Energy Research, Department of Geology and Geophysics
University of Wyoming
Hydrocarbons of the South Caspian Basin: How Exploitation Depends on the Understanding of the Neogene Paleoclimate
Hydrocarbon exploration is moving forward at an accelerated pace in the Neogene section of the South Caspian basin, driven by development of the giant “megastructure” field in the South Caspian basin and recent gas discoveries at Shah Deniz. In addition, there are large recent discoveries in the older strata of the North Caspian basin. As a result, major oil companies as well as national governments in the region are pushing plans for major export pipelines to the world market. This is a very opportune time to take at look at some of the remarkable aspects of the geologic history of the South Caspian basin that make it such an attractive petroleum province. Would you have guessed that Neogene paleoclimate is one such factor?
The latitude of the South Caspian basin put it in a “Mediterranean” zone where the climate has oscillated from wet to very dry every 20,000 years, in response to insolation changes driven by variations in the Earth’s orbit (Milankovitch cycles). This land-locked sea responded by major changes; the “sea”level changed probably more than 100 meters vertically and shorelines migrated probably several hundreds of kilometres laterally during each cycle. About a dozen such cycles occurred just during deposition of the Pereriva Suite, implying that there are (at least) an equal number of internal unconformities within just this interval of the reservoirs of the Productive Series. In addition, there is a major unconformity at the base of the Pereriva. There are probably about 50 unconformities within the Balakhany Suite. At first glance, this may induce a development geologist to just give up and apply some geostatistial tool to ‘get around’ the problem. A look at the sedimentology of outcrops and cores of the Productive Series, however, offer a lot of hope that the complexity can be resolved by understanding the architecture of the individual depositional sequences that resulted from these climate and ‘sea’ level oscillations.
The Caspian database also reveals that some fundamental paradigms in sequence stratigraphy require major revision before they can be applied to predict lacustrine reservoir architecture. Among these is the observation that the sequence boundary (lowstand exposure surface) essentially coincides with the maximum flooding surface because there is hardly any clastic sediments deposited in an enclosed lake during lake level fall - there is no fluvial discharge to get them there. Also, sequence boundaries in the South Caspian have no time relationship to those formed by global eustasy; they are neither in- nor out-of-phase. This is because levels of the Caspian Sea are driven by low-latitude insolation, which is mostly controlled by 20,000-year precession and 100,000-year eccentricity cycles. In contrast, global eustasy is controlled by polar ice volumes, which change in response to high-latitude insolation that is heavily influenced by obliquity changes on 40,000-year time scales. So, don’t attempt to date the Neogene section of the Caspian using global sea level charts.
Caspian reservoir architecture is not random; few things in geology ever are. Before order can be discerned and used effectively in exploitation, however, the factors controlling sedimentary architecture must be understood. Although many factors interact in the Caspian as elsewhere, the first-order control was the Neogene paleoclimatic cycles.
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