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U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY


South and North Barents Triassic-Jurassic
Total Petroleum System of the Russian Offshore Arctic
On-Line Edition

by

Sandra J. Lindquist

OVERBURDEN ROCK AND THERMAL HISTORY
Triassic rocks possibly reach a maximum of 8-9 km in thickness (rifting origin?), and facies range from continental and deltaic to submarine canyon (Ryabukhin and Zinin, 1993). Total thickness and facies vary significantly areally, and interbedded sills are common in the basin centers. The regional thermal gradient was probably highest during this geologic period because of rifting and the associated magmatic activity. Overburden sequences in basin depocenters contain approximately 2 km each of Jurassic and Cretaceous rocks mostly siliciclastics of shallow marine origin. Preserved post-Cretaceous strata are typically less than 1 km thick, but Cenozoic uplift resulted in significant thermal cooling and possibly as much as hundreds of meters of eastern Barents erosion. It is possible that there was Paleogene deposition and Neogene uplift and erosion. Prior to Cenozoic cooling, the oil generation window ranged from about 2 to 8 km deep in the basin centers (corresponding to 60° 150° C, fig. 2a) (Ostisty and Cheredeev, 1993). Considering the higher Triassic thermal gradient, hydrocarbon generation from Lower and Middle Triassic source rocks probably began by Late Triassic time.

TRAP STYLE
All discovered fields and wildcat wells are on structural closures, but many structural closures remain untested. Some basin-scale folds and faults relate back to the Uralian (Permo-Triassic) orogeny and the Early Kimmerian (Early Jurassic) orogeny. Penecontemporaneous folding and faulting during Triassic rifting and subsidence affected the facies distribution and initiated formation of the Ludlov Saddle. Uplift associated with the Cenozoic opening of the Arctic Ocean and/or with glacial isostacy resulted in the erosion of post-Neocomian strata and in further structural deformation. Uplift and pressure decrease also caused the expansion of existing gas accumulations, remigration of trapped hydrocarbons, possible loss of seal integrity, and local halting of hydrocarbon generation.

Jurassic reservoir sandstones are stratigraphically more continuous and generally better in reservoir quality than their Triassic counterparts. Thus, some Jurassic accumulations could be characterized as true structural closure or faulted structural closure traps. Ninety-seven percent of the known recoverable reserves in the eastern Barents region are in Jurassic reservoir rocks (Petroconsultants, 1996). 

Undertested are the undrilled structural closures and fault traps, drapes, stratigraphic onlaps, stratigraphic pinch-outs, stratigraphic and structural erosional traps, and diagenetic traps for all potential reservoir horizons in Triassic, Jurassic and Cretaceous sandstones.

Discovery History
Fields of the Triassic-Jurassic total petroleum system in the eastern Barents basins were discovered between 1982 and 1991 (table 1), but only one (Peschanoozer) has been developed. The first four fields, discovered between 1982 and 1988 in the southern regions, had Triassic reservoir rocks (fig. 3). Peschanoozer (1982) on Kolguyev Island in the northern Timan-Pechora offshore produces oil, gas and condensate from Lower Triassic sandstones. North Kildinskoye (1983), just west of the South Barents Basin province boundary, and Murmansk (1984) in the southern South Barents Basin contain dry gas in Lower to Middle Triassic sandstones. Tarkskoye (1988) on Kolguyev Island in the northern Timan-Pechora offshore produces oil from Triassic rocks.

In 1988, Shtokmanovskoye was discovered in the northwestern South Barents Basin (fig. 3). This field contains gas and some condensate in four Middle and Upper Jurassic sandstone horizons. Published reserve figures include 141 TCF (Oil and Gas Journal, 1990), 88 TCF (Dore, 1995) and 117 TCF (Malovitsky and Matirossyan, 1995), making it the largest province field in terms of reserves.

Ludlovskoye, discovered in 1990 on the Ludlov Saddle, tested gas and minor condensate from Middle and Upper Jurassic sandstones. Ledovoye, discovered a year later between Shtokmanovskoye and Ludlovskoye, tested gas from Middle Jurassic sandstones. Prisyazhniy (1995) reported 177 TCF reserves for all five offshore eastern Barents fields (excluding the two Kolguyev Island fields).


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U. S. Geological Survey Open-File Report 99-50N