SOURCE, RESERVOIR PROMISE SEEN IN MARATHON-OUACHITA OVERTHRUST

Ali Said Trabelsi
David K. Davies & Associates Inc.
Houston

The Permian Basin of West Texas is a prolific 7 oil and gas province that has been extensively explored, but the Marathon Ouachita overthrust area of Pecos County, Tex., is not fully explored.

Rocks of the Ouachita fold belt have been generally regarded by most petroleum geologists as metamorphosed and unsuitable for oil and gas accumulation.

Indications of the presence of hydrocarbons in Ouachita rocks have been reported from the earliest days of Permian Basin exploration. Goldstein and Flawn 1 indicated that in the subsurface Ouachita fold belt in Texas asphaltic materials are fairly common in sandstones and cherts.

At present, hydrocarbon (mostly gas) production has been well established in areas adjacent to the Marathon-Ouachita overthrust region, Pecos County (Grey Ranch, Downie, Pinon, and Thistle fields).

The Ouachita overthrust area in Texas has all the required elements for hydrocarbon accumulation and should be fully explored. This article gives a brief assessment of these elements (traps, source rocks, and reservoirs) in this area.

SURFACE GEOLOGY

Rocks exposed in the Marathon-Ouachita structural belt in Pecos and Terrell counties include strata of Pennsylvanian, Permian, Cretaceous, and Quaternary age. Pennsylvanian rocks are exposed in the Marathon region, and Permian rocks crop out in the Sierra Madera dome and in the Glass Mountains.

A large portion of the Ouachita overthrust area is covered by Lower Cretaceous rocks, Quaternary alluvium, and pediment gravel. Long anticlinal folds can be mapped in Cretaceous and Quaternary rocks. These folds generally trend eastward and plunge away from the Marathon-Ouachita uplift.

The Marathon uplift consists of many sharp anticlines trending northeastward that are mapped in Pennsylvanian rocks. The anticlinal folds reflected on the surface Cretaceous rocks could represent imprints of structural traps at depth. Cretaceous structural and geomorphological surface expressions have led to the discovery of Grey Ranch field. Large concentrations of fractures are associated with these Cretaceous anticlines. Surface fractures could reflect fractured reservoirs in the subsurface.

Because of the complex folding and faulting of the Ouachita overthrust, 3D seismic surveys are needed to identify structural features in detail. Conventional subsurface geologic and geophysical methods alone can be of limited value.

SOURCE ROCK EVALUATION

To assist in evaluation of source rocks in the area of study drill cuttings of three shale units (Wolfcampian, Woodford, and Simpson) from four wells (Fig. 1) were examined for organic richness (TOC), type of organic matter, and thermal maturity.

TOC was determined by the titration method, and values are reported in weight percent. The type of organic matter was identified by visual petrographic examination, and the thermal maturity of these shales was estimated using thermal alteration index (TAI).

Simpson shale samples from the Mobil 1 Cox well in Pecos Country were also analyzed for vitrinite reflectance (R.) values. A brief lithological description and results of these analyses of each shale unit are given below.

WOLFCAMPIAN SHALE

Wolfcampian shale in the area of study is black to grayish-black and contains a significant amount of silt-sized quartz grains. This shale registers about 97 API units on the gamma ray log and reaches several thousand feet in total thickness.

A considerable amount (about 9% of the rock volume) of cubic and framboidal pyrite and plant debris was detected in this shale. The dark coloration, and the abundance of pyrite in this shale, suggest deposition under reducing conditions.

WOODFORD SHALE

Woodford shale (Upper Devonian) is believed to be the source of at least 10% of the hydrocarbons in the Permian Basin.2

In Pecos County, this shale consists of distinctly laminated silty black intervals that average about 60 ft in total thickness. Woodford shale registers about 200 API units on the gamma ray log and contains a significant amount of organic matter, phosphatic nodules (Fig. 2), and cubic pyrite. The black color, the preservation of laminations, and the abundance of organic matter and pyrite suggest that this shale was deposited under anoxic water conditions.

SIMPSON SHALE

Simpson shale (Ordovician-Silurian) consists of laminated grayish-black and greenish-black intervals that register about 73 API units on the gamma ray log.

Simpson shale contains about 8% silt-sized grains of quartz, feldspar, and mica. Shale intervals are intercalated with Simpson sandstone and carbonate units.

TOTAL ORGANIC CARBON

Total organic carbon values for Wolfcampian shale range between 0.26 and 0.42 wt %; Woodford contains 2.44 to 4.31 wt % organic carbon; and TOC values for Simpson shale range between 0.21 and 0.54 wt %.

An empirical lower limit of 0.4 wt % organic carbon has been accepted by most organic geochemists as the minimum concentration required for large-scale hydrocarbon generation.'

During burial and hydrocarbon generation, the percentage of organic carbon continuously decreases because of the thermal effect.' In a mature source rock the measured concentration of TOC could be significantly less than the original concentration in the immature source rock.

Tissot3 suggested that in the deeper parts of sedimentary basins, a measured residual of 0.5 wt % TOC may reflect an original content of 1 wt % or more. Therefore, an acceptable TOC lower limit of subsurface rocks might be less than the accepted figure of 0.4 wt %. By this reasoning, Wolfcampian, Woodford, and Simpson shales which contain about 4 wt % TOC or more ' are organically rich enough to serve as source rocks for hydrocarbons in the area of interest.

MATURATION AND TYPE OF ORGANIC MATTER

Wolfcampian shale predominantly contains herbaceous, gas-prone organic matter (kerogen Type 1). This organic matter is represented by large concentrations of plant debris. Amorphous, marine organic matter is also present. Woodford shale contains oil-prone organic matter (alginite palynomorphs), and Simpson shale contains alginite and indigenous vitrinite. Alginite is more abundantly represented in this shale.

Based on TAI values, Wolfcampian shale in the area of study is thermally mature and should generate wet to dry gas. TAI values fall in the range of 3 - to 3 +, thus corresponding to a vitrinite reflectance (Ro) of 1.35 to 3.0.

Estimated TAI value for the Woodford shale reaches 3+. This value corresponds to a vitrinite reflectance of about 2.0, and hence bypasses the "oil generation window" to the dry gas zone (metacatagenic phase).

A vitrinite reflectance (Ro) of 2.57% was determined for this shale. This reveals that this rock is post-mature with respect to oil generation and is approaching the end of the gas generation phase. Gas generation window ends at an R(, value of approximately 3.0%.

POTENTIAL RESERVOIRS

Several potential hydrocarbon reservoirs exist in the area of study. They include from oldest to youngest: the Ellenburger dolostone, the Montoya cherty limestone and dolostone, and the Devonian chert. These rocks represent the major reservoirs in the fields adjacent to the Marathon-Ouachita overthrust area.

ELLENBURGER RESERVOIR

The Ellenburger rocks (Lower Ordovician) constitute a major reservoir in the Permian Basin of West Texas. Holtz et al.' subdivided this reservoir into three reservoir groups:

Ellenburger karst modified dolostone
Ellenburger ramp carbonate, and
Ellenburger tectonicly fractured dolostone.

The distribution of these reservoirs throughout the Permian Basin is discussed in detail by Holtz et al,' who found that in the Marathon-Ouachita area the Ellenburger consists of tectonicly-fractured dolostone. This reservoir group is subparallel with the Ouachita overthrust.4

The Ellenburger carbonates range in total thickness between 1,000 ft to the south and 1,500 ft in the central and northern part of Pecos County. To the north of the leading edge of the Ouachita overthrust, the top of the Ellenburger rocks was encountered between 15,000-21,000 ft; to the south of this fold belt, the Ellenburger was penetrated at a drilling depth of about 13,000 ft.

The Ellenburger rocks consists of limestone, dolomitic limestone, and dolostone. Dolostone units exhibit better porosity and permeability than their limestone counterparts. Estimated porosity from well logs (FDC/CNL and sonic) and from petrographic examination ranges from 4-5%.

The most porous dolostone units have up to 7% porosity and are 3-4 ft thick. Porosity consists of inter-crystalline pore and partially open microfractures (Fig. 3). Porosity and permeability improve near faulted zones because of intensive fracturing. Examples of Ellenburger collapse breccia zones are encountered in the Mobil 1 Cox well. This would suggest that Ellenburger karst-associated porosity also locally exists in parts of the Ouachita overthrust area.

MONTOYA RESERVOIR

The Montova formation (Upper Ordovician) consists of a suite of cherty carbonate rocks that are widespread in the subsurface of the Permian Basin. This formation produces from several fields such as Abell, South Pecos Valley, Heiner, and Lehn-Apco. 5

The Montova rocks were folded, faulted, and fractured during intense Permo-Pennsylvanian tectonic activities.5 During this time, reservoirs were developed and Montova traps formed. Wright 5 suggested that Ellenburger and Simpson hydrocarbons could have migrated upward along fault planes and fractures into Montoya reservoirs.

In the Marathon-Ouachita area, the Montova carbonates range in thickness between 140-270 ft. Based on log and petrographic analyses, the effective porosity averages about 5%. Many dolostone intervals (5-10 ft thick) within the Montoya formation have porosity as high as 7%. In cherty limestone intervals, porosity consists predominantly of microfractures, whereas dolostone intervals display intercrystalline porosity and anhydrite molds.

DEVONIAN CHERT

Devonian chert has good potential for hydrocarbons accumulation in the Marathon-Ouachita overthrust region. Large quantities of hydrocarbons have been produced from this rock (e.g. Pinon, Elsinore, and Thistle fields). This chert is light tan in color. It reaches up to 250 ft in thickness and thins to about 50 ft at the Sierra Madera dome (French 1 Sierra Madera well). The thinning of this rock unit may be due to truncation.

The Devonian chert is a porous reservoir. Porosity occurs primarily in leached molds of dolomite rhombs (Fig. 4), sponge spicules, and fractures. Average porosity of this reservoir is 7%. In Tunis Creek field, Pecos County, the porosity of this reservoir reaches up to 14%. Because fractures in this reservoir are tectonically induced, porosity and permeability are enhanced near faulted and folded zones.

CONCLUSIONS

All the shale units (Wolfcampian, Woodford, and Simpson) in the Marathon-Ouachita region constitute good source rocks for hydrocarbons. These source rocks are mature to Post-mature with respect to liquid hydrocarbon generation and are approaching the end of the dry gas generation.
Several structural features mapped in Cretaceous and Quaternary rocks in the are could represent imprints of structural traps at depth. Good 3D seismic data should confirm the existence of these traps.
The Ellenburger Montoya and Devonian Chert reservoir quality in the area of interest is adequate for hydrocarbon accumulation. Porosity and permeability improve near faulted and folded areas, due to intensive tectonic fracturing of these rocks.

ACKNOWLEDGMENT

I am grateful to Dr. David K. Davies for his enthusiastic support of this article. Special thanks to Michele Young for typing the final manuscript.

REFERENCES

Goldstein, Jr., A., and Flawn, P.T., Economic possibilities in the Ouachita system, Bureau of Economic Geology, The University of Texas at Austin, Publication No. 6120D, 1961, pp. 191-195.
Dow, W.G., Petroleum source beds on continental slopes and rises, AAPG Bull., Vol. 62, 1978, pp. 1,584-1,606.
Tissot, B., The application of the results of organic geochemical studies in oil and gas exploration, in Hobson, G.D., ed., Developments in petroleum geology: London, Applied Science Publishers, 1978, pp. 53-82.
Holts, M.H., and Kerans, C., Characterization and categorization of West Texas Ellenburger reservoirs in paleokarst, karat related diagenesis, and reservoir development, examples from Ordovician-Devonian age strata of West Texas and the Mid-Continent, Permian Basin Section-SEPM Publication No. 92-33, 1992, pp. 110-120.
Wright, W.F., Petroleum geology of the Permian Basin: West Texas Geological Society, 1979, pp. 166.