Oil Exploration Block Gurvanbayan lies in the southeast of People’s Republic of Mongolia, belongs to East gobi province in terms of administrative area, and structurally in the central East-Gobi basin. There are two sub-basins in the block, Zuunbayan and Unegt.
The petroleum exploration and development of Block are Gurvanbayan can be divided into three phases. The first phase is from the shallow well drilling in 1941 to the end of 1960s, managed by Mongolia and former Soviet Union, Zuunbayan and Tsagaan-Els oilfields were discovered based on gravity data, Zuunbayan oilfield was put into production and then depleted.
The second phase is from 1992 to 1997, managed by Occidental Petroleum Corporation, which is a modern petroleum exploration phase. Some old 2D data was re-processed, and some 2D data was newly acquired. From 1997 to 2001, Nescor Energy and ROC explored Zuunbayan sub-basin. Seven wells were drilled from 1994 to 2000.
In 2005, deployed 3D survey, which marked the start of the third phase. Up to December 2011, completes 97 exploration wells in Zuunbayan and Unegt sub-basins.
The oil testing and conventional production testing on the completion wells demonstrate exploration breakthrough. And commercial discoveries are obtained in the central uplifting zone in Zuunbayan sub-basin. Oil and gas shows were found in other zones, too. Three sets of oil-bearing formations were discovered in Zuunbayan sub-basin.
Petroleum-Initially-In-Place and Reserves in commercial discoveries are calculated as 2,961.75 * 104m3 or 186.30 * 106bbl and Total Possible Petroleum-Initially-In-Place 2,907.69 * 104m3 or 182.90 * 106bbl and in Total Proved+Possible Petroleum-Initially-In-Place 5,869.44 * 104m3 or 369.20 * 106bbl.
Source Rocks
The Early Jurassic in the Southern Mongolia has lacustrine shales with up to 3.5% TOC and geochemical analysis on samples collected at outcrop has shown that these shales would make excellent source material, with HI values over 600mg HC/g TOC.
Lower Cretaceous, lacustrine shales make up the primary source rocks in the East Gobi Basin. Stanford University proposes that source quality shales are likely distributed throughout the East Gobi Basin and possibly as far as Southern Mongolia [Hendrix, 1998; Graham, S.A., personal communication, 1999].
Potential source rock thicknesses over 100 meters have been documented for the East Gobi Basin [Sladen, 1991; Johnson and Graham, 1997]. Analytical results from outcrop samples collected by Stanford in areas away from Eastgobi basin show TOC as high as 14.6 percent [Johnson, 1998]. The better outcrop samples typically show a potential hydrocarbon yield of about 60kg/t, with one sample as high as 134 kg/t. Sladen [1991] states that lacustrine source rocks within the Zuunbayan Formation possess richness ranging as high as 53 kg/t.
Source-rock distribution and quality
Analyses of Lower Cretaceous coals and mudstones show that source potential is very variable. Many mudstone samples possess zero or negligible potential whilst others have potential in excess of 50 kg/t. There is a clear distinction between the coals which possess potential to generate gas, and the lacustrine mudstones which primarily have potential to generate oil.
Lower Cretaceous mudstones with potential to generate oil were deposited in lacustrine environments in the K1dz sequence. They possess richness ranging up to 53 kg/t. Lean mudstones typically show only potential for gas.
Of the various basins analyzed, samples from the Eastgobi basin area clearly contain the best quality oil-prone lacustrine source-rocks. It is also in the basin area that the thickest and most widespread mudrocks appear to be developed, with lacustrine mudrock sections in excess of 100m in some wells and at outcrop.
Maturation and Migration
Consensus has yet to be reached on the maturity of source rocks in the Eastgobi basin. Sladen [1991] suggested that Lower Cretaceous source rocks are largely immature and have not generated significant hydrocarbon volumes. He also suggested that a sediment thickness of over three kilometers was required to put source rock intervals into the oil window. Clarke [1991] interpreted Tsagaan-Els and Zuunbayan oils as being products of a source rock that had just reached maturity.
In contrast, Greene et al, [1999] reported that in Eastgobi basin oils were generated from a source rock in the early oil window and peak oil window respectively. EAIT [1990] placed the burial depths required for onset of oil generation at 1200 meters and peak generation at 1800 meters. Most outcrop exposures of source rock are thermally immature to marginally mature [Sladen, 1991; Greene et al, 1999; Johnson, 1998]. Outcrop exposures likely represent basin margin settings however, and one would expect that basin centers were buried much deeper and would be more mature. Prost [1999] modeled the hydrocarbon system and concluded that the lower part of the Zuunbayan Formation is “mature over large parts of the Unegt and Zuunbayan sub-basins and has probably generated substantial amounts of oil and some gas.”
