Silo field is located in the northern part of the Denver basin. Production is from the fractured Niobrara Formation at depths ranging from 7,600 to 8,500 ft (2,318 to 2,593 m). Cumulative production from 40 vertical wells at Silo is in excess of 1.3 million BO. Two wells combined have produced 466,000 BO. Recent drilling success with horizontal wells suggests much greater future production. Initial potentials from the new horizontal wells range from 216 to 2,026 BOPD. Ultimate production for horizontal completions is difficult to define by “early-life” decline curves because many of the wells are produced at low rates (choked back) to prevent formation damage. Cumulative production from vertical and horizontal wells through June 1992 is 2,008,146 BO and 1,366,975 MCFG and 221,589 BW. The dominant lithologies of the Niobrara are limestones (chalks) and interbedded calcareous and organic-rich shales. Niobrara thickness ranges from 280 to 300 ft (85 to 92 m). Four limestone intervals, averaging 30 ft (9.2 m), and three intervening shale intervals (averaging 47 ft [14.3 m]) occur regionally and are easily recognized on geophysical logs. The lower limestone is named the Fort Hays and the overlying units are grouped together as the Smoky Hill Member. The fractures are concentrated in the more brittle limestones. The main production is from the middle limestones of the Smoky Hill. The intervening shales have high organic matter content and serve as source beds. Open fractures systems are essential to Niobrara production because little matrix porosity or permeability exists in the limestones. Open fractures may be created by: folding; basement or listric faulting; solution of Permian evaporites; high fluid pressures associated with oil maturation; regional stress field; or a combination of the above. High resistivities are observed in limestone beds at Silo. These resisitivity anomalies appear to be related to the presence of a hydrocarbon accumulation delineated by isoresistivity mapping. Factors present at Silo will serve as a model for future Niobrara production in the Rocky Mountain region. These factors include: mature source rocks interbedded with brittle limestone; open fractures to form the reservoir; resistivity anomalies indicating accumulation; and technology to efficiently produce the reservoir.

Wattenberg field is a continuous-type gas accumulation. Estimated ultimate recovery from current wells is 1.27 tcf of gas from the Lower Cretaceous Muddy (J) Sandstone. Mean gas resources that have the potential to be added to these reserves in the next 30 yr are 1.09 tcf; this will be primarily through infill. drilling to recover a greater percentage of gas in place and to drain areas that are isolated because of geologic compartmentalization.Greatest gas production from the Muddy (J) Sandstone in Wattenberg field occurs (1) from within the most permeable and thickest intervals of Fort Collins Member delta-front and nearshore-marine sandstones, (2) to a lesser extent from the Horsetooth Member valley-fill channel sandstones, (3) in association with a large thermal anomaly that is delineated by measured temperatures in wells and by vitrinite reflectance contours of 0.9% and greater, (4) in proximity to the bounding Mowry, Graneros, and Skull Creek shales that are the hydrocarbon source rocks and reservoir seals, and, (5) between the Lafayette and Longmont. right-lateral wrench fault zones (WFZs) with secondary faults that act as conduits in areas of the field.The axis of greatest gas production is north 25 to 35degrees northeast g p g, which parallels the basin axis. Recurrent movement along five right-lateral WFZs that crosscut Wattenberg field shifted the Denver basin axis to the northeast and influenced depositional and erosional patterns of the reservoir and seal intervals. Levels of thermal maturity within the Wattenberg field are anomalously high compared to other areas of the Denver basin. The Wattenberg field thermal anomaly may be due to upward movement of fluids along faults associated with probable igneous intrusions. Areas of anomalous igh heat flow within the field correlate with an increased and variable gas-oil ratio.

This stratigraphic section extends from central Montana through the Powder River, Denver, and Raton basins to the San Juan basin of New Mexico. Four east-west sections across the Cretaceous basin, presented at this meeting, are tied together along the geographic center of the basin where the strata are largely marine in origin. Lithostratigraphic, chronostratigraphic, and sequence stratigraphic data are presented.

The Silo field is located in the northern part of the Denver basin. Production is from the fractured Niobrara Formation at depths ranging from 7,600 to 8,500 ft (2,318 to 2,593 m). Cumulative production from 40 vertical wells at Silo is in excess of 1.3 million bbl of oil. Two wells have produced 437,000 bbl of oil. Two wells have produced 437,000 bbl of oil. Recent drilling success with horizontal wells suggests much greater future production. Initial potentials from the new horizontal wells range from 500 to 2,000 bbl of oil/day. The dominant lithologies of the Niobrara are limestones (chalks) and interbedded calcareous and organic-rich shales. Four limestone intervals, averaging 30 ft (9.2 m), and three intervening shale intervals occur regionally and are easily recognized on geophysical logs. The lower limestone is named the Fort Hays, and the overlying units are grouped together as the Smoky Hill member. The fractures are concentrated in the more brittle limestones. The intervening shales have high organic matter content and served as source beds. Open fracture systems are essential to Niobrara production because little matrix porosity exists in the limestones. High resistivities are observed in limestone beds at Silo. These resistivity anomalies appear tomore » be realted to the presence of a large hydrocarbon accumulation delineated by isoresistivity mapping. Factors present at Silo will serve as a model for future Niobrara produciton in the Rocky Mountain region. These factors include (1) mature source rocks interbedded with brittle limestone; (2) open fractures to form the reservoir; (3) resistivity anomalies indicating accumulation; and (4) technology to efficiently produce the reservoir.« less

The Rocky Mountain region is known worldwide for its oil-producing sheepherder anticlines. Not as well known are the subtle traps for petroleum that were among the first fields discovered, and that now represent the largest future potential. Oil seeps were first recorded by explorers in Wyoming in 1832 (Dallas dome, Wind River basin) and 1847 (Absaroka thrust, southwest area). Oil, skimmed from springs, was used for medicinal purposes and sold for wagon lubrication. The first commercial well was drilled in 1862 by an oil seep 9 miles north of Florence, Colorado. Subsequent drilling led to the discovery of the Florence field which has now produced more than 15,000,000 barrels of oil. Production at this field is from fractured Cretaceous shale in a tilted graben with a fault trap on the updip side. Subtle traps are defined as those related to stratigraphic (facies) changes, unconformities, faults or fractures and diagenetic changes. Fluid pressure differential may play a role in each type trap. Early exploration concepts in exploring for subtle traps were to drill near oil seeps, or updip from oil shows in wells. The search for anticlinal traps dominated exploration prior to 1950, and the few subtle traps found were amore » fall-out of this effort. With the revolution of concepts in sedimentary geology since 1950, the search for subtle traps became more sophisticated because of better geologic models which incorporated facies changes, tectonics and sedimentation, unconformities identification of source beds, temperature and pressure fields, and development of improved well logging and seismic stratigraphic techniques.« less

The Society of Economic Paleontologists and Mineralogists, in collaboration with the International Association of Sedimentologists and the International Union of Geological Sciences Committee on Sedimentology, is developing a new international study under the provisional title of Global Sedimentary Geology Program (GSGP). Initially, three research themes are being considered: (1) event stratigraphy-the documentation of examples of mass extinctions, eustatic fluctuations in sea level, major episodes of volcanisms, and changes in ocean composition; (2) facies models in time and space-an expansion of the existing data base of examples of facies models (e.G., deltas, fluvial deposits, and submarine fans) and global-scale study of the persistence of facies at various times in geologic history; and (3) sedimentary indices of paleogeography and tectonics-the use of depositional facies and faunas in paleogeography and in assessing the timing, locus, and characteristics of tectonism. Plans are being developed to organize pilot projects in each of these themes.