FRACTURED ZONES DRAW HORIZONTAL TECHNOLOGY TO MARIETTA BASIN

M.G. Whitmire
Bandera Petroleum Inc.
Tulsa

Vertically fractured, low permeability reservoirs have long frustrated the efforts of oil and gas operators.

Oil men, risk takers by nature, cannot resist the challenge to try to beat the average, subeconomic well and get the better wells.

The advent of horizontal drilling technology gave new life to these hopes, and a number of drilling mini-booms developed in a diverse, widespread variety of vertically fractured U.S. reservoirs.

Results to date have been mixed. A few wells have been successful, but many have not.

The Marietta basin of southern Oklahoma may be the location of the next successful horizontal drilling play targeting the highly fractured Ordovician Viola formation.

BASIN SUMMARY

The Marietta basin is a narrow, elongated, northwest-southeast trending basin about 20 miles wide and 70 miles long.

It is frequently called the Sherman-Marietta basin because its deep axis lies along a line extending through Sherman, Tex., and Marietta, Okla.

The basin is sandwiched between the Criner Hills structural complex on the northeast and the Muenster arch on the southwest. It occupies much of Jefferson, Love, and Carter counties, Okla., and Cooke and Grayson counties, Tex. (Fig. 1).

The basin was formed by collision and subsequent rotation of the North American and South American plates which created a triple junction near the southeast end of this basin.

This compression squeezed the pre-Pennsylvanian rocks in this basin, forming several northwest-southeast trending structural fold belts. More importantly it caused severe vertical fracturing in these formations.

Two or more distinct fracture systems result in a "chicken wire" fracture pattern in a number of producing formations, including but not limited to Mississippian Sycamore and Woodford, Siluro-Devonian Hunton, Viola, and Cambro-Ordorvician Arbuckle.

Some of these formations often have primary porosity usually enhanced by fracturing, and if drilled within a trap are capable of major production. Individual wells may produce more than 500,000 bbl of oil.

Those producing primarily from fractured reservoirs are typically erratic. Production maps commonly show 200,000 bbl wells surrounded by 20,000 bbl wells.

The good wells happen to penetrate one or more good "cracks," and the poor wells penetrate the "less open," "less extensive," and "less communicated" cracks.

This of course immediately captures the imagination of the horizontal driller because drilling horizontally increases exponentially the odds of encountering the good cracks. The horizontal bore hole must be carefully planned and carefully executed.

HORIZONTAL DRILLING

All newly developing techniques seem to pass through three stages:

The initial stage in which enthusiasm and expectation are grossly exaggerated.
The intermediate stage in which frequent failures shatter those high expectations and a general distrust of the technology develops.
The final stage in which successes and failures are carefully analyzed and slow but meaningful technological advancement occurs.

The industry has probably entered the third phase, and the next few N,ears should show good results.

The Marietta basin has experienced a few horizontal drilling efforts. Two or three aborted attempts were made in the late 1980s, but the technology was new and by today's standards those attempts were ill conceived.

Two more recent attempts were completed for 250 b/d/well from Viola, but both had only a short lateral component when they stopped and were completed.

The most recent horizontal well drilled is the recently completed Esco Exploration Inc. 1-15H Dillard, in 15-5s-4w, Jefferson County, Okla. This well has a lateral component of about 1,500 ft, but it did not actually evaluate the primary objective.

Due to operational problems the well penetrated only the Upper and Middle Viola horizontally and did not reach the zone where maximum fracturing was expected.

The better production in the Viola usually occurs in the lower one third of the formation. However, the well was completed for 400 Mcfd of gas.

The optimum lateral component for drilling the Viola is probably in the 1,500-2,000 ft range. In this structurally complex area, the risk of geological or operational surprise increases rapidly with horizontal distance.

Cost of drilling and completing these horizontal wells is expected to be $500,000 to $1.5 million, making exhaustive analysis and planning a necessity.

GEOLOGY

The Marietta basin is separated from the Ardmore basin to the northeast by the Criner Hills structural complex, a classic "flower structure."

The folding and major faulting in this basin parallel the Criner Hills trend with an approximate North 45 West orientation.

One of the fracture systems present probably parallels this trend. Other systems may be at 30 or 60 angles to this primary system.

Drilling objectives include the following formations:

Sycamore: A shaly Mississippian carbonate formation, usually heavily fractured; can be an excellent producer.
Woodford: A black, organic shale, probably the best source rock in the Midcontinent area, highly fractured, and could be an excellent target. This is one of the underexplored objectives, probably because it is not the typical sand or carbonate reservoir.
Hunton: One of the most prolific producing formations in Oklahoma, a Siluro-Devonian carbonate, frequently produces large volumes in this basin from primary porosity in structural traps, particularly on a structure that has been sub-aerially exposed. It is absent due to erosion on some of the more prominent structures but can be preserved and trapped on the flanks of structures or in downthrown blocks. Single wells produce 500,000 bbl in such instances.
Viola: This formation probably will be the favorite objective for horizontal drilling. It is a highly fractured, massive limestone formation averaging 1,000 ft in thickness (Fig. 2). It has been drilled and completed in hundreds of vertical wells with results typical of the Upper Cretaceous Austin chalk and other vertically fractured reservoirs. It is difficult to drill a dry hole but impossible to get consistently good results with vertical drilling. Thus it becomes the prime objective for horizontal drilling.
Arbuckle: This is the least evaluated objective, probably because it is the deepest. It may be the most overlooked objective in the basin in spite of the fact that Healdton field, located along the northern rim of the basin, has produced more than 300 million bbl of oil from this formation. It is a massive limestone-dolomite formation, perhaps reaching 4,000 ft in thickness. It is believed to be highly fractured, much the same as the younger rocks. Like the other carbonate formations, it develops primary porosity due to dolomitization and, where sub-aerially exposed, develops porosity by weathering and karsting. There are several "Wilburton" style traps in this basin that have not been tested adequately. It is possible that this formation will ultimately produce more hydrocarbons in this basin than any of the younger formations.

VIOLA AS OBJECTIVE

The rest of this discussion will focus primarily on the vertically fractured Viola formation.

Planning the orientation of the borehole requires a thorough understanding of the structural configuration and the fracture patterns in the horizontal plane.

Crossing a fault unexpectedly or miscalculating the dip angle of the formation can result in failure. An abundance of well control and high quality seismic data make this a manageable risk.

In some areas the formations are dipping at such a high angle that slant hole drilling rather than horizontal drilling is a feasible approach. The orientation of fracturing can be established in some areas by running modern fracture-finding tools in existing boreholes.

There has been a tendency in horizontal drilling of vertically fractured reservoirs to "snuggle up" to vertical boreholes that have produced large volumes of oil. This can be a serious mistake, as the horizontal borehole may penetrate a depleted reservoir, introducing a broad range of problems.

An argument can be made for drilling in the midst of vertical boreholes that have produced smaller volumes, since the vertical wells may not have encountered the larger cracks.

When a massive, brittle carbonate formation such as the Viola is compressed and shortened, the resulting anticlines and synclines are accompanied by fracturing throughout the formation, but maximum fracturing occurs with maximum "bending" or "maximum curvature."

This usually occurs on the crest of anticlines and the bottom of synclines. The maximum opening of fractures would occur in the upper portion of the unit on anticlines and in the lower portion of the unit in synclines.

In this basin, like most basins, the synclinal areas are sparsely drilled while the anticlinal areas are heavily drilled.

SEISMIC ABUNDANT

This basin is blessed with an abundance of excellent quality seismic data that has one of the best signal to noise ratios of all the interior basins in the U.S.

This not only provides a basis for highly reliable structural mapping but a basis for identifying "fracture swarms" and orientation of fracturing.

Currently developing technologies such as amplitude vs. offset (AVO) and azimuthal anisotrophy (AA) have been effectively used with carefully processed, high quality data. This technology can be helpful in drillsite selection and borehole path planning.

LAND, RESERVES

It is possible to assemble drilling blocks in this basin.

A large percentage of the basin is open, a significant percentage is held by production by marginal wells, and some blocks are available by farmout.

Acreage prices vary broadly depending on proximity to existing production.

There is no real basis for reliably estimating reserves for horizontal wells, but recoveries of 500,000 to 1 million bbl would not be unreasonable.

In summary, the Marietta basin has the right ingredients for a major horizontal drilling play. Huge reserves remain in these vertically fractured reservoirs.

The production should be predominantly oil, except for the Arbuckle, which may be predominantly gas.

When the oil and gas industry recovers from its current shock, this basin will undoubtedly receive a great deal of attention from horizontal drillers.