FUSSELMAN PALEOTOPOGRAPHIC TRAP SEARCH PAYS OFF IN MIDLAND BASIN

Louis J. Mazzullo
Geological Consultant
Midland, Tex.

Carbonates of the Lower Silurian Fusselman formation of the Permian Basin have long been attractive hydrocarbon targets to many operators because they comprise some of the most prolific, deeper (sub-Woodford shale) targets in that area.

Exploration for the Fusselman has historically emphasized the application of seismic data to locate anticlinal and fault-bounded closures in the subsurface.

These structural leads are usually defined by mapping the base of the Woodford shale.

A major Fusselman play of the last few years has been to locate unconformity/structure traps along the regional pinchout of the Fusselman formation in the eastern Midland basin.

Several recent, significant discoveries both along the regional Fusselman pinchout and in the deeper Midland basin point out that there is still a lot of life left to this prolific reservoir.

FUSSELMAN FORMATION

In the deeper parts of the Midland basin, Fusselman is found beneath a cover of carbonates of the Upper Silurian Wristen and Devonian Thirtyone formations (Fig. 1).

In those areas, the search for sub-Woodford reservoirs has historically been aimed primarily at targets in the Wristen and Thirtyone sections, because these are immediately below the base of the Woodford seismic mapping horizon.

Fusselman discoveries west of the Fusselman subcrop belt were largely made as a result of drilling for deeper, Ordovician Ellenburger targets.

Fusselman reservoirs in these areas share many of the same features as those explored for along the regional pinchout but are harder to define because they lie beneath a cover of younger carbonates.

The distinction here between Fusselman and overlying Wristen carbonates is not readily resolved with seismic techniques, so it is not possible to deliberately search for Fusselman plays in the deeper Midland basin using conventional seismic exploration techniques.

GEOLOGIC DEVELOPMENT

The geologic development of the Fusselman formation in the Midland basin lends itself to the discovery of many more subtly-expressed paleotopographic traps.

Porous pay zones in the Silurian-Devonian section of the northern Midland basin portion of the greater Permian Basin vary stratigraphically, as well as structurally, with respect to their location beneath the pre-Woodford unconformity.

These lithologic variations can not be simply related to widespread pre-Woodford erosion and peneplanation. They are, instead, related to several episodes of exposure and tectonism within the sub-Woodford section.

Of particular importance to this discussion was a prolonged period of tectonism and erosion that occurred at the end of Fusselman (Lower Silurian) time. These events created local sites of deep erosion and karst development on the upper Fusselman surface, which were further modified within the Fusselman subcrop belt (and along the regional Fusselman pinchout) during another major erosional event prior to deposition of Woodford shale. These events were later overprinted by Late Mississippian-early Pennsylvanian tectonism.

The erosional relief on top of Fusselman is, in places, very pronounced and may be predicted in areas where deep well control is adequate to make reliable stratigraphic correlations using sample cuttings and/or core.

This paper presents two examples of Fusselman fields in the Midland basin where hydrocarbon traps were developed primarily as a result of paleotopographic relief on the top of the formation, with secondary structural overprints of much different magnitudes. These examples are from Patricia and Wells fields in Dawson County, Tex., both of which produce from Fusselman where it is overlain by varying thicknesses of Wristen carbonate.

These are two of several possible examples that can be cited to show how detailed lithologic mapping may be used to explain and predict the occurrence of Fusselman paleotopographic traps in the Midland basin. The wells in these field areas provide sufficient sample and well control to make reliable stratigraphic interpretations and correlations.

FIELD EXAMPLES

PATRICIA FIELD

Patricia field is in Moore CSL Leagues 267 and 268, southwestern Dawson County, Tex., in the central Midland basin about 25 miles east of the Central Basin Platform margin.

The field has produced more than 3.8 million bbl of oil from Fusselman. The field's producing wells were all perforated within 10 ft of the top of Fusselman.

Reservoir facies of the Fusselman in Patricia field include lime grainstones and dolomitic mudstones and wackestones. These are overlain across the field by a cover of Upper Silurian Wristen carbonates of varying thickness.

The contact between the non-cherty Fusselman carbonates and overlying cherty Wristen carbonates was determined from sample (cuttings and core) analyses of four key field wells and log correlations to the other wells. Despite the fact that most of the field wells did not penetrate beneath the base of Fusselman, sufficient section was drilled in enough wells so that lithologic/log correlations were effective in determining stratigraphic and paleogeomorphic controls here.

Patricia field is on an anticlinal closure (defined on the base of the Woodford) which appears to be bounded on either side by down-to-east faults (Fig. 2). Total structural relief at the base of Woodford across the field is a little over 50 ft from existing well control.

The closure on the base of the Woodford, however, does not reveal the paleogeomorphic relief present on top of the Fusselman.

WRISTEN CARBONATE

The thickness of the tight Wristen carbonate section varies from 46-150 ft across the field, with no direct correspondence between Woodford closure and Fusselman paleogeomorphic closure (Fig. 3).

The thinnest section of Wristen is found on the north end of the field, where it coincides with the plunge on the basal Woodford anticline. At that locale, there is as much as 104 ft of paleotopographic relief on top of Fusselman.

Wristen is 100-150 ft thick over the remainder of the field and thickens away from the field in all directions. It thickens abruptly on the downthrown side of the east bounding fault before it resumes its "normal" regional, eastward erosional thinning.

This abrupt thickening suggests movement along the fault prior to the pre-Woodford erosional event, which helped preserve the pre-Woodford section on the downthrown block and accelerate erosion on the upthrown block. Thickness variations on the upthrown block are not, however, entirely fault-related, because Fusselman paleotopography apparently influenced depositional thinning on at least part of the field.

Most of the oil reserves in the field are out of porous, dolomitic lime mudstones and wackestones, and where preserved, stratigraphically higher porous lime grainstones are secondary reservoirs. The erosional unconformity at the top of Fusselman brings porous reservoir rock in lateral and vertical contact with impermeable dolomitic and cherry limestones of the Wristen formation.

It is not likely that the amount of structural relief at the base of Woodford across this field could account for much of a seismically-recognizable anomaly by itself, considering the accuracy of conventional common depth point seismic data. Topographic relief at the top of Fusselman must have played a significant role in modifying the seismic expression of the sub-Woodford structure.

Preservation of a regionally-mappable porous Fusselman limestone on the crests of topographic highs account for the economic success of this field; clearly, severe erosion on Fusselman could just as well have destroyed more porosity than it preserved.

WELLS OIL FIELD

Wells field is centered in Taylor County School Land Survey League 2 in western Dawson County (Fig. 1).

The field was discovered in 1955 and has undergone several periods of development since that time. Wells field has produced more than 7.2 million bbl of oil from 22 wells, and although it is classified as a Devonian field, detailed lithologic studies indicate that production is actually from the Wristen and Fusselman formations.

Wells field is situated on a segmented anticlinal structure defined at the base of Woodford shale (Fig. 4). The presence of faults in the field is verified by fault cuts in at least two wells and by regional mapping in the northern Midland basin. There is a total of slightly more than 100 ft of closure on the field at the base of Woodford.

Production is from porous lime wackestones or packstones in Wristen and from porous grainstones and dolomities in Fusselman. Porosity is locally enhanced by secondary solution-enlarged joints and vugs.

Not all wells in the field penetrated below the top of Fusselman. Where deep control is available, Wristen varies from 0-106 ft thick and thickens to 125 ft east of the field and its eastern bounding fault.

The thickness of Fusselman in these wells varies independently of the thickness of the overlying Wristen section. The dramatic thickness changes of the Wristen section and the fact that it is Fusselman that actually subcrops below the Woodford in the southern part of the field are shown (Fig. 5).

The structure on Fusselman, however, does not necessarily coincide with that on the base of Woodford; for example, Fusselman subcrops in the Ard 1 Ed Dubose well on the downthrown side of a fault. Some of the structuring on Fusselman may have predated Wristen deposition, as suggested by the presence of an anomalously thin section of Fusselman beneath a thin section of Wristen in the Ard 1 J.W. Dubose well,

Subtle reverse structuring of pre-Woodford units is observed in the Ard 1 Joe Green well; that well, drilled early in the field's development, is located in a saddle between two closures (Fig. 4), well above the apparent original oil/water contact, but was a dry hole. Perforations in the upper part of the Wristen section in that well were swabbed to very high water cuts, perhaps suggesting that emplacement of oil pre-dated later (early Pennsylvanian) structuring.

CONCLUSIONS

It is often heard that the Permian Basin is a mature oil province in which all the large plays have been found.

There are a number of startling examples throughout this region, however, of more subtle, stratigraphicallyenhanced reservoirs that have produced large volumes of hydrocarbons. Discoveries in the last few years in new plays within Strawn (Lower Pennsylvanian) and Wolfcamp (Lower Permian), for example, are from stratigraphic traps that were largely overlooked in favor of more obvious, seismically expressed prospects.

Stratigraphic, or more precisely, paleotopographic traps are important in the Fusselman formation as well.

Fusselman has historically been treated as a structural play, although the examples presented in this report have shown how the orientation of the actual pay zones in large Fusselman fields are actually secondary to the structure mapped on the seismic marker horizon.

Topographic relief at the top of Fusselman in both field examples played a large role in the development of Fusselman reservoirs in both fields, and that relief contrasts sharply with lower magnitude structural relief at the base of Woodford, which is the mapping horizon relied upon to prospect for Silurian-Devonian plays in the region.

This geologic picture is repeated in other Fusselman fields throughout the region, so it is more the rule than the exception.

Opportunities still remain in the Permian Basin for discovery of many similar small to medium-sized Fusselman oil fields through integration of detailed lithologic studies, subsurface mapping, and, in some areas, seismic stratigraphy.

These reservoirs may not necessarily coincide with strong, positive structures, and because of this, they are mappable by conventional subsurface techniques.

To successfully explore for these subtle reservoirs, one must be aware of the significance of the pre-Woodford erosional and structural development of the rocks in addition to any post-Woodford structural modifications that may have taken place.

The search for these traps does not need to be confined to the Fusselman subcrop belt nor to the regional erosional pinchout, because sizeable paleotopographic highs and localized Fusselman subcrops beneath the Woodford shale could be developed at significant distances west of the Fusselman subcrop.

The best opportunities for paleotopographic Fusselman objectives may exist in areas where wells have been completed or abandoned only a few feet below the base of the Woodford shale, particularly where sub-Woodford (post-Fusselman) rocks are tight or not commercially productive.

Lithologic (sample cuttings and core) control from deeper (Fusselman or Ellenburger), strategically located wells in an area can be used to effectively explore for anomalous thickness changes in Wristen or within Fusselman itself. These changes, which can be determined through detailed lithologic analysis, might indicate proximity to potential Fusselman paleotopographic oil traps.

ACKNOWLEDGMENT

The author expresses appreciation to the West Texas Geological Society and the Permian Basin section of the Society of Economic Paleontologists and Mineralogists for permission to publish selected text and figures from their journals.

REFERENCES

Hills, J.M. and Hoenig, M.A., 1979, Proposed type sections for Upper Silurian and Lower Devonian subsurface units in the Permian Basin, West Texas; AAPG Bull., Vol. 63, pp. 1,510-1,521.
Krisle, J.E. and Carey, J.L., 1982, Wells Devonian Field; in Oil and Gas Fields in West Texas Symposium, West Texas Geological Society, p. 661.
Mazzullo, L.J., 1990 (in press), Implications of sub-Woodford geologic variations in the exploration for Silurian-Devonian reservoirs in the Permian Basin; West Texas Geological Society symposium, November 1990.
Mazzullo, L.J. and Mazzullo, S.J., 1989, Lithologic and geologic characteristics of the Silurian-Devonian and Woodford formations of the northern Midland basin, Texas (abstract); AAPG Bull., Vol. 73, p. 255.
Mazzullo, L.J., Mazzullo, S.J., and Durham, T.E., 1989, Geologic controls on reservoir development in Silurian and Devonian carbonates, northern Midland basin, Texas; Permian Basin Section SEPM Publ. 89-31, pp. 209-218.
Mear, C.E. and Dufurrena, C.K., 1984, Pre-Leonardian geology of Midland Farms field area, Andrews County, Texas; Trans. Southwest Sec. AAPG, pp. 111-123.