Tocito Sandstone Research Articles

Fossil fishes from a lag deposit within the Upper Cretaceous Mancos Shale in New Mexico, USA, with comments on correlative Turonian–Coniacian time-transgressive lags in the Western Interior Seaway of North America

A lag deposit between the Tocito Sandstone and Mulatto Tongue of the Upper Cretaceous Mancos Shale in Sandoval County, New Mexico, USA, contains a fossil assemblage of late Turonian–early Coniacian chondrichthyans and osteichthyans. This assemblage consists primarily of isolated teeth that derive from at least 26 taxa including: Meristodonoides sp.; Ptychodus mortoni; P. mammillaris; Scapanorhyncus raphiodon; Protolamna sp.; Cretodus cf. C. semiplicatus; Cretodus sp.; Cretalamna “appendiculata”; cf. Eostriatolamia tenuiplicatus; Archaeolamna cf. A. kopingensis; Squalicorax cf. S. falcatus; S. deckeri; Squalicorax sp.; Paranomotodon(?) sp.; Rhinobatos lobatus; Ptychotrygon triangularis; Texatrygon hooveri; Pseudohypolophus mcnultyi; Ischyrhiza mira; Chondrichthyes indet.; Micropycnodon cf. M. kansasensis; Pycnodontiformes indet.; Aspidorhynchidae indet.; Protosphyraena sp.; and Enchodus cf. E. gladiolus. The lag deposit formed along a series of outer shoreface and discontinuous sandbars in the southeastern corner of the San Juan Basin during eustatic sea-level fluctuation herein referred to as the Turonian–Coniacian Time Transgressive (TCTT) Event. This sea-level event and the concentration of fish remains into a lag deposit is also recorded at several other states within the Western Interior Seaway of North America. These stratigraphic properties have correlative potential across basins and states and provide a framework by which regional and eustatic sea-level events can be interpreted. Differences in coeval faunas found within these TCTT lags are bathymetrically controlled, related to the degree of taphonomic reworking, and proximity of the ancestral shoreline.

Depositional and sequence stratigraphic model of transgressive shelf sandstone: The Late Cretaceous Tocito Sandstone, San Juan Bain, New Mexico, U.S.A.

ABSTRACT There has long been debate about the origin of long, linear, isolated shallow marine sandstones. In the 1980s these were routinely interpreted as wave- or tide-modulated shelf sandstones. With the advent of sequence stratigraphy, many of these sandstones were reinterpreted as top-truncated forced-regressive to lowstand delta or shoreface, inviting the question regarding whether the concept of a shelf sandstones remain a viable depositional model. More recent syntheses demonstrate that shelf sands tend to be derived by reworking of underlying sediment through transgressive erosion and show varying levels of detachment and reworking into discrete ridges and bars by wave and tidal processes as they become more evolved. The Late Cretaceous Tocito Sandstone in the Western Interior Seaway, which overlies the regressive Gallup Formation, is a candidate for a transgressive shelf sandstone. In this study, we present detailed facies analysis, based on sedimentological sections measured from well-exposed outcrops of the Tocito Sandstone and its related stratigraphic units in northwest New Mexico, USA. With high-resolution sequence stratigraphic control, facies analysis, detrital-zircon anatomy, and petrographic analysis are integrated to investigate the sequence stratigraphic evolution and depositional processes of the Tocito Sandstone. Detrital zircons indicate that the Tocito, Torrivio, and Gallup deposits are derived from different source areas, and therefore imply separate sequence stratigraphic origins. The Tocito Sandstone is interpreted to have been deposited as shelf sand ridges in retrogradational estuarine to drowned-barrier-system environments with tidal and wave influence, characterized by intense bioturbation. The Tocito was deposited in an overall transgressive systems tract and sourced from reworking of the underlying Torrivio and Gallup units through processes of transgressive erosion.

Sequence stratigraphy and regional context of the Mancos-Niobrara in the northern San Juan Basin

In the northern San Juan Basin, the Niobrara Formation is represented by the upper half of the Mancos Shale (the Smoky Hill Member and Cortez Member). This section is generally equivalent to the Niobrara Formation along the Colorado Front Range. Although the Fort Hays Limestone is absent west of Pagosa Springs, the C Chalk and B Chalk are well-expressed as two resistant bench-forming calcareous units in the northern San Juan Basin. These two calcareous units have also been established as prospective hydrocarbon targets by operators in the area. Calcareous facies equivalent to the A Chalk were not deposited in the northern San Juan Basin due to siliciclastic dilution during the regressive latter half of the Niobrara marine cycle. The overall third-order Niobrara marine cycle includes these members of the Mancos Shale: the Juana Lopez, Montezuma Valley, Smoky Hill, and Cortez members. The Smoky Hill Member sits just above the basal Niobrara unconformity in most of the study area, and the entire section also has greater thickness and siliciclastic content than its equivalent farther east along the Front Range. Several extensive outcrop locations (in and around Pagosa Springs, Piedra, and Durango, CO) along with three new cores along the CO-NM border form the foundation for sequence stratigraphic interpretation of the Niobrara marine cycle in this study. All these locations and cores were tied back to the Mancos reference section at Mesa Verde National Park established by Leckie et al. (1997) which provides detailed description and biostratigraphy for the entire Mancos Shale. Establishing and applying a sequence stratigraphic framework to any section creates consistent reference standards for communication, research, and further correlation. Comparisons of chemostratigraphic data from equivalent strata between the northern San Juan Basin and Denver-Julesburg (DJ) Basin reveal significant differences in the timing and style of source-rock deposition (and associated low-oxygen conditions). The sequence stratigraphic framework also emphasizes tremendous lateral facies changes in the basal Niobrara section (i.e., Fort Hays Limestone to Tocito Sandstone). Once refined and applied, this stratigraphic framework can be used for predicting the distribution of reservoir properties, in addition to enhancing understanding of the Niobrara marine cycle and the Western Interior Seaway.

Low Net: Gross Reservoirs of the Transgressive Shelf Systems in the Upper Mancos Shale, El Vado Sandstone, San Juan Basin, New Mexico

The El Vado (EV) Sandstone Member (upper Coniacian) of the Mancos Shale has been considered both a conventional and unconventional hydrocarbon resource target (Ridgley, 2001) in the San Juan Basin (SJB) of northwestern New Mexico. Sandstones of the EV were deposited on the marine intracontinental shelf along the west margin of the Cretaceous Seaway. The EV is part of a transgressive-regressive wedge of rock that overlies a large, older transgressive wedge that contains the Tocito Sandstone. The EV is examined in outcrops in the northeast and the southeast parts of the SJB. In northern outcrops, the EV consists of five parasequences of interbedded sands and shales of similar thickness (∼4 m each) separated by marine-shale units that become progressively sandier upward. To the south, the EV Sandstone transitions from a shelf-deposited into a lower-shoreface sandstone. Well logs and core are used to map the subsurface distribution of the EV Sandstone in the SJB. It is reflected in logs as a thick (∼36.6 m; 120 ft) interval of low gamma ray and high resistivity, associated with highly laminated sand to silt interbedded with shales. A total of 13 cross sections are correlated and used to construct total-interval isopach and parasequence-scale isopach maps for the EV. Regional net-sand and net:gross maps, determined from 104 digitized well logs, highlight the regional distribution of sand in the formation. Thicks are oriented north to south and show compensated deposition suggesting that individual parasequences are not in communication across the basin, but opportunity exists to penetrate stacked parasequences with single vertical wells.

Geologically-based permeability anisotropy estimates for tidally-influenced reservoirs using quantitative shale data

The effect of the vertical to horizontal permeability ratio ( k v / k h ) on many displacement properties is significant, making it an important parameter to estimate for reservoir models. Simple ‘streamline’ models have been developed which relate k v / k h at the reservoir scale to shale geometry, fraction and vertical frequency. A limitation of these models, especially for tidally-influenced reservoirs, is the lack of quantitisative geological inputs. To address this lack of data, detailed shale characteristics were measured, using Lidar point clouds, from four different tidally-influenced reservoir analogues: estuarine point bar (McMurray Formation, Alberta, Canada), tidal sand ridge (Tocito Sandstone, New Mexico), as well as both unconfined and confined tidal bars (Sego Sandstone, Utah). Estuarine point bars have long ( x̄ = 67.8 m) shales that are thick and frequent relative to the other units. Tidal sand ridges have short shales ( x̄ = 8.6 m dip orientation) that are thin and frequent. Confined tidal bars contain shales that are thin, infrequent and anisotropic (x– = 16.3 m dip orientation). Unconfined tidal bars contain nearly equidimensional shales of intermediate length ( x̄ = 18.6 m dip orientation) with moderate thicknesses and vertical frequency. The unique shale character of each unit results in a different distribution of estimated k v / k h values. Estuarine point bars have lower average k v / k h values ( x̄ = 8.2 ×10 −4 ) than any other setting because of the long shales they contain. Tidal sand ridges have short, but frequent shales, which results in moderate k v / k h estimates ( x̄ = .011). Estimates of k v / k h are typically highest in confined tidal bars ( x̄ = .038), which contain anisotropic and infrequent shales. Unconfined tidal bars have moderate lengths and frequency resulting in k v / k h estimates averaging 0.004. The results of this study highlight the link between heterogeneity, reservoir architecture and flow parameters.

Abstract: A Tide-Dominated Delta System in the Western Cretaceous Seaway: The Coniacian Tocito Sandstone 

Abstract: A Tide-Dominated Delta System in the Western Cretaceous Seaway: The Coniacian Tocito Sandstone 

Permeability Structure of a Highly Heterogeneous Transgressive-Marine Complex: Tocito Sandstone, New Mexico: ABSTRACT

The Tocito Sandstone Member of the Mancos Shale is an Upper Cretaceous shallow-marine sandstone and mudrock complex deposited along the western margin of the Western Interior seaway. The Tocito is a major hydrocarbon producer in the San Juan Basin (approximately 117 million barrels of oil and 79 billion cubic feet of gas). Because of reservoir heterogeneity, ultimate Tocito oil recovery factors are low, generally between 10 and 20 percent. To enhance understanding of permeability heterogeneity in the Tocito, we have undertaken a detailed surface and subsurface investigation. A total of 2,697 permeability measurements have been made using minipermeameters. Permeability variation within the Tocito is controlled by two principal factors: lithofacies and burial/diagenetic history. Coarser grained and better sorted lithofacies have the highest permeability. The permeability values from outcrop and shallow subsurface cores are dramatically higher than those from deep subsurface cores. This is due to dissolution of grains and calcite cement, and decompaction that preferentially affected the outcrop and shallow subsurface. Correlation lengths for permeability values along horizontal transacts are typically less than 3 m, whereas those for vertical transacts are usually less than 0.6 m. These data suggest that small grid block sizes should be used during reservoir simulationsmore » if the investigator wishes to accurately capture the reservoir heterogeneity.« less

Offshore Bars or Incised Valleys? A Sequence Stratigraphic Approach - The Tocito Sandstone (Cretaceous), New Mexico: ABSTRACT

Offshore Bars or Incised Valleys? A Sequence Stratigraphic Approach - The Tocito Sandstone (Cretaceous), New Mexico: ABSTRACT

Organic matter variations in transgressive and regressive shales

Organic matter in the Upper Cretaceous Mancos Shale adjacent to the Tocito Sandstone in the San Juan Basin of New Mexico was characterized using organic petrology and organic geochemistry. Differences in the organic matter found in these regressive and transgressive offshore marine sediments have been documented and assessed within a sequence stratigraphic framework. The regressive Lower Mancos Shale below the Tocito Sandstone contains abundant well preserved phytoclasts and correspondingly low hydrogen indices. Total organic carbon values for the regressive shale are low. Sediments from the transgressive systems tract (Tocito Sandstone and overlying Upper Mancos Shale) contain less terrestrially derived organic matter, more amorphous non-structured protistoclasts, higher hydrogen indices and more total organic carbon. Advanced stages of degradation are characteristic of the phytoclasts found in the transgressive shale. Amorphous material in the transgressive shale fluoresces strongly while that found in the regressive shale is typically non-fluorescent. Data from pyrolysis-gas chromatography confirm these observations. These differences are apparently related to the contrasting depositional styles that were active on the shelf during regression and subsequent transgression. It is suggested that data from organic petrology and organic geochemistry provide greater resolution in sedimentologic and stratigraphic interpretations, particularly when working with basinward, fine-grained sediments. Petroleum source potential for the regressive Lower Mancos Shale below the Tocito Sandstone is poor. Based on abundant fluorescent amorphous material, high hydrogen indices, and high total organic carbon, the transgressive Upper Mancos Shale above the Tocito Sandstone possesses excellent source potential. This suggests that appreciable source potential can be found in offshore, fine-grained sediments of the transgressive systems tract below the condensed section and associated downlap surface. Organic petrology can be used to accurately predict petroleum source potential. The addition of simple fluorescence microscopy greatly enhances this predictive ability because non-generative amorphous material is generally non-fluorescent. Organic petrology must also be used to properly evaluate the utility of T max from programmed pyrolysis as a thermal maturity indicator. Organic matter dominated by autochthonous amorphous protistoclasts exhibits lower T max values than that which is composed of mostly phytoclasts.

Sequence Boundary Modification by Submarine Erosion, Upper Cretaceous, San Juan Basin

Unconformities within shallow-marine depositional systems are commonly interpreted as exclusively a result of erosion during falling relative sea level. However, significant submarine erosion also occurs during transgressions and is a complicating factor in the interpretation of these unconformities. A major unconformity present in the latest Turonian and Coniacian regressive-transgressive cycle in the northwestern New Mexico, separates the Gallup Sandstone (HST) from the overlying Tocito Sandstone (TST and rarely LST). The basinward portion of the unconformity contains no evidence of subaerial exposure and is contained entirely within marine strata. Also, the associated lacuna decreases paleolandward. Above the unconformity, although there is addition of coarser sand grains, overlying strata contain concretions, glauconite and phosphate nodules, and are more heavily burrowed than underlying strata. These relationships suggest that the preserved unconformity reflects shoreline transgression and a decrease in the rate of allochthonous sedimentation. The stratigraphic relationships are interpreted in the following manner. HST of the Gallup Sandstone prograded from southwest to northeast forming a series of wave-dominated shorelines. A subsequent fall in relative sea level resulted in a forced regression and the formation of coarse-grained delta 'plumes' (some Tocito sandbodies) and inferred lowstand deltas. Sea-level rise, following lowstand, resulted in regional transgression. Erosionmore » of previously deposited strata was accomplished by both shoreface erosion (ravinement) and inner shelf erosion during the initial stages of this transgression.« less

Organic Matter Variations in a Depositional Sequence: Implications for Use of Source Rock Data in Sequence Stratigraphy

Variations in the amount and type of organic matter preserved in shelf sediments are predictable within a sequence stratigraphic framework. These variations can be documented using routine petroleum source rock evaluation techniques. The type and preservation of organic matter is related to the rate of allochthonous shelf sedimentation. Each depositional systems tract in a sequence has a distinct depositional style that affects changes in the amount of terrigenous influx. Therefore, integration of source rock data with sedimentologic and regional stratigraphic results provides greater resolution in locating the critical surfaces that bound depositional systems tracts within the depositional sequence. This integrated sequence stratigraphic approach has been applied to conventional cores of Upper Cretaceous strata in the San Juan basin of New Mexico. Fine-grained marine sediments in transgressive systems tracts possess high total organic carbon and yield relatively high amounts of hydrocarbons during pyrolysis. Petrographically, this organic matter is composed primarily of amorphous macerals. Both the lower Juana Lopez and the Mancos Shale above the Tocito Sandstone provide examples of organic matter deposited in the transgressive systems tract. In contrast, marine depositional systems of both the lowstand and highstand systems tracts contain less total organic carbon and less pyrolyzable hydrocarbons. This methodologymore » not only improves systems tract identification in fine-grained, basinward intervals but also reveals those portions of the depositional sequence that are most favorable for marine petroleum source rock formation.« less

Characterization of coal-derived hydrocarbons and source-rock potential of coal beds, San Juan Basin, New Mexico and Colorado, U.S.A.

Coal beds are considered to be a major source of nonassociated gas in the Rocky Mountain basins of the United States. In the San Juan basin of northwestern New Mexico and southwestern Colorado, significant quantities of natural gas are being produced from coal beds of the Upper Cretaceous Fruitland Formation and from adjacent sandstone reservoirs. Analysis of gas samples from the various gas-producing intervals provided a means of determining their origin and of evaluating coal beds as source rocks. The rank of coal beds in the Fruitland Formation in the central part of the San Juan basin, where major gas production occurs, increases to the northeast and ranges from high-volatile B bituminous coal to medium-volatile bituminous coal ( R m values range from 0.70 to 1.45%). On the basis of chemical, isotopic and coal-rank data, the gases are interpreted to be thermogenic. Gases from the coal beds show little isotopic variation ( δ 13C 1 values range −43.6 to −40.5 ppt), are chemically dry (C 1/C 1–5 values are > 0.99), and contain significant amounts of CO 2 (as much as 6%). These gases are interpreted to have resulted from devolatilization of the humic-type bituminous coal that is composed mainly of vitrinite. The primary products of this process are CH 4, CO 2 and H 2O. The coal-generated, methane-rich gas is usually contained in the coal beds of the Fruitland Formation, and has not been expelled and has not migrated into the adjacent sandstone reservoirs. In addition, the coal-bed reservoirs produce a distinctive bicarbonate-type connate water and have higher reservoir pressures than adjacent sandstones. The combination of these factors indicates that coal beds are a closed reservoir system created by the gases, waters, and associated pressures in the micropore coal structure. In contrast, gases produced from overlying sandstones in the Fruitland Formation and underlying Pictured Cliffs Sandstone have a wider range of isotopic values ( δ 13C 1 values range from −43.5 to −38.5 ppt), are chemically wetter (C 1/C 1–5 values range from 0.85 to 0.95), and contain less CO 2 (< 2%). These gases are interpreted to have been derived from type III kerogen dispersed in marine shales of the underlying Lewis Shale and nonmarine shales of the Fruitland Formation. In the underlying Upper Cretaceous Dakota Sandstone and Tocito Sandstone Lentil of the Mancos Shale, another gas type is produced. This gas is associated with oil at intermediate stages of thermal maturity and is isotopically lighter and chemically wetter at the intermediate stage of thermal maturity as compared with gases derived from dispersed type III kerogen and coal; this gas type is interpreted to have been generated from type II kerogen. Organic matter contained in coal beds and carbonaceous shales of the Fruitland Formation has hydrogen indexes from Rock-Eval pyrolysis between 100 and 350, and atomic H:C ratios between 0.8 and 1.2. Oxygen indexes and atomic O:C values are less than 24 and 0.3, respectively. Extractable hydrocarbon yields are as high as 7,000 ppm. These values indicate that the coal beds and carbonaceous shales have good potential for the generation of liquid hydrocarbons. Voids in the coal filled with a fluorescent material that is probably bitumen is evidence that liquid hydrocarbon generation has taken place. Preliminary oil-source rock correlations based on gas chromatography and stable carbon isotope ratios of C 15+ hydrocarbons indicate that the coals and (or) carbonaceous shales in the Fruitland Formation may be the source of minor amounts of condensate produced from the coal beds at relatively low levelsof thermal maturity ( R m =0.7).

Stratigraphic Significance of Organic Matter Variations--Example from Mancos Shale and Tocito Sandstone, San Juan Basin, New Mexico: ABSTRACT

Investigation of the Upper Cretaceous (Coniacian) Mancos Shale and Tocito Sandstone has shown that both the amount and type of preserved organic matter varies in a predictable way according to depositional systems. Core and outcrop samples from the Mancos Shale above and below the Tocito Sandstone as well as mudstone interbeds within the Tocito were subjected to organic petrology and programmed pyrolysis measurements. Sedimentological studies have suggested that the Tocito was deposited as a transgressive shelf sand ridge. This analysis of the organic matter is consistent with that observation. The inferred transgressive shales above the Tocito Sandstone are much higher in both total organic carbon (TOC) and hydrogen index (HI) than regressive shales below. Kerogen in these transgressive shales is dominantly marine algal in origin (fluorescent amorphous protistoclasts), while the regressive shales below the Tocito possess considerable amounts of terrestrially derived kerogen (phytoclasts). Mudstone interbeds in the Tocito contain a mixed kerogen assemblage of mostly amorphous protistoclasts with minor amounts of amorphous structured and poorly preserved phytoclasts. This assemblage is a consequence of partial incorporation of underlying regressive shelf deposits into the transgressive ridge complex. Because the regressive Mancos Shale below the Tocito exhibits low TOC values (0.49-1.41%), low HImore » (159-255 mg/g C), and sparse fluorescence, little hydrocarbon source potential is expected. Transgressive shales above the Tocito possess sufficiently high TOC (2.14-5.29%), HI (353-623 mg/g C), and fluorescence to be considered potential hydrocarbon source rocks.« less

Sedimentary Architecture of Marginal and Shallow Marine Depositional Systems Tracts in Upper Cretaceous Series of San Juan Basin: ABSTRACT

The Upper Cretaceous series of the San Juan basin consists of five (or more ) depositional sequences bounded partially by subaerial unconformities and partially by correlative, basinward conformities. Three depositional systems tracts account for nearly all the preserved strata: highstand, lowstand, and transgressive. At relative sea level highstand, sediments are accommodated at the most landward sites within the basin (e.g., early Gallup Sandstone tongues). This systems tract is characterized by prograding bay-head deltas and successive open-marine deltas. During the ensuing sea level fall, much of the highstand system is removed by subaerial erosion. At relative sea level lowstand, sediments accumulate at the most basinal sites in the form of prograding strand plains, associated shelf sand sheets and plumes, and a minor amount of fluvial channel and overbank deposits. During transgression the coastal plain is aggrading and hence traps most terrigenous clastics before they reach the open coast. The attendant erosional retreat of the shoreface, however, releases coarse sediment (sand) to the shelf, where it accumulates in large-scale shelf sand ridges. These ridges are the principal petroleum reservoirs in the Cretaceous rocks of the basin (Tocito Sandstone Lentil, parts of the Twowells Tongue of the Dakota Sandstone, etc.). During the transgressionmore » a carbonate-dominated condensed section encroaches landward, stratigraphically above the transgressive depositional systems. This surface attains its maximum landward extent some time between peak rate of sea level rise and highstand. The three principal surfaces of the depositional sequence - the sequence boundary, the transgressive ravinement surface, and the condensed section - all have clearly defined sedimentological characteristics.« less

Architecture of a Transgressive Systems Tract: Tocito Sandstone, New Mexico: ABSTRACT

Architecture of a Transgressive Systems Tract: Tocito Sandstone, New Mexico: ABSTRACT

Revised Nomenclature of Mesaverde Group in San Juan Basin, New Mexico

Mesaverde group is substituted for Mesaverde formation throughout the San Juan basin, and the formations of the type locality, the Point Lookout sandstone, Menefee formation, and Cliff House sandstone, are also extended throughout the basin. Several names for units formerly called members of the Mesaverde formation in the southern part of the basin are retained as names of tongues or members of the formations of the Mesaverde group. The name Gallup sandstone replaces the name Tocito sandstone lentil of the Mancos shale. Crevasse Canyon formation of Allen and Balk (1954) is accepted for that part of the Mesaverde group between the Gallup and Point Lookout sandstones, with the Gibson coal member (restricted) at its top. A new name, the Cleary coal member of the Menefee form tion, is proposed for the beds formerly included in the upper part of the Gibson coal member of the Mesaverde formation.

The Doswell Oil Field, Rio Arriba County, New Mexico: ABSTRACT

The Doswell oil field lies near the geographic center of the San Juan basin, in the northern portion of Township 26 North, Range 6 West, Rio Arriba County, New Mexico, approximately 42 miles End_Page 962------------------------------ southeast of the town of Farmington. The field was discovered by the late Thomas W. Doswell and the late Todd M. Pettigrew and is now controlled by Lowry et al. Previous to the discovery, two non-commercial tests were drilled into the Morrison formation. The discovery well, Doswell-Pettigrew No. 2 Scott-Federal, NW/4 SE/4 of Section 9, Township 26 North, Range 6 West, was completed in July, 1951. It was drilled to a total depth of 6,700 feet, through the Tocito, and completed for an initial production of 404 barrels of oil daily, flowing through 12/64-inch choke. Average daily production from the No. 2 Scott-Federal to January 1, 1952, was 390 barrels of oil, and the Lowry et al. No. 4-13-132 Federal, the second producer, NE/4 NE/4 of Section 9, Township 26 North, Range 6 West, has averaged 186 barrels of oil daily since completion. Cumulative production of the field to January 1, 1952, was 64,054 barrels of oil. The limits of the field have not been defined; however, one mile south and updip from the discovery well, the Doswell-Pettigrew No. 1 State found the Tocito sandstone to be tight and shaly. The Doswell field probably represents a stratigraphic trap with no apparent structural uplift; however, subsequent drilling may indicate the presence of a structural trap, and sand conditions are the determining factor for oil accumulation. The producing formation, the Tocito sandstone lentil of the Mancos formation of Upper Cretaceous age, has been correlated with the Gallup sandstone, a tongue of the lower Mesaverde formation. The Tocito sandstone of marine deposition lenticular, and cross-bedded, has good development in the upper and fair development in the lower part. End_of_Article - Last_Page 963------------