Known Jurassic sandstone reservoir rocks are thickest and best in reservoir quality on paleohighs (Zakharov and Yunov, 1995). The three fields producing from Jurassic reservoir rocks have net pay thicknesses ranging from 8-76 m, porosity ranging from 15-25% and permeability ranging from hundreds of millidarcies to more than one darcy (Zakharov and Yunov, 1995; Petroconsultants, 1996). There are four stacked reservoir sandstones at the largest field, Shtokmanovskoye. Jurassic reservoir rocks are underexplored, with but little drilling to evaluate their potential.

Other Reservoir Rocks – Triassic and Cretaceous
Eastern Barents Triassic rocks record numerous transgressive/regressive cycles and possible erosional episodes (Johansen and others, 1993). Known reservoir rocks are more discontinuous than their Jurassic counterparts and are commonly overpressured. Sediment transport directions were predominantly westward and northward during a time of rapid basinal subsidence. A maximum Triassic thickness of 9 km characterizes the basin centers. Facies vary from fluvial/alluvial to deltaic to deep marine, and clinoforms on seismic data correspond to depositional water depths as great as 1200 m (Semenovich and Nazaruk, 1992).

Sandstone mineralogy is commonly lithic-rich, but still with "good" porosity (Oknova, 1993). The known fields produce from Lower Triassic sandstones (Charkabozh Formation) and are characterized by porosity ranging from 13-24%, permeability ranging from tenths to nearly 200 millidarcies, and net pay ranging from 3-12 m (Petroconsultants, 1996). Best prospectivity is at marine-to-continental facies transitions, but hydrocarbon recoveries are commonly low (20-30%) and reserves small (20-35 mmboe) from stratigraphic traps (Zakharov and Kulibakina, 1997). Triassic reservoir rocks are underexplored because of sparse drilling.

Cretaceous rocks (probably mostly Lower Cretaceous) have a maximum thickness of approximately 2 km in the eastern Barents region, and no economic accumulations have yet been found in them. They include widespread continental to shelf shales and sandstones that prograded southward in Neocomian time (Johansen and others, 1993). Thus, more proximal and sand-rich Cretaceous facies might be preserved in the sparsely drilled North Barents basin. Neocomian sandstones are important reservoir rocks in the West Siberian Basin east of the Novaya Zemlya archipelago. No information has been published regarding reservoir quality of Cretaceous sandstones in the eastern Barents region.

SEAL ROCK
Thick and widespread Mesozoic marine to continental shales are good to excellent local and regional seals in the eastern Barents basins. The major regional seal for this Triassic-Jurassic total petroleum system is an areally extensive, 400-600-m-thick Upper Jurassic to Neocomian marine shale with a mixed-layer clay mineralogy (Oknova, 1993; Zakharov and Yunov, 1995). Older Jurassic shales of local to regional areal extent have thicknesses of 50-300 m (Khain and others, 1993; Zakharov and Yunov, 1995). Triassic shales also provide good local seals (Semenovich and Nazaruk, 1992; Oknova, 1993; Zakharov and Kulibakina, 1997), as do Lower Cretaceous shales (Johansen and others, 1993).

ASSESSMENT UNITS
Three assessment units are used to characterize the South and North Barents Triassic-Jurassic total petroleum system for the purposes of resource prediction. No appropriate field growth function was identified for this petroleum system, and none was used in the assessment process.

The "Kolguyev Terrace" (#10500101) assessment unit is a 79,000-km² area in the northernmost, offshore Timan-Pechora Basin Province #1008 (including Kolguyev Island with 5% of the area), adjacent to and southeast of the South Barents physiographic basin and geologic province (fig. 1). Kolguyev Island contains two small fields producing oil and wet gas from discontinuous Triassic sandstone reservoirs in folded and faulted traps. Triassic and older source rocks are absent on the island, and lateral migration from the Triassic source rocks of the South Barents Basin is assumed. Stratigraphic traps also are expected. The area contains abundant and excellent Mesozoic shale seals. This frontier assessment unit is predicted to contain somewhat more gas reserves than oil reserves in reservoir rocks and traps similar to what have been discovered, but perhaps with additional Jurassic sandstone reservoirs. Significant lateral migration is required from the thermally mature shale source rocks in the South Barents Mesozoic depocenter. Water depths range from 0-80 m, and seasonal ice pack affects the area. 

The "South Barents and Ludlov Saddle" (#10500102) assessment unit includes the entire South Barents Basin Province #1050 (170,000 km²) and the northerly adjacent Ludlov Saddle Province #1059 (26,000 km²) (fig. 1). Four large, unproduced gas fields with Jurassic and Triassic sandstone reservoir rocks characterize this greater South Barents frontier assessment unit. Jurassic sandstones are more continuous, have better reservoir properties, and contain more known reserves than their Triassic counterparts; hence their inclusion in the total petroleum system name. Cretaceous sandstone reservoirs are also possible. Known traps are anticlines, and stratigraphic complexity is expected to also play a role in hydrocarbon accumulation. Source rocks are gas-prone and gas-and-oil-prone Lower Triassic shales thermally mature to oil on the basin margins and to gas in the basin center. Mesozoic shales are seals, with thick Upper Jurassic shales comprising a regional seal. Water depths range from 10-350 m, and seasonal ice pack affects the area. 

The "North Barents" (#10500103) hypothetical assessment unit includes the entire "boutique" North Barents Basin Province (#1060) (fig. 1). Reservoir rocks are presumed to be Mesozoic siliciclastics, with seals and traps similar to those in the South Barents and Ludlov Saddle assessment unit, but the Jurassic System might be shalier here than in South Barents. Thus, favorable rock attributes are at slightly greater risk than in South Barents and Ludlov Saddle, but source rock quality, thermal maturity history and migration scenarios are probably similar. Water depths range from 70-350 m, and seasonal ice pack affects the area.