Zuhair Al- Shaieb1 and Jim Puckette1
Search and Discovery Article #10023 (2002)
*Adapted from presentation to Tulsa Geological Society, September, 2001; poster-session presented at AAPG Annual Convention in Denver, CO, June, 2001, AAPG APPEX in Houston, TX, August 2001, and AAPG MidContinent Section Meeting, September, 2001; awarded as the outstanding poster at the last noted meeting. Abstract below is from poster-session presentations (Puckette, Al-Shaieb, et al., 2001).
1School of Geology, Oklahoma State University ([email protected]; [email protected]). The research group that conducted this study includes research assistants Melanie McPhail, Ken Rechlin, Julie Turrentine, Erin Van Evera, Chris Wiggers, and Amy Close.
Upper Morrowan valley-fill sandstones are major oil and gas reservoirs on the northwestern shelf of the Anadarko basin. Three major Morrowan lithofacies assemblages were recognized in cores and extrapolated from wireline log responses: marine, fluvial and estuarine. The primary marine lithofacies are dark fossiliferous shale and bioclastic limestone. Fluvial facies are characteristic of a braided stream – point bar channel sequence. This complex sequence contains sedimentary features such as trough cross bedding associated with stacked fining-upward sequences, low-angle cross beds, and fine- to coarse-grained sandstones with interbedded or laminated silty, shaly and coaly intervals. Estuarine facies consist of interbedded fine- to medium-grained sandstone and shale, with abundant trace fossils or burrows.
Incised valleys developed in response to major drops in relative sea level. Lowstand system tract (LST) deposits were not commonly preserved and are limited thin clay-clast conglomerates. Subsequent sea level rises resulted in valley filling with fluvial and estuarine facies of the transgressive systems tract (TST). As sea level continued to rise, sediment deposition shifted landward. Therefore, deposition of marine silt and mud represents the highstand systems tract (HST) sediment assemblage.
The sequence stratigraphic framework, coupled with compositional and textural parameters, controls reservoir quality. Braided stream—point bar channel sequences (F2, F3) deposited during the TST contain better reservoirs. Average porosity and permeability are 13.35% and 50.6 md, respectively. Marine sandstones (M2) contain abundant skeletal grains and carbonate cement that occluded porosity. Fine-grained estuarine sandstones are typically poor-quality reservoirs as a result of high detrital clay content and the affects of biogenic modification that destroyed primary porosity.
Figure 3. Map of fields producing from Morrow reservoirs, including fields investigated in this study--Hardesty, Northeast (or Northeast Hardesty), Eva, Northwest (or Northwest Eva), Carthage, and Burton.
• Definition of depositional facies, sequence stratigraphic framework and reservoir characterization of the Lower Pennsylvanian Upper Morrow reservoir.
• Application of the above to exploration and secondary recovery projects.
During late Morrowan time, the Anadarko basin was the site of deposition of deltaic, with transitional sediments derived largely from the north and northwest, marine units to the east, and coarse detritus as a fringe to the uplift(s) to the south (Figure 1).
Morrowan strata in the Oklahoma Panhandle contain a basal sandstone (Keyes) and an overlying section of shale/limestone, comprising Lower Morrowan (Figure 2A). Upper Morrowan contains a sandstone-rich interval (Purdy and Morrow A), along with shale and a thin limy sandstone.
Morrow reservoirs are primarily channel- fill fluvial sandstones within the complex valley fill deposits (Figure 2B). Valleys were cut into the underlying shelf muds during sea level lowstands, and filled during transgressive episodes.
Upper Morrow sandstone reservoirs have produced in excess of 280 million barrels of oil and 3.3 TCF gas from the northwestern shelf of the Anadarko basin and Hugoton embayment in Oklahoma, Texas, and Kansas.
Fields producing from Morrow reservoirs, including fields investigated in this study--Hardesty, Northeast (or Northeast Hardesty), Eva, Northwest (or Northwest Eva), Carthage, and Burton--are shown in Figure 3. The first three fields are in Texas County, Oklahoma, and the last is largely in the adjoining Cimarron County.
Northeast Hardesty, Northwest Eva, and Eva have produced 8% of the oil from Texas County, Oklahoma, and those fields have produced 4% of the gas from that county (Figure 4). In terms of relative size, Carthage has produced 74%, Northwest Eva has produced 23%, and Northeast Hardesty has produced 3% of the gas produced from those three fields
The Purdy Sandstone ranges in thickness from zero to more than 40 ft in an area that is 1-2 miles wide and some 4 ½ miles long in a north-northwesterly direction (Figure 5). This reservoir, which has produced approximately 10.2 MMBO and 1.6 BCFG, developed as incised-valley fill (Figure 6).
The Purdy Sandstone ranges in thickness from zero to more than 40 ft (Figure 7). It trends northeast along a belt that is less than ½ mile wide and more than 5 miles long. It bifurcates in the southwest, where one of the branches trends northwest and the other, more westerly. The reservoir is thought to have been deposited as part of an incised-valley fill (Figure 8). The field has produced more than 2 MMBO and 14 BCFG.
The lower Purdy Sandstone in the Carthage Field area ranges in thickness from zero to more than 50 feet along an arcuate trend that is convex to the west, more than 14 miles long, and 3 to more than 5 miles wide (Figure 9). The reservoir was deposited in an incised-valley fill (Figure 10). The field has produced almost 2 MMBO and more than 44 BCFG.
Figure 13. Four types of fluvial facies: core samples, along with positions and responses on well log. 1=lowermost paraconglomerate; 2=coarse-grained sandstone (braided stream); 3=ripple- to cross-bedded fine- to coarse-grained sandstone (meandering stream); 4=mudstone--silty/shaly interval (abandoned channel)
Figure 18. Photographs of lithofacies in core of lower Purdy Sandstone in Hendrix #3. F-1=fluvial paraconglomerate; F-2=braided-stream sandstone; E-2 (left) estuarine sandstone with ripple-like bedding; E-2 (center)=burrowed estuarine sandstone with flowage features and with deformed to disrupted dark gray shale laminae; M-2=fossiliferous sandstone with black shale; M-1=fossiliferous mudstone.
Figure 21. Photographs of lithofacies in Purdy Sandstone in Anadarko NE Hardesty Unit 11- 2. F-1=fluvial paraconglomerate; F-2=braided-stream sandstone; F-3=meandering-stream sandstone; M-1=fossiliferous black mudstone.
Figure 22. Diagram of lowstand systems tract, showing position of valley incisement and area for deposition, and incised valley containing thin veneer of fluvial sediments (F-1) on valley floor (after Wheeler et al., 1990).
Figure 23. Diagram of transgressive systems tract, showing area for (a) aggradation of fluvial sands (F-2, F-3, and F-4), (b) deposition of estuarine sediments (E-1 and E-2), and (c) formation of marine deposits (M-1 and M-2) (after Wheeler et al., 1990).
Figure 24. Diagram of setting for depositional sequence of Morrowan strata in the study area, with core samples representing various components of the sequence (diagram after Wheeler and others, 1990).
Interbedded sandstone and shale with trace fossils (mid-estuary)
Fine-grained sandstone with occasional silty/shaly/coaly interval (channel abandonment)
Ripple- to cross-bedded fine- to coarse-grained sandstone (meandering stream)
Coarse-grained sandstone with cross-bedding (braided stream)
Paraconglomerate (a high-current-energy stream)
Estuarine facies consist of interbedded fine- to medium-grained sandstone and shale with trace fossils (Figure 14) and similar interbedded sandstone and shale with thin coarse-grained sandstone. The former is considered to have been deposited in a low-energy mid-estuarine environment, and the latter in a variable-energy upper estuarine setting.
Marine facies are represented by dark gray shale/mudstone with abundant marine fossils (Figure 15) and fossiliferous sandstone. The former is thought to represent a low-energy, offshore shelf setting; the latter, high-energy, shallow-marine environment.
Facies in Petroleum Inc. Hendrix #3, Carthage Field are illustrated in Figures 16, 17, and 18. The lower two fluvial facies are represented, along with both estuarine and marine facies. In this well estuarine facies underlie the basal fluvial facies, and the braided-stream sandstone is overlain by marine facies
Lithofacies in Anadarko Petroleum NE Hardesty Unit 11- 2, NE Hardesty Field, are illustrated in Figures 19, 20, and 21. All fluvial facies are present in the cored interval; the basal fluvial conglomerate is underlain by marine shale.
The settings for deposits related to incised valleys--from lowstand systems tract, with valley incision to transgressive systems tract, with deposition of fluvial, estuarine, and marine facies--are shown in Figures 22 and 23.
In the study area, the framework for deposition of the valley-fill sequence, with associated sediments, in Morrowan strata included the following elements (Figure 24):
MFS=maximum flooding surface, which formed after . . . .
TSE=transgressive surface of erosion, or in Figure 24, top of sequence, which formed after . . . .
TST=transgressive systems tract, with deposition of estuarine and bay deposits after deposition of fluvial facies, which formed after . . . .
LST=lowstand systems tract, with deposition of channel lag on . . . .
SB=lower sequence boundary, a subaerial surface, which formed after . . . .
HST=highstand systems tract, with deposition of marine mud, which overlies . . . .
MFS=maximum flooding surface.
Figure 25. Evolution of upper Morrowan successions in study area. A. Depositional settings, in response to sea-level changes in cross section oriented down depositional dip (after Luchtel, 1999). B, C. Sequence as incised valley fill (LSE=lowstand surface of erosion; TSE=transgressive surface of erosion) (after Shepherd, 2000).
Figure 26. Upper Morrowan succession in Ferguson #1, NW Eva field. A. Idealized incised channel fill in cross section (after Shepherd, 2000). B. Petrologic log. C. Photomicrograph showing caliche in paleosol (from Harrison, 1990).
Upper Morrowan strata in the study area document geologic history during which sea-level changes were responsible first for erosion of incised valleys during lowstand and then for deposition of incised-valley fill and associated deposits during transgression resulting from sea-level rise. The upper Morrowan successions are illustrated in Figures 25 and 26.
Figure 28. Detrital constituents in upper Morrowan sandstones. A. Quartz (Q) sand grains. B. Sedimentary rock fragment (SRF). C. Plagioclase (PI) in fine- to medium-grained sandstone. D. Granitic rock fragment (GRF).
Figure 29. Diagenetic consitituents in upper Morrowan sandstones. A. Authigenic kaolinite (K) derived from dissolution of feldspars that partitions pore space. B. Kaolinite (K) “books” filling center of pores (in comparison to chlorite, which coats quartz grains). C. Bioclast filled with intraparticle glauconite (G). D. Thermal dolomite (D) with sparry calcite cement.
Figure 30. Photomicrographs of upper Morrowan sandstones showing porosity types. A. Enlarged intergranular porosity. B. Enlarged intergranular porosity. C. Intragranular porosity due to dissolution of feldspar. D. Microporosity in patch of kaolinite.
The upper Morrowan sandstones are quartz-rich, commonly subarkoses (Figure 27A); some samples are quartz arenites; and others are sublitharenite. Some of the detrital constituents are shown in Figure 28.
The diagenetic events/products and the sequence of their development are shown in Figure 27B. Diagenetic constituents (Figures 27B and 29) include quartz (as overgrowths), calcite, dolomite, kaolinite, and illite.
Porosity in upper Morrowan sandstones includes several types. They include intergranular, enlarged intergranular, intragranular, and microporosity (Figure 30). The relationship between porosity and permeability, according to Morrowan reservoir units (zones) are illustrated in Figures 31, 32, 33, 34, 35, and 36. Although permeability increases with increase in porosity, permeability is less than 25 md where porosity is less than 13 percent.
Figure 38. Pressure-depth plots for four different localities in Anadarko basin – Oklahoma Panhandle (locations shown in Figure 37). Overpressure is shown at point C; underpressure at point G; normal pressure at point A; slight underpressure in much of Pennsylvanian section at point F.
With normal pressure gradient considered to be 0.465 psi/ft, pressure gradients in underpressured Texas County in the Texas Panhandle, where pressure gradients in Morrow reservoirs are approximately 0.3 psi/ft (Figure 37). Pressure gradients in a large part of the deeper part of the Anadarko basin are greater than 0.6 psi/ft, with an area near the southern boundary of the basin showing gradient of 0.9 psi/ft. On the northern flank the gradients are normal or near normal (Figure 38).
The upper Morrow reservoirs in Texas County, Oklahoma, exhibit a variety of facies typical of incised valley systems.
Lowstand systems tract deposits are limited to clay-clast conglomerates (F-1).
Transgressive systems tract deposits are represented by fluvial (F-2, F-3, and F-4) lithofacies as well as the estuarine (E-1 and E-2) lithofacies.
Three major lithofacies were recognized: fluvial, estuarine, and marine.
Using textural, sedimentological, structural, and depositional parameters, each major lithofacies was subdivided.
Fluvial facies consist of F-1, F-2, F-3, and F-4.
Estuarine facies contain E-1 and E-2; marine facies, M-1 and M-2.
Petrographic, petrophysical, and core measured porosity/permeability data indicate that F2 and F3 fluvial facies are better quality reservoirs.
Authigenic kaolinite and clay matrix drastically reduce permeability in various lithofacies.
Carbonate cement, and to a lesser extent dolomite, reduce both porosity and permeability.
In summary, the F2 and F3 lithofacies have better potential of being high-volume oil and gas-producing reservoirs.
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