Petroleum Geoscience - Volume 3, Issue 4, 1997

Upper Cenozoic sequences have been studied in detail on 3-D seismic data, attribute displays and well logs, in order to illustrate the use of attributes in stratigraphy. Lowstand and uppermost highstand deposits are associated with high values of reflection strength, interpreted from logs as reflecting silty-sandy strata. Increasing values of reflection strength through the Upper Cenozoic are related to increasing sand content. Amplitude maps characterize sedimentary facies of systems tracts. An integrated study of timeslices, reflection intensity and logs reveal buried channels and their sedimentary facies. Shallow channels may represent glacial meltwater run-off during the interglacial Cromerian, while the succeeding more deeply incised valleys may be formed by glacioeustatic sea-level falls during the Saale and Weischel glaciations. The studied shallow section with good resolution may contribute to the method of using attributes in genetic facies analysis of subtle stratigraphic hydrocarbon traps, in deeper sections.

The Captain Field is located in Block 13/22a in the Western Moray Firth Basin of the UK North Sea, 80 miles northeast of Aberdeen in a water depth of 340 ft. Hydrocarbons are trapped in two geographical regions, the Main and Eastern closures, both with a significant stratigraphic pinchout component. The principal reservoirs are the Lower Cretaceous Upper and Lower Captain Sands, which are predominantly of turbiditic origin. Reservoir quality is uniformly excellent with in situ permeability averaging 7 D and porosity in the range 28-34%. The reservoir is largely unconsolidated sandstone with the depth to the crest of the field at "2700 ft TVDSS. The reservoirs contain a total oil-in-place of 1500 MMBO. The Upper Captain Sand has a small associated gas cap containing 29 BCF gas-in-place. The oil is heavy, by North Sea standards, with oil gravity ranging from 19 degrees and 21 degrees API and in situ viscosity between 88 and 47 cP, at the mean reservoir temperature of 87 degrees F. The fluid properties and offshore location necessitate the employment of innovative horizontal drilling methods, completion design and artificial lift technology in order to achieve an economically viable field development. Extended reach horizontal wells, with reservoir completion lengths of up to 6500 ft, are proposed for all oil producers and water injectors. Development risks were significantly reduced following two appraisal drilling campaigns in 1990 and 1993 culminating with the successful drilling and extended testing of a prototype horizontal field development well (13/22a-10). First oil production commenced from the Captain platform in early 1997.

The setting of the petroleum basins of Russia varies from Precambrian cratons to Tertiary active margins. Four economically and strategically important basins illustrate this diversity. The North Sakhalin Basin is a Tertiary strike-slip basin supplied with Oligocene-Pliocene reservoir sands, and seal- and source-forming mudrocks by the palaeo-Amur delta. Miocene diatomites are additional source rocks. Plio-Pleistocene structuring was crucial to hydrocarbon entrapment. The West Siberia Basin contains identified oil reserves of 60 billion bbl and 1400 tcf gas, respectively 47% of the oil reserves of the CIS and 77% of its gas reserves. The world-class Upper Jurassic Bazhenov source rock and a high impedance entrapment style are the keys to the productivity of the basin. The Timan-Pechora Basin contains Ordovician-Triassic reservoirs and a major Late Devonian source rock. Hydrocarbon preservation is good despite the basin's complex history of subsidence and inversion. On the Siberian Platform in East Siberia 12 billion BOE reserves are sourced, reservoired and sealed by Upper Precambrian-Cambrian rocks. Protracted preservation times there reflect a stable geological setting.

Internal deformation within the Eurasian continent during the Mesozoic and Cenozoic occurred in response to a series of discrete collisional events along its southern margin. Collisions are recorded from Late Triassic, Middle Jurassic, latest Jurassic, mid-Cretaceous and Late Palaeogene-Recent times. Periods of deformation recorded from sedimentary basins both close to and more distant from the southern margin of Eurasia can be related to these collisional events. The strain of each collision was accommodated both by intense deformation along the margin of Eurasia and by relative movement of continental blocks within the accretional collage of Eurasia along pre-existing heterogeneities. Additional mid-Jurassic and latest Cretaceous uplift events are not directly related to collisional events on the southern margin of the continent. Petroleum systems in both strike-slip dominated basins (such as the South Caspian and Turgay basins) and thrust-dominated basins (such as the Fergana and Afghan-Tajik basins) show the influence of deformation on trap formation and breaching as well as on the remigration and potential loss of hydrocarbons. Periods of deformation also have a direct influence on the timing of hydrocarbon generation.

Two strongly oil-prone source intervals are present in the West Siberian Basin, the Togur Formation (Early Toarcian, Early Jurassic) and the Bazhenov Formation (Volgian-Berriasian, Late Jurassic to Early Cretaceous). Clay sediments of the Togur Formation were deposited in a system of intracontinental sub-basins in the southern half of the West Siberian Basin. The marine deposits of the Bazhenov Formation accumulated throughout the entire basin. Two types of extractable bitumens and oils - marine and non-marine - may be identified from their isotopic carbon composition, sulphur content, bulk composition, and biomarker hydrocarbons (normal alkanes, pristane/phytane, C (sub 27-30) steranes, hopanes and homohopanes). Marine oils, accounting for 80% of oil resources, are sourced from the Bazhenov Formation. The genetic relationship between oils in Upper Jurassic and Cretaceous reservoirs and the organic matter from the Bazhenov Formation is indicated by great similarities in their biomarker hydrocarbon compositions and their maturation levels. Non-marine (lacustrine) oils are sourced from the Togur Formation and account for 11% of oil resources.

The Tunguska basin, eastern Siberia, contains 3.5-8 km of Late Precambrian to Triassic sedimentary and igneous rocks. Source-reservoir-seal systems are present throughout the Upper Precambrian to Permo-Carboniferous interval. Hydrocarbon generation and accumulation largely preceded the formation of the Siberian traps, a Late Permian to Middle Triassic association of effusive and explosive extrusives and intrusive dolerites. The intrusives occur mainly in Palaeozoic strata and have profoundly affected hydrocarbon accumulation. The major process is of destruction of hydrocarbon accumulations, owing to the fact that substantial volumes of the Palaeozoic basin fill has been heated to 150 degrees C plus. At lower temperatures experienced further from the contacts between the intrusions and the country rocks, organic matter thermal maturation levels may significantly exceed those related to burial alone. Water-mineral-hydrocarbon interactions in association with magmatic heating have produced a range of effects, including the generation of hydrocarbons rich in sulphur compounds such as mercaptans.

The South Kyrtayel oilfield lies on the western margin of the Pechora-Kozhva Swell, a major inverted Palaeozoic rift element of the Timan- Pechora basin. The field was discovered in 1976 but is only now being developed by Bitech-Silur who obtained a licence over it in 1995. Like many fields nearby, the South Kyrtayel field contains modest volumes of oil and gas in Middle Devonian sandstones capped by Upper Devonian shales: current estimates of hydrocarbons in-place are 70 MMBBLS of oil and 70 BCF of gas. The fluviatile and near-shore marine Middle Devonian reservoir sandstones were deposited in a rift basin. The trap is an anticlinal structure bounded by a north-south trending fault. The trap developed in the Devonian and was modified during the Carboniferous to Jurassic. There has been considerable uplift of the field since maximum burial and oil charge in the Triassic.