The geothermal formation mechanism in the Gonghe Basin: Discussion and analysis from the geological background

摘要

The Gonghe Basin,a Cenozoic down-warped basin,is located in the northeastern part of the Qinghai-Xizang (Tibetan) Plateau,and spread over important nodes of the transfer of multiple blocks in the central orogenic belt in the NWW direction.It is also called “Qin Kun Fork” and “Gonghe Gap”.The basin has a high heat flow value and obvious thermal anomaly.The geothermal resources are mainly hot dry rock and underground hot water.In recent years,the mechanism of geothermal formation within the basin has been controversial.On the basis of understanding the knowledge of predecessors,this paper proposes the geothermal formation mechanism of the “heat source–heat transfer–heat reservoir and caprock–thermal system”of the Gonghe Basin from the perspective of a geological background through data integrationintegrated research-expert,discussion-graph,compilation-field verification and other processes: (1) Heat source: geophysical exploration and radioisotope calculations show that the heat source of heat in the basin has both the contribution of mantle and the participation of the earth''s crust,but mainly the contribution of the deep mantle.(2) Heat transfer: The petrological properties of the basin and the exposed structure position of the surface hot springs show that one transfer mode is the material of the mantle source upwells and invades from the bottom,directly injecting heat;the other is that the deep fault conducts the deep heat of the basin to the middle and lower parts of the earth''s crust,then the secondary fracture transfers the heat to the shallow part.(3) Heat reservoir and caprock: First,the convective strip-shaped heat reservoir exposed by the hot springs on the peripheral fault zone of the basin;second,the underlying hot dry rock layered heat reservoir and the upper new generation heat reservoir and caprock in the basin revealed by drilling data.(4) Thermal system: Based on the characteristics of the “heat source-heat transfer-heat reservoir and caprock”,it is preliminarily believed that the Gonghe Basin belongs to the non-magmatic heat source hydrothermal geothermal system (type II21) and the dry heat geothermal system (type II22).Its favorable structural position and special geological evolutionary history have given birth to a unique environment for the formation of the geothermal system.There may be a cumulative effect of heat accumulation in the eastern part of the basin,which is expected to become a favorable exploration area for hot dry rocks.