Lexam Explorations - New Cretaceous Basin Discovered in the Rockies

Media - New Cretaceous Basin Discovered in the Rockies Lexam's oil shows during gold exploration leads to the discovery of a cretaceous sub-basin within the tertiary san luis basin of southern colorado. It is positioned between the san juan, piceance, denver-julesburg, and raton basins and contains cretaceous source rocks at depths between 7000' and 17,000'. San Luis Basic Project Saguache and Alamosa Counties, Colorado In 1992 and 1993 Lexam Explorations (U.S.A.) Inc. encountered strong shows of oil in 27 of 42 shallow gold exploration wells that it drilled along the west flank of the Sangre de Cristo mountains in south central Colorado. It also happens to be the east flank of the Tertiary San Luis basin, an area that conventional wisdom rejected for oil exploration due to the erroneous interpretation that all of the Paleozoic and Mesozoic sections were eroded away during the Laramide. Well, the oil was identified as Cretaceous and outcrops of Mancos shale, Dakota sandstones, and Morrison shale have been located. Not only did palynology studies confirm the black shales as Cretaceous, but the TOC measurements ranged from 1.63% up to 7.31% by weight. Since then, Lexam has acquired 30 miles of 2-D seismic and 550 gravity stations to add to its regional framework of seismic, gravity, and magnetic data. The structural style of the sub-basin is not as previously reported and neither are the sediments. Tom Watkins, minerals geologist for Lexam, has been intimately involved in this developing play for 5 years. During that time he has reviewed publications and presentations that contradict Lexam's findings, but he prefers to let the d ata speak for itself. This July he will lead a field trip, sponsored by the Rocky Mountain Association of Geologists, to the Mesozoic outcrops on the east side of the San Luis basin. There, he will let about 30 geologists pound on the rocks, review the oil shows and palynology, and otherwise convince themselves that the geologic history of the San Luis basin is being rewritten.
The San Luis Basin is part of the northern Rio Grande rift (Figure 1) and is structurally analogous to oil producing, extensional basins in Nevada and a number of highly productive rift basins around the world. Hydrocarbon bearing Cretaceous rocks have been found in the Albuquerque and Espanola basins on the New Mexico side of the Rio Grand rift. As demonstrated by Lexam, they are present in the San Luis basin too. The development of traps in prerift Cretaceous rocks is associated with rift-related transfer zones. Two types of structural traps, closed structures and fault blocks, have been mapped seismically. Stratigraphic traps are anticipated, but lack control. All surrounding basins with Cretaceous source/reservoir rocks below 7000' contain basin centered hydrocarbon accumulations which are productive. Previous interpretations of the San Luis basin have postulated a regional uplift between the San Juan sag on the west and the Raton Basin on the east that rose during the Laramide about 55 to 60 mybp.The interpreted	Click on image to enlarge
uplift persisted until about 35 mybp when Tertiary sediments and volcanics began to accumulate on the denuded Precambrian basement. During the last 30 my tectonic drive has been provided by the extension of the northern Rio Grand rift. The San Juan basin consists of two half grabens, the western Monte Vista graben and the eastern Baca Graben, separated by a central horst block. Tertiary fill was interpreted to range from 1700' over the central basin "Alamosa Horst" to over 15,000' in sub-basins on the flanks of the horst. The rifting represents a markedly different structural style when compared to Laramide compression and uplift. What has been absent (and is understandably difficult to assemble) is a clear interpretation of the tectonic interaction of Ancestral Rockies, Laramide, and finally, Rio Grande deformation. It was so much easier to interpret when all of the pre-Tertiary evidence was interpreted to have been eroded away. What has been found is an oily Mesozoic section with Jurassic through Cretaceous outcrops along the east flank of the Sangre de Christo range and a seismic character in the Baca graben that is remarkably similar to the Mancos-Dakota-Morrison section in the Raton basin.
The Baca graben lies east of the Alamosa horst and is defined by gravity and seismic. An east-west cross section (Figure 2) is based on Lexam's most recent seismic and shows the preserved Cretaceous sub-basin below the Tertiary fill. A new interpretation of the Mesozoic basin history starts after a period of erosion beveled the region prior to the deposition of the Jurassic through Cretaceous sequence. Based on regional correlations in the Cretaceous, it is estimated that the Mesozoic package was 3500' to 4500' thick. The key difference in this basin history is that during the Laramide orogeny, a foreland basin formed to the east of the	Click on image to enlarge
Alamosa horst. Fifteen to twenty million years of erosion exposed the crystalline basement on the horst, but left about 3000' of Cretaceous sediment in the adjacent basin. Rifting began about 30 mybp and caused the basin to subside. At least 7000' of Tertiary fill blanketed the Baca Graben During the extension and subsidence, a low angle, detachment fault boundary between the basin and the Sangre de Cristo range left blocks of Mesozoic sediment attached to and smeared out along the fault zone. These blocks form the outcrops highlighted in the July, 1997, RMAG field trip.
The low angle detachment fault (Figure 3) is also a change from previously published structural styles which postulated high angle normal faults for the eastern basin boundary. The new detachment surface is seen on seismic and was penetrated by two wells which Lexam drilled in 1995. Although these wells only found remnants of Mesozoic rocks along the fault zone, they also had shows of hydrocarbons and confirmed the interpreted position of the crystalline basement. Tying the wells to Lexam's seismic data also indicates that a more complete Cretaceous section is widely preserved in the center of the sub-basin.	Click on image to enlarge
Based on samples, shows, palynology, oil typing, outcrops, gravity, and a seismic correlation borrowed from the Raton basin, 35 miles to the SE, it appears that the Baca graben contains a preserved Mesozoic package approximately 3000' thick at depths of 7000' to 17,000'. This package is up to 45 miles long, 18 miles wide, and covers about 500 sq mi. (Figure 4, ) It is truely an unexplored Cretaceous sub-basin in the heart of the Rockies.
The burial history of the Cretaceous section and estimated temperature gradient places these prolific source rocks within the oil/gas generation window beginning 10 to 15 mybp. A regional hydrocarbon saturation is anticipated by analogy to the San Juan, Piceance, and D-J basins where blanket saturations are seen in several Cretaceous reservoirs. This probability is enhanced by the Crestone sub-basin's greater depth. Lexam has identified three Dakota prospects in the San Luis Basin of south-central Colorado at depths ranging from 7,000' to 14,000'. The reserve potential of the three prospects is estimated to be 72 MMBO recoverable. In addition, ten exploration lead areas have been identified in the 40-mile-long Crestone segment of the Baca graben. Lexam Explorations (U.S.A.) Inc. has assembled 150,000 acres of fee, State and Federal land along the northeastern margin of the basin.	Click on image to Enlarge
References Rio Grande Rift, New Mexico Geological Society, Northern New Mexico Guidebook, Baldridge, W.S., Dickerson, P.W., Riecker, R.E., and Riolek, J., eds., 1984. Basins of the Rio Grande Rift: Structure, Stratigraphy, and Tectonic Setting, Geological Society of America, Special Paper 291, Keller, G.R. and Cather, S.M., eds., 1994. "An Oil Find That Was Good As Gold", AAPG Explorer, July, 1995. Author Biographies John Morel has been an oil & gas geologist and geophysicist for 22 years with Amoco, Davis Oil Co, Gary-Williams Oil Producer, Basin Exploration, and his own consulting company, Foxpark Oil & Gas. He has worked throughout the Rocky Mountain region, the midcontinent, onshore west coast, and a few foreign concessions. Special interests include seismic stratigraphy, over pressured gas, and fractured reservoirs. Recent developments in 3-D seismic have focused him on reservoir delineation studies. He holds an M.S. and Ph.D. from the University of Wyoming. Tom Watkins has been a mineral exploration geologist for 22 years with Exxon, Coastal, Cruson and Pansze, Abermin, Lexam Explorations and as a consultant. He has worked extensively in Colorado as well as portions of New Mexico, Utah, Nevada, Arizona and West Africa. He has coordinated Lexam's oil & gas exploration in the San Luis basin since the discovery of oil shows and Mesozoic sediments in 1992. The primary focus of his work is exploration for epithermal gold deposits. He holds a B.Sc. and M.Sc. from the Colorado School of Mines.