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Introduction
Priobskoye, is an onshore oilfield located in the Khanty-Mansi Autonomous Okrug region of Russia and is located 65 km east of the capital city of the region, Khanti-Mansiysk. The field falls within the Middle OB Hydrocarbon region within the Wester Siberian Basin .The field occupies an area of 5,466 $$km^2$$ and a portion of the field is located on the banks of the Ob River. The oilfield is located within the Southern portion of the Western Siberian Basin, which is a major hydrocarbon basin in Russia. The Priobskoye field was first discovered in 1982, but was not drilled for production till 2000. The hydrocarbons in Priobskoye field unlike many other hydrocarbon fields is trapped within stratigraphic traps, instead of structural traps and is one of the largest stratigraphic trap hydrocarbon fields in the Western Siberian Basin. Initially projected to have recoverable reserves amounting close to 565 billion tons of oil.

History
The field was first discovered in 1982, after seismic surveys were undergone in the area from 1978-1979. But due to complicated underlying geology and environmental factors, i.e. floodplains of the Ob River and permafrost in the region, working in the area was not easily accessible. But in 1999 the Priobskoye oilfield started to really produce some output of oil and bring a rise in income to the Russian energy company YUKOS. YUKOS saw increases in income with the help of increased oil output from Priobskoye. The company became the #1 producer of oil in Russia for a few years. In 2003, the company reported that it doubled in its oil production, producing close to 6 million barrels per day from its oilfields. A portion of this success is attributed to the "Giant-Priobskoye" which is described as the "ace in the hole" due to it having been very underdeveloped prior to YUKOS involvement. The great reign of YUKOS with Priobskoye's success came to an end once the Oligarch and head of the company and at one point Russia's richest man, Mikhail Khodorkovsky, was arrested on accounts of tax evasion and money laundering, accusations of laundering around $27 Billion dollars. Due to this and other legal issues within the company, assets of the company were eventually all auctioned off. The asset of interest, Priobskoye oilfield, is now currently owned by two companies. The Northern and larger portion of the field is owned by the #1 current Hydrocarbon producing company in Russia, Rosneft, and the Southern smaller portion of the field is owned by a Russia Energy company, Gazprom Neft.

Basin Formation and Fill
Priobskoye field is in the Western Siberian Basin, which is an Intra-Cratonic basin and one of the largest in the world covering approximately 3.5 million km squared. The basin basement formed during upper Paleozoic after the collision of the Russian Platform, Eastern Siberian Platform and the KipChak Arc complex. This collision formed the Ural Mountains. Following the collision and forming of the Ural Mountains, the Siberian Flood Basalts were deposited all across the basin around 250Ma. These flood basalts are associated with large Mantle Plumes that developed under Eurasia, which resulted from believed "global plate boundary reorganization". Following the deposition of the flood basalts, during the early-mid Triassic period the basin rift-graben system developed with a north-south orientation. This active rifting stage came to an end in the Late Triassic. At this point the plumes are believed to possibly be related to lithospheric thinning and "magmatic thinning" of the crust. This thinning led to periods of great subsidence starting during the Jurassic period and continued till Tertiary. During the Jurassic period the basin saw increased rates of sedimentation. It was during the late Jurassic period, the basin experienced much higher sea level and becomes a deep water marine basin, it was at this point that the Hydrocarbon regional source rock formation, the Bazhenov Formation, was deposited. Following the the Jurassic the basin saw a decrease in water level and certain portions of the basin became deltaic environments and the deposition of the Neocomian age sandstone, siltstone and shale interbedded formations are deposited. Following the deposition of the Neocomian age sediment, the basin saw many other transgressive and regressive phases and had great periods of sedimentation fill mainly deposition of marine and continental deposits Aptian-Eocene period. The Oligocene till more recent sedimentation has mainly been composed of continental and lacustrine deposits.

Regional Source Rock
At Priobskoye Field one source rock for the hydrocarbon is the Late Jurassic deposited Bazhenov Formation. The Bazhenov Formation was deposited during a period of high seas when the basin was experiencing a deep water marine basin. The Bazhenov Formation is comprised of mainly thinly laminated to massive, siliceous, carbonaceous shales. The environment and corresponding lithofacies associated with Bazhenov Formation deposition can be seen in the Lithofacies and Isopach map of Bazhenov Formation and stratigraphic equivalent units figure.The Bazhenov Formation in the area of Priobskoye field has high Total Organic Content, stemming from plankton and bacteria in the sediment, in the central part of the basin near Priobskoye levels of TOC mainly range from 7-15% and can sometimes be higher. The Bazhenov Formation in the area produces kerogen of type II.



Source and Reservoir
The Neocomian age sediment is the main reservoir and source at Priobskoye field. This sediment is deposited after the basin stopped exhibiting deep water marine environments. The area experiences an overall loss in sea level and the Neocomian age sandstones are deposited during this overall period of regressions. The area of Priobskoye field went through many rapid changes between Highstand System Tracts(HST) and Lowstand System Tract(LST) phases leading to the clinoformal geometry of the beds during high sea level would bring about deposition of organic rich shales and then after deposition environment would shift. These small transitions gave way to the source and reservoir that is present at Priobskoye field. The general trend for the Neocomian age sediment is a transitioning from deep water basin to shoreline and deltaic environment with deposition of mainly sandstones, siltstones, shales, and as well turbidites. Along these clinoform beds is where the hydrocarbon lies, at the toe of the beds above the Pimskaya shale are the turbidites, the "Achimov Formation". The Achimov sediment serve as one important reservoir and source from the Neocomian age deposited sediment. It was deposited during a LST conditions as submarine fans and interbedded shales that are very organic rich .Working up the clinoforms other Neocomian age strata are composed of siltstones, shales and occasional sandstone beds which represent the slope deposits along the clinoform. The topsets of the clinoforms represent Neocomian sandstones and shale interbeds. These sandstones, turbidites and occassional organic rich shale beds deposited from these HST and LST transitions along these clinoforms act as the source and reservoirs for the Priobskoye Field. The porosity of the sandstones within these clinoforms ranges from 15-25%, while permeability ranges from .1-100 millidarcies, but on average the area is experiences around the 1-10 Millidarcy range. The Neocomian age sediment is capped on the bottom by the massive Transgressive phase deposition of the Pimskaya Shale and is capped on the top by the massive transgressive phase flooding Bystrin shale.

Stratigraphic Traps
The Oil from the Bazhenov is considered to be "tight oil" or unconventional and requires use of hydraulic fracturing for best rates. These hydrocarbon reservoirs are deposited during the regressions of the Neocomian period as mentioned before. This led to the deposition of the clinoforms when the delta system is prograding. The source and reservoir Neocomian age sandstones and turbidites and the source of Bazhenov formation can be read in the above sections. The trapping mechanism for these hydrocarbon of Priobskoye field occur as marine ravinement events occurred as the along these clinoforms during periods of transgression and regressions. These ravinement events along the clinoforms erode sediment and create seal as erosion occurs, getting rid of porous sandstone reservoirs and depositing impermeable shale beds. These hydrocarbons are trapped in lenses of sandstone along these clinoform beds as depicted in figures 3 and 4 and are trapped in these clinoforms due to transgressive flooding shales like the Pimskaya and Bystrin shale. The Bazhenov formation is as well a source of hydrocarbons at Priobskoye, the hydrocarbons unconventionals and are pools of oil due to the Bazhenov Formation creating pools of oil. The Neocomian age sediment is the most explored and utilized sources in Priobskoye's operating history.