BASEMENT PALEOTOPOGRAPHY KEY IN COLORADO MORROW CHANNEL OIL PLAY
Gilbert Thomas
Thomas & Associates
Denver
The Lower Pennsylvanian Morrow channel sandstone play in southeast Colorado has proven to be a lucrative one.
With wells to 5,000-6,000 ft, oil recoveries of 150,000-300,000 bbl/well and relatively inexpensive drilling costs, investment returns are some of the highest to be found in the U.S. onshore. At present prices, payout of drilling and completion costs can be achieved in less than six months.
Delineating the meandering Morrow channels, however, has proven difficult. They can vary in width from one well to four wells and often take unexpected turns and twists.
Such was the case with large Sorrento field (Fig. 1), discovered in 1979. By 1984, development of the field along a linear southeasterly trend (the regional paleoslope for Morrow channels) had slowed because of a half dozen channelless dry holes drilled where operators thought the channel should be.1 It was later found that the channel had taken an abrupt turn to the north and then to the east.
Besides the risk of sudden channel turns when developing a field, finding a channel in the first place can be tricky. Most operators use seismic to determine where Morrow channels are cut into the Mississippian unconformable surface and then find sand buildups in the channels by various seismic reprocessing techniques. The geophysics are combined with detailed subsurface log studies for final drilling site selection.
Still, dry holes are prevalent in the wider play. Wildcat success ratio has been estimated at only about 10%.2
One of the problems in delineating Morrow channels and developing channel fields in Southeast Colorado may be the industry's widespread belief that in the Morrow's fluvial plain paleoenvironment of extremely low slope, channel path is a random process with meanders or sudden turns being related only to the mechanics of water wandering down an almost-flat regional slope and therefore, unpredictable (some Morrow unit deposition slopes are estimated at only 4 ft/mile-in comparison, the present day Arkansas River displays a 35 ft/mile slope).
One purpose of this article is to show that such a belief is fallacious, being more the result of academic abstract reasoning than of actual observational data. And far from being unpredictable, paleostream channels with all their turns and twists are positionally influenced by an ever-present factor too often ignored, i.e., basement paleotopography.
To support this premise, the relationship between basement paleotopography in Southeast Colorado and such Morrow channel fields as Sorrento, Stockholm, Flank, and Interstate (in Southwest Kansas) will be examined.
PALEOTOPOGRAPHY
This subject has long been mentioned in oil and gas papers, although rather vaguely as a possible cause of hydrocarbon accumulations. Exact causative mechanics are usually never mentioned.
Other terms such as paleogeography, paleostructure, or paleohills are sometimes used to mean generally the same thing as paleotopography but imply a shallower paleohorizon in the sedimentary section instead of the basement level used in this article.
Thomas & Associates has found in analyzing surface topographic patterns on thousands of 1:24,000 scale topographic maps that it is possible to delineate basement paleotopographic features that more times than not are associated with hydrocarbon accumulations.
Fig. 2 illustrates the premise of how basement paleotopographic features can be detected at the surface via the ever present process of differential compaction of the sedimentary rock section over the noncompactable basement features.
Whether the basement feature is a hill, ridge, hogback, fault scarp, or whatever, if high enough it will be subjected to differential compaction that in turn produces a "compaction fold" (or monocline) over the feature. If subjected to an extended period of erosion as shown in Fig. 2 for the Mississippian horizon, the compaction fold can be breached topographically. The same topographically breached pattern is seen now at the surface (although more subtle) if the basement feature has sufficient paleorelief to allow the differential compaction strain to reach the surface.
In Southeast Colorado, Fig. 2 represents one of the basement paleotopographic strike-ridge complexes comprising the ancestral Las Animas arch complex.
Although some believe the arch to have first formed in middle to late Pennsylvanian and later, reactivated strongly during the Laramide orogeny, the fact that the arch is part of the north-northeast trending Transcontinental arch (other parts are the Sierra Grande, Sioux, and Minnesota arches) which in turn is part of the basement Proterozoic system of arches in the U.S. (Nemaha/Seminole, Cincinnati/Algonquin are others), indicates that the Las Animas arch was originally a Proterozoic basement arch reactivated in the Pennsylvanian and also later during the Laramide.
As a basement Proterozoic arch, the Las Animas arch should be comprised of numerous relict strike ridges just as ancient anticlinoriums and synclinoriums are today on the Canadian shield. It is these basement relict paleotopographic highs that affect the overlying stratigraphic rock section as shown in Fig. 2.
In this way too, basement, higher strike-ridge complexes form significantly higher "compaction folds" locally in the stratigraphic section. In Southeast Colorado, these higher folds are recognized as the Eads and Brandon axes of the overall Las Animas arch. Tectonic reactivation in the area affected both the basement features and the overlying "compaction folds" producing the compaction/structural folds seen today.
At the Mississippian level on which Morrow channels essentially flowed, north-northeast trending "compaction folds" developed over the basement paleotopographic highs. When exposed to erosion, many "folds" were breached topographically as sketched in Fig. 2. With a gentle southeast slope to the Mississippian erosional surface, any Morrow channels flowing southeastward should have either been deflected around these Mississippian "hills" or flowed through the "hills" where breached openings existed. In the former case, Morrow channel, abrupt right angle turns can be explained as paleotopographic deflections instead of random meandering channel turns.
It is another purpose of this article to show that these paleotopographic deflections of the Morrow channels are exactly what happened in Southeast Colorado.
CLIFFORD-BLEDSOE RANCH-SORRENTO CHANNEL
Fig. 3 shows the relationship between Morrow channels and paleotopography commonly found in Southeast Colorado. The Morrow channel here is representational of the "E" valley-fill system of Wheeler et al.3 The paleotopographic features were mapped in 1988.
Starting west of Clifford field (1), the channel "bowed" across a north-northeast paleotopo high before encountering, an apparently higher Mississippian paleohill (2) at which point it made a right-angle turn to the south-Clifford field oil production occurs in the southward-deflected channel. The channel then returned to its easterly course, crossing over a paleohill apparently where a break in the hill existed (3).
It next encountered a north-northeast trending paleohill about three miles to the east (4) where another "bow" deflection took place. Another mile east (5), the channel turned to pass through a possible saddle between northwest and north-northeast paleotopo highs. At both minor deflection points, Morrow minor production occurs.
After the saddle at locality 5, the channel resumed a southeasterly course (the regional paleoslope) until it encountered a north-northeast high at point 6. Here a right-angle deflection/turn happened; channel oil production occurs in this deflection/turn at Castle Peak and Bledsoe Ranch fields. At point 7 a saddle in the paleohill was encountered and the channel resumed its regional southeasterly course.
At locality 8, a large but apparently low paleorise was encountered and an easterly "bow" deflection took place. No production has been found in this segment to date. At point 9 the channel crossed a well-exposed ("starred") narrow paleohigh that presumably was breached at the Mississippian level because the channel passed right through the apex of the feature.
The "starred" feature outline is the surface expression of the basement paleotopo high that also appears at the Mississippian level as a compaction fold. It is this fold that shows on seismic and was used as a guide in drilling the Mississippian "anomaly" that eventually produced the surprise 1979 discovery of oil in a Morrow channel.
It is at locality 10 where the Morrow channel took an abrupt right-angle turn to the north-northeast that developmental drilling ran out of channel as earlier mentioned. The deflection/turn happened around a large paleotopo high that is part of the Eads axis of the Las Animas arch. From points 10 to 11, the channel first "bowed" around a small north-northeast paleohigh and then deflected to the north-northeast around a last paleotopo high at the upper right corner of the map.
As mentioned previously, the two sets of information displayed in Fig. 3 were arrived at independently; the channel deflections are real. Equally important is the deflection-production relationship. There can be no denying that each time the Morrow channel was deflected by a paleotopographic high from its southeasterly regional course to a position that parallels regional structural strike (at the Pennsylvanian level), favorable oil bearing channel sands were formed. The question "Why?" has to be asked. One deflection in Fig. 3 might be coincidental, but three right-angle deflections just as a paleotopographic high is encountered and all three deflection localities containing oil has to be more than just coincidence.
CHANNEL DEFLECTION MODEL
To answer that question, Fig. 4 was constructed based on the observations shown in Fig. 3. Four stages are postulated for this deflection/deposition model. First, where a Morrow channel flowing down the regional southeasterly paleoslope encountered a north-northeast paleotopographic high on the eroded Mississippian surface, deflection of the channel occurred.
An abrupt change in stream course direction, especially on a very low slope, always produces a slowing of the stream's velocity. At the same time the affect of the water wanting to continue down the regional slope but instead being deflected by the paleotopo obstacle, produces sharp undercutting of the bank parallel to the obstacle. This undercutting second stage can be so pronounced as to make it appear in drill holes that a fault has to be present to account for the abrupt lithologic change between the channel sands and the bank material.
Thirdly, along the deflected course of the channel, the abrupt decrease of the stream's velocity produces deposition of its debris load as point bars. This deposition will continue along the deflected course until a point is reached where the stream can pass over or through the obstacle to resume its path down the regional slope (4).
In essence, the Deflection Model is saying that Morrow channel-sand production of any extent is really only possible where paleotopography deflects the channel to deposit point bars. Random-walk channel meanders are not of sufficient length nor develop adequate velocity impedance to produce point bars of any significant size.
Again it should be emphasized that the Deflection Model is based on actual observations along the Clifford-Bledsoe Ranch-Sorrento channel. Small Smoky Hill field north of Sorrento (Fig. 1) is another example of Morrow channel production occurring precisely along the deflected course of a channel around a north-northeast paleotopographic high. Even larger Morrow channel fields, far removed from the Sorrento area, display the same deflection/production relationship with paleotopography.
FLANK-INTERSTATE FIELDS
A recent opportunity to compare independently mapped paleotopographic highs with a subsurface-defined Morrow channel was provided in Baca County (Fig. 1).
Here (Fig. 5), Bolyard4 presented a Morrow "B" sandstone channel that made a "U-turn" and two distinct right angle turns that overall seem rather bizarre if considered as the result of a random-walk, meandering channel. When compared with paleotopographic features mapped on surface topographic maps, however, the "bizarre meanders" are seen to be abrupt deflections over and around the paleotopographic highs and make a great deal of sense.
The Morrow channel enters Fig. 5 at the northwest corner and flowed down the regional southeasterly slope to point 1, where a minor deflection around the nose of a north-northeast paleotopo high took place. The channel then flowed south-southeasterly until it encountered the "starred" (well-expressed) paleotopo high (2) where it was deflected southward before it found a low enough spot to cross over the southwesterly plunging nose of the high. Flank field occupies the deflected course of the channel.
At locality 3, the channel encountered a strong north-northeast alignment zone (probable basement fault zone) and a cluster of well-expressed ("starred") northwest trending paleotopo highs. This complex of features deflected the channel north-northeastward some six miles (hence the "bizarre U-turn" decidedly away from the regional southeasterly paleoslope) until it ran into an "elbow" intersection (4) between north-northeast and northwest paleotopo highs. Here the channel was forced to take a right-angle turn to resume its regional southeast course but not before favorable sands were deposited at the deflection site that became Midway gas field.
Finding itself between two northeast paleotopo highs with no cross north-northeast paleotopo high obstacles impeding its path, the channel flowed down the regional slope for some nine miles before encountering the major north-northeast paleotopo high (double "starred") at locality 5. Again, an abrupt right-angle turn happened against the flank of the north-northeast feature and again, channel oil production occurs in the deflected course, i.e., large Interstate field.
The major north-northeast paleotopo high (5) apparently was a major paleohill on the Mississippian erosional slope on which the Morrow channel was flowing because the width (1.5 miles) and length (4 miles) of the deflected stream is exceptional. Once the major paleotopo high was left behind, the Morrow channel continued southward, confined to a north-northeast alignment zone, "bowing" over a "starred" northwest high (at Interstate South field) to locality 6, where the channel abruptly broke away from the influence of the paleotopographic features and turned at right angles to resume its regional course. In essence, the course of the Lower Pennsylvanian Morrow channels can be likened to a pinball machine in which the steel ball rolling down the "regional slope" is constantly deflected by the "bumper" obstacles in its path.
If the three deflection/production sites of Fig. 5 are added to the three deflection/production sites along the Clifford-Sorrento channel previously discussed, it becomes evident why the Deflection Model is believed to show the primary reason for Morrow channel production in Southeast Colorado.
ARAPAHOE-FRONTERA-STOCKHOLM CHANNEL
If more evidence is needed to convey the importance of paleotopography in the Morrow channel play, it can be found in the Arapahoe-Frontera-Stockholm fields along the Colorado/Kansas line.
Fig. 6A shows the conditions present in this area in late 1988: the paleotopography had just been mapped by Thomas & Associates for a client and the three Morrow channel fields were being developed. A probable Morrow channel was inferred at the time from knowledge of the regional paleoslope and the tenets of the Deflection Model (Fig. 4).
Starting with Stockholm field (a deflected channel segment against the flank of a well-expressed north-northeast paleotopo high note the almost right-angle turn of the channel), the inferred channel was constructed to wind its way through the paleotopographic highs, passing through saddles or "bowing" across highs. Tributaries were added from the north (better developed than from the south because of the regional slope advantage) where the paleotopo pattern suggested them. In short the Morrow channel course was inferred primarily from just the paleotopographic highs that were mapped quickly and inexpensively on surface topographic maps.
In June 1991 an opportunity arose to go back to the area and add all the developmental wells drilled since 1988. Fig. 6B shows the added wells in relation to the 1988 conditions (the 1988 wells are larger symbols than the 1991 wells).
The combined pattern of all the wells clearly confirms the position of the 1988 inferred channel. Only in locality "X" south of the word "Arapahoe" did the actual channel diverge appreciably from the inferred channel. Instead of running directly southeasterly through the one 1988 well (A) as inferred, the actual channel appears to have been deflected eastward around a "starred" north-northeast high (southwest of X). In all other places, the actual channel and the inferred channel are essentially the same. For this article, another inferred channel (dashed line) has been added using the regional slope/paleotopo deflection premise. To our knowledge, no well has yet been drilled into this possible channel.
SUMMARY, CONCLUSIONS
The comparison of independently mapped basement paleotopographic highs and Lower Pennsylvanian, Morrow channel-sandstone oil fields strongly suggests that basement paleotopo highs produced paleohills on the Mississippian erosional surface (via differential compaction), on which the Morrow channels flowed. These Mississippian paleohills in turn acted as obstacles to the random meandering flow of the Morrow channels, deflecting the channels locally to form point bar sandstones favorable for hydrocarbon accumulation.
This Morrow channel development scenario is based on the observation of the relationship of paleotopographic highs with such Morrow channel fields as Clifford, Castle Peak, Bledsoe Ranch, Sorrento, Flank, Interstate, Arapahoe, Frontera, and Stockholm.
The Morrow channel-paleotopography relationship is considered so strong that it can only be concluded that exploring for Morrow channels without knowledge of the paleotopography is akin to throwing darts at a lease map when selecting Morrow drill sites. If nothing else, knowledge of the paleotopography can decrease geophysical survey costs appreciably.
REFERENCES
- Bowen, D.W. et al., Geology and reservoir characteristics of the Sorrento-Mt. Pearl field complex, Cheyenne County, Colo., Rocky Mountain Association of Geologists Guidebook: Morrow Sandstones, 1990, pp. 67-77.
- Moriarty, B.J., Stockholm northeast extension: Effective integration of geochemical, geological and seismic data, Rocky Mountain Association of Geologists Guidebook: Morrow Sandstones, 1990, pp. 143-152.
- Wheeler, D.M. et al., Stratigraphy and depositional history of the Morrow formation, southeast Colorado and southwest Kansas, Rocky Mountain Association of Geologists Guidebook: Morrow Sandstones, 1990, pp. 9-35.
- Bolyard, D.W., Upper Morrow "B" sandstone reservoir, Flank field, Baca County, Colo., Rocky Mountain Association of Geologists Guidebook: Morrow Sandstones, 1990, pp. 191-203.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.