Gilbert Thomas
Thomas & Associates
Denver
With the advent of horizontal drilling, fractured shale and chalk reservoirs have become important targets in the search for oil and gas deposits.
Equally important in the search has been the corollary question of just where in the fractured reservoir do the most fractures occur?
While reservoir fracturing is generally thought to be either the result of mechanical stress (tectonic) or hydrostatic stress, another cause of fracturing not generally recognized is differential compaction stress over paleotopography.
It is the purpose of this paper to explain the general principles of compaction fracturing and by examples from Silo field in the Denver basin show the production significance of such fracturing.
FRACTURE GENERATION
Cretaceous Austin chalk, the prime objective for horizontal drilling in Pearsall field of Texas, has been fractured by "regional extensional stress acting on brittle chalk units sandwiched between plastic shale and marl, according to Galloway et al.1 Such stress of regional extent is commonly tectonic in origin.
Another example of tectonic fracturing in reservoirs is offered by Fischer and Rygh for the Mississippian-Devonian Bakken formation in the Williston basin.
The Bakken, with a middle silty limestone member sandwiched between organic-rich black shales, is the prime fractured objective of recent horizontal drilling in the basin. The tectonic mechanical stress generating the fractures is believed to be caused by basement block adjustments along major linear fault trends.
A second type of fracturing in the Bakken, recognized by Fischer and Rygh, is microfracturing that "may be related to hydrostatic stress, forming as hydrocarbons are generated." They further note that while the tectonic macrofractures contribute to overall production, they are not as essential for production as the hydrostatic microfractures.
A third type of reservoir fracturing, differential compaction fracturing, can be seen in Silo field 15 miles northwest of Cheyenne in Laramie County, Wyo.
Silo is a fractured shaly limestone and limey shale reservoir3 in the Upper Cretaceous Niobrara formation, which is overlain by the Pierre shale and underlain by the Carlile shale.
Niobrara is approximately time equivalent to the Austin chalk of Texas.
Niobrara production is 8,000 ft deep, and 42 wells have produced a cumulative combined 1,276,465 bbl of oil and 970.114 MMcf of gas through Dec. 31, 1989, since the discovery well in 1983.4
Experience with many fractured reservoirs indicates that differential compaction fracturing in reservoirs is more widespread than commonly believed.
Indeed, it is probably the main cause of the notorious discontinuous nature of fractured "sweet spots" in such large reservoirs as Pearsall.
DIFFERENTIAL COMPACTION FRACTURING
The mechanism behind the formation of compaction fracturing is the ever-present force of gravity.
A schematic section of the rock column at Silo field illustrates the conditions under which compaction fracturing takes place (Fig. 1).
Wherever a basement paleotopographic feature with sufficient relief occurs beneath a substantial body of sediment, the force of gravity (via the weight of the sediment) differentially compacts the sediments over the paleotopo feature: greater compaction off and on the flanks of the feature and less over the feature. This process in effect sets up a mini-shear zone over and around the feature. The differential stress on the rocks within the mini-shear zone eventually fractures the rocks.
Both shear fractures and extensional fractures are produced within the mini-shear zone, with the latter being more numerous because of the stretching of the rocks between the differential compaction loci.
Dependent upon the vertical relief of the paleotopo feature and the amount of sediment overburden, the gravity generated mini-shear zone is propagated upwards through the overburden.
Although no exact measurements of the extent of this upward propagation of differential compaction stress and fractures are known by the author from the literature, it is obvious that the differential stress and hence compaction fractures will diminish upwards away from the causative paleotopo feature.
Compaction fractures, also, do not extend continuously upward through the rock section.
Actual fracturing of the stressed rocks not only is a function of the amount of stress but also of the thickness and lithologic strength of individual rock units.
Thus a thick, competent limestone at the 2,500 ft level above the basement would not fracture as extensively as a thin limestone subjected to the same stress at the same level.
Thick shale units may not fracture at all but rather flow plasticly in response to the same stress. So, although the gravity generated differential compaction stress is present (in diminishing intensity) from basement to surface, continuous fractures are not.
Diminishing of compaction stress upwards can also be seen in the diminishing of "structural" relief upwards as in Fig. 1. Again, no exact figures are known to the author for the upward decrease in relief, but experience in many basins suggests that for every 2,500 ft of overburden, a 50% decrease of the original paleotopo relief occurs.
Hence, in Fig. 1, a 400 ft high paleotopo feature on the basement surface is reflected by 200 ft of convex "structural" relief at 2,500 ft above the basement. This is because rocks compressed downwards above a central point resisting compression naturally take on a convex form that will diminish in convexity as the height above the resistant center increases.
At 10,000 ft above the basement feature in Fig. 1, only 25 ft of "structural" relief is present, a 94% total reduction from the relief of the basement feature.
It should be noted that a 4x vertical exaggeration factor has been used in Fig. 1 for illustrative purposes. Actual basement paleotopographic relief at Silo is probably nearer 100 ft based on the amount of "structural" convexity at the Niobrara datum indicated by well control.
A 100 ft figure is also compatible with the nearest basement paleotopographic relief figures of 100-200 ft in Saskatchewan, where the basement surface disappears beneath sedimentary rocks of the Williston basin.
SILO FIELD PALEOTOPOGRAPHY
Fig. 1 also illustrates how basement paleotopo features are commonly reflected at the surface for mapping purposes.
Concentrated erosion occurs on the convex "structural" relief features at the surface, producing topographically breached patterns (topographic flattening and stream deflections are also common surface indications of paletopo features).
The concentrated erosion breached pattern takes place even with only 6 ft of "structural" convexity as found at Silo. Less than 5 ft of "structural" relief on the Gulf Coast produce breached topographic patterns, also.
A study of the surface topographic patterns at Silo produced the results seen in Fig. 2, compared with the Niobrara datum structure map. The dominant north-northeast strike of the paleotopo features is consistent with the basement topographic and structural trends of Saskatchewan as well as with the overall north-northeast trend of the Proterozoic arches and troughs of the U.S. (Sioux arch, Nemaha ridge, etc.).
It is interesting to note that the main structural nose of Fig. 2 coincides with the largest north-northeast paleotopo feature, suggesting the nose to be caused by differential compaction over the paleotopo feature.
Observation of numerous coincidences between paleotopo features and structural noses, monoclines, and even anticlines suggests that these long-recognized structural features are not "structural" in origin at all but rather due to differential compaction.
Gay has also recognized that gravitational compaction is the real cause of numerous structures, referring to them as graviclines.5 The term "compaction" nose might be the most appropriate of all for the Silo nose.
The coincidence of the Niobrara noses with the independently mapped paleotopo features is firm evidence for the presence of paleotopo features beneath Silo field. Equally firm evidence is the effect that the paleotopo features have on production rates of individual wells in the field, as will be discussed shortly.
HYDROCARBON ACCUMULATION POCKETS
Before describing the effect of differential compaction fracturing on hydrocarbon production, it is necessary to explain how the paleotopo features affect hydrocarbon migration and accumulation.
Wherever paleotopo features converge updip, there is the possibility that "pocket" barriers to hydrocarbon migration will occur, thereby causing localized hydrocarbon accumulations. Fig. 3 shows the interpretation of these possible "pockets" in the Silo area as well as paleochannels and a paleoescarpment.
Because Silo is located on the east flank of the Denver basin and northeast of a basinal low point, hydrocarbon migration should have been northeasterly in the Silo area. Under such conditions, localities A, B, C, D, and E of Fig. 2 would be considered special hydrocarbon accumulation sites in the Niobrara reservoir.
SILO COMPACTION FRACTURING
Added to the entrapment possibilities at Silo would be gravity generated compaction fractures as explained earlier.
Fig. 4 illustrates this for the largest paleotopo north northeast feature mapped at Silo. Based on the "halo" mini-shear zone occurring over and around a basement paleotopo feature as shown in Fig. 1, a flank compaction "halo" of fractures has been inferred in Fig. 4.
As a means of showing the significance of differential compaction fracturing on reservoir production, the average annual yield of each producing well within or near the fracture zone is also indicated.
It is thought extremely significant that of the 18 wells in the field that have produced more than 3,000 bbl/year of oil, 11 or 61% occur within the inferred fracture "halo" or contiguous to it. The other seven wells occur in fracture "halos" related to other north-northeast paleotopo highs.
Of equal significance is the fact that the two most prolific wells (greater than 30,000 bbl/year) not only occur in the inferred fracture zone but also in "pocket" C, an inferred special site for greater hydrocarbon accumulation.
While it can be argued that the two most prolific wells are caused by a contiguous fault suggested by the Niobrara structure contours, comparable faults are also suggested on the east flank of the paleotopo high (paleoescarpment zone, Fig. 4), but production is nowhere near as prolific as on the east flank.
The big difference in flank production rates does not appear to be the fault-fracture factor but rather the presence of a closed "pocket" to entrap extra oil.
Gas production shows the same increase as oil within the flank fracture "halo" of Fig. 4.
The two most prolific gas wells are in "pocket" C of the flank fracture zone, as is the fifth most productive well (7.936 MMcf/year). The third (10.157 MMcf/year), the sixth (6.247 MMcf/year), and the tenth most productive wells (4.219 MMcf/year) are all within the flank zone.
Of the top 10 gas wells of Silo field, six are in the fracture "halo" of Fig. 4. The other four top producers occur within flank fracture zones and-or barrier "pockets" related to the other paleotopo highs.
It is interesting to note that none of the most productive wells at Silo occurs along "structural" noses coincident with paleotopo crest axes. All of them occur in paleotopo flank positions where differential compaction fracturing is greatest.
Indeed, the wells along or near "structural" noses/paleotopo crests are either dry or very poor producers.
CONCLUSIONS
The comparisons of Fig. 4 clearly illustrate the significance of paleotopographic differential compaction in localizing zones of increased fracturing in fractured.
Further, Figs. 2 and 4 show that increased fracturing is not the only factor in localizing increased production locales in fractured reservoirs. Updip, paleotopographic barrier "pockets" to hydrocarbon migration can be a greater factor in large hydrocarbon accumulations than fracturing.
In Silo field, seven of the 10 wells that produced more than 5,000 bbl/year of oil occur in barrier "pockets," while the other three occur only within flank compaction fracture zones.
It is concluded, therefore, that the delineation of the paleotopography beneath reservoirs, whether in a vertical or horizontal drilling program, is necessary to define the best production localities for the most economically effective drilling.
REFERENCES
- Galloway, W.E., Ewing, T.E., Garrett, C.M., Tyler, N. and Bebout, D.G., Austin/Buda fractured chalk: in Atlas of Major Texas Oil Reservoirs, 1983, pp. 41-42.
- Fischer, D.W. and Rygh, M.E., "Overview of Bakken Formation in Billings, Golden Valley, McKenzie Counties, N.D.," OGJ, Nov. 20, 1989, pp. 71-73.
- Wyoming Geological Association, Silo field, in Oil & Gas Fields, Southeast Wyoming symposium, 1984, pp. 122-123.
- Geological Survey of Wyoming, Oil and Gas Division: Personal communication, 1990.
- Gay, Jr., S.P., Gravitational compaction, a neglected mechanism in structural and stratigraphic studies...: AAPG Bull., Vol. 73, No. 5, 1989, pp. 641-657.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
