HOMING IN ON 'SWEET SPOTS' IN CRETACEOUS AUSTIN CHALK

Gilbert E. Thomas Thomas & Associates Denver Frank P. Sonnenberg Consulting Geologist Denver In discussing the nature and causes of fracturing in the Cretaceous Austin chalk of south central Texas, many geologists and operators involved in horizontal drilling of the chalk consider regional rock stress as the probable main cause of the fractures. Galloway et al.,1 for example, states that the "fractures resulted from regional extensional stress acting on brittle chalk units sandwiched between

Nov. 29, 1993

6 min read

Gilbert E. Thomas
Thomas & Associates
Denver

Frank P. Sonnenberg
Consulting Geologist
Denver

In discussing the nature and causes of fracturing in the Cretaceous Austin chalk of south central Texas, many geologists and operators involved in horizontal drilling of the chalk consider regional rock stress as the probable main cause of the fractures.

Galloway et al.,1 for example, states that the "fractures resulted from regional extensional stress acting on brittle chalk units sandwiched between plastic shale and marl. "

Another school of thought, while recognizing regional, coastward subsidence of the rock section as a factor in producing tensional fractures, also recognizes the role of deep structure or basement topography in producing Austin chalk fractures.

Stapp,2 for example, notes that buried ridges, old faults, or salt pillows in the Lower Cretaceous and Jurassic may not affect the dip rate at the chalk level, but may serve to make local differential compaction or strain fractures in the Austin and underlying Buda.

In either case - regional stress alone or regional and local stress combined - resultant localities of greatest fracturing that are most conducive for maximum hydrocarbon production have proved difficult to find. Stapp recognized this fact for vertical wells long before any horizontal wells were drilled.

Also, as he mentions, "Production from the chalk and Buda lime along the trend is inconsistent" and declines rapidly within a year. In short, maximum production localities in the fractured chalk reservoir are not only difficult to find but the average production rate declines rapidly.

If Austin chalk fractures are mainly the result of regional extensional stress without localizing factors, then fractured "sweet spots" are randomly distributed and successful exploration is more or less a matter of luck, usually dependent upon the coincidental placement of a seismic line. Under such conditions, discovery of localities of maximum fracturing and production ("sweet spots") would be serendipitous at best.

But if local, deep-seated structure or basement topography are the main causes of "sweet spots," then a successful exploration method would be to first delineate the basement paleo structure or topography and secondly, place a seismic line to confirm the delineated features. Finding localities of maximum fracturing and production would then be based on scientific logic rather than luck.

It is the purpose of this article to present the results of an examination of these alternative causes for the Austin chalk fracturing in the hope of determining the most cost effective exploration method for the fractured chalk reservoir.

INVESTIGATION

To find out if buried structure or topography in south central Texas affects the fracturing in the Austin chalk and hence, the resulting magnitude of production, Thomas & Associates carried out an experimental mapping study with the cooperation of a client who generously supplied the horizontal well production data in Pearsall and Giddings fields.

Using a method developed by the authors, surface topographic maps at 1:24,000 scale were examined in both areas for clues to any subsurface paleo structure or topography present.

Once subsurface features of various degrees of expression were delineated, the horizontal well data were processed to provide cumulative average barrels of oil per month for each well. Gas data showed average production in thousand cubic feet per month.

The production figures were then contoured and compared to the previously mapped paleo structural or topographic features. Some of the comparisons are shown in the following illustrations. Because it is not always possible to differentiate buried structure from topography, the mapped subsurface features in the illustrations are referred to as paleotopographic highs.

PEARSALL FIELD

Fig. 1 covers an area on the southern flank of the large Pearsall anticline, striking northeast, in La Salle County, Tex. The mapped paleotopo features (dashed lines with screen pattern) strike north-northeast.

Note the first overall close association of the maximum production localities with the paleotopo highs.

Secondly, the highest rates of production occur, for the most part, on the flanks of the buried highs. This is probably because differential compaction fracturing over buried topographic features is usually greatest over the flanks of the feature.

And as Stapp noted, "Microfractures result from compaction and subsidence..." in the chalk.

It is the degree of micro-fracturing that determines the interconnectiveness of the fractures and consequently the amount of hydrocarbons available for production in the locality. Without an interconnected fracture system, production cannot be sustained at initial rates and declines rapidly.

Maximum, flank micro-fracturing between two paleotopo highs might explain the highest, sustainable production rate for a horizontal well in the Fig. 1 area (heavy arrow), 22,000+ bbl/month of oil during 21 months to June 1992.

Fig. 2 shows just how closely related the maximum production trends (solid contour lines) are to the buried features, this time striking northeast. (These may be buried, small anticlines on the southern flank of the Pearsall anticline because their strike is the same).

Five northeast-striking paleotopo highs are shown along with the highest producing oil trends. Note that here, most of the oil trends occupy the crests of the paleotop highs rather than the flanks as in the previous illustration.

While this crestal relationship appears to be the rule for northeast-striking buried features (flank relationships predominant for north-northeast features), exceptions do exist as the central paleohigh Fig. 2 shows. There is definitely a flank relationship here that actually curves around the southwestern plunge of the high.

GIDDINGS FIELD

Similar paleotopo high-maximum production locality, associations as found in the Pearsall area were also found in Giddings field, Brazos County, Tex.

One example of these is shown in Fig. 3 for both oil and gas. In both cases, maximum production sites occupy the flanks of the north-northeast high.

CONCLUSIONS

It is obvious from the three preceding illustrations that buried basement structure or topography do indeed have a causative affect on the Austin chalk highly fractured "sweet spots," producing maximum inter-11 connectiveness of the fractures and hence maximum production.

Fig. 4 shows the premise best. Three horizontal wells are shown in relation to a north-northeast striking paleotopo high in the Pearsall area. The two wells away from the flanks or crest of the north-northeast high have produced decidedly less oil than the third well located on the crest of the high and drilled downdip along the flank of the high.

This type of relationship between horizontal well sites and buried highs was found consistently throughout the Pearsall and Giddings areas, indicating strongly that the most profitable exploration method for the Austin chalk tarend is the delineation of buried structural or topographic features, followed by seismic confirmation of the features.

REFERENCES

Galloway, W.E., Ewing T.E., Garrett, C.M., Tyler, N., and Beboat, D.G. Austin/Buda fractured chalk, in Atlas of Major Texas Oil Reservoirs, 1983, pp. 41-42
Stapp, W.L., What to look for when exploring the Austin Chalk, World Oil. Feb. 1, 1978, pp. 55-60.