NEW MEXICO ELEVATOR BASINS-2 - Petroleum systems described in Estancia, Carrizozo, Vaughn basins

Jan. 15, 2001
This is the second part of a three part article on late Paleozoic elevator basins of the southern ancestral Rocky Mountains.

This is the second part of a three part article on late Paleozoic elevator basins of the southern ancestral Rocky Mountains.

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The Estancia basin of central New Mexico (Fig. 1) is an asymmetric, north-south trending structural depression.11

The present-day basin covers 1,500 sq miles. It is bordered on the east by the late Paleozoic Pedernal uplift, on the west by the Tertiary-age Los Pinos, Manzano, and Sandia Mountains, on the north by the Espanola basin, and on the south by Chupadera Mesa. The Rio Grande rift lies west of the Los Pinos, Manzano, and Sandia Mountains.

The eastern flank of the basin is formed by late Paleozoic tectonic elements and the western flank is formed by Tertiary tectonic elements. On the western flank of the basin, Pennsylvanian and Lower Permian strata crop out on the eastern flank of the Los Pinos, Manzano, and Sandia Mountains and dip eastward into the basin.

Depth to Precambrian ranges from 8,700 ft in an elevator basin in the east to less than 1,000 ft on a shelf west of the elevator basin.

Perro sub-basin

The Perro sub-basin is an elevator basin along the eastern flank of the Estancia basin (Fig. 8). It trends north-south and is bordered on the east by the Pedernal uplift. The shelf of the Estancia basin lies to the west. The Perro sub-basin is 20 miles long and 5 miles wide.

Reflection seismic data indicate that bounding faults are high angle with a normal sense of movement.31

Strata that infill the Perro sub-basin are Morrowan (Early Pennsylvanian) through Wolfcampian (Early Permian) in age. Morrowan strata rest unconformably on Precambrian basement. Most of the basin fill appears to be Morrowan to Atokan in age,11 indicating that the basin formed primarily during the Early Pennsylvanian.

The Morrowan and Atokan sections consist of black shales, fine-grained micaceous sandstones, and coals. Sandstones were derived from erosion of the Precambrian metasediments and igneous rocks that form the core of the Pedernal uplift to the east.

Precambrian metasediments that lie below the Pennsylvanian on the western shelf also provided detrital material for the lower parts of the basin fill.

In contrast to the Cuervo sub-basin, the Perro sub-basin apparently was filled with siliciclastic sediments as quickly as it subsided. With sedimentation exceeding rates of subsidence, sands were deposited not only in the sub-basin but were transported across the sub-basin and onto the shelf to the west.11 Significant carbonates are not encountered on the western shelf until one is 10 miles west of the western boundary fault of the Perro sub-basin.

The deeper parts of the Perro sub-basin are the main hydrocarbon kitchen in the Estancia basin. Black shales and coal in deeper parts of the Perro sub-basin are thermally mature source rocks of gas.11 The black shales have TOC values of 1-2%, sufficient for hydrocarbon generation. Kerogens in the black shales are dominantly woody, gas-prone types. Coals generated gas upon maturation.

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Source rocks are thermally mature in the deeper parts of the Perro sub-basin where TAI values exceed 2.6 (Fig. 9). Outside of the Perro sub-basin, potential source rocks in the Estancia basin are sub-mature except in the westernmost updip part of the basin where high heat flow from the Rio Grande rift has placed Pennsylvanian strata in the oil window11 (Fig. 9).

Petroleum system

The petroleum system in the Estancia basin is similar to the petroleum system in the Tucumcari basin. Pennsylvanian shales and coals in the sub-basin have generated gas. Some of this gas may have been trapped in sandstones in the sub-basin. Shows in this sparsely drilled area support this concept.11

Gas may also have migrated vertically upward along basin-bounding faults and into shelf sediments east and west of the Perro sub-basin. Shelf sediments to the east are mostly sandstones in the Abo and Yeso formations (Permian).

No updip seals are present to the east; therefore it is unlikely that gas may be trapped in that direction. On the west, however, gas could have migrated updip through sandstones in the Pennsylvanian section until it became trapped in an updip position in structures or by porosity pinch- outs in Pennsylvan- ian sandstones and, farther west, in reefal limestones.

Carrizozo basin

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The Carrizozo basin of central New Mexico (Figs. 1, 10, 11) is a north-south trending elevator basin that originated during the Pennsylvanian. Although itis not well defined because the region is sparsely drilled, gravity data indicate the basin covers 500 sq miles. It is 30 miles long and 15 miles wide.

The Carrizozo basin is bordered on the east by the late Paleozoic Pedernal uplift and on the west by a Paleozoic shelf area that underlies the present-day Chupadera Mesa. The Estancia basin lies to the north. The Oscura anticline, a large north-south trending surface structure of Tertiary age, lies 8 miles west of the basin. The boundary to the south is poorly constrained by existing data, but it is thought that the Carrizozo basin is connected to a northern arm of the Orogrande basin.

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The deepest parts of the Carrizozo basin have not been drilled, but gravity modeling by G.R. Keller of the University of Texas at El Paso suggests it may be as deep as 10,000 ft.32 What is interpreted from gravity data as an intrabasinal horst has been drilled by the Manzano 1 Cat Head Mesa well (Fig. 11). Depth to Precambrian on this horst is 4,200 ft. Depth to Precambrian on the Pedernal uplift to the east and the shelf area to the west of the Carrizozo basin ranges from 2,000-4,000 ft. A recently acquired seismic reflection survey has confirmed the presence of a deep basin bounded by high-angle faults.

Basin fill sediments were encountered in the Cat Head Mesa well and in wells drilled in the northern arm of the Orogrande basin. Basin fill appears to consist mostly of Pennsylvanian sandstones and black shales, Wolfcampian (Lower Permian) red sandstones, conglomerates, and shales of the Abo formation, and Leonardian (Lower Permian) salts, sandstones, shales, dolostones, and limestones of the Yeso formation.

The Standard of Texas 1 Heard well, drilled in the northern extension of the Orogrande basin (Fig. 10), encountered Precambrian basement at a depth of 7,754 ft. The Pennsylvanian section in that well is 1,344 ft thick and ranges in age from Atokan (Early Pennsylvanian) through Virgilian (Late Pennsylvanian).33 The Atokan strata rest unconformably on Precambrian The Bursum and Abo formations (Wolfcampian; Early Permian) overlie the Pennsylvanian section and are 1,940 ft thick. The Yeso rests on the Abo and is 4,265 ft thick in the well (Foster, in Griswold34), 2,700 ft thicker than in other wells in the area. The extra thickness of the Yeso is partially explained by 900 ft of salt that is not present in other wells in the area. This salt was apparently deposited in a relict basin that formed initially during Pennsylvanian but had continued existence as a topographic basin through the Leonardian.

The only other petroleum exploration wells in the area that contain identifiable salt in the Yeso are the Manzano 1 Cat Head Mesa well and the Texaco 1D Federal well (Fig. 10). In the Cathead Mesa well, the red beds of the Abo formation (Early Permian) rest unconformably on 330 ft of Atokan (Early Pennsylvanian) sediments (Fig. 11). This Atokan section consists of black shales and sandstones. Middle through Upper Pennsylvanian strata are missing.

A much thicker section of Atokan and post- Atokan Pennsylvanian strata as well as a thicker section of Yeso salt may well be preserved in adjoining, deeper areas of the basin. In the Texaco 1D Federal well, the interval of Yeso formation that contains identifiable salt on the well logs is 1,400 ft thick.35

Petroleum system

Atokan shales are mature source rocks of oil and gas in the Cat Head Mesa well. The TAI in the black shales ranges from 2.5-3.5. These values are in the oil window. The highest values of TAI appear to be located next to Tertiary-age igneous sills.

Because the Pennsylvanian has been buried more deeply in the surrounding basin than on the intrabasinal horst, levels of maturity should be higher in the deep basin, the lower part of which could conceivably be in the thermogenic gas zone (Fig. 11). TOC exceeds 1%, sufficient for petroleum generation. Both oil-prone and gas-prone kerogens are present. Oil-prone kerogens are dominant in different shale beds than the gas-prone kerogens are; therefore, some shales are oil generative and other shales are gas generative.

Salt in the Yeso provides a vertical seal to migration. Widespread bedded salt provides an excellent seal in a basin.36 Therefore, hydrocarbons generated in the deep basin should be preserved in the subsalt section. Possible subsalt reservoirs are the sandstones in the lowermost Yeso, sandstones in the Abo, and sandstones in the Pennsylvanian.

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The Cat Head Mesa well encountered an excellent but enigmatic gas show while drilling through Precambrian basement (Fig. 12). Two explanations were put forward for this show. The show had to have come from the pre-Abo section because the Abo formation had been cemented behind casing when the show was encountered. Therefore, the show represented either delayed entry of gas into the borehole from underpressured basal Atoka sandstones or it represented entry of gas into the borehole from fractures in the Precambrian crystalline basement.

It is possible that the gas came from the basal Atoka sandstones. The well was drilled with fairly heavy mud (about 10 lb/gal). The Atoka sandstones appear to be porous and permeable when cuttings are examined with a binocular microscope. Unfortunately, a density porosity log is not available for the well. Drillstem tests in the region indicate the basin is somewhat underpressured. The relatively heavy mud would have invaded a permeable underpressured reservoir while the reservoir was being drilled, thereby delaying entry of gas into the borehole.

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The gas may also have come from a fracture system in Precambrian. Precambrian in the well consists of granodiorite and diabase. A drillstem test in the Precambrian recovered water, indicating that a fracture system is present in the Precambrian of the horst block. The overlying Atokan shales and the Atokan shales in the basin adjacent to the horst block may act as seals for the fracture system. The Pennsylvanian shales in the adjacent basin may be source rocks. Very similar situations form some of the larger oil fields in the Gulf of Suez (Fig. 13).37 38

Vaughn basin

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The Vaughn basin is an elevator basin in north-central New Mexico (Fig. 1). A south-projecting arm of the Pecos shelf, it splits the northern part of the Pedernal uplift into two segments (Fig. 14). The main axis of the Pedernal uplift lies west of the Vaughn basin. To the east is an eastern limb of the Pedernal uplift, the San Ignacio platform of Reynolds18 and Reynolds and Reynolds.19 To the north, the Vaughn basin connects with the Pecos shelf; which has been mapped by Kottlowski39 and Baltz and Myers.14

The Vaughn basin has not been drilled. Its existence is postulated from a combination of three data sets:

  1. Subsurface data in wells that have been drilled east, west, and north of the basin.
  2. Regional aeromagnetic intensity data.40
  3. Surface geology.41
An interpretive structural cross section across the basin shows two wells on the Pedernal uplift that indicate strata dip gently to the east with the Permian resting unconformably on the Precambrian (Fig. 15). On the east side of the cross section is a well that also has Permian resting unconformably on Precambrian. Between these areas is a minimum on the aeromagnetic intensity profile. This magnetic low is seen on the regional aeromagnetic intensity map 40 to connect northward with a magnetic low that coincides with the Pecos shelf.

The east side of the magnetic minimum underlies a major north-south trending fault (the Vaughn fault) that was mapped at the surface by Kelley.41 The Vaughn fault offsets Tertiary and Quaternary sediments at the surface. Vertical offset at the surface may be as large as 1,000 ft.41

Many surface faults in the region appear to be reactivated late Paleozoic and Precambrian faults.11 13 42 43 44 This observation, in conjunction with the mapped magnetic low that projects southward from the Pecos shelf, suggests that a Pennsylvanian elevator basin is present in the subsurface west of the Vaughn fault. If so, analogy with other elevator basins discussed in this article suggests it may contain shale source rocks in the Pennsylvanian section.

Because these strata may be buried to depths comparable to other elevator basins discussed in this article, they may hold mature source rocks of oil or gas. Reservoirs may be Pennsylvanian sandstones deposited in the basin. Alternatively, hydrocarbons generated in the Vaughn basin could have migrated vertically along the bounding faults and into permeable Paleozoic strata west or east of the elevator basin. Hydrocarbons could also have migrated northward into shelf strata on the Pecos shelf.

Summary of studied basins

Elevator basins are defined as long, narrow, and structurally deep troughs bounded by high-angle faults. They are typically 20-50 miles long and 5-15 miles wide. Bounding faults have vertical offsets that can exceed 5,000 ft.

Several elevator basins of Pennsylvanian age have been identified and studied in central and east-central New Mexico. These include the Cuervo, Trementina, Trigg Ranch, and Quay sub-basins of the Tucumcari basin, the Perro sub-basin of the Estancia basin, the Carrizozo basin, and the Vaughn basin.

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These elevator basins began forming during the Early to Middle Pennsylvanian and saw continued tectonic activity into Early Permian. These basins separate Pennsylvanian-age uplifts from Pennsylvanian-age marine shelves. Most of these elevator basins are elongate parallel to the bordering uplifts but some cut obliquely across the boundaries of these uplifts.

The elevator basins acted as traps for siliciclastic sediment derived from erosion of the Precambrian cores of the adjoining uplifts. Basin fill consists of black shales, arkosic conglomeratic sandstones, and in some cases coals. The shales and coals are thermally mature, kerogen-rich hydrocarbon source rocks. Kerogens in the shales in some of the studied elevator basins are gas prone; in other studied basins the kerogens are oil and gas prone.

Source rocks are concentrated in the elevator basins. Deeper burial in the elevator basins as compared to the adjoining shelves has rendered the Pennsylvanian strata more mature in the basins than on the shelves. TOC values in the shales generally exceed 1% and can exceed 9%. Overlying Permian strata are generally thermally immature and contain insufficient TOC to be source rocks.

A single but complex type of petroleum system is common to all of the studied elevator basins. Oil or gas generated in the basins may be trapped in sandstones in the basin fill. For these accumulations, migration involved only short distances from source rocks into interbedded reservoirs.

Fields similar to Broken Bone field of Cottle County, Tex.,7 may be present. However, hydrocarbons may also have utilized basin-bounding faults as migration pathways. Those hydrocarbons were expelled from source rocks and carrier beds in the basins and moved vertically upward along the faults. From the faults they migrated laterally into permeable Pennsylvanian, Permian, and Triassic reservoirs adjacent to the elevator basins. In this way, hydrocarbons could be trapped and accumulate in shallow, thermally immature strata.

In the Tucumcari basin, recurrent fault movement during the Laramide orogeny resulted in the upward propagation of faults through the Triassic section. Hydrocarbons generated in Pennsylvanian shales moved vertically upward along faults and into Triassic sandstones where they accumulated in combination structural-stratigraphic traps.

Next (conclusion): Elevator basin models, implications for exploration.