PETROLEUM POTENTIAL OF THE WEATHERED, FRACTURED, AND HYDROTHERMALLY ALTERED BASEMENT RESERVOIRS: CASE STUDY FOR UKRAINE

Abstract

The weathered crystalline crust (WCC) is a transitional geobody occurring between the intact basement and sedimentary cover and representing the result of complex geological and geochemical processes called weathering that impacted the topmost part of the crystalline bedrock. WCC studies are important for useful ore minerals and hydrocarbon exploration. Until present, around 450 of the discovered petroleum fields in the world are solely or partly attributed to the weathered, fractured and altered basement rocks of different consolidation age and known for all continents excluding Antarctica. Much more petroleum production comes from weathered and altered volcanics, and metasedimentary Precambrian, Paleozoic, and Mesozoic rocks, usually considered incorrectly as barren ones. Giant oil and gas fields produce crudes from crystalline basement and granite wash in the USA, Venezuela, Brazil, Libya, Kazakhstan, Russia, China, Yemen, Vietnam, etc. The commercial discoveries of oil and gas took place during the last decades in the weathered basement along the buried Northern flank of the Late Devonian Dnieper-Donets paleorift as well in Ukraine. Some basement reservoirs are characterized by unique geochemical features and may contain commercial reserves of noble gases like helium. A sophisticated new technique applying a joint inversion of seismic and gravity data and adjusted by other geological and production data allows building of a geodensity model of a WCC reservoir and confident delineation of prospective basement reservoirs.