U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
The Timan-Pechora Basin Province of Northwest Arctic Russia: Domanik – Paleozoic Total Petroleum System
Thermal Maturity of Source Rock
The primary Domanik source rock is thermally mature in most of the Timan-Pechora Basin Province (Figure 5 and Figure 6), but there is considerable disagreement as to exactly when thermal maturity was first reached in different parts of the province. In areas with deepest burial histories (e.g., foredeeps or west side of Pechora-Kolva Aulacogen) or where the thermal gradient has been higher, oil generation began earliest.
According to Ulmishek (1982) and Martirosyan and others (1998), earliest generation was late Early Permian (Figure 8), but Shimansky and others (1995) suggest Late Carboniferous and Pairazian (1993) promotes Early Carboniferous (Figure 7a, Figure 7b, Figure 7c, Figure 7d, Figure 7e, and Figure 7f). Gas generation began as early as Late Permian to Early Triassic time (Ulmishek, 1982). In contrast, on more stable platform areas, oil generation might not have begun until the late Triassic to early Jurassic periods, with the process halting during later uplift and never reaching the stage of gas generation (Ulmishek, 1982; Dedeev and others, 1994; Schamel and others, 1994; Abrams and others, 1999). Most authors believe that Domanik hydrocarbon generation post-dates or overlaps the timing of trap formation resulting from Carboniferous-Triassic structural inversions and orogenies, but Pairazian (1993) proposes significant Early Carboniferous Domanik generation – at least partly pre-dating major trap formation. Oil generation had probably peaked by middle Jurassic time (Schamel and others, 1994; Abrams and others, 1999).
Lower Permian potential source rocks have only reached peak-generation stage to oil in the Uralian foredeeps (Ulmishek, 1982) and perhaps in parts of the Pechora-Kolva Aulacogen. Potential Silurian source rocks generated oil as early as Late Devonian to Early Carboniferous time, post-dating or coinciding with only the earliest inversion and trap formation (Schamel and others, 1994). By the time of the Hercynian and Early Cimmerian orogenies, deepest Siluro-Ordovician source rock would have been generating wet gas.
TRAP STYLE OF OIL AND GAS FIELDS
Trap Style, Characteristics, and Development
Nearly 90% of Timan-Pechora known ultimately recoverable reserves are reported to be in structural traps (Petroconsultants, 1996) –
|including 95.6% of reserves in sandstones
and 84.6% of reserves in carbonates (Table
2) – but stratigraphic targets are underexplored. The pre- and post-Middle
Devonian unconformities provide significant potential trapping mechanisms
Hercynian (latest Paleozoic) Ural foredeep basins and Early Cimmerian (early to middle Mesozoic) Pay-Khoy counterparts contain thrusted, blind-thrusted and duplexed anticlines. Total shortening in the thrusted regions ranges from 60 to hundreds of km, and prospective subthrust anticlines can extend 15 km or more beyond the westernmost surface expressions of the thrust belt (Sobornov and Bushuyev, 1992). Mostly, only hanging-wall anticlines with surface expressions have been tested with wells, and they produce hydrocarbons at depths ranging from 1.5 - 4.5 km (Sobornov and Rostovshchikov, 1996), including the largest Timan-Pechora producing gas field, Vuktyl. Folded and faulted, pre-Artinskian (middle Early Permian) shelf carbonates – potential reservoirs at 1-5 km depths – underlie their westward-thrusted, slope-facies, age equivalents. Detachments occur in Artinskian and Kungurian shales and evaporites. Those shales are thermally mature locally and provide both source rock and seal potential. Additional detachments in Upper Devonian to Lower Carboniferous shales and in Upper Ordovician evaporites create more complex structural deformation in the Kosyu-Rogov and Korotaikha foredeeps to the northeast.
Structural inversion on the margins of the Pechora-Kolva and Varandey-Adzva Aulacogens began by middle Carboniferous time and continued intermittently throughout the Hercynian and Early Cimmerian orogenies. Such activity ultimately created anticlines with as much as 1000 meters of relief (Schamel and others, 1992; Schamel and others, 1994). Numerous, large producing fields – including the two largest oil fields in the province, Usa and Vozey – produce from these anticlinal trends. The Pechora-Kolva Aulacogen is also prospective where it projects southeastward under the Uralian foredeep.
Shallow structural closures in the more stable Izhma-Pechora Depression have 20-100 meter amplitudes, and one of the largest closures there is 4x20 km in area (Ulmishek, 1982). Paleozoicand Early Mesozoic stratigraphic and combination traps against old normal faults, paleo-horsts, and several basement arches (e.g., Bolshezemelsky Arch, Lay Swell) are expected throughout the province (Dedeev and others, 1994; Aleksin and others, 1995; Sobornov and Yakovlev, 1996).
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