Introduction, Purpose, and Scope, continued
Petroleum in the Northern Mid-continent Region
The occurrence of petroleum in the Northern Mid-continent Region is influenced by unique cratonic-tectonic and stratigraphic settings. Original Precambrian shield areas of the earth commonly are surrounded and overlain by belts of sedimentary rocks. The edges of the old Precambrian plates are downwarped, and at margins extremities, tectonic mobility has created belts of mountains and zones of relatively intense deformation. Cratonic provinces lie astride the marginal, distal portions of the ancient plates, but inland from the mobile belts. The ancient plates are now called the stable "cratons" and the mobile belts are termed geosynclines or geoclines. The Northern Mid-continent region is part of the ancient Canadian Shield, and occupies much of the southwestern-central craton of North America. It is typified by vertical tectonics, continual adjustment and reactivated of structural zones, shallow-water sedimentation, and mild deformation.
Along the edge of the craton the Precambrian crust is broken by numerous faults, many of which have frequently been reactivated. Many of the movements were small magnitude, and are reflected in the diagenesis of petroleum reservoirs (Gerhard et al, 1982, 1988, 1991 ). Individual structural and petroliferous basins, such as the Williston, Kennedy, Denver-Julesburg, Forest City, Salina, and other basins differentiated from these epierogenic movements. Anticlines such as the Las Animas arch, Central Kansas uplift, Nemaha ridge, Cedar Creek anticline, and Nesson anticline sustain petroleum production as major uplifts within or between basins (Fig. 2). Many of the traps in the Northern Mid-continent are combination or stratigraphic traps, in contrast to the numerous structural traps in mobile mountain belts. Shallow water and peritidal sedimentation, abundant subaerial unconformities, and complicated flooding patterns have created complex traps. Consequently, heterogeneity tends to be quite high and few fields are easy to analyze. However, the opportunities for additional recovery are correspondingly high.
Petroleum production in the region remains nationally significant despite the national downturn in drilling and increased abandonment rates. In 1991 the region produced over 702 BCF of gas and over 130 million barrels of oil. The region's states nationally rank number 8th, 9th, 10th, 12th, 20th, and 27th in oil production for 1991. Estimates of additional recoverable oil in the region are difficult to assess because they are not broken out in sub-state units. USDOE estimates that 4.663 billion barrels of unswept mobile oil and 16.119 billion barrels of immobile oil are recoverable in the six-state region. Kansas ranks highest of the group with 2.385 billion barrels mobile and 8.705 billion barrels immobile oil. The other states are incompletely represented in the TORIS data
A fuller treatment of the petroleum resources of each state may be gained by study of the literature cited in this proposal.
Mid-continent oil fields have been some of the most productive in the country and can still play an important role in stabilizing domestic production. The Kansas petroleum industry is typical of that of the entire Mid-continent region. Numerous small operators dominate (there are over 3000 registered oil and gas operators in Kansas), with approximately 85% of the production coming from fields operated by independent companies. Unlike major oil companies, these operators have few technological resources or technical support at their disposal.
Kansas is also one of the most mature oil provinces in the nation, with virtually all producing wells (99%) falling into the stripper category (i.e., 10 BOPD or less), with these stripper wells accounting for over 90% of total oil production (Arthur D. Little Report to Kansas Inc., 1991). The number of abandoned wells and fields has increased significantly over the last decade. The rate of well abandonment from 1987 to 1990 was above the national average, with 1500 to 1900 net wells plugged per year (USDOE, 1989). Average production per well in Kansas is approximately 3.3 BOPD (1990, 55.4 MMBO from 45,470 wells), and represents a 25% decline in production from the 75 MMBO produced from 51,888 well in 1985 (Average per well production 4 BOPD). Approximately 50% of the remaining resource in Kansas (10.6 billion BO) is either abandoned or is, at today's prices, rapidly approaching the economic limit (USDOE, 1989).
One of the most effective ways to slow the current slide in mid-continent oil production, outside of improvement in the price of oil, is by the development and application of new technology and information to existing fields and development plays. Subtle traps and bypassed oil are the future of new exploration and production in the region; finding and exploiting these resources will hinge on sophisticated geological and engineering interpretations of reservoir rock distribution and makeup. The small independent operators of the nation need this technical backing. Careful reservoir characterization and engineering analysis of wells continuously from initial production through abandonment may result in the recovery of many more barrels of oil than was believed likely.
Kansas Geological Survey, Digital Petroleum Atlas
Updated June 1996
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