The Controlling Of Faults On Hydrocarbon Accumulations In Deep Basin And Its Mechanism In Kuqa Depression

As an important part of unconventional oil and gas resources,tight sandstone gas in the deep basin is of considerable potentials,and could be important areas for hydrocarbon exploration in the future.In recent years,it is demonstrated by the exploration of typical tight sandstone gas reservoir that the structure background is relatively complex.Specifically,faults due to multiple tectonic movements play an important controlling role in the complicated occurrence of tight sandstone gas.However,traditional theories fail to explain the mechanism of tight sandstone gas reservoirs in complex geological backgrounds,and thus fail to predict the occurrence.Based on comprehensive analysis of typical gas reservoirs,positive roles of faults in controlling the accumulation of tight sandstone gas are analyzed,which brings the controlling factors and principles respectively corresponding to construction.Then,methods are proposed to evaluate the positive roles of faults.After that,negative roles of faults in controlling the accumulation of tight sandstone gas are analyzed,especially the zone destroyed by fault in tight sandstone gas reservoir and its controlling factors.With the principles analysis,methods are put forward to determine the destroy zone caused by faults.Based on above works,favorable and unfavorable zones controlled by faults in Dibei area are assessed.Three critical geological conditions are demonstrated to promote the accumulation,namely faulting strength,spatial configuration of faults and seal rocks as well as the distance of tight reservoirs to the fault surface.In the condition that faulting rate is less than a critical value with the presence of effective cap rocks,gas should be more enriched in the place near faults,forming �sweet points�.The dual media formed by fault reduces the filling resistance(capillary pressure),improve the permeability in tight sand,and the secondary filling points deveopled in tight sand transform the planar filling with low-efficiency into the high-effective composite filling model with both source-reservoir boundary and faults,which makes the buoyancy and gaseous tension work together in fault.The model used to evaluate the acceleration of tight-sand gas with fault suggests the configuration of faulting strength,spatial configuration of faults and seal rocks as well as the distance of tight reservoirs to the fault surface determine the gas saturation in tigh-sand reservoir.Also,this model can be used to predict the gas saturation,and predict the favorable area and �sweet points� as well.In fact,the damage of fault to tight-sand gas reservoir is to destory the critical buoyancy threshold.The relatively high pore throat radius and permeability in deformed zone can't meet the conditions for the critical buoyancy threshold,which destroys the tight-sand gas.The fault throws control the range of damaged zone in tight-sand gas reservoir,and larger the fault throw,wider the damaged zone.The range of damaged zone can be defined with the width between fault surface and the points where the permeabilities of fault and rock are equal to each other.It is identified that faults contribute to deplete reservoirs by destroying the lower limit of buoyancy effect.When it comes to the fault displacement,larger displacement corresponds to stronger destruction.When the permeability of fault belts equals to that of wall rocks,the width of corresponding fault belts is the range that fault can destroy the critical buoyancy threshold.Favorable areas for tight-sand gas are distributed in the slope bbetween F1 fault and F3 fault,eastern F5 fault and F7 fault.Due to the late high activity rate and absence of seal rocks,accumulated tight sandstone gas reservoirs are destroyed by F1 fault and F2 fault.Taking the F1 fault as the example,the destruction range around Yishen 4 well is roughly 1200 m,which is verified by the drilling.Therefore,proposed evaluation models can objectively evaluate the controlling influence of faults on tight sandstone gas reservoirs in the deep part of basins.