|Fig. 1: USGS map of heavy oil resources in the Orinoco. Blue: Orinoco Oil Belt Assessment Unit. Red: East Venezuela Basic Province.  (Courtesy of the U.S. Geological Survey.)|
Most assessments of the world's oil sources tend to consider only conventional resources and exclude the unconventional resources including extra heavy oil and bitumen. Heavy oil resources are increasingly becoming commercially producible both in the Athabasca province of Alberta, Canada and more recently, in the Orinoco Oil Belt (OOB) of Venezuela. The purpose of this paper is to describe the current status of Orinoco bitumen development, to estimate the Original Oil in Place (OOIP) and recovery factor of OOB, and to evaluate the future contribution to worldwide oil reserves offered by these unconventional resources of extra heavy oil and bitumen.
Venezuela contains billions of barrels in extra-heavy crude oil and bitumen deposits, most of which are situated in the Orinoco Oil Belt. The Orinoco Belt located in south-eastern Venezuela is probably the largest essentially untapped oil accumulation in the world. North of the Orinoco River, the Belt stretches over a length of approximately 700 kilometers from east to west, with an area about 54,000 km2.  OOB is composed of very soft, unconsolidated reservoir sands of Mioceno age. The reservoirs are typically either meandering channel sands formed within a mayor deltaic deposit or fluvial deposits dominated by braided streams. Within the meandering channel sands in particular, lateral and vertical continuity of the sands body is sometimes limited resulting in laterally discontinuous sands bodies. OOB is one of the largest known accumulations of bitumen in the world; estimated to be around 1200 billion barrel oil (BBO), approximately equal to the world's reserves of conventional oil.  The OOB is currently divided into four exploration and production areas, Boyaca (before Machete), Junin (before Zuata), Ayacucho (before Hamaca), and Carabobo (before Cerro Negro). 
Most of the available estimates of recoverable oil resources in OOB rely mainly on published geologic and engineering data for reservoir (net oil-saturated sandstone thickness), petrophysical properties (porosity, water saturation and formation volume factors), recovery factors obtained from pilot projects, and estimates of volumes of original oil-in-place. The data used by USGS are summarized in Table 1. Petroleos de Venezuela S.A. (PDVSA) has performed a comprehensive study and announced the magnitude of the original oil-in-place to be 1180 billion barrels of oil, a commonly cited estimate for the OOB. PDVSA has recently updated the OOIP to 1300 BBO. Given the uncertainty of regional sandstone distribution and oil saturation, a range from 900 to 1400 BBO is considered for OOIP. Recovery factor (percentage of the OOIP that is technically recoverable) is estimated from the current technology for recovery of heavy oil in the OOB, and other areas such as west Texas and western Canada.
The minimum recovery factor is estimated to be 15 percent (recovery expected for cold production using horizontal wells)
The median recovery factor is estimated to be 45 percent (with the assumption of widely use of horizontal drilling and thermal recovery methods)
The maximum recovery factor is estimated to be 70 percent (on the assumption that other recovery processes, in addition to horizontal drilling and steam-assisted gravity drainage, will eventually be applied on a large scale in OOB). 
|Table 1: Key input data for assessment of OOB. |
Given the data and the uncertainty ranges in table-1, the estimate of volumes of recoverable oil in OOB is in the range from 380 to 652 BBO with the mean of about 513 BBO. USGS has not investigated the economically recoverable resources or reserves in OOB. These results do not imply anything about rates of heavy oil production and no time frame is implied other than use of reasonably foreseeable recovery technology. 
We discussed different type of uncertainties for estimating original oil-in-place of OOB, and eventually announced a range of OOIP and recoverable oil. Expansion and development of enhanced oil recovery schemes for OOB is the key point to increase recovery factor of this extra-heavy oil field. The large range of recovery factor illustrates the strong coupling between technological maturities with the successful field implementation of Enhanced Oil Recovery (EOR) processes.
© Amir Salehi. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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