Sedimentological and sequence stratigraphic characterization of the transitional-marine reservoirs of the Centenario Formation at the Rio Colorado Heavy Oil Belt, Neuquén Basin, Argentina

CEVALLOS, M.F.; RIVERO, M.T.; VEIGA, G.D.; GINGRAS, M.K.; ESCRIBANO, D.O.; RONCHI, D.; ANGELOZZI, G.N.

Congreso; 18th International Sedimentological Congress; 2010

The NE edge of the Neuquén Basin, northward from the Colorado River, was poorly known due to a paucity of well-bore data (less than 1 well per every 75 km2) and the absence of economic discoveries. The recent discovery of more than 400 million barrels of oil in place (19°API) in the Centenario Formation (Hauterivian-Barremian) trigged aggressive exploration and development activities. A new dataset consisting of 1440 sq.km of 3D seismic, 700 wells and 400 m of cored intervals from 17 wells, has helped to improve the geological knowledge of the area. In this part of the basin the thickness of the Centenario Fm. varies from 120 m to zero due to erosive truncation of the Intersenonian Unconformity. The contrast with the thickness of over 800 m that this unit shows at the basin axis suggests that clastic input, sourced from the ESE, was focused along that direction. Meanwhile north of the Río Colorado region a lower subsidence area developed in a lateral and distal position from the sedimentary source. Two third-order sequences have been identified in the Centenario Fm. where only the transgressive (TST) and highstand system tracts (HST) are developed. Both sequences coarsen upwards and the change from the basal shaly-marly sections to the uppermost sandy units is generally transitional. Nevertheless, there are some sharp contacts that reflect high-order local regressive events. On the basis of vertical and lateral facies arrangement, trace fossil associations and body-fossil occurrence, different sedimentary environments were identified. The distal parts of the unit accumulated in a prodelta to wave-dominated delta front and interdistributary bay environments, with the proximal counterparts represented by a coastal plain setting. TST sections show sedimentary structures, body-fossil and trace-fossil assemblages characteristic of an open-marine depositional setting: e.g. (1) wave-generated sedimentary structures; (2) the presence of the fossils Amnobaculites sp.cf. A. subcretaceous Cushman y Alexander, Eogutulina sp.cf. E. liasica (Strickland), Trochammina depressa Lozzo, Epistomina australis Masiuk y Viñas y Haplopragmoides sp.; and (3) trace fossils such as Diplocraterion, Chondrites and Rhizocorralium. In particular, the TST of the Upper Centenario Sequence shows evidence of a rapid transgression event. The base of HSTs were deposited at a brackish, transitional-marine, tidally dominated setting, interpreted from the presence of agglutinated foraminifers (Trochammina depressa Lozzo, Silicosigmoilina sp., Psamminopelta sp., Ammobaculites sp.cf. A. alaskaensis Tappan) and marine-continental palinomorphs, scarce root traces and a low diversity / diminutive assemblage of trace fossils comprising Skolithos and Planolites. Scarce and poor quality reservoirs are present in this section. The uppermost part of HSTs contains sand bodies (the main reservoirs) that were mainly deposited within fluvial and locally tidally influenced channels in the coastal plain. The presence of rare Planolites, Skolithos, and Thalassinoides suggests that these channels were at times brackish-water in nature. Reservoirs correspond to very fine and fine grained feldespathic litharenites, well sorted, fairly consolidated, with excellent petrophysical properties (porosity from 25 to 38%, and permeability from 0.5 to 4 D). In spite of the degree of sorting, subtle changes in grain size distribution strongly impact pore-throat sizes, thus affecting the production performance and recovery factor. These reservoir types show different levels of hydrocarbon saturation, down to irreducible oil saturation in the poorestquality reservoir class. A variety of enhanced oil recovery techniques have been tested in the trend. These include: heavy-oil production with sand, water and steam flooding, horizontal wells and ASP injection. Geological characterization played a key role in meeting the many challenges involved in the exploration and development of the trend.