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= Carabobo Field =

= West Siberian Basin = The West Siberian Basin is a rifted basin that formed around the Permian-Triassic boundary. Geographically, it occupies the West Siberian Plain located in modern day Russia. The basin is bounded on the west by the Ural belt, on the east by the Siberian craton and Taymyr uplift, on the south by the Kazakh and Altay-Sayan uplifts, and on the north by the North Siberian sill. It is the largest flat land area in the world at roughly 1.3 million square miles, with elevations rarely exceeding 100m. It has a maximum sedimentation thickness of about 15km in the northern part of the basin, with an average thickness of 3-10km. It is well known for being the largest hydrocarbon basin in the world.

Tectonic Setting
The basin is a relatively undeformed Mesozoic sag that overlies the Early Triassic rift system. As an intracratonic basin, the history of its development is complex. However, study of the basin's basement, flood basalts, and stratigraphy have led researches to an understanding of the complex tectonism. From the research, it is understood that there was an abrupt change in the tectonism around the Late Cenozoic, likely due to the colliding Indian and Eurasian plates. This compression was important not only for the extensive folding and faulting that resulted, but also resulted in slight uplift of the basin.

This uplift drastically reduced sedimentation rates in the basin, as the basin was raised to sea level. While there was an extremely thick Mesozoic section of sediment deposited in the basin, the recent sedimentation rates have deposited thinner sections. As rifting in the basin ceased around the Late Jurassic, the tectonism of the basin has been characterized by post-rift subsidence, continuing to modern times. The basin-wide subsidence is a product of thermal re-equilibration of the lithosphere-asthenosphere system due to the mantle plume that resulted from rifting in the basin.

Stratigraphy
Wells have shown the basement to be composed of Late Carboniferous-Permian buried arcs, accretion complexes and microcontinents, similar to the geology of the Urals and  Altaid mountains surrounding the basin, rather than Precambrian shield geology of the neighbouring cratons. Exposures of Late Paleozoic-Mesozoic strata have been examined, found to contain mostly fluvial conglomerates and breccia. From these sediments, it was determined that the majority of sediment derived from the Altaid mountains to the South. The sedimentary succession in the basin is composed of fluvial conglomerates and breccia from the Middle Triassic to Tertiary. Middle Triassic sediments can only be seen in the northern part of the basin, while the southern areas are overlain by Toarcian and younger sediment. This progressive aging in sediment suggests that the depositional area gradually expanded southward to the present day basin boundary. Owing to the nature of the basin-floor subsidence in the basin, the stratigraphy is largely influenced by a repeated pattern of marine transgression and regression. Thus, sandy marine clinoforms were repeatedly blanketed by marine shales. These cycles produced very productive hydrocarbon provinces, which are generally classified in three major stages.

The most important stage in the stratigraphic development of the West Siberian basin was the formation of a deep sea in Volgian-Early Berriasian time. The Bazhenov formation was deposited during this time period, which is a highly organic-rich siliceous shale. In the Neocomian, a period of regression ensued, which deposited prograding clastic rocks that served as the primary reservoir for the large amount of oil reserves in the basin.

Another important transgression event occurred in the Turonian which deposited another organic-rich shale, the Kuznetsov formation. This formation is overlain by sandstone beds that are very gas-productive, and capped by a predominantly-shale Berezov formation.

The third most productive formation is the Tyumen formation, which was deposited in the Lower Jurassic. Deposited towards the beginning of the basin-floor subsidence, it is not as productive as the other two formations.

Evolution History
There are 4 major tectonic events associated with the formation of the basin. The general consensus is that the basin began rifting around the Permian-Triassic boundary. The basin evolution has been modeled in the tectonic maps below.

Permian Event
In the Late Permian, the collision of the Siberian, Euramerican and Kazak plates formed the Eurasian portion of the supercontinent Pangaea. This collision formed the Ural orogenic belt, however the northern portion of the continent was much thinner due to the squeezing of the Kazak plate underneath, which ultimately became the West Siberian platform, a deep-water basin with relict oceanic crust of the paleo-Asiatic ocean.

Early Triassic Event
Due to the breakup of Pangaea during this time, the West Siberian platform began colliding with the arctic to the North. The result of this North-South collision was East-West extension along the platform, due to the thin, weak lithosphere of the region. This extension led to thinning of the lithosphere and rifting, which in turn led to the rise of mantle plumes, formation of grabens and dolerites, and extensive faulting along the platform. Analyses estimate the extensional beta factor of the rifted basin to be around 1.6.

Jurassic Event
At the end of the Jurassic, the rifting began to cease due to tectonic processes in the Arctic Ocean. Due to large amounts of sedimentation into the basin from the uplifts around the basin margins, post-rift subsidence resulted in a very thick Mesozoic section. The Mesozoic section is multiple kilometers in thickness.

Cenozoic Event
The Late Cenozoic was marked by extensive folding and faulting in the basin. The tectonics of this time period is widely considered a direct cause of the Indian plate's collision with the Eurasia continent around 50 million years ago. The folds of this time period become crucial to hydrocarbon exploration, as the anticlinal structures in the basin serve as the primary trapping mechanism.

Siberian Flood Basalts
The Siberian flood basalts form the world's largest igneous province (LIP) in the world. Eruption began in the Late Permian-Early Triassic, depositing over an areal extent of 5x10^6 square kilometers. An example of passive rifting, the rifting in the West Siberian basin led to the rise of mantle plumes. Modeling has been conducted to determine the size of the mantle plume. To do this, subsidence patterns from one-dimensional conductive heat flow models were compared to observed subsidence form backstripping studies. The results determined that the plume head had an initial thickness of 50km, with an initial temperature of 1500°C. It also found that it was situated around 50km below the surface with an initial crustal thickness around 34km.

The flood basalts were deposited in massive beds, as well as in grabens generated throughout the basin. Radiometric aging of the volcanic rocks suggest the eruption lasted a very short duration, with most estimates around 1 Ma. Meanwhile, magnetostratigraphy of the lavas estimates that the duration was much longer, with a possible duration of several million years.

Since the eruption of these flood basalts coincides with the Permian-Triassic extinction event, many people have suggested that it is a possible cause of the extinction.

Hydrocarbon Exploration
While most of the oil in the West Siberian basin has been discovered, it remains as Russia's richest petroleum province. 355.6 billion barrels of oil equivalent (BBOE) have already been discovered in the basin, and is ranked 1st among 128 provinces designated for reappraisal by U.S. Geological Survey. The basin contains several giant oil and gas fields, including the Samotlor Field with nearly 28 billion barrels of oil in place, and Urengoy Field with 350 TCF original gas in place. There are three major hydrocarbon complexes in the West Siberian basin: Jurassic, Neocomian, and Turonian.

The Jurassic complex encompasses any hydrocarbons originating from the Togur and Tyumen formations. Containing 8.4 percent of the basin's reserves, it mainly contains oil with around 11.8 billion barrels. Most revervoirs are clastic sandstones underlying the Tyumen formation, as the sandstones above the Tyumen formation have lower porosity. Thus, most reservoirs are sealed by the Tyumen formation, with the rest sealed by the Bazhenov formation that lies above it.

The Neocomian complex is the most productive of the three. Encompassing any hydrocarbons originating from the Bazhenov formation, it contains more than 80 percent of the oil reserves in the basin at 117.5 billion barrels of oil and 97.6 TCF of gas. Most reservoirs are contained in the Neocomian sands, but recently unconventional reserves in the Bazhenov are gaining steam through hydraulic fracturing. The seal for these reservoirs is generally the Kuznetsov formation.

The Turonian complex encompasses any hydrocarbons originating from the Kuznetsov formation and contains mostly gas. Most reservoirs are contained in Turonian sands, sealed by the nonproductive Berezov formation.