Wayne M. Ahr

Early Mississippian climate based on oxygen isotope compositions of brachiopods, Alamogordo Member of the Lake Valley formation, south-central New Mexico

Stable isotope compositions of brachiopod shells from the Alamogordo Member of the Lake Valley Formation (south-central New Mexico) provide information about Early Mississippian paleoclimate. The Alamogordo Member was deposited on a ramp at the southwestern edge of the North American craton, at lat 20°S. It consists of cherty lime-mudstone to lime-packstone (predominantly mudstone and wackestone) strata and scattered mud-cement (Waulsortian) mounds. The brachiopods analyzed lived at water depths estimated to have been between 100 and 300 m. The δ 1 8 O values of well-preserved shells of a single species of brachiopod average -2.3‰ relative to PDB (the Peedee belemnite isotope standard). This average represents a temperature of 21-26 °C if the isotopic composition of the seawater (δ 1 8 O s w ) was between 0‰ and -1‰, as inferred from Pleistocene and Holocene δ 1 8 O s w and other considerations. Sea-surface temperature would have been 26-31 °C (after we applied a +5 °C correction for water depth). This tropical climate for an Early Mississippian low-latitude site agrees with previous isotopic and climate-modeling results, but contrasts with a temperate climate inferred from comparison of the lithology and biota of the Alamogordo Member with those of modern temperate carbonates.

Early diagenetic microporosity in the Cotton Valley Limestone of East Texas

Abstract The Upper Jurassic, Cotton Valley Limestone was deposited on a mature ramp where monotonous, regional slopes were punctuated by salt-generated structures and basement topography. The strandline and the crests of paleobathymetric highs were blanketed by oolitic and palletoidal grainstones. The ratio of grainstones to mudstones increases in the upper Cotton Valley, reflecting a regional shallowing phase. Thinner, shoaling-upward sequences are present but they do not correlate easily, especially from basement highs to salt domes, probably because salt movement occurred during deposition of Cotton Valley rocks. The complex diagenetic history of the Cotton Valley, inferred from cross-cutting features observed in thin section and from trace-element and stable-isotope content, includes the origin of “chalky” microporosity, especially in ooids. This intraparticle, intercrystalline porosity occurs in a fabric of equant, subhedral to euhedral, low-Mg calcite micro-rhombs which appear to have developed at the expense of an acicular precursor. Such microporous ooids are present mainly on the crests of paleobathymetric highs; nearby, offstructure ooids contain a mixture of micritic and well-preserved ooids. However, these micritic ooids are different from the micro-rhombic, microporous ones on the highs. The well-preserved low-Mg calcite ooids from offstructure positions exhibit relict acicular microstructures in some of their lamellae. The “chalky” microporosity is crosscut by virtually every other diagenetic feature in the Cotton Valley Limestone. The ϵ13C values from individual microporous ooids range from +1.65 to +2.76% PDB, which is not in the range of values associated with precipitation in a hydrocarbon-rich environment. The formation of microporosity was followed by the formation of embayed grain contacts, pore-filling cementation, grain compaction, stylolite formation, replacements by quartz and rhombic dolomite, fracturing, fracture-filling cementation, saddle dolomite, late leaching, and formation of authigenic sulfides. The microporosity is interpreted to have been formed in the near surface diagenetic environment, early in the burial history of the Cotton Valley. As there is no evidence of vadose diagenesis, the ooids on the crests of calcarenite shoals must have been placed is disequilibrium with their surroundings by a change in water chemistry, probably as a consequence of regression and an influx of fresh water. The introduction of hydrocarbons appears to be contemporaneous with such late diagenetic features as saddle dolomite and authigenic sulfides; however, the extent to which those fluids affected the micro-rhombic calcite crystals appears to be negligible.

Upper Jurassic updip stratigraphic trap and associated Smackover microbial and nearshore carbonate facies, eastern Gulf coastal plain

The development of Little Cedar Creek field in the eastern Gulf coastal plain of the United States has shown that the current exploration strategy used to find hydrocarbon-productive microbial and high-energy, nearshore carbonate facies in the Upper Jurassic Smackover Formation requires refinement to increase the probability of identifying and delineating these potential reservoir facies. In this field, the petroleum trap is a stratigraphic trap characterized by microbial boundstone and packstone and nearshore grainstone and packstone reservoirs that are underlain and overlain by lime mudstone and dolomudstone to wackestone and that grade into lime mudstone and dolomudstone near the depositional updip limit of the Smackover Formation. Reservoir rocks trend from southwest to northeast in the field area. The grainstone and packstone reservoir is thickest in the central part of the field. The boundstone reservoir is thickest in local buildups that are composed of thrombolites in the southern part of the field and is absent along the northern margin. These reservoir facies are interpreted to have accumulated in water depths of approximately 3 m (10 ft) and in 5 km (3 mi) of the paleoshoreline. In contrast to most other thrombolites identified in the Gulf coastal plain, these buildups did not grow directly on paleohighs associated with Paleozoic crystalline rocks. The characterization and modeling of the petroleum trap and reservoirs at Little Cedar Creek field provide new information for use in the formulation of strategies for exploration of other Upper Jurassic hydrocarbon productive microbial and related facies associated with stratigraphic traps in the Gulf coastal plain.

The Sedimentologic and Paleoecologic Significance of Lithotrya, a Rock-Boring Barnacle

ABSTRACT The rock-boring lepadomorph barnacle Lithotrya is a vigorous agent of bioerosion in the littoral zone of tropical seas, but few geologists are aware of its existence or its environmental significance. Lithotrya is commonly found in patchy exposures of beach rock around the island of Icacos, Puerto Rico, where it burrows upwards into overhanging ledges. The organism creates easily recognizable oval burrows and is important in the production of bioclastic sediments. Barnacles of the family Scalpellidae range back to the Triassic, yet no fossil examples of Lithotrya borings have been reported.

Constituent composition of Early Mississippian carbonate buildups and their level-bottom equivalents, Sacramento Mountains, New Mexico

Abstract The Alamogordo Member of the Lake Valley Formation (Lower Mississippian-Tournaisian to early Viséan) was deposited on a homoclinal ramp at depths below the fair-weather wave-base and mainly below the photic zone. Depth increased toward the present southwest, and the level-bottom biota of the Alamogordo Member is present in assemblages that parallel the depth gradient (Jeffery & Stanton this volume). Waulsortian-like mounds are abundant in the Alamogordo, Nunn and Tierra Blanca Members of the Lake Valley Formation on the distal portion of the homoclinal ramp. These mounds contain essentially the same biota as the Waulsortian buildups of Europe. Alamogordo mounds are low, tabular features composed mainly of spiculiferous micrite, while Nunn and Tierra Blanca mounds are more domical and are composed largely of fenestrate bryozoan cementstone. Lithology and biota of coeval, level-bottom beds and mounds differ significantly because of habitat differences (cavities and hard substrates in and on mounds), and because of taphonomy, an example of which is the preservation of fenestrate bryozoan sheets in radiaxial fibrous calcite cements on mounds as compared with comminuted and dispersed fenestrate hash in level-bottom muds. Lake Valley mound constituents, although essentially identical to those in the European Waulsortian, do not occur in regular growth ‘phases’ like those described by Lees & Miller (1985). The difference is interpreted to mean that the New Mexican mounds lacked enough vertical relief above the sea floor in deep water to have been affected by any environmental gradients that may have existed across the entire platform.

Long-Lived Pollutants in Sediments from the Laguna Atascosa National Wildlife Refuge, Texas

DDT and some heavy metals are long-lived pollutants which may be retained in sedimentary layers and may be relocated by post-depositional biological or mechanical processes. Because DDT has a half life of up to 17 yr and it is concentrated in higher ecological levels, even a few parts per billion in the sedimentary substrate can be harmful to wildlife. The potential danger and the persistence of many long-lived pollutants create a genuine need for environmental studies made by geologists.

Depositional framework and reservoir potential of an upper Cotton Valley (Knowles Limestone) patch reef, Milam County, Texas

ABSTRACT The Knowles Limestone is an upper unit of the Cotton Valley Group, and in Milam County, Texas, is approximately 350 feet thick and consists of shales, terrigenous dolomitic limestones, grainy limestones, and algal boundstones with stromatoporoids and corals. The boundstones represent an elongate, wave resistant, encrusted skeletal patch reef which probably developed on a subtle salt-generated topographic high. The reef appears to be slightly more than 1 mile across in its narrowest lateral dimension as determined by facies correlations of three cored wells in the study area. Principal reef framebuilders included massive and columnar stromatoporoids, solitary or dendroid corals, and encrusting Tubiphytes and Lithocodium algae. Dendroid stromatoporoids, digitate corals, Tubiphytes oncoids, echinoids and various mollusks were the principal contributors to the detrital infill of the reef. Increased dip and dip reversal of draping beds suggest that the Knowles reef achieved relief above the sea floor. Evaluation of dipmeter logs indicates that the beds dip 5° northwest on the updip reef flank and 10° southeast on the downdip reef flank. Reef core boundstones and reef talus were consistently present downdip, and lagoonal to tidal flat facies were common updip throughout Knowles deposition. The reef organisms eventually became overwhelmed with terrigenous sediment transported downdip as the tidal flat environment prograded over the lagoonal, reef talus and reef core facies at the end of Knowles deposition. Early dolomitization of tidal flat and lagoonal facies has created local porous zones in some of these rocks. However, the reef, per se, is cemented by sparry calcite and is not a potential reservoir facies.

Burial Diagenesis and Reservoir Development in North Haynesville (Smackover) Field, Louisiana: ABSTRACT

ABSTRACT Smackover carbonates were deposited on a regional ramp which was locally affected by salt-generated paleotopography and basement structures. The paleobathymetry at North Haynesville Field was a salt-generated high on which oolite grainstones accumulated. These shoals consisted of tide-dominated sand waves that were flanked by algal-rich grainstones and packstones which, in turn, were surrounded by open marine, peloidal wackestones. The sand shoals were lithified primarily in the marine phreatic environment but, since they had accumulated significant depositional relief, they became exposed during minor regressions. Consequently, the shoals were affected by early meteoric phreatic diagenesis. Blocky calcite cements and inversion of metastable allochems marked this diagenetic episode. Subsequent burial diagenetic history can be charted from early to late by the sequential appearance (in order) of the following characteristics: microstylolites, dolomitization, macrostylolites, poikilotopic calcite cements, baroque dolomite cements, and late leaching. Whole-rock trace element analyses indicate that Mg, Fe, and Mn strongly correlate with dolomitized horizons, Sr correlates with algal-encrusted grains, and Al correlates with tight, argillaceous micrites. To an extent, the algal-encrusted grains are also correlated with late, dissolution-enhanced intergranular porosity. The North Haynesville reservior is partly selective for certain depositional microfacies. Selection is for those sand shoals that had the highest primary porosity and permeability and that were affected by dissolution enhancement in the subsurface. However, the same late dissolution processes affected both micrite and allochems in the non-facies-selective sectors of the reservoir.

ABSTRACT: Determining Reservoir Quality by Combined Stratigraphic, Petrographic and Petrophysical Methods As Part of Optimized Recovery Programs: Womack Hill Smackover Field, Clarke and Choctaw Counties, Alabama

Objective of the Study The objective of this study is to describe depositional and diagenetic characteristics of the Oxfordian (Jurassic) Smackover Formation in the Womack Hill field, Alabama, as part of an integrated reservoir description program. In order to understand the distribution of reservoir units, this study utilized an integrated array of data from core lithological descriptions, borehole logs, core reports, thin section petrography, porosity and permeability measurements on core plugs, and mercury injection capillary pressure (MICP) measurements. These data made it possible to establish reliable measures of reservoir quality by comparing pore geometry with pore type; then determining which pore types correspond with highest porosity-permeability paired values. Pore aperture (throat) median sizes measured by mercury capillary pressures were tested for correspondence with porosity, pore type, permeability, and saturation in order to establish quality rankings for the reservoir units.

Reconstructing Paleoenvironments of Jameson (Strawn) Reef Field, Coke County, Texas: ABSTRACT

The Jameson reef is an elongate, strike-trending mud buildup that occurs within the Middle Pennsylvanian (Desmoinesian) Strawn reef trend of west-central Texas. It consists primarily of carbonate wackestones, mudstones, and packstones with smaller units of Chaetetes framestones and grainstones. Microfacies analyses indicate that the buildup was deposited on a warm, shallow-water carbonate platform characterized by downslope mud accumulations and upslope calcarenite beaches and islands. The shape of the buildup was controlled by ocean currents, which spread the sediments laterally and downdip. The sediments were exposed to early marine and meteoric diagenesis as well as late subsurface diagenetic processes. Marine diagenesis includes primarily micritization of component grains. Leaching occurred in the meteoric phreatic environment following micritization. The leaching produced intergranular, intragranular, moldic, and vuggy porosity. A later transgression placed the buildup in the meteoric phreatic environment that induced cementation, partially filling pore spaces. The grainier rocks were cemented to a greater degree than the wackestones. Dolomitization occurred in the mixing zone where dolomite rhombs generally filled pores in the micrite matrix. Diagenesis culminated with stylolitization in the deep subsurface reducing porosity. The middle zone of wackestone makes up the best reservoir rock mainly because selective cementation of the grainier rocks occurred in the meteoric phreatic environment. End_of_Article - Last_Page 1423------------