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OKLAHOMA’S MORROW GAS PLAY |
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Based on a workshop co-sponsored by PTTC's South Midcontinent Region and the Oklahoma Geological Survey on February 23, 2000 in Tulsa, Oklahoma and February 24, 2000 in Oklahoma City.
The Morrow Sandstone of the shelf and deeper parts of the Anadarko Basin in western Oklahoma has produced significant natural gas since the late 1950s. Regional geological mapping and production data summarized from contrasting settings at Milder Field, Arapaho Field, and Cheyenne West Field can be used to define sub-plays within the basin.
Large reserves are present in marine and nonmarine strata at moderate depth and significant development potential remains in many of the fields. Log correlations indicate that channels oriented north to northeast cut across predominantly west-to northwest-oriented marine-influenced sandstone trends. The highest vitrinite reflectance values, greatest weight percent of organic carbon, and greatest depth of burial coincide with an area of high temperature methane. Overpressuring in the Morrow occurs only in the deep Anadarko Basin. Shallower parts of the basin are characterized by mature oil generation and wet gas production. Authigenic kaolinite and chlorite in Morrow sandstones can cause severe production problems. Kaolinite content decreases and chlorite content increases southward into the basin.
Anadarko Basin, Clay plugging, Morrow Formation, Overpressured gas, Primrose Sandstone
Morrow Overview and Other
Richard D. Andrews, Oklahoma Geological Survey
Arapaho Field
Walter J. Hendrickson, IHS Energy Group
This workshop focused on Morrow sandstone reservoirs of western Oklahoma that are prone to gas accumulation in the deeper parts of the Anadarko basin and shelf. Morrow strata in these areas are characterized by shallow-marine delta-front and channel deposits. Regional maps and cross sections show the distribution and character of the gasbearing sands.
In western Oklahoma, the Morrow is discontinuous to the east and northeast toward the Nemaha fault zone. It thickens continuously to 4,000-5,000 ft to the south at the base of the Wichita uplift. No Morrow outcrops are present adjacent to the Anadarko basin margins within the area. The Morrow play has been important because of the large reserve base and moderate drilling depths. Most of its production comes from levels shallower than 12,000 ft. At least 7 TCF of gas has been produced from the Morrow during the past 20 years. Some Morrow fields have been fully developed, but many others have significant development potential.
Morrow drilling during the 1950s and 1960s discovered high-volume gas wells in the Mocane-Laverne field in Beaver and Harper Counties of northwest Oklahoma and the Panhandle. Drilling rapidly progressed into the deeper part of the basin to the south and southeast as the Watonga-Chickasha trend was developed during the 1960s and 1970s. Morrow fluvial-dominated deltaic (FDD) oil production in the Canton area was developed in the mid to late 1970s. During the same time frames the Morrow chertconglomerate fan-delta facies was developed in the deepest part of the Anadarko. Drilling activity has steadily decreased since 1982.
The Morrow produces primarily from sandstones of both marine and nonmarine origin. Log interpretations indicate that north-to northeast-oriented channels often cut across the otherwise predominantly west-to northwest-oriented sandstone trends. These patterns of Morrow deposition were caused by regional changes in sea level.
Sediment Source Areas. Detrital sediments composing the lower Morrow interval originated from areas to the north and northeast (Central Kansas uplift). Specific corridors of sediment influx have been identified as incised channels trending north and northeast. Distinctive progradational sequences can be identified in the subsurface that extend basinward to the south and southwest. In a basinward direction, the environment of deposition changes from subaerial fluvial floodplain to estuarine-deltaic to delta front marine. Well log cross sections illustrate this transition of environments to the south and southwest.
Most of the upper Morrow detrital materials in the Oklahoma and Texas Panhandles originated from Colorado and western Nebraska, with a smaller component coming from the Sierra Grande Uplift of northeastern New Mexico and Southeastern Colorado. The upper Morrow chert conglomerate deposited in the very southern part of the Anadarko basin has a southern provenance (Wichita uplift). Fine-grained marine deposits of the upper Morrow in Custer, Washita, and Caddo counties were deposited in a shallow marine shelf environment. Marine bars may have originated from redistribution of channel sands by tidal and longshore currents.
Stratigraphy. Morrow sediments are Early Pennsylvanian in age, and have not been formally divided in western Oklahoma. For this work, the Morrow Formation was divided into an upper and lower unit. Division between the upper and lower Morrow is picked at the top of a series of regionally extensive limestone beds in the middle part of the formation. The upper Morrow is normally dominated by a thick marine shale section. Most sandstone reservoirs in the upper Morrow occur above the thick shale and may be marine or fluvial in origin. Lower Morrow is dominated by a series of sandstone sequences. The basal sandstone unit of the lower Morrow is called the Primrose. The Morrow is unconformably bounded above by the Atoka Formation and is unconformably bounded below by the Springer Formation. Because of the unconformable relationship with the underlying Springer, many informal Springer units contact the base of the Morrrow.
Dominant Lithologies. The five main rock types in the Morrow include:
Chemical Parameters. A map of vitrinite reflectance of Morrow sediments in the Anadarko Basin shows that the highest Ro values (more mature) are in the deepest part of the basin, where they exceed 2.5% R o. More or less concentric hydrocarbon zones, truncated to the south by the Mountain View fault system, have the highest vitrinite reflectance values in an area of high temperature methane. Areas with R o values less than 1.5% plot in the mature oil window, although considerable gas has also been generated or has migrated into these areas.
The highest weight percent organic carbon is also in the deepest part of the Anadarko Basin, although sample locations are much less dense than for the vitrinite reflectance data. Based on the amount of organic carbon, possible local sediment influx was from a southwestern source, which is probably true for the upper Morrow.
Authigenic carbonate is an important consideration when completing a well. Many parts of the Morrow play have little carbonate cement (calcite or dolomite), such as the western part of the play. Shallow basinal areas with more than 15% carbonate cement in Morrow strata have porosity impediments.
Authigenic kaolinite and chlorite in Morrow sandstones can cause severe production problems. Kaolinite dislodged during fluid surges can plug pore throats, thereby reducing permeability. Such damage can often be overcome by standard massive fracture treatments. Kaolinite content decreases into the deeper part of the Anadarko Basin. Chlorite is probably the most harmful and most abundant authigenic clay in the Morrow. It reacts with HCl to form an insoluble gel that can ruin the permeability of a reservoir.
Pressure Gradients. Gas reservoirs in the Morrow are highly influenced by variable pressure conditions that affect reserves and drilling protocols. In shallower parts of the Anadarko Basin, normal pressure gradient is about 0.465 psi/ ft, however, deeper in the basin, the pressure gradient for the Morrow increases to about 1.0 psi/ ft. Overpressuring is generally recognized when pressure gradient exceeds 0.6 to 0.7 psi/ ft. In overpressured zones, gas reserves are greatly enhanced per unit volume. Drilling in overpressured areas requires "mudding up" to higher mud weights to prevent blowouts. Overpressuring in the Morrow occurs only in the deep Anadarko Basin.
Gas Density. At moderate depths in the Anadarko Basin, Morrow gas has a relatively high density (> 0.65 g/ cm3) owing to the amount of liquid or heavier hydrocarbons in the gas. Under these conditions, the gas is "wet" and has a tendency to produce some liquid condensate during pressure drawdown. Deeper in the basin, Morrow gas is less dense (< 0.6 g/ cm3). Under these conditions, such as in Cheyenne West Field, the gas is "dry" and produces little or no condensate.
Three Oklahoma fields (Milder, Arapaho, and Cheyenne West) were discussed in detail to show different cases of gas production from contrasting stratigraphic levels and lithologies.
Milder Field, Ellis County, Oklahoma, produces from lower Morrow and Primrose sands along the northern edge of the Anadarko Basin. The main Morrow reservoirs produce from nearshore-marine and possibly fluvial-and/ or tidal-channel deposits. At Milder field, the Morrow thins to the north whereas sandstone becomes increasingly thicker but "wet" to the south. Reservoirs are probably compartmentalized, even within the same facies. Cumulative gas production is about 36.2 BCF with 24.9 BCF coming from the lower Morrow B sand and 11.3 BCF from the lower Morrow Primrose sand. Maximum gross sand thickness is 57 ft for the B sand and 79 ft for the Primrose, but due to authigenic clays and cementation, net reservoir sandstone is generally much thinner. Both sands in this field seem to have been affected by tidal or nearshore depositional processes.
Many early wells were drilled in Milder Field during the mid-to late 1960s on 640-acre spacing. These wells were the best in the field, with cumulative production of 1 to >4 BCF per well. Subsequent development occurred mostly during mid-to late 1990s on 320-acre spacing. These later wells generally have poorer production history and reduced pressures, indicating partial depletion. By July 1999, there were a total of 47 producing wells, many of which were multiple-zone completions, making performance determination of individual zones difficult.
Arapaho Field, Custer County, Oklahoma, produces primarily from upper Morrow and lower Morrow Primrose sands in the deep part of the Anadarko Basin. The upper Morrow has produced nearly 62 BCF and 21 MBO/ well from 26 wells. The upper Morrow reservoir was originally developed on 640-acre spacing, followed by some infill drilling to about 320-acre units. Two upper Morrow units have been identified, although there may be several independent reservoirs. Because there is no downdip water leg in any of the upper Morrow reservoirs in this field, the drive mechanism is pressure depletion. Trapping is stratigraphic.
The second major producing zone in the field is the Primrose sandstone. Cumulative production from 18 wells in this lower Morrow zone is >32 BCF, with a per-well average of nearly 1.8 BCF. Upper and lower Primrose units are recognized in the field. The lower unit consists of a number of isolated, stratigraphically-controlled reservoirs. The lower unit has been developed on 320 to 160-acre spacing because of reservoir heterogeneity. The field also produces from a younger Pennsylvanian unit, the Skinner sandstone.
Cheyenne West Field, Roger Mills County, Oklahoma, produces from upper Morrow chert-conglomerate in the southern, deep part of the Anadarko Basin. The southerly source area contrasts with most other Morrow settings which were sourced to the north and northwest. Although production is primarily from the upper Morrow, additional production exists from younger Pennsylvanian units including the Marmaton, Cherokee (Red Fork), and Atoka. Morrow sandstone/ conglomerate reservoir rocks thin to the north and have an identifiable gas-water contact along the southern limit of the field. Pressure communication occurs over large areas. Cumulative gas production is about 332 BCF from 25 wells, or about 13 BCF per well although some wells produced over 30 BCF. The producing reservoir reaches a thickness of 46 ft but thickens to more than 60 ft south of the field in the downdip water leg. The reservoir appears to have been deposited in a fan-delta system.
Morrow Gas Play in the Anadarko Basin and Shelf of
Oklahoma,
Special Publication 99-4,
Available for a nominal fee from the Oklahoma Geological
Survey's Publication Sales Department
Phone 405-360-2886, Fax 405-366-2882
Richard D. Andrews
Oklahoma Geological Survey, The University of Oklahoma
100 E. Boyd, Rm. N-13, Norman, Oklahoma 73019
Phone 405-325-3031, Fax 405-325-7069, E-mail Rdandrews@ou.edu
Walter J. Hendrickson
IHS Energy Group
101 N. Robinson Ave, Suite 400, Oklahoma City, OK 73102
Phone 405-232-3229, Fax 405-231-2517, E-mail walthendricks@ihsenergy.com
For information on PTTC’ s South Midcontinent Region and its activities contact:
Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, E-mail cjmankin@ou.edu
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