An Introduction to the

Mississippian to Permian Carbonate

Rocks of the Sacramento Mountains, New Mexico

by Peter Scholle and Robert Halley

 

 


This site offers a continuation of a three-day field trip to the classic Permian reef complex and other geologic features of the Guadalupe Mountains. The Sacramento Mountains part of the trip extends the Guadalupe Mountains trip from Carlsbad to Artesia, Cloudcroft, Tularosa, and Alamogordo and then returns to El Paso, Texas. This site contains the information needed to make that trip — an introduction plus two roadlogs with diagrams, and photographs, as well as an extensive bibliography in order to provide support material for a geology student audience. Although the work is copyrighted, feel free to us any parts of it for teaching purposes as long as you acknowledge the source. To use portions for other purposes, please contact Peter Scholle at: scholle1@nmt.edu.

 

OUTLINE:

INTRODUCTION

Exposures of Upper Paleozoic strata in the northern Sacramento Mountains offer a superb opportunity to view varied carbonate lithologies, local facies changes, and the products of diagenesis within a variety of shelf and slope carbonate buildups. Similar buildups occur in the subsurface in nearby New Mexico, Texas and Utah basins and are known to be excellent hydrocarbon reservoirs. They have been targets of exploration in the area for the last quarter century.

We will visit three types of mounds and discuss their similarities and differences in the field. On Day IV, we will study phylloid algal mounds, structures which are widespread throughout the United States (Wray, 1968). We will compare a Virgilian mound (Pennsylvanian), which is largely a carbonate mud accumulation, with a Wolfcampian mound (Permian) that contains copious amounts of submarine cement. On Day V, we will visit an Osagean (Mississippian) buildup composed of a carbonate mud-fenestrate bryozoan-marine cement core facies and crinoidal sand flank beds. This mound is similar to Lower Carboniferous mounds of Europe, known as Waulsortian mounds and named from occurrences near Waulsort, Belgium (Bolton et al., 1982; Lees, 1964; Lees, 1982; Lees et al., 1985; Lees and Miller, 1985). Comparable mounds are also found in Pennsylvanian and even Permian carbonates in many areas of the world (Davies et al., 1988; Füchtbauer, 1980; Hurst et al., 1988; Smith, 1981 — see supplemental bibliography #1 for many additional references).

The exposures in the Sacramento Mountains provide a cross-sectional view of the rocks, but it is not possible to develop a regional picture of facies relationships in a few days as may be done for the Permian basin. The Guadalupe Mountains part of this field course visits an area where erosion and evaporite solution produced outcrops that may be relatively easily related to a paleogeographic framework. In contrast, strata in the northern Sacramento Mountains dip into the subsurface a few miles to the east of the outcrops, and they are downfaulted below the Tularosa Valley to the west.

The northern Sacramento Mountain area was closer to sources of terrigenous clastic sediments than the Carlsbad area during the Late Paleozoic. The Pedernal land mass (Fig. xxxx) repeatedly shed material south and west to the Alamogordo area. Some of the tectonism which occurred during this time is evidenced in the Sacramento Mountains by Late Paleozoic faulting. Some tectonism may also be reflected in the sediments themselves, which show evidence of repeated, relative sea level changes, probably of both tectonic and eustatic origin.

In the northern Sacramento Mountains we will continue to investigate many of the themes developed in the Carlsbad area, but now in a considerably different setting. These themes include facies relationships, faunal and lithologic variation, reef models, marine cementation, subaerial exposure, porosity and permeability development and preservation. They are themes which are increasingly incorporated into modern exploration scenarios and are well illustrated by the outcrops we will visit.

 

SUMMARY OF SIGNIFICANCE TO PETROLEUM EXPLORATION

The general geology of the northern Sacramento Mountains has been worked out by Pray (1952, 1961) and Otte (1959b), who provided the framework for many later, more detailed studies. Pray (1959) summarizes work in the area before 1950. Excellent general field guides to the area have been published by Pray (1959) and Butler (1977). Pray (1975) edited a field guide to shelf-edge and basin facies limestones in the Sacramento Mountains; Stanton (2000) edited a field guide to the Pennsylvanian and Mississippian (Carboniferous) carbonate buildups in shelf and ramp/slope settings. Figure xxxx indicates the position of our field stops on a generalized stratigraphic section for the northern Sacramento Mountains.

Several processes discussed and developed at outcrops in the Permian Reef complex will again be evoked to explain observations on these older bioherms. The significance of these processes varies from buildup to buildup, and the internal structure and composition of the bioherms reflect these differences. Some buildups are cement-rich, some mud-rich, some contain shallow-water fossils, some deep-water fossils. We will try to extract as much interpretive data as possible from bioherm outcrops. Such observations will help to interpret similar rocks in the subsurface.

In contrast to the Capitan reef facies, which (unlike its back-reef and basinal equivalents) does not produce substantial oil in the subsurface, bioherms similar to those we visit in the Sacramento Mountains do form excellent reservoirs. Phylloid algal limestones, like those at Virgil and Yucca mounds, form reservoir rocks at Aneth Field (Elias, 1963; Irwin, 1963; Peterson and Ohlen, 1963), Ismay Field (Choquette, 1983; Choquette and Traut, 1963), New Lucia Field (Toomey and Winland, 1973), Lusk Field (Thornton and Gaston, 1968), several fields in the ³Horseshoe Atoll2 (Schatzinger, 1983; Vest, 1970), and Saunders and Conley fields (Kerr, 1969). These studies show, in some cases, direct association of subsurface porosity and the presence of phylloid algae. Porosity takes the form of shelter pores beneath algal blades in mudstones and wackestones, intergranular porosity in algal plate grainstones, and secondary porosity in leached algal plate mudstones. In some cases, porosity and permeability are provided by fracturing or dolomitization in this facies.

Several of the associated lithologies also provide excellent reservoir rock, some of which are oolitic, crinoidal and fusulinid grainstones. It is significant that production from many fields appears to be from the shelf-edge buildups themselves and not from fore-reef or back-reef facies. Fields along the Abo Trend (LeMay, 1972; Mack and James, 1986; Wright, 1962) and the Kemnitz-Townsend Trend (Malek-Aslani, 1970) occur at the shelf edge, a position occupied by the tight Capitan Limestone in younger units to the south. Early submarine cementation is a major factor in the lack of oil production from the younger (Capitan) reef. In addition, many of the clastic terrigenous units interspersed with the carbonate buildups are act as reservoir rocks (e.g. Broadhead, 1984 ).

The retention of porosity in the subsurface is still a topic of considerable study. We see little matrix porosity in outcrops of Late Paleozoic mounds (although vugs are characteristic of the lower Virgil mound). The original porosity in these carbonate sediments was very high (40-85%), and the processes involved in such great porosity loss have not been well delineated. One of the best documented cases of porosity loss in carbonate sands comes from studies of the crinoidal facies of the Lake Valley Formation (Meyers, 1974; Meyers, 1978; Meyers, 1988; Meyers et al., 1982; Meyers and Lohmann, 1985).

Hydrocarbon reservoirs in rocks similar to those that occur in the Lake Valley appear to be uncommon but not non-existent. Pray (1958a) reported that cores from the La Pan Field of Clay County, Texas, are similar to lithologies associated with Muleshoe bioherm. La Pan field may therefore be a buildup similar to Muleshoe bioherm. Other production has come from numerous small fields in the Chappel Limestone (Ahr and Ross, 1982; Armstrong. 1989) and from the European mid-Permian Zechstein (Peryt, 1986); porous zones in such Permian bioherms have also been reported from unexplored strata in Greenland (Scholle et al., in manuscript).

Meyers (1974) showed that cementation and porosity loss in Lake Valley non-biohermal sediments are linked to periods of subaerial exposure. As much as 60% of the original porosity was lost within about five million years of sediment deposition. Almost all the rest was lost within 20-30 million years. Cementation took place during two episodes of subaerial exposure (Meyers, 1978). It is interesting to note, however, that early subaerial exposure is credited with producing leached porosity in many phylloid algal and bryozoan-cement mounds (Scholle et al., in manuscript; Surlyk et al., 1986; Wilson, 1975a). Apparently, exposure and fresh-water diagenesis act as a double-edged sword (i.e., under some circumstances exposure helps produce reservoir rocks); in other cases, exposure destroys reservoir properties. The particular circumstances which control the products of exposure are not well understood. Factors that probably exert considerable influence on the diagenetic history of these rocks include rate of transgression or regression, duration of exposure, climate, original sediment mineralogy and local paleohydrology.

Even less well understood are later diagenetic processes which may affect these limestones in the subsurface. The Holder and Laborcita mounds had been buried to at least 750 m (2,500 ft) and the Lake Valley bioherms as deeply as 2,000 m (6,500 ft) by the end of the Paleozoic. Processes, such as compaction, pressure-solution cementation, fracturing, cementation by dolomite and anhydrite, are known to occur at depth but are undocumented in these rocks.

Finally, one wonders what has been the effect of uplift and the current episode of exposure on these rocks. Might some of the differences between the rocks we see in outcrop and their subsurface counterparts be due to their Cenozoic uncovering? Or was the character of these rocks essentially fixed during their burial?

Again, it should be emphasized that we will not develop a regional picture of sedimentary facies in the Sacramento Mountains as we do in the Guadalupe Mountains. Generalized paleogeographic maps for the Osagian and Virgilian Stages of the area are outlined in Figure xxxx. We will review principles of carbonate deposition as they apply to late Paleozoic bioherms and formulate new questions which have particular significance to petroleum exploration.

Bibliograpy of Paleozoic Geology in the Sacramento Mountains

last revised: 11 May 2000

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