Thermal regime of a geothermal system considerably depend on availability or absence of free fluid convection in it. Other side geothermal systems are characterized by significant heterogeneity and, specifically, permeability and other parameters may change from layer to layer . So, estimations of permeability coefficients of geothermal systems, accordmg to reference data, differ by several orders [3] . Usually for estimation of convection onset in these systems the value of the critical Rayleigh
number 4n2 is used, talang place for uniform layer [2]. In case of significantly heterogene layers the values of the critical Rayleigh number, calculated by mean permeability, as will show below, may considerably differ from 4n2. In connection to indicated and other factors the problem about onset of fluid convection in a geothermal stratum, consisting of two horizontal sublayers with different permeabilities is considered. The parametrical investigation of the probem in dependance on permeability and thickness of layers is carried out. It is necessary to mark that permeabilities of real geothermal strata may have and essential anisotropy. Influence of uniform anisotropy in the simplest case was investigated by Ephene [l]. Understanding of quntitative and qualitative influence of indlcated and other factors on origin and structure of a stream in a geothermal system will assist to interpretate the geophysical experimental data, specifically, by heat flow. As it has been shown in [3], convection in geothermal systems is not exclusion, but aides and induces a positive anomaly of heat flow on the Earth's surface above a geothermal system.

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