The Lena-Vilyuy Region extends over an area of about 300.000 sq km of the northeastern margin of the Siberian Platform and lies wholly within the Yakutsk-Sakha Republic of the Russian Federation. It is part of the Khatanga-Vilyuy Petroleum Province and extends over the middle and lower portions of the valley of the River Lena as well as over the lower courses of its two largest confluents - the Vilyuy and Aldan Rivers. The Lena-Vilyuy Region coincides with the Lena-Vilyuy basin - a composite geological feature consisting of two distinct tectonic elements lying in the east and west of the Lena River. To the east, the Predverkhoyansk megatrough represents a foredeep of the Verkhoyansk Range which was folded in Cretaceous times, whilst to the west, the Vilyuy syneclise is a large and shallow embayment which makes a deep encroachment into the Siberian Platform. The Lena-Vilyuy basin contains important gas and gas-cond ensate accumulations, and geological and geophysical exploration activities have led to the discovery of ten fields which are considered to contain reserves of proved economic potential. Of these ten fields, two lie at the margin of the Predverkhoyansk trough and the others are located in the Vilyuy syneclise. The geological succession of the Lena-Vilyuy basin begins with an infrabasm platform sequence of shallow-water carbonates, ranging in age from the Riphean to the Lower Carboniferous and resting on a crystalline basement. Above the carbonates, the main basin fill is composed of a rather monotonous sequence of alternating elastics with conglomerates, sandstones, siltstones and shales being deposited in a range of environments which include shallow-marine, paralic, coastal-deltaic, fluvial and paludal-continental. Main sources for the detrital material making up this sedimentary infill are considered to have been elevated portions of the Siberian Platform lying to the west and south of the Lena-Vilyuy basin and - from earliest Cretaceous times - a contribution from the Verkhoyansk Mountain Range which was rising to the east. Abundant carbonaceous matter and frequent coal beds constitute excellent source material for gas, with the highest concentrations of the essentially humic organic material found in the Permian. Adding to such conditions favourable for hydrocarbon generation, a pattern of sedimentation developed which led to repeated interfingering of predominantly argillaceous source rocks with coarser clastic reservoir rocks. Sealing units also characterize such sequences. Most hydrocarbon accumulations in the Lena-Vilyuy basin are therefore found as multi-pool fields. The most important of the reservoir rocks are those which developed as part of the clastic sequences of Late Permian, Early Triassic or Early Jurassic age. Most characteristic trap type is one showing a gentle anticlinal nature. Normal faulting appears to have played only an insignificant role in the development of hydrocarbon accumulations in the Lena-Vilyuy basin. The first field discovered in this basin was the Ust'-Vilyuy accumulation which was found in 1968. It marked the start of a series of subsequent discoveries which to date has resulted in 15.0 billion cu m gas being produced in the region. Gas and condensate production continues in the Talon-Mastakh field and in the Middle Vilyuy field. Gas production from these two accumulations during 1991 was 1.245 billion cu m gas. Since the overall hydrocarbon potential of the Lena-Vilyuy basin exceeds the needs of the Yakutsk-Sakha Republic, however, exportation of produced gas to the Russian Far East and to Southeast Asian countries would be a future possibility. To achieve this, considerable investment in long-distance pipeline construction would be required.