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Devonian Shelf to Basin Facies Distributions and Preliminary Shale Geochemistry for South-central and Southwestern New Mexico*
By
William D. Raatz1
Search and Discovery Article #10074 (2004)
*Adapted from extended abstract of presentation at Southwest Section, AAPG, Annual Meeting, Fort Worth, Texas, March 1-4, 2003
1New Mexico Bureau of Geology and Mineral Resources, a Division of New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 ([email protected])
Abstract
The often
poorly exposed Devonian section in southern New Mexico contains complex vertical
and lateral ramp-to-basin facies changes that culminate in an elongate trough
filled with up to 76 m of
black
shale. The trough trends east-west for over 350
km, and ranges from a width of ~30 km near Las Cruces to ~100 km near Deming. To
the north, Middle-Late Devonian-aged ramp carbonates, sandstones, siltstones,
and shales of the Oñate (Givetian), Sly Gap (Frasnian), and Contadero (Frasnian-Fammenian)
formations crop out in the San Andres and Sacramento mountains. These formations
grade southward into the Percha Formation
black
shale facies. Due to the largely
barren nature of the shales, it is difficult to determine exact correlations.
The Percha Formation is divided into two members, the
black
, fissile, barren
Ready Pay and the more calcareous and fossiliferous
Box
. Fossils from the upper
Box
Member indicate a Fammenian age. South of the trough, the oldest Devonian
units comprise the cherty shelf carbonate Canutillo Formation (Middle-Late
Devonian), which is both overlain by, and apparently a partial lateral facies
equivalent to, the Percha
black
shales.
Existing 1960’s
vintage isopach and facies maps are updated with more recent outcrop and
subsurface data, and integrated with geochemical data and basin analysis models
to characterize better the areal extent, volume, richness, and maturity of the
black
shale facies. Public-domain geochemical data show potential for the
Devonian shales in south-central and southwestern New Mexico to act as source
rocks: TOC values locally reach 3.7% and thermal maturities are consistently in
the mature to very mature range.
Three goals exist for this ongoing project:
-
Better constraints on formation correlations and facies relationships
-
Improved characterization of Devonian shales for hydrocarbon source rock potential (richness and maturity trends, kerogen types, and expulsion timing)
Description of the shales for shale gas reservoir potential.
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Table and Figure Captions
IntroductionThe
Devonian succession in southern and southwestern New Mexico has a long
history of stratigraphic study (e.g., Nelson, 1940; Stevenson, 1945;
Laudon and Bowsher, 1949; Kottlowski et al.,1956; Pray, 1961; Kottlowski,
1963; Rosado, 1970; LeMone, 1982, 1996; Sarouf, 1984; Day, 1988, 1998);
however, only a limited number of reports place the complex facies into
a regional context (Kottlowski, 1963; Bowsher, 1967; Raatz, 2002) or
discuss organic geochemistry and thermal history (Broadhead, 2002). This
report updates and re-evaluates stratigraphic data and integrates
organic geochemistry, burial history, and other aspects relevant to the
Devonian System’s hydrocarbon potential. Table 1 lists named well and
outcrop locations used in the study, with numbers keyed to locations in
Figure 1. A total of 151 locations were used to construct a Devonian isopach map (Figure 2), and from those data points with sufficient
information available, a Percent
StratigraphyThe
Middle to Upper Devonian formations present in southern New Mexico
unconformably overlie the Silurian Fusselman Formation and underlie
Mississippian units. They are composed of thin, locally fossiliferous gray to brown shales, siltstones, sandstones,
carbonates, and barren anoxic
Interpretation of the environment of deposition responsible for the
anoxia and subsequent Devonian formations are well established within individual mountain ranges, although members continue to undergo revision, and correlation between ranges is not always clear (see Kottlowski et al., 1956; Seager, 1981; Sorauf, 1984; Kottlowski and LeMone, 1994). The oldest Devonian strata present in south-central New Mexico may be the Canutillo Formation (Nelson, 1940) in the Organ/Franklin Mountains. It unconformably overlies the Fusselman Formation and underlies and is a partial lateral facies equivalent to the Percha Formation, which in the southern area includes shaley facies correlative northward to the Oñate and Sly Gap Formations (Seager, 1981). The Canutillo is composed of a lower dolomitic siltstone and an upper cherty carbonate. The formation thins northward from 26 m in the Franklins to 6 m at Bishop Cap to 1 m in the southern San Andres Mountains. The Oñate Formation (Stevenson, 1945) of the Sacramento and San Andres mountains is of late Givetian age and unconformably overlies the Fusselman, while to the south in the Organ/Franklin Mountains its shaley facies is incorporated within the Percha Formation. In the Sacramento Mountains the Oñate consists of open-marine shelf deposits composed of gray silty dolomite, dolomitic siltstone, and minor sandstone with bryozoans, brachiopods, and local chert (Pray, 1961). It thins from 18 m in the south-central Sacramentos to 6 m in the far northern and southern reaches of the range (Pray, 1961). In the Hueco Mountains, 32 m of sparsely fossiliferous shales, silty shales, and silty limestones may correlate to the Oñate, Sly Gap, and Percha (Kottlowski, 1963). The lower Oñate in the San Andres Mountains is similar to the Sacramento Mountain sections, augmented in the central range by nondolomitized wackestones containing corals, crinoids, brachiopods, and bryozoans. The upper Oñate in the San Andres Mountains is regressive and clastic-rich, with siltstones, shale, and an upper cross-bedded sandstone unit documented (Kottlowski et al., 1956; Sorauf, 1984). The formation thins to the north and south from its maximum of 26 m in San Andres Canyon, becoming sandier to the north and shalier to the south. The Sly
Gap Formation (Stevenson, 1945), of Frasnian age, disconformably
overlies the Oñate (Pray, 1961; Day, 1988). It is present in the
northern and central Sacramento Mountains, the entire San Andres
Mountains, and is incorporated as part of the Percha Formation in the
Organ/Franklin Mountains (Pray, 1961; Seager, 1981; Sorauf, 1984). It is
interpreted to represent a transgressive-regressive succession deposited
in shelf (Sacramento Mountains, northern San Andres Mountains) to basin
(southern San Andres Mountains) environments (Day, 1988). In the
Sacramento Mountains the Sly Gap Formation contains interbeds of
calcareous shale, thin-to-nodular fossiliferous lime mudstone, and
lesser The Contadero Formation (Stevenson, 1945) is recognized in the northern San Andres Mountains and originally incorporated all strata between the Sly Gap and the Mississippian, but it was revised by Flower (in Kottlowski et al., 1956) to include what were originally upper Sly Gap units and also exclude upper dark shales with Fammenian fauna, which were placed in the Percha Formation. Sorauf (1984) revised Flower’s member nomenclature to include: the Salinas Peak Member (sea-level highstand shale to sandstone, with upper coral-bearing nodular limestone); Thurgood Sandstone Member (regressive, fine-grained, well indurated sandstone with calcareous cement and brachiopod fragments); and Rhodes Canyon Member (Fammenian-aged shales and burrowed siltstones with brachiopods, correlative to the Ready Pay Member of the Percha Formation). The Contadero Formation, which is not recognized in the Sacramento or the Franklin Mountains, may have formed in a narrow structural re-entrant largely limited to the San Andres Mountains area (Day, 1988). In the
Organ/Franklin Mountains the Percha Formation is used to include all
Middle-Upper Devonian shales above the Canutillo Formation, including
strata that are age-equivalent to the Oñate, Sly Gap, and Contadero
formations (Seager, 1981). To the north, when used at all, the Percha is
constrained to only those dark shales of Fammenian age. The Percha is
divided into two members: the Ready Pay ( In
southwestern New Mexico Devonian strata are composed of dark fissile and
carbonaceous shale facies from the basin environment and are considered
the Percha Formation regardless of age. Barren dark shales of the Ready
Pay Member overlie Fusselman carbonates and, in turn, are overlain by
dark green to
Organic GeochemistryOrganic
geochemistry data for Devonian units in south-central and southwestern
New Mexico is spotty and of varying vintage and quality (New Mexico
Bureau of Geology and Mineral Resources Digital Data Series- Database
DDS-DB2; New Mexico Bureau of Geology (Mines) and Mineral Resources Open
File Reports 92, 153, 202, 206, 237, 263, 328, 362, and 456).
Figure 4
posts public domain Devonian TOC values on a %
Burial HistoryBurial history and basin analysis studies are not common for this region. Two major problems exist in constructing accurate models: (1) data quantity and quality, and (2) the complex heat-flow history of this area that has experienced Ancestral Rocky Mountain tectonism, the Laramide orogeny, and Rio Grande rifting. Although a fair number of well penetrations exist (Table 1, Figure 1), most are of pre-1980 vintage (many significantly older) and contain generally poor log suites. Detailed bolson thickness data and structural styles are poorly constrained; lithologies and lithic percentages can often only be estimated; and formation and age picks are usually performed without the aid of biostratigraphic data or core. Geochemical and thermal data are rare. Estimate of heat flow through time is a major variable in any burial history model. The Rio Grande Rift area contains one of the more complicated thermal regimes in the world. High-quality regional present- day heat flow maps exist (Reiter et al., 1975), but they must smooth some of the natural heterogeneity derived from spatially complex intrusions and faults. For example, the regional map (Reiter et al., 1975) illustrates a heat flow range of 1.4 to 4.7 hfu over the study area; however detailed local measurements reach as high as 17 hfu (equates to over 700 mWM/m2; New Mexico Bureau of Geology and Mineral Resources Open File Report 456). The fact that this area has some of the best geothermal energy potential in the United States, including a number of existing successful projects, bespeaks to its high and complex heat flow. Even under ideal circumstances, basin models offer nonunique solutions that fit known data. Due to the data complexities discussed above, a single “best estimate” model is misleading for this area, since a greater than normal number of parameters are poorly constrained. I, therefore, provide two best fit end member models that hopefully bracket much of the area (Figures 5, 6, 7, and 8): (1) Grim Mobil-32 #1, a deep well test below 20,000 feet containing thick Tertiary Rio Grand Rift bolson valley fill but heat flow within regional norms, and (2) McGregor GDP 51-8, with much thinner bolson deposits but anomalously high heat flow.
Petroleum PotentialNumerous oil and gas shows and one significant gas discovery (Harvey E. Yates 1Y Bennett Ranch well, Sec. 14, T.26S., R.12E.) in the study area indicate an active petroleum system exists. Of 83 exploratory wells drilled in the Tularosa Basin, 25 contain shows. Despite this proven potential, the area’s large size, the multiple source and reservoir facies, and the complex structural history offering numerous trapping mechanisms, few integrated petroleum systems studies have been undertaken (e.g., Broadhead, 2002). A regional, comprehensive interpretation of this area’s petroleum system is needed to understand better its potential and to high-grade primary opportunities.
Source RocksSource
rocks have been documented for Devonian, Mississippian, Pennsylvanian,
and Permian strata (Broadhead, 2002), and for Cretaceous shales and
coals in central New Mexico. This paper has concentrated on the thick
Devonian
Expulsion timingFor the Ancestral Rocky Mountain Orogrande and Pedregosa basin areas (e.g., central Tularosa Basin between the Sacramento and San Andres Mountains, and in southwestern New Mexico in Hidalgo and southwest Luna counties) oil-prone source rocks of Lower Paleozoic and possibly Upper Paleozoic age became early to moderately mature during Late Pennsylvanian through Permian time. Outside of the Ancestral Rocky Mountain basin depocenters, it is less likely thermal maturities reached levels for major expulsion. It is difficult to ascertain the effect of the Laramide orogeny on the eastern area since little Mesozoic section is preserved and no detailed vitrinite profiles have been located in the public domain to quantify the extent of Mesozoic deposition and subsequent erosion. In the western area, thick preserved Cretaceous deposits may have caused a second expulsion event. The Rio Grande Rift, with its elevated heat flows, igneous intrusions, and thick bolson deposits brought all graben-area source rocks to maturity or post-maturity, resulting in minor(?) oil and potentially major gas expulsion beginning ~28 Ma and continuing today. Horst areas vary from immature to post-mature, depending on the stratigraphic interval and extent of uplift associated with specific blocks. It is likely that over the large study area hydrocarbons have been continually expulsed from Late Paleozoic time until Recent, with pulses centered around the three major orogenic events.
ReservoirsNumerous potential reservoir facies exist, including: fractured Precambrian basement, Cambro-Ordovician sandstones (Bliss Formation), karsted Ordovician carbonates (El Paso Formation), Silurian dolomites (Fusselman Formation), Devonian sandstones (northern area), Devonian shales (southern area), Mississippian carbonate bioherms (including large Waulsortian mounds), Pennsylvanian (Morrowan/Atokan) sandstones, Upper Pennsylvanian phylloid-algal mounds (correlative to the Holder Formation outcrops in the Sacramento Mountains), Lower Permian (Wolfcampian) basin margin mounds and breccia debris flows, Upper Permian (San Andres and Yeso Formation) backreef limestones and dolomites, Cretaceous sandstones (Dakota Formation) and coalbed methane, and Tertiary (Eocene) fractured igneous sills (the major reservoir for the Otero Mesa Harvey E. Yates gas discovery). Recovered fluids from existing wells include oil, gas, saline water, and fresh water.
Traps/sealsThe large study area has undergone multiple tectonic episodes and also has numerous documented stratigraphic pinch-outs, creating a wide range of trapping styles and mechanisms, many analogous to the neighboring prolific Permian Basin. Ancestral Rocky Mountain block faults, many reactivated during Rio Grande Rift extension, potentially juxtapose reservoir facies against units with low permeability or against fault planes with clay smear/cataclasis effects. Structural roll-on horsts near major normal faults also add dip closure. Low-angle Laramide thrust faults and rollovers are documented in outcrop (e.g., Pray, 1961) and the subsurface; for example, a major thrust-fault-induced rollover was encountered in the McGregor 51-8 well. Stratigraphic traps include Devonian shale gas, large biohermal mounds in the Mississippian, Pennsylvanian, and Permian sections, carbonate debris flows off basin margins sealed (and potentially sourced) with basinal shales, Pennsylvanian, and Permian stratigraphic pinch-outs onlapping Ancestral Rocky Mountain uplifts, Lower Paleozoic pinch-outs of strata onlapping the Transcontinental Arch, Cretaceous coalbed methane, and fractured Eocene igneous sills intruded into tight carbonates and shales. Seal integrity is a concern in the eastern Sacramento Mountain uplift area. Major fracture systems have breached some horst block units, flushing reservoirs with fresh water. This negative does create an opportunity for fresh-water exploration in this growing, water-starved area.
Preliminary Comments on Most Prospective AreasIntegrating previous studies and new work, the most prospective area for Devonian-sourced hydrocarbons appears to be the south-central portion of the study area, bounded approximately by the latitudes of Hatch to the north and El Paso to the south, and longitude of Deming to the west and extending eastward beyond the study area. This area has adequate organic richness, thermal maturity, reservoir intervals, and trapping mechanisms to create a viable petroleum (predominantly gas) system. To the north, Devonian organic richness lessens, due to the influx of shelf clastics; to the west source-rock richness decreases, due to unknown reasons and maturities increase to post mature. To the south richness decreases, due to a facies change into the cherty carbonate Canutillo Formation.
Conclusions
South-central New Mexico contains the necessary ingredients for economic
discoveries of hydrocarbons sourced by Devonian
AcknowledgmentsPRA Inc. BasinMod 1-D was used for basin modeling. The New Mexico Bureau of Geology and Mineral Resources, a Division of New Mexico Institute of Mining and Technology (New Mexico Tech) provided data and time to perform this ongoing study.
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