) The hydrodynamic after faulting in Kelasu thrust belt have multiple low and high potential zones. The low gas potential zone of structural high part in north moves towards the south. As a result, the gas potential of north is higher than the south. The overpressure transfer by faults results in enhancing the regional gas potential and reducing the gas potential of strata in which the overpressure is transferred by faults.3) In the transient flow field during faulting, the upwarp areas of high gas potential are the releasing areas of fluid and the sinking areas of low gas potential in the shape of letter "U" are the areas of pressure releasing and fluid suction. The potential gradient gas is decreased along fault.4) Influenced by the strength of tectonic compression, the flow coefficient of under-salt faults in the north is greater than the south and the coefficient in Kelasu area is greater than Dabei area. Between the two areas, the coefficient in Dabei area is determined by hydraulic conductivity and the coefficient in Kelasu area is determined by both hydraulic conductivity and hydrodynamic.5) By comparing the difference of gas accumulation influenced by faults with Dabeil and Kela2gasfield, the characteristic is that the gas distribution of Dabeil gasfield is controlled obviously by the parts of vertical transportation and the gas has the characteristic of vertical migration and accumulation, and the gas of Dabeil gas oilfield mainly migrate and accumulate laterally. The pump sucks function generated from transient flow field enhances the efficiency of fault transformation and hydrocarbon accumulation. The weighted average number of the flow coefficient of faults related with the formation of each gas field indicates that the Kela2gas field is three orders of magnitude higher than Dabei1gas field, which is probably primary cause of the difference in gas accumulation efficency and proven gas reserves. When the fault cut off the salt detachment strata which are above the depth in conversion of plastic to stiffness, the reservoir would be destroyed, and the hydrocarbon accumulation efficency would be decreased.