The Presence of Weathering Crusts in the Top of the Pre-Jurassic Basement of the South of Central Uvat Is One of the Key Risks Affecting the Success of Drilling Exploratory and Appraisal Wells

The thesis of the report contains brief results of the study of variable genesis weathering crust in the south of the central Uvat. The case study of two wells where rhyolites were tapped in one well, and basalts were tapped in the second well, shows that the formation of metasomatic weathering crusts on different initial substrate (acidic or basic composition rocks) leads to great varietes in the mineral-petrographic composition and, most importantly, to varied thickness to date. The weathering crusts in the acidic lithology rocks, which have density and velocity characteristics similar to those of the sedimentary cover. Owing to this, in the wave field they form several independent reflections similar in their dynamics to the reflections of the sedimentary cover which allows predicting productive layers where they are actually absent. In this regard, to reduce risks when drilling wells in these areas the key issue is to map zones of weathering crust development along acidic rocks in the inter-well space in order to account problem zones during seismic correlation.