LOUISIANA HORIZONTAL WELL TAPS OIL IN AREA OF SALT RELATED FRACTURING

James H. Skelton
Independent Geologist
Houston

The successful drilling and completion of the world's deepest horizontal well in Louisiana confirms seismic indications that the brittle Upper Cretaceous Austin chalk has been highly fractured by nearby halokinetics (salt movement).

Cliffs Oil & Gas Co., Houston, 1-A Roy Martin in Avoyelles Parish flowed 2,566 b/d of oil from the chalk at a true vertical depth of 15,339 ft.

Previous studies of fractured formations have identified two types of fracturing: tectonic and differential compaction.1

This paper states a new theory relating to a third type of fracturing called halokinetic fracturing. While it examines the exploration implications of this new commercial target, a halokinetic fracturing model is also presented here that may be applied to similar geologic settings worldwide.

INITIAL DRILLING

Cliffs central Louisiana well was a redrill of the vertical 16,632 ft Gulf 1-A Martin (Fig. 1).

Gulf drilled the Lower Cretaceous Tuscaloosa sand test in the late 1970s when the Tuscaloosa trend was a hotbed of deep drilling activity.

Spurred by climbing oil and gas prices and Natural Gas Policy Act price incentives for deep gas production, many of these deep Tuscaloosa wells encountered drilling kicks while drilling the lower member of the Austin chalk.

Some of the kicks were so severe that high gravity oil and rich gas flowed to the pits for several weeks. Finding many of the Tuscaloosa sands too thin to be of commercial value, operators completed several of the wells in the area several hundred feet up the hole in the basal Austin chalk formation, where paying production was established.

Originally the vertical Gulf 1-A Roy Martin was completed for 50 b/d of oil, and it produced only 2,778 bbl of oil. An offset well, the vertical Gulf 2-A Martin, flowed 520 b/d of oil from Austin chalk. The 2-A well ultimately produced nearly 500,000 bbl of oil.

REDRILL, SALT PROXIMITY

Cliffs in late 1991 drilled a 2,030 ft horizontal leg away from the original 1-A Martin hole in a south-southwesterly direction.

The drilling intersected a section of oil and gas filled fractured chalk that produced bulk hydrocarbons while drilling. Cliffs reported that after 2 months' production the well had stabilized at a rate of 1,000 b/d of oil.

The North Bayou Jack trend has a unique geologic setting when compared with other fractured chalk producing areas in the respect that it is located only a dozen miles east-northeast of two piercement salt domes, Eola and Cheneyville.

The Upper Cretaceous Austin chalk was deposited thinly over these domes. The formation's productive capability was demonstrated in the late 1970s on the east flanks of these domes at Bunkie and Bayou Rouge fields and on structural trend with Moncrief field and Masters Creek field.

Besides being in close proximity to these salt domes, North Bayou Jack is on the forereef side of the Lower Cretaceous reef buildups.

SALT MOVEMENTS

Herrmann, in a comprehensive study of the Sligo reef complex in Louisiana, described an arcuate deep structural hingeline that coursed through Avoyelles Parish from the southeast.

The structure produced a hingeline arc in Rapides and Grant parishes and continued west-southwest through Sabine and Vernon parishes.

Major Lower Cretaceous reefs of the Sligo and Mooringsport (Glen Rose) built up on this structure.

The significance of this structural arc is that it effectively divided the North Louisiana Salt basin from the major salt deposits of the Gulf of Mexico basin.

As the rigid Austin chalk and later overburdening sediments were laid down basinward, underlying halokinetic withdrawal accelerated southward, forming the northernmost salt domes Cheneyville and Eola. Subsequently, other salt stocks such as Pine Prairie, Reddell, and others in the Gulf basin began vertically developing to the north.

According to the halokinetic theory,3 the overall result of the withdrawal of such large volumes of underlying salt was the creation of a rim syncline at the periphery of the basin.

HALOKINETIC FRACTURING

In the case of the Bayou Jack area, the rim of the syncline is the forereef face of the pre-Austin chalk reef buildup. In front of the reef face is one place where halokinetically-induced faulting and fracturing occur.

The halokinetic faulting extends upward into the Eocene Wilcox formation as seen in seismic sections. Other seismic indicators show intense halokinetic fracturing occurring throughout the Austin chalk and downward into the Tuscaloosa sand wedge.

From this evidence, a halokinetic fracturing theory can be stated: Wherever brittle rock strata are situated within a sequence of ductile rock layers that all overlie a substantial volume of mobile salt layers or salt structure, intense forms of halokinetic fracturing will occur within the brittle strata at distinct flexural areas within the salt basin.

The halokinetic fracturing model developed by the author denotes the different types of salt related fracturing (Fig. 2).

The Austin chalk hydrocarbon production in the North Bayou Jack trend is thought to be from parallel halokinetic fractures near the Lower Cretaceous basin synclinal rim. These oil filled fractures may also be intersected by dip-oriented flexural fractures from lateral regional sag or more likely from differential localized lateral sagging on the fault face.

The model also depicts concentric and radial halokinetic fracturing that may be found in brittle chalks near domal rim synclines.

SOURCE ROCKS

The sources of hydrocarbons in the North Bayou Jack Austin chalk trend is believed to be the underlying Eagle Ford shale, Tuscaloosa sands, and associated Tuscaloosa shales.

Seismic fracture indicators, which are similar to those described by Kuich4 in Giddings Austin chalk field of Texas, show potential halokinetic fractures providing the pathway for migration of hydrocarbons into the fractured chalk with the ductile upper Austin marl providing the reservoir seal.

McCulloh5 et al., in a study of Tuscaloosa sandstones, identified geopressured Eagle Ford shales between underlying normal pressured Tuscaloosa sandstones and overlying normal pressured Austin chalk.

It is possible that the oil filled fractures in the Austin chalk may be somewhat geopressured while the matrix chalk remains normally pressured.

The exploration implication of halokinetic fracturing is that by applying the halokinetic fracturing theory and model to underexplored and previously untargeted halokinetic areas, domestically and globally, some new and possibly giant producing trends likely will be discovered.

Horizontal drilling technology can now economically reach fractured reservoirs in brittle lithologies that were marginally productive through vertical completions. Good quality seismic data are already available or may be easily acquired for halokinetic fracture identification.

REFERENCES

Thomas, G.E., Effects of differential compaction fracturing shown in four reservoirs, OGJ, Feb. 3, 1992, pp. 54-57.
Hermann, L.A., Lower Cretaceous Sligo reef trends in central Louisiana, Gulf Coast Association of Geological Societies Transactions, Vol. 21, 1971, pp. 187-198.
Kupfer, D.H., Crowe, C.T., and Hessenbruch, J.M., North Louisiana Basin and Salt Movements (Halokinetics), Gulf Coast Association of Geological Societies, Vol. 26, 1976, pp. 94-110.
Kuich, N., Seismic Fracture Identification and Horizontal Drilling: Keys to Optimizing Productivity in a Fractured Reservoir, Giddings Field, Texas, Gulf Coast Association of Geological Societies Transactions, Vol. 39, 1989, pp. 153-158.
McCulloh, R.P., and Purcell, M.D., Hydropressure Tongues Within Regionally Geopressured Lower Tuscaloosa Sandstone, Tuscaloosa Trend, Louisiana, Gulf Coast Association of Geological Societies Transactions, Vol. 33, 1983, pp. 153-159.