TECHNOLOGY EXPLAINED Laptev Sea: A Frontier Arctic Basin The Laptev Sea Basin is a frontier Arctic basin explored by regional seismic only. Its thick sedimentary cover is thought to contain numerous potential reservoirs, seals and mature source rocks, and structural traps are probably abundant. The Lena River Delta is of special interest for hydrocarbon entrapment. Edward Dongarov, IHS Energy mouth of the Khatanga River eastward to in the basin. In 2004-2006, SMNG and MAGE The Laptev Sea Basin – covering an area the Omoloy River. It includes part of the acquired additional regional datasets. comparable to the southern part of the Lena River delta, which extends over one Norwegian Barents Sea – lies almost entire- hundred kilometres into the sea. Geological Setting and Basin Evolution ly on the continental shelf of Laptev Sea, The Laptev Sea Basin's sedimentary The basement of the Laptev Sea Basin off the northern coast of Russia. It is situ- fill (including the Precambrian-Cambrian comprises a number of heterogeneous ated between the Taymyr Peninsula in the sequence) is estimated to be up to 13 blocks that were welded together during west and the islands of the Novo sibirsk km thick in the basin's deepest parts. the mid-Mesozoic (Late Jurassic) "Cimme- (New Siberian) Archipelago in the east. Its The basin's structure includes a series of rian" collision of the Siberian Platform and southern border is roughly coincident with rift troughs of north-western strike filled the pre-existing continent of Arctida (or the Siberian coast, and it extends from the with Upper Cretaceous-Miocene sedi- Hyperborea). These blocks have differing ments overlain by a Pliocene-Quaternary pre-Cimmerian sequences, but their off- cover. shore boundaries are not obvious on seis- Water depths in the basin typically are mic data, as the area has been tectonically Basin Boundaries less than 50 m, but along the northern mar- overprinted by Cenozoic rifting. The basin's western boundary is formed by gin of the basin there is a steep continental The most important structural elements the Taymyr Fold Belt. The fold belt is subdi- slope, where water depths increase to over comprising the basement include the off- vided into two systems, i.e. the North Taymyr 1,000 m. shore extension of the Siberian Platform with metamorphosed Upper Proterozoic clastic in western Laptev Sea (also known as the and volcanic formations; and the South Taymyr No Offshore Wells Laptev Fold System), and the offshore with Upper Paleozoic-Lower Mesozoic clastics The Laptev Sea Basin has been explored extension of the Verkhoyansk and the Nov- and Permian-Triassic volcanics. by geophysical methods only. Detailed air- osibirsk-Chukotka Mesozoic fold belts in To the south, the Olenek fold zone separates borne magnetic surveys were carried out the East Laptev. the Laptev Sea Basin from the Lena- Anabar over the islands, but at a much smaller From Ordovician to Early Carboniferous Trough and Yenisey-Khatanga Basin. East of scale offshore. The first refraction seismic time, the northern East Siberian continental the Lena Delta, the Verkhoyansk Fold Belt surveys were run in 1973, and later in 1979 margin evolved in a stable tectonic regime. forms the basin's eastern margin. Near the and 1985-1988. Ordovician-Middle Devonian sequence in coast, the north-south striking fold belt splits First CDP surveys were run in the mid the Laptev Sea Basin is thought to be com- into several branches running north-west and east. 1980’s. Between 1984-1990, a series of posed largely of shallow marine carbon- To the east, the basin's margin is formed regional lines were shot across the entire ates. Upper Devonian-Early Carboniferous by the Lomonosov-Svyatoy Nos uplift zone. It offshore. These and subsequent profiles sequence is represented by shallow marine extends from Cape Svyatoy Nos to the Lya- were shot on an irregular and sparse grid. clastics and carbonates. khovskiye and the Anzhu group of islands on The seismic data were interpreted using Towards the end of the Devonian, the the shelf and then on to the Lomonosov Ridge correlations with five stratigraphic test Cherskiy block broke off the eastern mar- Photo: Halfdan Carstens in the ocean. The Anzhu group of islands are wells drilled onshore close to the basin’s gin of the Siberian Platform initiating an part of the Kotelnicheskiy median massif. southern margin. oceanic basin east of the Siberian Platform. The northern boundary of the basin is In 1993, 1994, 1997 and 1998, Germa- A major submarine fan sequence known as defined by the shelf edge and continental ny’s Federal Institute for Geosciences and the Verkhoyansk Complex was deposited slope as it opens into the Eurasia basin of the Natural Resources in cooperation with in this oceanic basin between the Early Arctic Ocean. the SevMorNefteGeofizika (SMNG) Trust Carboniferous and the Early Cretaceous. In acquired a large data set of regional seismic Late Jurassic-Early Cretaceous time, it was t 36 GEO ExPro September 2007 folded by the collision of Siberia and vari- ous exotic terranes (Cimmerian collision). © HIS Laptev Sea: The initial phase of rifting causing the opening of the Eurasia Basin to the north of the Laptev Sea Basin took place in the Late Paleocene-Eocene. The spreading occurred along the Nansen-Gakkel ridge. A vast marine transgression took place in the Early-Middle Eocene. In the Oligocene-Early A Frontier Arctic Basin Miocene, the plate rotation pole shifted to the southern margin of the Nansen-Gakkel Ridge and a compressional episode took place. Extension resumed in the Middle Miocene-Middle Pleistocene. Petroleum Geology Source Rocks The main potential source rocks in the Laptev Basin are believed to occur in Permi- an and Triassic formations. As no wells have been drilled in the basin, all rock intervals described are from outcrops or wells on the mainland or from nearby islands, all of which lie outside the basin. The giant Olenek bitumen deposit in Per- mian formations is located on the Olenek high onshore northern East Siberia. In the early 1950’s, a gas show from a Permian interval was registered in an exploration well drilled near the River Olenek's mouth. Non-commercial flows of heavy oil from Permian-Triassic sediments were registered in wells near the Khatanga Bay. Triassic rocks from wells in the neigh- bouring Lena-Anabar Trough are largely Gross sedimentary thickness and structural elements of the Laptev Sea Basin. shales, with thin algal limestone interbed- ded. TOC and bitumen A content in Middle and Upper Triassic rocks are considerably and mudstones, Lower-Middle Devonian potential. The thick sedimentary cover, higher than in the underlying formations, carbonates and mudstones and Upper locally exceeding 10,000 m, is thought to with TOC values usually exceeding 2%. The Jurassic-Lower Cretaceous clastics. contain numerous potential reservoirs, kerogen is mixed humic-sapropelic. It is An active petroleum system in the Laptev seals and mature source rocks at various mature which is evidenced by the high Sea Basin is confirmed by surface seepages stratigraphic intervals. The presence of carbon content and low heterogeneous as well as seismic data. The composition Cenozoic rift zones is associated with high element content in bitumen. and stable carbon isotope data of adsorbed heat flow, favourable for hydrocarbon gen- Offshore, the Upper Paleozoic-Lower hydrocarbons from near-surface sediments eration. Structural traps are thought to be Cretaceous sequence is between 500-3,000 display a signature of thermally generated widely developed, and the Lena Delta and m thick, and its top occurs at 2,000 m or gas from a marine source rock within the paleo-delta are of special interest for hydro- deeper suggesting that potential source maturity range of peak petroleum gen- carbon entrapment. rocks in the sequence were able to reach eration. Seismic data show zones of poor maturity. Generally, Triassic rocks offshore reflectivity and chimneys suggesting there The Precambrian-Lower Cretaceous are thought to comprise marine clastics is ongoing hydrocarbon generation and Potential Precambrian-Lower Creta- deposited in reducing geochemical envi- migration at depths down to at least 2-3 ceous reservoirs could be associated with ronments, while Permian rocks comprise seconds twt. the regional unconformity surfaces. Such shallow marine clastics and volcaniclastics unconformities have been identified within with minor carbonate interbedded. Exploration potential the Precambrian, top Cambrian carbon- Other potential source rocks include Analogies with adjacent sedimentary ates, and at the boundary between Middle Precambrian1 -Cambrian clastics and car- areas suggest that the Laptev Sea Basin Devonian carbonates and Upper Devonian bonates, Ordovician-Silurian carbonates could have a substantial hydrocarbon clastics. 1 In Late Precambrian — a time period now called the Ediacaran that lasted from about 650 to 540 million years ago — macroscopic fossils of soft-bodied organisms can be found in a few localities around the world. Given the right geological conditions, horizons with abundant organic material may have turned into source rocks. t GEO ExPro September 2007 37 GEOSCIENCEV ignett EXPLAINED voir/seal combinations. © HIS No large structures/potential traps have so far been identified within the rift troughs. Potential traps here could be represented by drape basement structures. The best trapping potential appears to be associated with the trough flanks and the slopes of the highs separating the troughs. Expected mechanisms are onlaps, pinch-outs and 70% of the world is covered by facies changes. These are likely to be com- bined with structural control as there are numerous normal faults present in Upper Wavefi eld Inseis, the rest is land Cretaceous-Tertiary sequence. Potential traps associated with delta- ic and fluvial systems could be of major importance in the basin. Five long-living river systems, i.e.